Negative priming between pictures and words in a selective attention ...

Memory & Cognition /988. /6 (I). 64-70

Negative priming between pictures and words in a selective attention task: Evidence for semantic processing of ignored stimuli STEVEN P. TIPPER Mount Allison University, Sackville, New Brunswick, Canada and JON DRIVER University of Oxford, Oxford, England This study examined the processing of ignored pictures and words when attention was directed to a different picture or word. Previous work by Tipper (1985) demonstrated that the priming effect of an ignored picture on a subsequent categorically related picture is inhibitory. This effect was termed negative priming. Tipper concluded that ignored pictures achieved abstract categorical levels of internal representation, and that these representations were inhibited during selection of a simultaneously presented picture. This conclusion, however, was premature. Observation of the figures used by Tipper suggests that objects within a category have greater structural similarity than do objects in different categories. The negative priming effect could therefore be at a structural level of representation. The present study examined priming across symbolic domains (pictures and words) where there was no structural relationship between objects. Negative priming was again observed and was equivalent to the negative priming observed within symbolic domain. These data suggest that ignored drawings and words do achieve abstract categoricallevels of representation, and that the mechanisms underlying negative priming operate at, or beyond, this level. The study of visual selective attention is concerned with how organisms select particular objects for action and successfully ignore other objects potentially capable of controlling action. This problem can be subdivided into two issues. The first issue is the role that attention plays in the perceptual processes that produce internal representations of the visual environment with which the organism interacts. Two opposing theories try to explain this role. The precategorical view (Broadbent, 1982; Johnston & Dark, 1986) holds that attention is critical for the perceptual processes involved in object recognition. Thus, if an object is ignored, only low-level features such as color are encoded, and the object's identity is not internally represented. The alternative, postcategorical selection theory (Allport, 1980; Deutsch & Deutsch, 1963; Van der Heijden, 1981), holds that attention plays a limited role in object recognition. If an object is familiar, with well-established internal representations, the perceptual system will encode the identity of the object whether or not it is attended. The second issue concerns the mechanism of selection. Precategorical selection theories were predicated upon We gratefully acknowledge the assistance of Maxine Cresswell and Wendy Perkins with data collection, and Glenda MacQueen and an anonymous reviewer for comments on a draft of this paper. The research was supported by M.R.C. Grant G831391ON. Requests for reprints should be sent to S. Tipper, Department of Psychology. Mount Allison University, Sackville, NB EOA 3CO, Canada.

Copyright 1987 Psychonomic Society, Inc.

filter models (Broadbent, 1958, 1971, 1982) in which objects were selected on a physical basis and only then received further "semantic" processing. Kahneman and Treisman (1984) argued that postcategorical selection theories underspecify the mechanisms that allow an organism to direct its actions to one object from a set, because equivalent information is available for both relevant and irrelevant objects. Two models of postcategorical selection have been proposed. In the passive decay model, the activation level of the representation of the relevant object is maintained until irrelevant activation has passively decayed. In this model selection is selective remembering (Van der Heijden, 1981). Alternatively, Neill (1977) proposed that representations of ignored objects may be actively inhibited. Tipper (1985) broached these two issues. To examine whether passive decay or active inhibition occurs, it was necessary to observe the fate of the representations of the ignored object subsequent to the initial encounter with that object. A priming paradigm was therefore employed. Subjects were presented with two pictures (the prime display) and told by means of a physical cue (color) which to select and name and which to ignore. Both pictures were above threshold, and thus both were capable of controlling action. Shortly afterward a probe display, similarly containing two pictures, was presented. In the crucial condition, the selected picture in the probe display was identical to the ignored picture in the previous prime display. Tipper 64

NEGATIVE PRIMING predicted that if the representations of ignored pictures are inhibited during selection, then processing of a subsequent picture requiring these same representations would be impaired. This prediction was supported; there was a reaction-time cost in naming the probe picture for the condition described, relative to the control condition. This effect was termed negative priming. Negative priming was also observed when the ignored picture was not physically identical to the probe picture, but was in the same category (e.g., cat-dog). This was tentatively attributed to a process of spreading inhibition in semantic memory (Roediger & Neely, 1982). In terms of the postcategorical selection theories, the categorical negative priming was interpreted as evidence that ignored objects are processed beyond the level of physical properties to that of categorical representations, and that inhibition takes place at or beyond this level. (See Schneider & Fisk, 1984, for similar views of automatic categorical processing and inhibition of ignored information.) The claim that categorical internal representations are achieved independent of attention may, however, be premature. Rosch, Mervis, Gray, Johnson, and BoyesBraem (1976) and Sperber, McCauley, Ragain, and Weil (1979) pointed out that pictures within the same semantic category have many features in common (see also Snodgrass & McCollough, 1985). Examination of Figure 1, which contains examples of the figures employed by Tipper (1985), would tend to support these suggestions. Clearly the pictures depicting objects within the semantic category of tool have features and overall shape in common, relative to objects in other categories, such as furniture. It is feasible, therefore, that the influence between pictures within a category is at the physicalfeature level of representation rather than at an abstract categorical level.

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The present experiment attempts to identify the level of internal representation achieved by ignored objects, while controlling the confounding variable of physical properties. To this end, priming effects are observed between objects represented in different symbolic domains (pictures and words) that have no features in common. As Anderson (1980) pointed out, pictures are an analog representation that reflect the physical properties of the objects they represent. Words, on the other hand, are arbitrary symbolic representations. Therefore, any priming effects observed between words and pictures must be beyond the physical level of internal representation; the effect must be at some abstract semantic level. Such priming effects would support postcategorical theories of attention, and suggest that inhibition is late in the sequence of processes from perception to action (Tipper & Cranston, 1985).

METHOD The method for the experiment described here is based on that of Tipper (1985, Experiment 3). The essential difference is that in this study priming effects were observed across symbolic domain (from pictures to words, and from words to pictures), preventing any possible contribution from physical similarity. For comparison, we included within-domain priming conditions (from pictures to pictures, and from words to words) in which physical similarity between prime and probe clearly exists. As in previous research (Allport, Tipper, & Chmiel, 1985; Tipper, 1985), priming effects were observed both for ignored stimuli and for stimuli selected for response. In the latter case, any priming effects are typically facilitatory (e.g., Sperber et al., 1979). The subjects' task was to give the superordinate category of the selected stimulus (e.g., animal in response to dog). This categorization response was chosen in an attempt to maximize the chance of observing negative priming across symbolic domains. In the facilitatory priming literature (D. I. Irwin & Lupker, 1983), priming between words and pictures is more robust with this categorization response than with naming. Moreover, we have evidence that negative priming between words is obtained with categorization but not with naming.

Subjects Fifty-six subjects (36 females) from the long-term University of Oxford subject pool were each paid £1.50 to participate. They ranged in age from 18 to 32, and had normal color vision (as determined by the Ishihari color test) and normal or corrected-to-normal visual acuity in both eyes.

Figure 1. Examples of figures employed by Tipper (1985)demonstrating the structural similarity between pictures within the same semantic category.

Apparatus and Materials A six-field tachistoscope having two three-field power units (Electronic Developments Ltd.) was used for stimulus presentation. A hand-held microswitch was used by subjects for starting each trial. A voice key and a millisecond timer (Behavioral Research and Development Electronics Ltd.) were used to measure oral categorization latencies. The 10 pictures used to create the experimental stimuli were adjusted with a reducing photocopier to be approximately equal in size. Visual angles ranged between 4. rand 7.9°. They were drawn with Bic biro pens. The related pairs were cat-dog, chair-table, trumpet-guitar, hammer-spanner (wrench), and hand-foot. The selected picture was drawn in red and superimposed over the igGored green picture. The superimposed pictures of a given display were always in different categories. (For examples of superimposed

66

TIPPER AND DRIVER selected probe word. Ignored semantic (lS)-The ignored prime picture was in the same category as the selected probe word. Ignored repetition (IR)-The ignored prime picture had the same name as the selected probe word. There were 20 trials in each condition. Each probe display appeared once in each condition. Stimulus presentation was randomized for each subject. Each experimental prime display appeared twice. Figure 2 provides examples of the stimuli and conditions in the picture-word group.

objects in selective attention tasks, see Goldstein & Fink, 1981; J. Irwin, 1979; Rock & Gutman, 1981; Tipper, 1985.) Each picture was superimposed over every other picture employed in the experiment, except for the other picture in its own category, yielding 80 displays. Twenty of these displays were selected to be the probe displays; thus each picture appeared twice as the selected picture. This limited set of probes was employed so that each priming condition was observed within the same stimulus display. The remaining displays constituted the prime displays, 10 of which were used for practice trials and 50 for experimental trials. The IO word stimuli were simply the names of the pictures, and prime and probe displays were prepared in a similar way. The selected red word partially overlapped the ignored green word such that it was equally likely to be above and to the right, above and to the left, below and to the right, or below and to the left. The visual angles of the words were 1.3 0 vertical and 3.3 0 to 7.5 0 horizontal.

Procedure The subjects were initially shown a series of five cards, each containing two pictures or two words in the within-domain groups or both sets of materials in the between-domain groups. The pictures and words were in their associate pairs on each card (e.g., catdog). Subjects were required simply to name each stimulus. Tachistoscopic presentation was used to establish the appropriate masking stimulus onset asynchrony (SOA) for each subject. These durations were approximate minimum viewing times required to identify the selected picture and/or selected word in the absence of any other interfering or intervening task. Brief exposure durations and pattern masking were employed to reduce the possibility of switching attention to the ignored stimulus after selection of the attended stimulus, and also for compatibility with previous research (Allport et aI., 1985; Tipper, 1985). When the subject pressed the microswitch, a fixation cross appeared for 600 msec, followed by the superimposed selected and ignored pictures (or words); both were presented monoptically to the left eye. This was followed by a pattern mask to the left eye for 100 msec. All presentation fields in the tachistoscope were adjusted to be of equal luminance. SOA was titrated for picture and word displays in separate blocks of trials for the subjects in the between-domain groups. Within each group, half the subjects underwent picture SOA setting followed by word SOA setting, and vice versa for the other half. Subjects in the within-domaingroups received SOA setting only for the stimulus domain they would experience. Stimulus-mask SOA began at IO msec and was increased using the method of ascending limits by 10-msec steps. The same picture (or word) display was repeatedly presented until subjects correctly named the red stimulus. The

Design The experiment manipulated three independent variables. The first was between subjects: half of the subjects received between-domain priming, that is, picture displays as primes and word displays as probes (picture-word) or word primes and picture probes (wordpicture), and half received within-domain priming (word-word or picture-picture). The second between-subjects variable was whether the probe was a word or a picture. In all four groups the subject was to select and categorize a red stimulus, while ignoring an overlapping green stimulus. The difference between the groups was whether the subjects selected pictures, words, a picture and then a word, or a word and then a picture. Assignment of the subjects to these four groups was random. There were 14 subjects in each group. The third independent variable, priming condition, was within subjects. The following description is of the picture-word group. The conditions were as follows (except as mentioned, other components of the displays were unrelated): Attendedrepetition (AR)The selected prime picture had the same name as the selected probe word. Attended semantic (AS)-The selected prime picture was in the same category as the selected probe word. Control (C)-The selected and ignored prime pictures were both unrelated to the

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Figure 2. Examples of the priming and probe displays in the picture-word group. The lines depicted as solid were red (selected), and those depicted as broken were green (ignored).

NEGATIVE PRIMING longest SOA for the five displays presented was recorded, and

5 msec was added to this figure. The resulting SOAs were then used throughout the experimental trials. Mean SOA was 118 msec (range 80-2(0) for the pictures and 125 msec (range 80-230) for the words. The next stage consisted of the experimental trials. When the subject pressed the microswitch, he/she saw a series of events as follows: (I) A fixation cross was presented to the subject's left eye for 600 msec. (2) The prime display containing either a red line drawing superimposed on a green drawing or red word over a green word was displayed. This also was presented to the subject's left eye. (3) A pattern mask was presented for 100 msec to the left eye. (4) A fixation cross was presented to the right eye for 1,100 msec. (5) The probe display containing either pictures or words was presented to the right eye. (4) Finally, a pattern mask was presented for 100 msec to the right eye. The subjects were informed that the stimuli would alternate between eyes. This alternation of presentation was included for compatibility with previous research (Tipper, 1985). The subjects in the picture-word group were then given instructions concerning the stimulus sequence. They were told that they should correctly identify the first red picture, as they would have to recall its category shortly afterward. (For example, if the picture was of a dog, the subjects were to report the superordinate category of animal. The category labels to be used were explicitly stated: animal, furniture, music, tool, and body.) However, it was stressed that they should not overtly categorize the picture when it was presented. When the word probe appeared, the subjects were requested to categorize the red word as fast as possible. They were then asked to recall the category of the selected prime picture. The recall of the category of the selected prime was requested to ensure that the subjects attended to the red picture. The subjects were informed that trials counted only if they could correctly report the category of the red (selected) prime. Furthermore, the subjects were informed that green stimuli would also be present in the displays; they were instructed that these were irrelevant to the task and should be ignored. The same instructions were given to the subjects in the other three groups, accounting for the changes in stimuli. The intertrial interval was approximately 15-20 sec, during which time the subjects noted their own reaction times (RTs), thus receiving feedback on categorization latency performance; these were veri-

fied by the experimenter. The subjects were also informed of any errors. The initial 10 trials in this procedure were practice, although subjects were not informed of this. The final stage, adapted from the procedures used by Rock and Gutman (1981) and Tipper (1985), was a test for the subjects' awareness of the ignored priming picture or word. On the last trial only, there was no probe picture or word to be categorized. It was replaced by a white card. As this blank white card was presented, subjects were asked the surprise question of what the previous green picture (or word) had been. Thus they were asked to recall any information that may have been available for conscious scrutiny at about the time when the probe would have been presented (see Tipper, 1985, for further details).

RESULTS AND DISCUSSION One subject who appeared to be unable to carry out the selection task (34% errors) was dropped from data analysis. In the catch trial recall task, 21 % (3 subjects) were able to recall the ignored picture and 14 % (2 subjects) recalled the ignored word in the between-domain groups. Similarly, 14% recalled the ignored picture and 21 % recalled the word in the within-domain groups. As discussed by Tipper (1985), with this small set of stimuli repeatedly presented, there is an 11 % chance of guessing the ignored stimulus. The observed recall in these groups is not above this chance level based on Fisher's exact test. Such failure to recall the ignored stimulus supports previous findings (Allport et al., 1985; Tipper, 1985) using the same catch trial methodology, and is consistent with previous research demonstrating that subjects cannot recognize ignored stimuli from such displays (Goldstein & Fink, 1981; Rock & Gutman, 1981; Tipper, 1985). Errors for both recall of the selected prime and response to the selected probe are shown in Table 1. Analysis of the prime recall errors produced no significant effects for

Table 1 Mean Reaetion Time to Categorize Probes and Percent Errors in Recalling Primes and Categorizing Probes Ignored Attended Attended Ignored Repetition Repetition Semantic Cuntrol _S_e_m_a_n_tic_ _~--,-~_ Picture Prime-Word Probe Reaction Time (msec) % Errors-Prime % Errors-Probe

754 .7 .7

Reaction Time (msec) % Errors-Prime % Errors-Probe

837 1.8 2.1

Reaction Time (msec) % Errors-Prime % Errors-Probe

714 1.8 1

Reaction Time (msec) % Errors-Prime % Errors-Probe

701 2.1 .7

767 1.07 2.86

998 1.4 2.5

1023 1.8 1.4

1038 .70 1.79

985 2.5 3.57

1005 .4 1.4

972 1.8 2.1

996 2.5 2.5

875 3.2 .5

873 4.3 3.9

Word Prime-Picture Probe 809 2.5 1.1

956 2.5 3.2

Word Prime-Word Probe 752 3.9 1.8

953 2.9 1.8

Picture Prime-Picture Probe

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745 828 1.8 .7 2.5 ..- - - - _ . 1.4 __._-_._ .._-----_

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TIPPER AND DRIVER

any of the factors: between-within domain [F(I,52) = 2.6], word-picture probe [F(l,52) = .831], and priming conditions [F(4,208) = .758]. Analysis of probe errors produced a similar pattern: between-within domain [F(l,52) = .257], word-picture probe [F(1 ,52) = .047], and priming condition [F(4,208) = 1.803]. Further analysis compared the control condition with each of the priming conditions using the Wilcoxon test. No significant contrasts were revealed, which discounts a speed-accuracy interpretation of the RT data to be discussed. Trimmed mean RT (minus the 5.2% extreme scores above two standard deviations) are also represented in Table 1. RT was analyzed in a three-way mixed ANOVA. It should be noted that the analysis of RT with outliers included produced the same pattern of results as those to be reported. The between-subjects factor of within-between domain was significant [F(l,52) = 4.18,p < .04]. Overall mean RT was longer when subjects had to switch domain from prime to probe than when they remained within a symbolic domain (917 msec and 841 msec for between and within domains, respectively). The second betweensubjects factor of word-picture probe was nonsignificant [F(l,52) = .90]. The within-subjects priming factor was highly significant [F(4,208) = 196.183,p < .001]. The only interaction that was significant was between wordpicture probes and priming [F(4,208) = 13.16, P < .001]. To examine the priming effects for each group, comparison of each of the priming conditions with the relevant control condition was undertaken by Wilcoxon test (the number of subjects demonstrating the effect from the total of 14 in each cell is also reported). All of the selected primes produced substantial facilitatory priming. In the picture-word group AR and AS were significant (p < .01: AR = 14 and AS = 14 subjects, showing facilitation), as they were for the word-picture group

(p < .01: AR = 14 and AS = 12 subjects), the wordword group (p < .01: AR = 14 and AS = 13 subjects), and the picture-picture group (p < .01: AR = 14 and AS = 13 subjects). The ignored primes produced clear evidence for negative priming. In the picture-word group both IS and IR were significant (p < .02: 11 and 12 subjects, respectively, showing negative priming). In the word-picture group IR produced significant negative priming (p < .01: 13 subjects), whereas IS was marginally significant (p < . 1: 10 subjects). In the picture-picture group both IR and IS produced significant negative priming (p < .01: 12 and 11 subjects, respectively). Finally in the word-word group IR was significant (p < .01: 11 subjects), but IS was nonsignificant. No other comparisons were significant. Figure 3 summarizes all the above priming effects. The observation of facilitatory priming between symbolic domain for attended pictures and words in the present experiment is consistent with previous research (D. I. Irwin & Lupker, 1983; Sperber et al., 1979), which reported such priming to be a robust phenomenon at short interstimulus intervals with a categorization response. Similarly, the within-domain facilitatory priming observed here has previously been well established (Meyer & Schvaneveldt, 1971; Warren & Morton, 1982). The consistency of the attended (facilitatory) and ignored (inhibitory) priming effects reported are equivalent, but the overall size of the attended priming effect is much larger than that of the ignored primes. This can plausibly be attributed to response repetition, for the categorization response is identical for prime and probe displays in the attended priming conditions only. The overall attended priming effect for the word probes, however, was substantially larger than that for the picture probes (228 msec vs. 122 msec; see Figure 3). This accounts for much of the unpredicted interaction in the

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