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“Are there lexicons?” A study of lexical and semantic processing in word-meaning deafness suggests “yes” Tobias Bormann* and Cornelius Weiller Neurologische Universita¨tsklinik Freiburg, Germany
article info
abstract
Article history:
A controversial issue in the cognitive neuroscience of language is the question whether
Received 17 February 2011
independent lexical representations need to be included in cognitive models. Recent
Reviewed 23 May 2011
models claim to account for the available data without including phonological or ortho-
Revised 1 June 2011
graphic lexicons. These models base their lexical decision (“Is it a word or not?”) either on
Accepted 3 June 2011
familiarity of the input string or alternatively, on semantic information. These two alter-
Action editor Roberto Cubelli
natives were evaluated in a series of experiments with an individual suffering from word-
Published online 20 June 2011
meaning deafness. This is a rare disorder of auditory word comprehension which affects mapping of a word’s phonology to its meaning. The participant, BB, was unaffected by the
Keywords:
‘word-likeness’ of nonwords with comparable accuracy for plausible and abstruse
Word-meaning deafness
nonwords. She was further able to make lexical decisions despite her severe impairment in
Auditory comprehension
comprehending the word’s meaning. Lexical and semantic processing were assessed on an
Lexical decision
item-specific basis providing a methodological advancement over previous studies. The
Lexicon
comprehension tasks involved word-picture matching as well as definition tasks. The
Meaning
results suggest that BB’s lexical decisions are based neither on familiarity of the input string nor on semantic information, which was largely unavailable. The only alternative are lexical representations on which she could base her decisions. ª 2011 Elsevier Srl. All rights reserved.
1.
Introduction
Individuals with neurological impairments may present with strikingly selective, often unexpected cognitive deficits. Often, their impairment can be described in the framework of a model of normal cognition. On the other hand, the impairments of these subjects may provide important constraints for models of normal cognitive processes. That means, a cognitive model needs to be able to explain an individual’s behavioral symptoms by means of ‘functional lesions’ within its architecture. One example of such a ‘functional syndrome’, albeit an extremely rare one, is the condition known as “word-meaning
deafness”. This is a striking impairment of auditory word comprehension in the face of intact hearing and sound processing. Even more peculiarly, pre-semantic phonological processing is preserved. In the most impressive reports on this disorder, subjects exhibited unimpaired pre-lexical phonological processing, lexical decision, repetition, and writing to dictation (Bramwell, 1897; Franklin et al., 1996; Hall and Riddoch, 1997). Rather than simply assigning these individuals an aphasic syndrome (e.g., “Transcortical-sensory aphasia” in case of those subjects with fluent speech), a cognitive view on their impairment locates the functional deficit prior to semantic access but subsequent to phonological processing (Kohn and
* Corresponding author. Neurologische Universita¨tsklinik Freiburg, Breisacher Street 64, D-79106 Freiburg, Germany. E-mail address:
[email protected] (T. Bormann). 0010-9452/$ e see front matter ª 2011 Elsevier Srl. All rights reserved. doi:10.1016/j.cortex.2011.06.003
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Friedman, 1986). The disorder has to be distinguished from ‘word-sound deafness’ (i.e., impaired phonological processing; Auerbach et al., 1982; Saffran et al., 1976) and ‘word-form deafness’ in which pre-lexical processing is preserved yet access to the phonological lexicon is affected (Howard and Franklin, 1988). Few case reports on word-meaning deafness are available in the literature (Bramwell, 1897; Franklin et al., 1994; Franklin et al., 1996; Hall and Riddoch, 1997; Heilman et al., 1981; Kohn and Friedman, 1986; Tyler and Moss, 1997; Ziehl, 1896). The cognitive impairment is viewed as involving the mapping of a correctly processed phonological stimulus to semantic representations corresponding to the word’s meaning (e.g., Franklin et al., 1994, 1996; Kohn and Friedman, 1986). The cognitive impairment in ‘word-meaning deafness’ is of central relevance for models of auditory word processing. For many years, a cognitive model has been ‘common sense’ in neuropsychology and neurolinguistics which incorporates at least three stages in word processing: Pre-lexical phonological processing, lexical access, and semantic processing (e.g., Caplan, 1992; Franklin, 1989; Morton, 1969; Patterson and Shewell, 1987). These models distinguish between a level of pre-lexical phonological processing which serves as input to a phonological input lexicon which, in turn, is independent from representation of a person’s semantic knowledge. The mental lexicon is conceived of as a permanent storage of words that a person knows with one entry per word (e.g., Coltheart, 2004). Since each word matches an individual entry in the lexicon, this position can be viewed as “localist”. Recently, however, a number of researchers have promoted cognitive models of language processing without an independent lexicon. This has been done in the field of reading aloud (Dilkina et al., 2008; Plaut et al., 1996; Seidenberg and McClelland, 1989; Woollams et al., 2007), visual lexical decision (Dilkina et al., 2010; Plaut, 1997), naming (Lambon Ralph et al., 2000), short-term memory (Patterson et al., 1994), writing (Jefferies et al., 2007), verb morphology (Joanisse and Seidenberg, 1999; Patterson et al., 2001), visual object processing (e.g., Rogers et al., 2004) and syntactic processing (e.g., Elman, 2009). The majority of studies has been carried out in the area of reading but all these models share the basic assumption that processing of linguistic (or visual) stimuli may be explained by processing of surface information (phonemes, graphemes, visual details) in close interaction with semantic information. Processing of regular or typical stimuli (e.g., words with regular, unambiguous pronunciation or spelling; objects with more typical features) is carried out ‘at the surface’ with little support from semantic representations while atypical and exceptional stimuli require support from a second set of, usually semantic, representations (Patterson et al., 2006; Rogers et al., 2004). Advocates of these models have presented a wide array of findings in support of their conclusions. The majority of studies has been carried out with participants with semantic dementia. This is a progressive selective impairment of semantic knowledge (Snowden et al., 1989) with initially spared processing of ‘surface information’. Early on, however, subjects begin to exhibit difficulties on supposedly ‘non-semantic’ tasks such as lexical decision, object decision, verb past tense generation and others. Patterson et al. (2006) studied individuals with semantic dementia with a series of tasks including reading aloud, visual
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lexical decision, writing to dictation, verb past tense generation, object decision, and delayed copying of object drawings. Performance on the ‘atypical’ stimuli in every single task was impaired and correlated with scores on tests of semantic knowledge suggesting semantic input to be critical for processing irregular stimuli in the different domains. The semantic input in these tasks is considered to be direct and not mediated by independent lexical representations. Phonology, orthography and semantics are also considered “primary systems”, thus this family of accounts is sometimes summarized under the label of the ‘primary systems’ account (Jefferies et al., 2007). We will use the terms “primary systems” and “distributed models” interchangeably. With respect to auditory word processing, a number of computational models have been implemented and published. Gaskell and Marslen-Wilson (1997) as well as Plaut and Kello (1999) presented connectionist models of phonological processing free of independent localist lexical representations. Gaskell and Marslen-Wilson’s (1997) model consists of a layer of phonetic input units which, in parallel, map activation onto semantic units and phonological units. In Plaut and Kello’s (1999) model, the phonetic input maps onto semantic units via phonological units. From a theoretical point of view, these models are more parsimonious than those cognitive models incorporating lexical representations, and if they are able to account for the available data they are to be preferred. These models, however, have been found to have problems simulating quite an easy, common task in cognitive studies, the “lexical decision” task. In this task, subjects ‘simply’ judge whether a given stimulus is a word of their language or not. People are quite accurate in this task. The lexical decision task can easily be explained in models with lexical representations: A stimulus either leads to the activation of an entry which is compatible with the input in which case the subject’s response is “yes”, or the stimulus does not match an entry in the lexicon in which case, probably after a certain deadline, the subject’s response will be “no”. In contrast, the lexical decision task is more difficult to simulate in models without independent lexical representations. Since these models cannot base their decisions on lexical representations in their architecture, the decision has to be based on other representations. In the domain of visual lexical decision, Seidenberg and McClelland (1989) implemented a decision mechanism based on the familiarity of the orthographic input or the phonological output, and their model, by this means, could reject unfamiliar nonwords. An alternative way to implement the lexical decision is to base it on semantic information, and this has been the preferred mechanism (already discussed by Seidenberg and McClelland, 1989). For example, Plaut (1997) argued that (visual) lexical decision was based upon semantic information in his model, and Plaut and Kello (1999) referred to this work in their treatment of phonological processing. A similar approach has been taken by Gaskell and Marslen-Wilson (1997) who suggested that lexical decision in their model was based on output from semantic units. Note, though, that these authors also consider alternative ways to make lexical decisions. The use of semantic representations to support lexical decisions was motivated because of the findings of Besner et al. (1990) and Fera and Besner (1992). Besner et al. (1990) argued that in
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a lexical decision task, the original connectionist model had a rate of false alarms much higher than human subjects. In addition, Fera and Besner (1992) observed that subjects were unaffected (both in accuracy and latency) by the degree of overlap between words and nonwords while the connectionist networks were: If the nonwords were more ‘word-like’, the model’s accuracy in the lexical decision dropped considerably (cf. Plaut, 1997). Abandoning a lexical layer from cognitive models, on the other hand, has been quite a radical step and has triggered responses from other cognitive researchers who have argued in favor of independent lexical representations and sets of syntactic or morphological rules (e.g., Coltheart, 2004, 2006; Coltheart et al., 2010b; Miozzo, 2003; Miozzo and Gordon, 2005; Tyler et al., 2004). With respect to word-meaning deafness, the disorder which is investigated here, the relevance is obvious: Lexical decision is possible while semantic access is impaired. This can be explained by assuming that these individuals access their phonological input lexicon normally despite their considerable difficulties in processing the meaning of the auditorily presented stimuli. The present study lends further support to the view that lexical decision can be made independently of semantic access. Any such study, including well-documented single-case studies, is justified for several reasons: As people with so selective an impairment are reported rarely, each individual case study is of particular relevance. A single-case study of a rare disorder carries more weight than studies of more frequent phenomena, for example, agrammatic sentence comprehension. Single-case studies are even more relevant since several questions in the field of cognitive processing will not be resolved by functional imaging studies or behavioral studies with unimpaired subjects. In several areas, patient data is the only available evidence, and the question of whether independent lexical representations exist is particularly dependent upon cognitive case studies. There are further reasons for doing single-subject studies: Advocates of the primary systems account have countered the presentation of deviant single-case reports mainly with two different arguments: First, they argued that these data may constitute statistical extremes of a normally distributed phenomenon. That means, these results did not constitute qualitatively different performance but extreme statistical variation (Grodzinsky et al., 1999; Patterson et al., 2006). Second, it has been argued that single-case studies are more difficult to evaluate because different studies employed different materials and tasks which makes comparison of the studies more difficult. For example, Woollams et al. (2007) argued that Blazely et al. (2005) had employed rather unusual nonwords in their lexical decision task. Their subjects with semantic impairment were, thus, able to make lexical decisions independently of their semantic knowledge. (Note, however, that one subject still exhibited impaired lexical decision). These objections to the single-case method have been discussed previously (e.g., Caplan, 2001; Coltheart, 2006). They may be countered first, by providing a large enough data base, that is, repeated observations with large enough item numbers. If above-chance performance on a specific task, which is not predicted by a specific cognitive model, is observed repeatedly, the argument that each individual
finding was the result of statistical factors is rendered implausible. Second, if the same items are used in both lexical decision and comprehension tasks, the chance is minimized that the materials used in the different tasks had a major influence upon the results. This, on the other hand, would make it necessary to include subjects for whom comprehension might be assessed easily and with standard comprehension tasks. Previously reported subjects with word-meaning deafness had specific difficulties processing abstract words (Franklin et al., 1994, 1996). However, in these cases it is more difficult to assess comprehension since abstract words cannot be included in word-picture matching task. If, however, the comprehension impairment affected concrete words, this could be assessed in standard wordpicture matching tasks. This was the case for our subject BB. Therefore, our study will provide a number of methodological advances and will go beyond previous case reports: First, we will compare easy and difficult lexical decisions. Varying the degree of word and nonword overlap, however, is a critical test of those connectionist models which base lexical decision on familiarity of the input stimulus (e.g., Seidenberg and McClelland, 1989). As discussed above, lexical decisions based upon familiarity of the input string should be affected if nonwords exhibit similarity to words (cf. Fera and Besner, 1992). In addition, it has been demonstrated for healthy subjects that in a lexical decision task in which word-like nonwords were included effects of concreteness may emerge (e.g., James, 1975). Furthermore, our study will assess lexical decision and comprehension on an item-specific basis. It is, therefore, easier to compare performance on these two tasks. If it could be demonstrated that an individual could make lexical decisions for the same set of words for which he or she exhibited severe comprehension impairments, the objection that different materials had been used does not apply (Blazely et al., 2005). In addition, in classical cognitive neuropsychological reasoning, “word-meaning deafness” could have two underlying functional impairments: It could, first, consist of a permanent disconnection between the phonological input lexicon and semantic representations or alternatively, it could be an access disorder in which case we might expect inconsistent performance across different trials (e.g., Crutch and Warrington, 2005; Warrington and McCarthy, 1983; Shallice, 1987, 1988; see Rapp and Caramazza, 1993, and Caplan, 1992, for critique). Although this argument applies less to studies employing large numbers of stimuli (Franklin et al., 1996), assessing lexical decision and comprehension at an item-specific basis offers a better control in case the deficit is one of accessing semantic representations. The outline of the present article is as follows: We shall first present the background information on our subject, BB, with whom we have been working for several months. We shall pay particular attention to her ability to process auditory stimuli at different levels, that is, pre-lexical phonological processing, lexical access and comprehension at the semantic level. We will then report four experiments which assessed in detail her ability to judge the lexical status of phonological stimuli and to process the meaning of the same words.
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2.
Methods
2.1.
Medical history
BB was 44-year-old (born 1965) when she was admitted to the hospital because of difficulties to breath in May 2009. Because of suspected posterior wall infarction, a coronary angiography was performed. During this intervention, BB suddenly exhibited severe aphasia and a weakening of the right arm. She was admitted to the Department of Neurology, Freiburg where an acute embolic stroke affecting the left middle cerebral artery (MCA) as well as a subacute embolic stroke of the right MCA was diagnosed. High resolution MR scans from six months after her initial admission show left parieto-temporal lesions and right parietal lesions along with a lesion to the left thalamus (Fig. 1).
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Following admission to the intensive care unit (ICU) of the Department of Neurology, she suffered from renal failure. The diagnosis of unspecific collagenosis was made. BB had to stay at the ICU for three weeks. Six months later (November 2009), BB suffered a sudden anarthria which led to the second admission to our department. The reason was, most likely, a seizure. During her stay, she was brought to the attention of the authors because of her severe aphasia. A brief neurolinguistic evaluation was carried out. She initially had profound anomia (2/20 picture naming correct) along with severe comprehension impairments and poor repetition. Her digit span was one though repetition of even a single digit was occasionally impossible. Auditory lexical decision was good (38/40 correct). Semantic comprehension of spoken words was severely impaired while comprehension of the very same words from visual input was preserved. The Token Test could not be
Fig. 1 e Scans showing BB’s lesions.
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administered because of BB’s severe impairment in following even short commands (0/10 correct). The present study was begun nine months post-onset. Impairments remained unchanged between November 2009 and a follow-up eleven months later. In all tasks assessing auditory language processing and repetition, the experimenter’s mouth was covered with a sheet of paper to prevent lip reading.
2.2.
Neuropsychological assessment
A brief neuropsychological assessment, carried out in May 2010, revealed slow performance on the Trail Making Test A (82 sec, percentile rank 1). BB complained about hypesthesia in her right arm, and a clumsiness of the right hand was noted. Block tapping span forward was five blocks, backward span was four blocks. On the Tower of London-test of planning/ problem solving (Tucha and Lange, 2004), she was correct on 14 of 20 trials corresponding to normal performance (percentile rank 30). Her nonverbal episodic memory (Diagnosticum fu¨r Cerebralscha¨digung, DCS; Lamberti and Weidlich, 1999) was in the normal range with seven of nine patterns recalled after the fifth learning trial (percentile rank 30) and a total of 23 correct patterns across five learning trials (percentile rank 30). In the block design subtest of the Wechsler Adult Intelligence Scale (WAIS), her raw score was 33 which corresponds to average performance. BB was mildly apraxic.
2.3.
Assessment of language functions
2.3.1.
Naming
Subtest 30 (“Oral naming”) of the neurolinguistic assessment battery “Lexikon modell-orientiert (LeMo)” of De Bleser et al. (2004) was administered. BB was correct for 12 out of 20 targets. Errors consisted of two phonemic errors, two formal errors, two circumlocutions, one perseveration of a previous response and one omission. Three errors occurred for high-frequency words, five errors occurred for low-frequency targets. In a set of 80 pictures taken from Bormann et al. (2008), BB was correct on 38 targets, produced eight semantic errors, two mixed errors (formally and semantically related to the target word), three formal errors, 18 phonemic errors and failed on eleven items.
2.3.2.
Nonverbal semantic processing
Nonverbal semantic processing was studied with a variety of different tests. BB was unimpaired in all five subtests of the Bogenhausener Semantik-Untersuchung (BOSU; Glindemann et al., 2002). In subtest 1, BB was unimpaired in identifying one out of four pictures not matching the sketch of an everyday event (e.g., a kitchen scene; pan, pot, stove, comb). In subtests 2 and 3, performance was in the unimpaired range when identifying the odd-one-out from four pictures (10/10 and 9/10 correct, respectively). In subtest 4, she was able to identify the odd-one-out from four printed words (10/10 correct; unimpaired performance). In subtest 5, she correctly identified the correctly colored object from a choice of four colored objects (e.g., a crocodile printed in green, blue, yellow and red; 10/10 correct). In subtest 12 (“Association Match”) of the Birmingham Object Recognition Battery (BORB; Riddoch and Humphreys, 1993), she performed without error (30/30 correct). BB was also unimpaired on the picture version of the Pyramids and Palm
Trees Test of Howard and Patterson (1992) where she scored 51/ 52 correct. Her only error was on the rocket item which we have found ambiguous even for healthy individuals. Overall, BB’s semantic processing was preserved. In all tests of nonverbal semantic processing, her performance was within the normal range. Additional support for BB’s preserved semantic processing comes from her unimpaired performance on tasks involving written stimuli (see below). This is critical evidence given than BB’s condition involves impaired matching of an auditory stimulus to semantic representations. The diagnosis of “word-meaning deafness” depends upon the intact comprehension of written and picture stimuli, otherwise a more general impairment of semantic processing would be the most likely impairment (cf. Hillis et al., 1990).
2.3.3.
Repetition and short-term memory
Repetition of digits yielded a digit span of one item if the digits between 1 and 9 were used. However, BB occasionally made errors even with single digits. She was further severely impaired repeating single numbers higher than 10 (i.e., digits between 11 and 30) where she made several errors. In 30 trials of two digits (single digits only), BB was correct on seven trials. In 16 trials, she was able to repeat one digit, usually the initial one (n ¼ 15). Repeating two one-syllable words was almost impossible. She was correct on one pair of phonologically unrelated words (1/10 trials correct) and was unable to get any pair of phonologically related words correct (0/10 correct). If only one word could be reproduced, it was usually the first one. Repetition of sentences was almost impossible even for three-word sentences (e.g., “The lion roars” repeated as “The lion . don’t know”). BB was correct on two out of ten sentences. Repetition of single words was also impaired. BB’s difficulties were those reported for individuals with deep dysphasia: Nonword repetition was next to impossible (1/40 correct) with errors being phonologically related or unrelated neologisms. Repetition of single words was assessed with “LeMo” subtests 9 and 13. Subtest 9 controls for frequency and concreteness and revealed severe difficulties: BB was correct on nine out of 40 items. There was an effect of frequency [9/20 vs 0/20 correct; c2(1)¼11.61, p < .01] and a marginally significant effect of concreteness [7/20 concrete vs 2/20 abstract words correct; c2(1)¼3.58, p < .06]. “LeMo” subtest 13 controls for word class (nouns, adjectives, function words). BB was correct on 6/30 nouns, 8/30 adjectives and 7/30 function words (21/90 correct). Her errors consisted of 40 failures to respond, seven formally related lexical errors, nine semantic errors (including three function word substitutions, Table 1), six perseverations, three phonemic errors, three unrelated lexical responses and one mixed error [formally and semantically related, “du¨rr” e “du¨nn” (skinny e slim)]. In a further comparison of 24 high-frequency concrete nouns versus 24 function words, BB was correct on 20 nouns versus 16 function words [c2(1) ¼ 1.78, n.s.]. There was no evidence of abnormal decay of the phonological trace as the cause for BB’s deep dysphasia (cf. Ablinger et al., 2007): If single digits had to be repeated after a delay of 4 sec, performance was comparable to immediate repetition (immediate repetition: 16/18 correct; delayed repetition: 15/18 correct). Likewise, she was correct on 16/30 words which she could repeat without delay and achieved 18/30 correct
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Table 1 e BB’s lexical and semantic errors in single word repetition. Target
BB’s response
Strick (rope) Sohn (son) Wein (wine) Roh (raw) Zar (czar) Schnur (cord) wir (we) zu (to) an (on)
➔ ➔ ➔ ➔ ➔ ➔ ➔ ➔ ➔
die Rolle (the cabin roll) Mann (man) Wasser (water) Fleisch (meat) Burg (fortress) Rolle (cabin roll) sich (him/herself) sich (him/herself) den (theacc.)
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the initial stimulus was lost when the second stimulus appeared. Therefore, the tests were repeated with one repetition of stimuli granted. This resulted in performance in the lower unimpaired range (nonwords: 67/72 correct; words: 70/72 correct).
2.3.5.2. AUDITORY
LEXICAL DECISION (LEMO SUBTEST 5). A word or nonword was read to BB who had to decide whether the stimulus was an existing German word or a nonword. Again, the mouth of the experimenter was covered during this task. No repetition of the stimulus was granted. BB performed in the unimpaired range (77/80 correct, 97%; unimpaired range 73e80).
2.3.5.3. WORD-PICTURE MATCHING. On the formal comprehension
responses on the same set after a delay of 4 sec. There was one week between both administrations. Overall, BB exhibited a severe repetition impairment. Her verbal short-term memory span was limited to one digit though repetition of even single digits was highly unreliable. Repetition of word pairs proved impossible for her, repetition of single words was characterized by effects of concreteness and frequency with the majority of errors being omissions, formally related or semantic errors (e.g., “raw” e “meat”). Nonword repetition was close to impossible. This pattern of performance matches the description of deep dysphasia (e.g., Ablinger et al., 2007; Jefferies et al., 2007; Katz and Goodglass, 1990). It is, most likely, the result of an additional impairment of the retention of phonological information since frequently BB would be able to repeat words she did not comprehend and vice versa.
2.3.4.
Reading and writing
Reading of 30 words (15 regular, 15 irregular) resulted in 14 correct responses, three formally related lexical responses, twelve phonemic errors and one failure to respond. There was no difference between stimuli with regular and irregular graphemeephoneme correspondence. Of 18 nonwords, BB read five correctly, produced lexicalizations for five stimuli and eight phonemic errors of which six differed from the target by one phoneme. Writing to dictation was affected by BB’s severe repetition deficit. Therefore, writing was assessed in a written naming task. Since, however, BB spoke the name of the object first, this task is not a pure task of written naming but includes phonological information from spoken output which may be used via sublexical phoneme-to-grapheme conversion mechanisms. Written naming (“LeMo” subtest 31) required the writing of the names of 20 objects the same which had been used in oral naming. BB was correct on eleven target words with five omissions and four incomplete responses with deletion of word-final letters.
2.3.5. Auditory and visual word comprehension 2.3.5.1. DISCRIMINATION OF WORDS AND NONWORDS (LEMO SUBTEST 1 AND 2). BB heard two stimuli (nonwords or words) from the “LeMo” assessment battery. She had to judge whether the stimuli sounded the same or different. She was correct on 55/72 nonwords (unimpaired range 66e72) and on 63/72 words (unimpaired range 66e72). Performance on both tests was in the mildly impaired range. Performance was, however, affected by her severe short-term memory impairment. BB complained that
task of Blanken (1999), the participant has to identify a spoken word from an array of three pictures. The names of the three objects are similar in sound. BB was correct on 142 of 150 trials (95%). On the comprehension test of “LeMo” (subtest 23), BB was correct on 15 out of 20 trials which were clearly below the unimpaired range (19e20 correct). Only concrete target words were used. If the same stimuli were included in a written version of this task (“LeMo” subtest 24), BB was unimpaired (20/20 correct). In a further comprehension task, the participant has to identify a picture from an array of six semantic categorymembers (Blanken, 1996). BB was correct on seven out of 20 trials (35% correct). No feedback on her choice was given. The same task was repeated seven days later with written stimuli. BB was correct on 18 of the 20 trials (90% correct) which was significantly better than her performance on the auditory version (McNemar test, p < .01). Word comprehension was further assessed with a variant of the Pyramids and Palm Trees test of Howard and Patterson (1992). Instead of a picture stimulus, a spoken or written word had to be matched to two associatively related pictures (e.g., the word “pyramid” had to be associated with the picture of a palm or a fir tree). The target words from the English version were translated to German. In one session, the two pictures were presented to BB, and the word was read to her. If required, one repetition was granted. In the written version of the test, the written stimulus was presented to her for about 2 sec and then covered. If necessary, the stimulus was presented a second time for about 2 sec. The two sessions were separated by two weeks. BB was correct on 34/52 trials when the word was spoken to her and on 51/52 trials, when the word was written. The difference as assessed by McNemar’s test was highly significant ( p < .01). The same significant different was between spoken target word and picture targets in the three picture version (51/52 correct, see above). The word semantic association test of the Psycholinguistic Assessment of Language Processing in Aphasia (PALPA, Kay et al., 1992) was translated and administered with spoken and written words. In this task, the subject has to identify a word closest in meaning in response to a stimulus word. There were 16 trials with highly imageable and 16 trials with low imageable words. Between the written and the spoken version lay seven days. With written input, BB was correct on 14 of 16 low imageable trials and on 16 of 16 trials for highly imageable stimuli. In contrast with spoken input, she was correct on 2 and 9 trials, respectively. The difference between the spoken and written version was highly significant (McNemar’s test, p < .01)
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as was the difference between low and highly imageable trials in the spoken modality [c2(1) ¼ 6.79, p < .01]. Finally, to assess the role of word frequency for comprehension of auditory words, two comprehension tasks were devised, a semantic and a formal comprehension task. In the semantic task, two lists were compiled with 22 high and 22 lowfrequency targets. These target words were read to BB once, and she then had to identify the target from an array of five pictures. The mean frequency of the high-frequency target was 133.9, the mean frequency of the low-frequency targets was 2.5. No effect of frequency was observed for auditory comprehension of single words. She correctly identified ten high-frequency target words and eight low-frequency target words [c2(1) < 1.0, n.s.]. Overall, she was correct on 18 of 44 trials in the spoken modality. When the same task was repeated with written stimulus words two weeks later, she was correct on 42 of 44 trials, a performance significantly superior to the spoken version (McNemar test, p < .01). No effect of frequency emerged in the written version of the test. A similar comparison was also carried out in a formal comprehension task in which BB had to identify a spoken word from three formally related pictures [e.g., “Huhn” (hen) from “Huhn (hen) Hut (hat) Hund (dog)”]. The 30 highfrequency targets in the formal comprehension task had a mean frequency of 655.9, the 30 low-frequency targets had a mean frequency of 4.1. BB was correct on 29 high-frequency targets (with two correct trials requiring a repetition of the stimulus) and on 27 low-frequency targets [c2(1) < 1.1, n.s.]. Although the targets had different frequency values and different numbers of distractors in the semantic and the formal comprehension task, word frequency did not affect BB’s comprehension of the stimuli in either task. To summarize, BB suffered from a severe impairment of comprehension when words were spoken to her. Pre-lexical phonological processing as well as lexical processing (lexical decision; word comprehension with formally related foils) was well preserved. Her difficulties were in the matching of a spoken word to its respective semantic representations. In all tasks employing semantically related distractor words, BB was severely impaired. According to Franklin (1989), ‘word-meaning deafness’ should be diagnosed in those individuals who have intact lexical decision and preserved comprehension of written words. This is exactly what BB exhibits: She has good abilities in phonological discrimination tasks irrespective of the stimuli being words or nonwords. Her severely impaired short-term memory span of roughly one item posed a serious obstacle for some of the matching tasks yet the ability to discriminate minimal pairs (words or nonwords) was preserved. Her performance was not affected by word frequency but was affected by a stimulus’ concreteness. This has been demonstrated in other subjects (Franklin et al., 1994). In addition, however, BB’s difficulties also affected concrete words.
3.
Experimental investigations
3.1.
Control subjects for experimental investigations
eight healthy subjects of comparable age and educational background served as controls (mean age: 49.0 years, range 43e59 years). The control subjects underwent all four experiments in a single session which lasted approximately 1 h. They were reimbursed by an allowance of 15 Euro for their participation. Controls’ performance will be reported for each experiment prior BB’s data. In the lexical decision tasks, we calculated d0 , a measure from signal detection theory, which intregrates hit rate and rate of false-positive responses and, thus, compensates for response bias. In some tasks, control subjects did not commit errors in which case d0 cannot be calculated. The tenth of an error on a single trial was included in the calculation of d0 for these subjects.
3.2.
Experiment 1: Easy versus difficult lexical decision
3.2.1.
Procedure and materials
There were two different versions of the lexical decision task with 30 words and 30 nonwords each. In the easy auditory lexical decision task, words had a frequency of over 100 based on Celex (mean 1287.3). Nonwords were generated from rare consonantevowel combinations with few orthographic neighbors (two stimuli had one neighbor each while the rest had none; mean: .07 neighbors). In the difficult version, word stimuli had a Celex frequency of 0 while the nonwords were generated from words with a single consonant exchanged. Thus, nonwords were word-like and had an average of 3.9 neighbors (as provided by WordGen; Duyck et al., 2004). Since German has a transparent orthography, orthographic neighbors provide a valid measure for a word’s lexical neighbors even if the task involved processing of spoken stimuli. Based on the observation from computational models, lexical decision should be harder with word-like nonwords (Fera and Besner, 1992). It could further be argued that there should be a difference between high and low-frequency words with the latter being much easier to identify. By combining word-like nonwords with very low-frequency words, this task should be difficult. The stimuli were recorded digitally and played to the subject who had to make an auditory lexical decision. No repetition of a stimulus was granted. The easy task was carried out before the difficult version of the task.
3.2.2.
3.2.3.
In the following, we shall assess BB’s lexical and semantic processing in closer detail in four experiments. A group of
Results
On average, controls had 58.0 correct on the easy version of the task (standard deviation 1.60) and 53.0 correct on the difficult version of this task (standard deviation 2.07). Mean d0 for control subjects were 4.28 and 2.48, respectively (standard deviations .59 and .39). The difference between the two tasks was highly significant (for correct responses: t ¼ 5.64, p < .01; for d0 : t ¼ 6.55, p < .01). BB performed perfectly (100% correct) on the easy task and correct on 58 of 60 trials of the ‘difficult’ auditory lexical decision task. There was one false-positive error (false alarm) and one miss (d0 ¼ 3.67). The difference in her performance on both tasks was not significant.
Discussion
The first experiment explicitly addressed the several of the computational models, namely lexical decision could be carried out based on surface information. Fera and Besner (1992)
prediction of that auditory familiarity of showed that
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normal subjects were, by and large, unaffected by making visual lexical decision more difficult. In contrast, the computational model of Seidenberg and McClelland (1989) exhibited difficulties when there was a large overlap between words and nonwords, and several researchers suggested that their models would use additional information, for example activation of semantic representations, as a basis for their decision (cf. Plaut, 1997; Gaskell and Marslen-Wilson, 1997). A similar observation to that of Fera and Besner (1992) was made in the present study which employed auditory lexical decision in the context of a single-case study. The subject, BB, exhibited a small, non-significant trend toward better performance on the easier task but performed in the normal range in both tasks. This suggests that she could base her lexical decision on the phonological code independently of semantic representations. If she had used semantic information for the difficult lexical decisions, she would have performed more poorly on the difficult decision task in which nonwords were highly similar to words and words were of extremely low frequency. The results do not support this hypothesis. Yet, the results from the healthy controls which demonstrate a highly significant difference between the two tasks, suggest that the manipulation of task difficulty was, indeed, successful. We can, thus, be confident that our use of low-frequency words and word-like nonwords made it difficult for the participants to make their auditory lexical decision. BB, however, who could not make use of semantic information, performed in the normal range on both versions of the task.
3.3.
Experiment 2: Lexical decision and comprehension
3.3.1.
Procedure
BB had to make an auditory lexical decision on a set of words (N ¼ 80) with the same number of nonwords. A week later, the same set of words was then used in an auditory comprehension task. In this task, BB had to make a word-picture matching task in which the target picture was presented along with two foils. In half of the trials (n ¼ 40), distractors were semantically related, in the other half (n ¼ 40) the distractors were formally related to the target word. BB also completed the word-picture matching tasks with written words a further week later.
3.3.2.
Results
In the lexical decision task, there were 160 decisions to make. On average, the controls had 155.5 correct (between 149 and 158 correct), d0 ranged between 3.17 and 4.48 with an average d0 of 4.01 (standard deviation .49). BB was correct on 79/80 words and 77/80 nonwords (156 correct in total; d0 ¼ 4.02). Her performance was significantly superior to chance performance. In the semantic comprehension task, the controls achieved an average of 39.0 (.93) correct responses on the semantic identification task and an average of 37.88 (.83) on the formal identification task. BB was correct on 31/40 stimuli (77.5%) of the semantic task and correct on 39/40 (97.5%) of the formal comprehension task. Her performance on the phonological matching task was significantly superior to performance on the semantic matching task [c2(1) ¼ 7.31, p < .01]. BB performed 100% correct (80/80 correct) on the written version of the word-picture matching task which is significantly superior to her performance after auditory presentation.
3.3.3.
301
Discussion
Experiment 2 replicates the observation from the background assessment and Experiment 1, namely that BB was unimpaired in lexical decision despite her severe comprehension impairment. She performed in the normal range on the lexical decision task with d0 in the normal range. She was further in the unimpaired range on the formal comprehension task in which she had to match a spoken word to one of three pictures with formally related foils. In contrast, her performance on the semantic task was dramatically impaired if compared to the controls’ average performance. Her performance was more than seven standard deviations below the controls’ mean. Controls had an average of one error while BB had an error rate of nine. Note that the semantic decision task was a one-out-of-three selection task with concrete target words which most aphasic subject find not very demanding. When comprehension for written stimuli was assessed seven days later, BB’s performance was much superior and significantly so. She achieved 100% correct responses on the matching tasks if the target words were presented for reading. This second experiment, along with the background assessment, replicates the study of Franklin et al. (1996) who demonstrated preserved lexical decision in the face of impaired comprehension at the semantic level in their subject. This strongly suggests that subjects with impaired access to a word’s meaning can be unimpaired in their ability to carry out a lexical decision. Clearly in BB, the lexical decision must have been based on information independent of the word’s meaning. This is readily explained by assuming lexical representations which are independent of semantic representations. One advantage of the present experiment is the use of the same set of stimuli for both the lexical decision and the comprehension tasks. It was, therefore, possible to document BB’s good lexical decision for exactly those stimuli which she did not comprehend reliably. Previous studies had employed different sets of materials for the lexical decision and the semantic comprehension tasks which makes it difficult to compare the subjects’ performance on both tasks. Using the same set of words in a lexical decision task and a word-picture matching task, however, was only possible because BB’s comprehension deficit affected concrete words and was not limited to abstract ones. This makes the present case special. Previous reports of comparable disorders documented pronounced deficits for abstract words which cannot be employed so easily in comprehension tasks. By using the same set of words in a written comprehension task closely matched to the auditory task (written presentation limited to 2 sec) and documenting BB’s perfect performance, we can show that her semantic processing per se was uncompromised. Since BB did not demonstrate improvements between November 2009 and July 2010, it is highly unlikely that her better performance on the written version of the task was due to improved semantic processing.
3.4. Experiment 3a: Lexical decision, then word-picture matching with semantic foils 3.4.1.
Procedure
BB was given two tasks. She first heard a word or nonword for which she had to make a lexical decision. Immediately after
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a positive response (i.e., correct “word” decision), she saw two pictures from which she had to choose the one matching the word. Eighty words had to be identified with the distractor being a closely semantically related object. She received no feedback about her decision. The word-picture matching was directly followed by presentation of the written stimulus. BB then had to make the choice between the two pictures again.
3.4.2.
Results
For the auditory lexical decision task, the controls achieved an average of 155.37 correct responses (97%) with a standard deviation of 3.06. Mean d0 was 4.01 (standard deviation .52). In the subsequent semantic comprehension task, the controls achieved an average performance of 97.7% correct responses (standard deviation 2.05%). BB was correct on 72 words and on 78 nonwords (150 correct; d0 ¼ 3.42). The 72 words correctly identified as an existing word were followed by a semantic comprehension task. In this, BB was correct on 28 trials (39%). In contrast, her semantic comprehension was highly superior after written presentation (78/80 correct; McNemar test, p < .01).
3.5. Experiment 3b: Lexical decision, then word-picture matching with formally related foils 3.5.1.
Procedure
Another set of 80 words had to be identified with the distractor being formally related. The 80 target words were again combined with 80 nonwords in random order.
3.5.2.
Results
For the auditory lexical decision preceeding the formal comprehension task, controls achieved an average rate of 155.9 correct responses (97.4%). The standard deviation was 3.09. Mean d0 was 4.09 (standard deviation .46). In the subsequent formal comprehension task, the control subjects achieved an average of 95.47% correct (2.58%). BB made to correct lexical decision for 75 words and for 78 nonwords (153 correct; d0 3.49). The 75 words with correct lexical decision were followed by the word-picture matching task with a formally related distractor. BB made 60 correct choices (80%). The difference in BB’s performance on the semantic and the formal comprehension task was highly significant [c2(1) ¼ 29.15, p < .01]. Written comprehension of the words was unimpaired (79/80).
3.5.3.
Discussion Exp. 3a and b
The results of Experiment 3a and 3b replicate the previous observation, namely that BB was able to carry out an auditory lexical decision despite being impaired in the comprehension of the word’s meaning. Her accuracy in the lexical decision task was in the normal range in terms of correct responses and just below the normal range in terms of d0 . Yet, her score still fell in the range of the controls’ scores. In contrast, her performance on the semantic comprehension task was severely impaired while controls performed with above 95%-accuracy. BB performed below chance performance, even significantly so, which, on first sight, is implausible. Yet on several trials, she frankly refused to answer and could not be convinced to guess. She reported that she could not make a decision between the
two pictures. We had no other chance but to score these items as incorrect. This, however, happened exclusively in the semantic comprehension task. Even if we assumed that she would have made a guess and would have reached chance performance (50% correct), she would still be far below the controls’ range of performance. In contrast, her performance on the formal comprehension task was better preserved albeit not in the normal range. BB scored 80% correct while controls achieved about 95%. This might suggest that access to the phonological lexicon was impaired in some situations. However in Experiment 2, BB had been unimpaired in the formal matching task. Alternatively, introducing the delay of roughly 1 sec between lexical decision and word-picture matching might have affected her performance on the matching task. The semantic comprehension task from Experiment 2 was, therefore, repeated with a delay of 3 sec (in comparison to no delay in the original administration of the test). If the delay between stimulus presentation and decision was crucial, the delay of 3 sec should affect BB’s performance dramatically. However, she was correct on 33 of 40 decisions in comparison to the original score of 31. The difference between both conditions was not significant and suggests that introducing a delay of 1 sec between stimulus presentation and wordpicture matching task did not cause BB’s severe impairment. This perfectly matched her intuitions: She stated repeatedly that she was either able to summon a picture in response to a given word or not but that she did not feel that she was losing a phonological trace which made the decision more difficult. The results of Experiment 3a and 3b demonstrate that despite her severe difficulties in the semantic interpretation of the auditory word, BB was unimpaired in the preceeding lexical decision for the same material. The present results, thus, replicate the findings from the previous experiments. The present experiments offer one further advantage: Semantic comprehension was directly compared to comprehension of the word form. It is thereby possible to rule out alternative accounts of BB’s performance. Most notably, the lexical decision task differs from word-picture matching in several dimensions. It has, therefore, been argued that word-picture matching might be a more sensitive task to assess comprehension (Franklin et al., 1996). However, task complexity should be comparable in both semantic and formal word-picture matching with the only difference being the competing information (semantic vs word form information). Both tasks are comparable in that they involve a spoken word followed by two pictures to which the subject has to respond. Thus, if task complexity per se had been the reason for impaired processing, BB should have performed poorly in both matching tasks. Likewise, word-picture matching was delayed by roughly 1 sec. It could have been possible that this short delay was enough to cause decay of the phonological trace which, then, affected word-picture matching. Lexical decision, in contrast, could be made immediately after presentation of the stimulus when the phonological trace was still available. However, the significantly superior performance on the formal comprehension task rules out this alternative explanation. In this task, BB’s performance was significantly superior to her semantic decision. In addition, the effect of delay was explicitly tested and was found not to be responsible for the dissociation. Finally, it should be noted that unimpaired subjects usually find the semantic tasks easier than
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the formal comprehension task. To summarize, Experiments 3a and b demonstrate that BB was able to make lexical decisions independently of her ability to comprehend the same stimuli. This was, again, assessed at an item-specific level.
3.6.
Experiment 4: Lexical decision, then definition
3.6.1.
Procedure
BB was presented a task in which she had to make a lexical decision in response to a spoken word or nonword. There were 160 targets (80 words, 80 nonwords). The word stimuli were controlled for frequency and concreteness. Seven days later, BB was asked to provide a definition of the word’s meaning. The quality of the individual definitions was assessed independently by two experienced speech therapists. A fairly liberal criterion was adopted due to BB’s anomia. The therapists were instructed to judge whether the definitions gave evidence of comprehension of the word given that the subject was aphasic (“adequate/inadequate”). A further five days later, BB was presented the same words in writing and was asked to provide a definition. The words were presented to her for 2 sec. The definitions were, again, rated for evidence of semantic comprehension. The raters received no information as to whether the definition had been provided in response to a written or an auditory stimulus. Hence, they should have applied the same criteria to stimuli.
3.6.2.
Results
BB was correct on 153 out of 160 auditory lexical decisions (95.6%). Of the 80 target words, 76 were accepted as a word, 77 nonwords were correctly rejected (95%, d0 ¼ 3.43). Immediately after her positive decision, BB was able to provide an acceptable definition for 20 of the 76 words (26%). All 20 correct definitions were for concrete words [difference between concrete and abstract words: c2(1) ¼ 26.67, p < .01]. There was no difference for frequency. The majority of her inacceptable definitions consisted of failures to provide a definition (e.g., “I didn’t get it.”; “I can’t do it.”; “Don’t know what it is. I know it’s a word but I don’t know which one.”; cf. Table 2). In contrast, 68 of her definitions for the 76 written target words were judged acceptable by the naı¨ve raters (90.7%). The difference between auditory and written presentation was highly significant (McNemar test, p < .01). The consistency between the two raters’ judgment was highly reliable (contingency coefficient C ¼ .698, p < .01; Phi ¼ .973, p < .01).
3.6.3.
Discussion
Experiment 4 employed a different method yet its results point into the same direction: BB was able to make an auditory lexical decision quite accurately without being able to provide an adequate definition of the stimulus word. A common response of hers was something like “O, this is a real one, it is a word, but I don’t get what it is. you will have to write it down for me, then I know what it is”. Her definitions were judged by two independent experienced speech therapists who were asked to adopt a fairly liberal criterion. That is, any hint suggesting comprehension should lead to a positive vote of the raters. The raters agreed significantly. Although the definition task was not administered to the control subjects, we are confident that these would have provided adequate definitions. Since there was no limit to the length of the definition, healthy controls would have provided much longer definitions which would have been easy to identify for the independent raters. Therefore, BB served as her own control since she was administered the task with written target words. BB’s definitions from written words were rated superiorly which suggests that it was the mode of input (spoken vs written word) which mattered and it was not a general impairment of producing definitions. We think, therefore, it is safe to conclude that semantic knowledge per se was preserved in BB which made it possible to produce adequate definitions if the semantic representations could be accessed properly. Again, the tasks show that BB’s cognitive impairment involved access of semantic representations from spoken lexical input. This, however, did not affect her ability to make lexical decisions. Experiment 4, thus, replicates the findings from Experiments 2 and 3.
4.
General discussion
The disorder known as ‘word-meaning deafness’ is an extremely rare condition with only a limited number of case studies published (Bramwell, 1897; Franklin, 1989; Franklin et al., 1994, 1996; Hall and Riddoch, 1997; Kohn and Friedman, 1986; Ziehl, 1896). The disorder is one of mapping phonological input onto semantics. Consequently, ‘word-meaning deafness’ should be diagnosed in those individuals who have preserved lexical decision and preserved comprehension of written words (e.g., Franklin, 1989). Some subjects have been able to repeat and write words they did not comprehend
Table 2 e Examples of BB’s definitions following auditory and written word presentation. Target
After auditory presentation
After visual presentation
Rooster
“Like a hen, it’s the male . there are many hens but only one of these.” (correct) “I don’t know. I can’t say it, I don’t know what it means” (incorrect) “I don’t get it. I know it’s good [i.e., a word] but I don’t know which one, I mean, what it is.” (incorrect) “Oh, another one which I didn’t get . You will have to write it down for me otherwise I can’t say anything about it.” (incorrect)
“Yes, the rooster. It is only one, there are many of the others, the females.” (correct) “A bad feeling, if someone’s got more than you and you want it, too” (correct) “Ah, yes, when someone is furious (correct)
Envy Anger Zone
“Well, this is difficult to describe. how shall I put it? My father used to work in an . with aircrafts, and they had dogs out there, so nobody would enter at night. It belonged to the army.” (incorrect)
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(Bramwell, 1897; Franklin et al., 1996; Hall and Riddoch, 1997; Ziehl, 1896) while others were unable to do so (e.g., Franklin et al., 1994). Our subject was impaired in repeating single words as well as digits and sentences. We view this as an independent deficit of short-term memory as BB exhibited dissociations between intact repetition and impaired comprehension (“‘Jar’, ‘jar’, what is a ‘jar’?”). We will discuss the implications of BB’s repetition impairment further below. We shall first briefly summarize the results of the assessment of word comprehension as well as the experiments. Along with anomia and the mentioned repetition deficit, BB exhibited word-meaning deafness. She was well able to make phonological same-different judgments on words and nonwords. If her severe impairment of short-term retention of verbal stimuli was compensated for, she performed in the unimpaired range. She was unimpaired in lexical decision. In contrast, her ability to identify the respective picture matching a spoken word was severely impaired. While in a definition task, there was a difference between concrete and abstract stimuli, her comprehension impairment affected both concrete and abstract words. Most word-meaning deaf subjects, thus far, exhibited comprehension impairments predominantly affecting abstract words. We, however, observed severe impairments in tasks in which concrete nouns had to be matched to their respective pictures. For example, BB was severely impaired in the respective version of the Pyramids and Palm Trees Test (i.e., spoken target word, two pictures). Likewise, she was unable to identify a picture in response to a spoken word (e.g., animals, tools, means of transportation) even if these involved only concrete words/ concepts. If, however, alternative input modalities were used, either written words or pictures (in the picture version of the PPT), BB’s performance was flawless. Therefore, her semantic processing was unimpaired, and her deficit involved a selective impairment of matching a spoken word to its corresponding semantic representation. Franklin (1989) suggested that impaired lexical access should lead to formally related word forms being activated. One could expect subjects to give definitions to formally related words (e.g., ‘pardon’ being defined as ‘garden’; cf. Howard and Franklin, 1988). This, however, was not observed in BB. Furthermore, we explicitly assessed her ability to identify a spoken word from formally related foils (e.g., Experiments 2 and 3). She was much better able to do so than distinguishing a target word from semantically related foils. We would, thus, conclude that BB did not suffer from Franklin’s (1989) word-form deafness. The first experiment assessed the role of phonological overlap in BB’s lexical decision. In the model of Seidenberg and McClelland (1989), a lexical decision is based upon an error score of the activation pattern resulting from the current input in comparison to a re-activated pattern based on learning experience. The model predicts that lexical decisions get more difficult with word-like nonwords and low-frequency words (cf. Fera and Besner, 1992). In contrast, with high-frequency words and nonwords dissimilar to existing words, lexical decision should be easier. Crucially, some authors suggested that with difficult lexical decisions, the cognitive system has to refer to other source of information, including semantic representations (Plaut, 1997). A similar observation has been made by James (1975) who reported concreteness effects when
word-like nonwords were used. Applied to the present situation, this hypothesis predicts particular difficulties of our participant with word-like nonwords and low-frequency words since for these stimuli, she has to rely on support from semantics. This was explicitly tested in Experiment 1 which, however, yielded no impairment in the difficult lexical decision task. A similar finding had been reported with healthy subjects in visual lexical decision (Fera and Besner, 1992). An alternative method of simulating lexical decisions based on the familiarity of the phonological output (Gaskell and Marslen-Wilson, 1997) is equally doomed to fail: We observed a severe repetition impairment in BB which also suggests impaired mapping of phonetic input to phonological (output) representations. Nonword repetition was close to impossible with ‘word-like’ nonwords provoking lexicalization errors. In word repetition, she produced semantic errors as well as formal and segmental phonemic errors. Thus, her mapping of the auditory input onto phonological output representations was highly unreliable. Moreover, a direct comparison of ‘word-like’ nonwords (i.e., nonwords derived from high-frequency words by exchanging one phoneme) and ‘strange’ nonwords (i.e., very different from German words and without any phonological neighbors, materials from Experiment 1) revealed that lexicalization errors (n ¼ 7) appeared exclusively for word-like nonwords. This, however, was not mirrored by a larger number of false-positive responses in the respective lexical decision task. It is, thus, unlikely that activation in a phonological output layer served as the basis for BB’s lexical decisions. Having first demonstrated that BB was unaffected by degree of formal overlap between words and nonwords, we ran three further experiments to investigate whether lexical decision was independent of comprehension. All these experiments studied lexical and semantic processing on an item-specific level. That means, for each target item, BB made a lexical decision and carried out a word-picture matching or a definition task either immediately following her lexical decision or some days later. Performance on both tasks was, thus, easier to compare than in previous reports, and it was not affected by details of the stimulus lists. In Experiment 2, BB had to make a lexical decision and, one week later, had to perform a comprehension task on the very same stimuli. She was highly accurate in her lexical decisions (above 98% correct; d0 ¼ 4.02) while being severely impaired in the semantic comprehension task. Comprehension of written stimuli was, however, at ceiling and identifying a spoken word among formally related foils was highly superior. In Experiment 3, the subject performed a lexical decision and immediately afterward, had to identify the spoken word from a pair of two pictures. The distractors could be semantically or formally related. Her accuracy in lexical decision was above 90% (d0 above 3.4) while comprehension was severely impaired. It was, however, impaired if a semantic distractor was present, not as much if a formally related distractor was present. Again, BB was unimpaired when a written word had to be matched to one of two semantically related pictures. The results of this experiment are even more striking as semantic comprehension involved matching a spoken word to one of two pictures, not to one of four or six. The task, thus, was even easier than standard tasks of comprehension which involve matching to one of four (e.g., De Bleser et al., 2004). In addition,
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the formal condition provides a close control condition to the semantic task and allows us to rule out alternative explanations such as the delay of the second task about 1 sec after BB’s correct lexical decision. There was a highly significant difference between her performance on the formal in comparison to the semantic version of the task. In Experiment 4, BB finally was unable to provide definitions for most of the auditorily presented words while she was able to define written words. This difference was obvious even when a fairly liberal criterion was employed. Yet, despite her difficulties in defining words she heard, BB was able to make a lexical decision. Again, accuracy was well above 90%. Lexical and semantic comprehension were assessed on two different occasions so that delay should not have played a role in the semantic task. Since we compared BB’s performance after auditory presentation to performance after written presentation of stimuli, BB also served as her own control. In addition, the written stimuli were presented for a maximum of 2 sec. Decay of phonological information, thus, could not explain her dissociation between performance on the auditory and written versions of the matching tasks. Moreover in Experiments 2 and 4, the stimuli were presented on two different occasions, thus invalidating any objection that duration of the delay between stimulus and the respective task was the major determinant of performance. An impairment of decay might be held responsible for impaired activation of semantic representations: It might be argued that activation is available for phonological processing in order to support lexical decisions but that it decays too rapidly to allow access to semantics. This would, however, leave the results of Experiment 1 unexplained. Furthermore, at several different occasions, both comprehension and repetition tasks, delay was included without yielding significantly different performance. The careful control conditions employed across the experiments provide, in our view, a considerable methodological advancement in comparison to previous case reports. Some of these previous studies did not report whether they controlled for duration of stimulus presentation or, in some cases, did not employ a comparison of formally and semantically related distractors. Thus, a subject may exhibit better performance on written stimuli simply due to a short-term memory deficit and/or lapses of attention which affect the transitory speech signal much more than written stimuli. The critical comparison is that of lexical decision and comprehension on the same set of items. An essential feature of the connectionist models described in the introduction is that lexical decisions are based on the activation pattern of semantic units (Gaskell and MarslenWilson, 1997). This is exactly why these models may be more parsimonious than those models incorporating lexicons: During lexical decision, exactly those representations are activated which are also involved in processing the word’s meaning. Therefore, if access to semantic knowledge is impaired, so should be lexical decisions. There may be task differences but a severe semantic impairment should inevitably lead to severe impairments in lexical decision. This, however, was not observed in our study. In line with previous case reports, the assessment of our subject BB demonstrates that lexical decision may be preserved despite severe comprehension impairments. None of the proposed alternative mechanisms is able to account for BB’s pattern of performance except those incorporating lexicons.
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Note that Plaut and Booth (2006) recently demonstrated that with semantic impairment in their model, lexical decision was still possible. Yet, the way the semantic impairment was implemented, a deletion of 30% of semantic units of the network, is by no means adequate for the peculiar impairment underlying word-meaning deafness in which the semantic representations themselves are unaffected. Furthermore, it remains to be seen whether the semantically lesioned model is still able to distinguish between highly similar words and nonwords since Plaut and Booth (2006) assessed these questions in two independent simulations and not in a single simulation. Recently, Dilkina et al. (2010) simulated semantic damage in a model without independent lexicons. They observed that lexical decision may be impaired on items which are correctly understood in other tasks and vice versa. However, damage to the model affected performance on both tasks while in the present study, BB’s lexical decisions were unimpaired despite her severe semantic deficits. Furthermore, Dilkina et al.’s (2010) visual lexical decision task was a two-alternative forced choice task in which a low-frequency word with low average bigram frequency is presented along with a pseudohomophone distractor consisting of highly familiar combinations of letters (bigram frequencies). It has been argued that this variant of the lexical decision task is confounded with neighborhood density (Coltheart et al., 2010a). In the present auditory lexical decision task, we explicitly varied neighborhood demonstrating that BB was unaffected by lexical neighborhood. BB’s condition involves impaired mapping from phonology to semantic representations which, in theory, could be implemented by weakening the connections between phonology and semantics or adding noise. However, this lesioning of the connections should lead to impaired performance on word-like nonwords which should erroneously map on closely related conceptual representations (i.e., those concepts from which the nonwords were derived by changing a phoneme). Word-like nonwords should therefore lead to false-positive responses. Overall, the results suggest that phonological and ‘lexical’ processing can be independent of the ability to comprehend the stimuli. This had been concluded from previous, wellcontrolled and detailed case studies (e.g., Franklin et al., 1994, 1996; see Coltheart, 2004, for review). In addition to our methodological advances, any report of such a rare condition like ‘word-meaning deafness’ helps to counter the objection that single-case studies are prone to artifact either due to the materials employed (cf. Woollams et al., 2007) or due to representing an extreme case of a normal distribution (Grodzinsky et al., 1999; cf. Patterson et al., 2006). The present results support models incorporating a permanent storage of known word forms, i.e., a lexicon. The data, thus, contradict those models without independent lexical representations. Furthermore, a single level of pre-semantic processing is at odds with two other variants of word-deafness reported in the literature, ‘word-sound deafness’ and ‘word-form deafness’ (cf. Howard and Franklin, 1988). Subjects with wordsound deafness are impaired in phonological processing despite intact processing of nonspeech sounds while subjects with word-form deafness have intact pre-lexical phonological processing but impaired lexical access. Often, these subjects give definitions of formally related words in
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response to spoken words (Howard and Franklin’s subject mentioned above). It is clear that these two groups of subjects exhibit qualitatively different functional lesions and that this double dissociation requires two pre-semantic levels of processing. While the study focused on phonological processing and is relevant for different models of auditory language processing (Gaskell and Marslen-Wilson, 1997; Plaut and Kello, 1999), the conclusions apply to a wide range of different models which all, more or less, follow the initial framework of the connectionist “triangle model” of reading (Seidenberg and McClelland, 1989). As described above, the framework covers cognitive functions such as reading aloud, naming, verb morphology processing, visual object processing, repetition and short-term memory as well as spoken language comprehension. In all these models, preserved semantic processing is required in order to reliably perform supposedly ‘non-semantic tasks’. Our results, thus, call into question the whole group of models summarized under the “primary systems” label (cf. Jefferies et al., 2007). The present study is even more important because the central issue of whether lexicons need to be incorporated into cognitive models will, most likely, not be resolved by studies of unimpaired subjects or by means of functional imaging. In any study with unimpaired subjects, lexical processing will always also involve semantic processing. It is impossible to separate these stages of processing in subjects other than those rare cases of word-meaning deafness or word-meaning blindness (Lambon Ralph et al., 1996). Therefore, reports of subjects with rare selective cognitive impairments provide not only a central but the only available evidence. We explicitly acknowledge the wealth of data that proponents of the primary systems account have put forward (e.g., Patterson et al., 2006; Woollams et al., 2007). Nevertheless, dissociations are to be favored over associations in classical cognitive neuropsychological reasoning (Coltheart, 2001; Shallice, 1988), and we fully assent to this position. Given the strong dissociation between preserved segmental and lexical phonological processing and impaired access to a spoken word’s meaning which BB exhibits, we conclude that only cognitive models incorporating separate lexical representations can account for the present data.
Acknowledgments We are indebted to BB and her sister for their time and support and the volunteers who served as control subjects. Max Coltheart provided very helpful comments on an earlier version of this manuscript. Jochen Baumm and Christel Nagel kindly rated the definitions. Parts of this research were presented at the 29th European Workshop for Cognitive Neuropsychology in Bressanone (January 2011).
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