A Comparison Between Schizophrenia Patients and

A Comparison Between Schizophrenia Patients and Healthy Controls on the Expression of Attentional Blink in a Rapid Serial Visual Presentation (RSVP) Paradigm by Vinci Cheung, Eric Y.H. Chen, Ronald Y.L. Chen, Ming F. Woo, and Benjamin K. Yee

The expression of attentional blink (AB) in 24 schizophrenia inpatients was compared to 22 healthy subjects in a dual-target rapid serial visual presentation (RSVP) paradigm in which a sequence of discrete stimuli was presented in rapid succession. Correct identification of the first target led to poorer detection of the second one when they were interspersed by distractors. This second-target deficit constitutes the AB effect, which is most pronounced between 200 and 500 ms after the offset of the first target stimulus and steadily decays as the number of intervening distractors increases. Despite relatively poor performance in terms of target identification within RSVP streams, schizophrenia patients expressed an AB effect that was as clear as that seen in healthy subjects. Moreover, there was evidence for an enhanced AB effect in schizophrenia patients. This outcome contrasts with the robust finding that schizophrenia patients are attenuated in the expression of prepulse inhibition, another paradigm believed to assess attentional control. The present results add to the extensive literature on the nature and specification of attentional dysfunction implicated in schizophrenia. Keywords: Attention, psychosis, RSVP, schizophrenia. Schizophrenia Bulletin, 28(3):443-458,2002 The continual interest in the characterization of cognitive deficits in schizophrenia can be traced back to the early descriptions by Bleuler (1911) and Kraepelin (1919). Theories developed in the past 4 decades have attempted to identify some core cognitive deficiency that might underlie certain clusters of symptoms, or even a wider spectrum of psychopathology, associated with schizophrenia (e.g., Chapman and Chapman 1973; Goldstein 1978; Nuechter-

Send reprint requests to Dr. E.Y.H. Chen, Department of Psychiatry, Queen Mary Hospital, University of Hong Kong, 102 Pokfulam Road, Hong Kong, P.R.C.; telephone: +852 2855 1345; fax: +852 2855 4486; e-mail: [email protected].

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lein and Dawson 1984; Gray et al. 1991; Braff 1993; Andreasen 1999). One particular emphasis has been on the neuropsychological significance of schizophrenia patients' attentional dysfunction (e.g., McGhie and Chapman 1961; Mirsky 1969; Hemsley 1977; Schneider 1978; Gray et al. 1991; Mirsky et al. 1992; Cornblatt and Keilp 1994; Swerdlow et al. 1994; for a review, see Braff 1993). The alternative theories differ in the aspects of attentional control hypothesized to be impaired in schizophrenia. The present report focuses on the expression of a form of visual attentional modulation known as AB (Raymond et al. 1992). When two target stimuli (Tl and T2) are intermixed with distractor stimuli within an RSVP sequence, successful identification of Tl can impair the identification of T2 when there is at least one distractor separating the two targets (Broadbent and Broadbent 1987; Raymond et al. 1992; Chun and Potter 1995). This effect is referred to as attentional blink, and it is particularly pronounced within a temporal window of 200-500 ms following the offset of Tl (Raymond et al. 1992; Chun and Potter 1995; Giesbrecht and Di Lollo 1998). Among recent theories, a two-stage model has been proposed to explain the AB effect (Chun and Potter 1995). The first stage refers to the detection of the presence of the defining features of a target, but not of the target's identity. This is followed by the second stage, a capacity-limited process that enables identification of a target (Broadbent and Broadbent 1987). Accordingly, identification of Tl is interfered with by the succeeding (distractor) stimulus, which in turn delays the completion of this capacity-limited process. The excess delay thus incurred then leads to the poor processing of T2, which cannot be initiated until

Abstract

Schizophrenia Bulletin, Vol. 28, No. 3, 2002

V. Cheung et al.

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Although it has been shown that such P50 suppression is probably immune to certain forms of attentional manipulation (e.g., Jerger et al. 1992), it serves as another example demonstrating the temporal control of stimulus processing or associated response emission, or both. On both the PPI and P50 suppression paradigms, it has been reported that the response inhibition toward the second stimulus (induced as a result of the preceding presentation of the first stimulus) is weaker in schizophrenia patients than in healthy controls (e.g., Braff et al. 1978; Nagamoto et al. 1989, 1991). Such impairments have been demonstrated in chronic schizophrenia patients regardless of whether they were under the direct influence of medication during testing (Nideffer et al. 1971; Braff et al. 1992; Judd et al. 1992; Boutros et al. 1993). These findings have been taken as evidence for a deficiency in some forms of attentional control in schizophrenia patients such that they attend to stimuli that control subjects tend to ignore or not to attend to. The present experiment examined whether a deficit of a similar nature—namely, a reduced AB effect— would be seen in the AB paradigm. Despite the apparent similarities between AB and PPI/P50, there are some key differences (see Filion et al. 1999). Hence, the possibility that they might be preferentially taxing different aspects of attention-related processes must not be dismissed. One major difference is that a conscious identification of first target stimulus is essential for the demonstration of AB but not of PPI or P50. Thus, a significant PPI effect can be obtained regardless of whether the subjects were instructed to attend to and to identify the prepulse stimulus (DelPezzo and Hoffman 1980; Dawson et al. 1993; Filion et al. 1993), even though such attentional manipulation can influence the expression of prepulse-induced startle modification under certain test parameters. On the other hand, if subjects were instructed to ignore the first target (Tl) within a dual-target RSVP stream, identification of the second target (T2) would not be affected. Indeed, such a procedure was included by some researchers (e.g., Raymond et al. 1992) as a control condition for the demonstration of AB. This differential sensivity to verbal instruction to ignore the distractor stimulus between PPI and AB might imply that AB is more specifically related to the cognitive processes involved when the subject's controlled attention is under severe strain. If proven correct, this interpretation is of significant interest with respect to the suggestion that attentional deficit in schizophrenia is particularly severe in information processing with a conscious and effortful demand (Callaway and Naghdi 1982; Mori et al. 1996). In addition to the model of AB proposed by Chun and Potter (1995) outlined above, other existing theories (e.g., Raymond et al. 1992, 1995; Shapiro et al. 1994, 1997) also agree that the expression of the AB effect is closely related


Tl is identified. On the other hand, if T2 follows Tl immediately (within 200 ms), then both target stimuli are processed together without any detrimental effect upon the identification of T2 (Raymond et al. 1992; Chun and Potter 1995). The possibility that the expression of AB might be compromised in different psychopathological states or conditions has been little explored. One recent study evaluated this possibility in patients with unilateral neglect and observed that such patients exhibited a more severe and protracted AB compared to controls (Husain et al. 1997). This finding has been taken as evidence for an expression of visual neglect in the temporal domain, complimenting the more common form of its expression in the spatial one. The present study is the first attempt to evaluate such form of temporal attentional control in schizophrenia by studying the expression of AB in schizophrenia inpatients using an RSVP paradigm adopted from Chun and Potter (1995). The known impairment on sustained attention and high distractibility in schizophrenia (Nuechterlein and Dawson 1984) would lead one to expect a general reduction of target identification rate within RSVP sequences as such. However, it should not prevent us from assessing AB, because the AB effect is demonstrated in a within-subject manner, with each subject providing his or her own baseline level for comparison. Nonetheless, a separate block of single-target RSVP streams has been included in the present study to allow a more direct measure of single-target identification within RSVP streams, free from the extra demand involved in a dual-target identification task needed for the demonstration of AB. Processing of stimuli in close temporal proximity has been investigated with several paradigms in schizophrenia. These studies often involve a serial presentation of two stimuli in which the response to the second stimulus is reduced relative to the response to the same stimulus presented alone. One example of such response inhibition is prepulse inhibition (PPI) of the acoustic startle response (Graham 1975; for a review, see Filion et al. 1998), whereby a weak "prepulse" stimulus reduces the magnitude of the response induced by a subsequent startle-eliciting "pulse" stimulus. PPI is typically demonstrated with a prepulse-pulse interval between 80 and 500 ms (Berg and Balaban 1999; Blumenthal 1999), comparable to the temporal window for the expression of the AB effect. Similarly, when two identical stimuli are presented in close temporal proximity, the amplitude of the P50 waveform to the second stimulus is markedly reduced in healthy subjects, to approximately 50 percent of their P50 response amplitude toward the first stimulus (Freedman et al. 1987). This reduction is usually demonstrated with an interstimulus interval of 100-800 ms, and it peaks at about 500 ms (Freedman et al. 1987; Nagamoto et al. 1989).

Attentional Blink in an RSVP Paradigm


name all 26 letters of the English alphabet correctly. An additional three patients refused to participate. No subjects were excluded because of known hearing loss, uncorrected vision, or color blindness as assessed by the Ishihara (1990) test. The final size of the schizophrenia group was therefore 24 (12 male and 12 female). The mean (± standard error of the mean [SEM]) age was 34.3 ±1.8 years, with a range of 21-51 years, and the mean (± SEM) years of education was 12.2 ± 0.6 years. The mean (± SEM) age of illness onset was 26.4 ±1.5 years, and the mean (± SEM) duration of illness was 7.9 ± 1.9 years. The first language of acquisition of all accepted patients was Cantonese-Chinese. Among the 24 schizophrenia patients, 18 were maintained on conventional antipsychotic medication (e.g., haloperidol) with an average (± SEM) dose of 721.6 ± 192.0 milligrams chlorpromazine equivalent per day (Davis 1974). Three patients were maintained on atypical antipsychotic medication (clozapine, olanzapine, or risperidone), and one patient was maintained on both conventional and atypical antipsychotic medications. Two patients were previously maintained on conventional antipsychotic medication but were drug-free for at least 4 days. Mean (± SEM) total score on the Positive and Negative Syndrome Scale (Kay et al. 1987) was 68.5 ± 3.9, with the mean (± SEM) scores for positive and negative symptom scales being 16.4 ±1.5 and 14.3 ± 1.4, respectively. The general pathology scale averaged 33.4 ± 1.9, and the mean (± SEM) score on the social aggressiveness scale was 4.5 ± 0.5. Healthy controls. Twenty-two healthy volunteers were recruited as control subjects, consisting of 10 males and 12 females, with a mean (± SEM) age of 32.1 ± 2.4 years and a range of 18-54 years. The mean (± SEM) years of education was 13.0 ± 0.5, with a range of 7-16 years. All subjects were first asked to complete a self-report questionnaire in Chinese, based on which the screening criteria for selection of control subjects (as described below) were applied. The questionnaire also included questions on the subjects' handedness, daily tobacco and weekly alcohol consumption, and necessary demographic data (sex, date of birth, level of education, profession). This was followed by administration of the Ishihara test of color blindness and assessment of the subjects' ability to name all 26 letters of the alphabet correctly. Subjects with a personal or family history of psychiatric illness, a history of drug or alcohol abuse, or a history of neurological illness (such as stroke, meningitis, or epilepsy), along with subjects who had taken psychiatric medication, were excluded. Second, subjects who had

Methods Subjects Patients. Thirty-three inpatients satisfying DSM-IV criteria for schizophrenia (American Psychiatric Association 1994) were screened from the acute psychiatric unit in Queen Mary Hospital, Hong Kong. They all underwent a clinical interview, administered by one of two psychiatrists (R.Y.L.C. and E.H.Y.C), designed to evaluate current mental and medical state, psychiatric symptoms, course of illness, and personal medical history. Patients with a history of substance abuse, neurological illness (such as epilepsy, stroke, or meningitis), or mental retardation were excluded. Four subjects were excluded on this basis. Two more patients were excluded after failing to

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to the demand on limited capacity processing, although they might disagree on the precise psychological nature or functioning of the limited capacity system. Accordingly, AB (i.e., reduced accuracy of T2 identification) stems from a strain on this limited capacity system induced by the demand of Tl identification and processing. Given the general finding that schizophrenia is associated with reduced attentional span or sustained attention (Nuechterlein and Dawson 1984) as well as impairments in working memory (e.g., Goldman-Rakic 1994; Fleming et al. 1997; Goldberg et al. 1998), a potentiated and protracted AB might therefore be expected to be seen in schizophrenia patients. This is because the deleterious effect of Tl identification upon T2 identification would be more pronounced in schizophrenia patients than in controls. The two contrasting predictions described above have illustrated two characterizations of the psychological nature of the attentional dysfunction in schizophrenia. While the former hypothesis described this dysfunction as a form of overattention, the latter hypothesis emphasizes the lack of sustained or effortful attention (whether it stems from reduced processing capacity or impaired working memory). The present study would be instrumental in distinguishing the relative significance of these two aspects of attentional dysfunction on the AB paradigm, even though they might contribute equally to the genesis of schizophrenia symptoms. The present study employed solely medicated patients in the schizophrenia group. Thus, the effects of the disease cannot be distinguished from the effects of antipsychotic medication. It is therefore limited in its scope of interpretation compared to studies in which never- or nonmedicated patients are recruited. Nevertheless, the results obtained should be conducive for comparison with reports investigating other attentional paradigms in comparable patient samples and should pave the way for future studies of AB and schizophrenia.

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Stimuli and RSVP Sequences. There were two distinct sets of stimuli: distractors and targets. They were always displayed in the center of the monitor screen. The distractor set comprised eight numeric stimuli: 2, 3, 4, 5, 6, 7, 8, and 9. The target set comprised eight letter stimuli: A, C, E, J, K, R, T, and Y. They were presented in the black "system font" against a grey background (80% white and 20% black). All stimuli were approximately 15 mm high and 12 mm wide. At a viewing distance of 70 cm, they subtended a visual arc of 1.23° vertically. A white plus sign, "+," of the same font, subtending a visual arc of 0.41°, served as the fixation point. Each RSVP sequence comprised 16 to 20 stimuli, among which either one or two stimuli were drawn from the target set, with all remaining stimuli drawn from the distractor set. The distractors were generated randomly with the constraint that no numeral distractor could repeat itself within five consecutive serial positions in an RSVP sequence. The targets were randomly drawn from the target set with the constraint that no letter target could appear more than once in a block of four consecutive trials. Each stimulus within an RSVP sequence was presented for a duration of 100 ms. Stimuli were separated by an interstimulus interval (ISI—the time interval between the offset of one stimulus and the onset of the next) of 16.67 ms. This time interval corresponded to the minimum ISI afforded by our cathode ray

Procedures. On the day of testing, the subjects were first invited to sit comfortably in a quiet room where their ability to name all 26 letters of the alphabet was ascertained. They were then informed that the task they were going to participate in involved the use of a computer and a monitor and that the nature of the experiment was to evaluate visual perception and attention. Neither speed nor accuracy was explicitly emphasized; the subjects were encouraged to take their time to respond and to guess if they felt uncertain. Preliminary session. The subjects were instructed that there would be a series of stimuli appearing in the middle of the screen in rapid succession and that their task was to identify any letter target stimulus or stimuli that might appear, then report the stimulus or stimuli to the experimenter at the end of the sequence. The initiation of a trial was controlled by the subjects via an external keypad, which triggered the appearance of a fixation point in the center of the screen. This point remained on the screen for 180 ms. The stimulus sequence commenced 500 ms following the offset of the fixation point. At the end of the sequence, subjects were required to report verbally on the identity of any targets) and the order of their appearance if more than one target was detected. The experimenter then recorded the response via an external keyboard. The subjects were then allowed to initiate the next trial at their own pace.

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Apparatus. The stimuli were generated by a portable IBM-compatible personal computer with a clock speed of 150 MHz (Slimnote-9150TZ, Twinhead Co., Taiwan) and displayed on a 14-inch color monitor (Phillips, Holland) with a retrace cycle of 60 Hz. The computer program was prepared using the software Experimental Run Time System, version 3.18 (BeriSoft Cooperation, Frankfurt, Germany). The experimenter employed an external keyboard to key in the verbal response of the subject. The subject was given a separate keypad (BeriSoft Cooperation, Frankfurt, Germany) to initiate an RSVP sequence.

tube monitor—that is, at one retrace cycle. Hence, the sequencing of an RSVP stream could be represented as Stimulus n (100 ms) -»ISI (16.67 ms) -> Stimulus n + 1 (100 ms) -> ISI (16.67 ms) -» Stimulus n + 2 (100 ms) —> ISI (16.67 ms), and so on. The position of the first target (Tl) within an RSVP sequence varied among five possible serial positions: 3, 4, 5, 6, and 7. The first target would be the only target if the given RSVP sequence was scheduled to contain one target only. For dual-target sequences, the serial position of the second target (T2) lagged behind Tl by 1, 2, 3, 4, 5, 6, 8, 10, or 12 serial position(s). Hence, there were nine possible T1-T2 lag conditions, with the number of intervening distractors between Tl and T2 ranging from a minimum of 0 (i.e., Lag 1 condition) to a maximum of 11 (i.e., Lag 12 condition). T2 was always followed by at least one distractor stimulus (Chun and Potter 1995, p. I l l ) , to ensure reliable generation of the AB effect (see Giesbrecht and Di Lollo 1998). In the present study, instead of stopping at Lag 8 like Raymond et al. (1992) and Chun and Potter (1995), we decided to increase the maximum lag length to 12. This was done to accommodate the possibility that any aberration of the AB effect in schizophrenia patients might involve a delay or a protraction in its expression.

received any medication in the past week; who had known hearing loss, unconnected vision, or color blindness; or whose first language was not Cantonese-Chinese were further excluded. No controls were excluded on the basis of these criteria. The two groups were highly comparable in terms of age (F < 1.0, nonsignificant [ns]) and sex ratio (%2 = 1.09, df= 1, ns). They also did not differ in terms of years of education or the raw score on the revised Wechsler Adult Intelligence Scale, Information Subscale (WAIS-R-I, Hong Kong version, Hong Kong Psychological Society 1989) (both F's < 1.0, ns).



get identification as a function of the presentation rate. This session consisted solely of trials with RSVP sequence embedded with one target only (i.e., single-target trials). Subjects were informed that the sequences would be appearing at different rates, that each sequence contained only one target, and that their task was again to identify the target stimulus. This part also comprised two blocks, separated by a 2-minute interval. There were eight possible rates (rate = 1/period) of stimulus presentation, as represented by the length of time that each stimulus remained on the display monitor. These were 16.7, 33.3, 50.0, 66.7, 83.3, 100.0, 133.3, and 150.0 ms. The ISI was always the minimum (i.e., 16.7 ms, or one retrace cycle). The target stimulus could appear in one of five possible serial positions. The presentation rates and target stimulus locations were fully counterbalanced in a factorial design with 4 X 10 = 40 trials per block, arranged in a pseudorandom order of presentation. Among the 46 subjects, 2 schizophrenia patients failed to complete the second part of the experimental session and therefore contributed no data to the analysis of that part of the experiment. Statistical Analysis. With the exception of Shakow's cooperativeness scores, which had a highly skewed distribution and were therefore subjected to nonparametric analysis, all data sets were submitted to parametric analysis of variance (ANOVA) with the appropriate design. These analyses were conducted using Statistical Package for Social Sciences (SPSS) for Windows, version 8 (SPSS Inc., Chicago, Illinois). Post hoc pairwise comparisons between individual means were carried out following the emergence of significant main effects or interaction using Tukey's HSD (honestly significant difference) procedure as described by Howell (1997). Additional a priori t comparison using the Dunnett's procedure and Pearson's parametric linear correlation analysis was also carried out.

Results The experimental session consisted of two parts that were designed to evaluate the expression of AB and the performance on single-target identification, respectively (see Procedures). The first part consisted of single- as well as dual-target trials, whereas the second part consisted of single-target trials alone. Reports of the data collected from the first part are organized into two sections below: (1) the expression of AB, derived from the performance of T2 identification on dual-target trials; and (2) the performance on Tl identification on both single- and dual-target trials presented in

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The practice session was divided into two parts. The first part familiarized the subject with the procedure and the demands of the task. It consisted of sequences containing 1 target and 14 distractors, with the temporal location of the target randomized among the five possible serial positions described earlier (see Stimuli and RSVP Sequences). The subjects were allowed to practice until they had responded correctly in five out of the preceding ten trials. They were allowed a maximum of 30 such trials. In the second part of the practice session, the subjects were again presented with an RSVP sequence per trial, but now each sequence consisted of two target stimuli. There were 20 such trials on this dual-target identification task. The location of the first target was again randomized among the five possible serial positions, and the lag of T2 relative to Tl (i.e., T1-T2 lag) was varied randomly among the nine possible conditions (see Stimuli and RSVP Sequence, above). The subjects were allowed to proceed to the experimental session if they could identify either or both targets of a sequence in 10 out of the 20 trials. All subjects attained criterion performance. Experimental session. The experimental session also comprised two separate parts. At the end of each part, subjects were rated on a five-point scale of cooperativeness (Shakow 1977). In addition, at the end of the first part, subjects were instructed to take a short rest prior to being evaluated on the WAIS-R-I. The task demand and procedure throughout the experimental session were largely identical to that of the practice session. The subjects were again informed that a series of stimuli would appear in the middle of the screen in rapid succession and that their task was to identify any letter target stimulus, then report it to the experimenter at the end of the sequence. The initiation of a trial was again under the direct control of the subject via the same external keypad as in the practice session. The first part of the experimental session was designed to assess the AB effect. It consisted of two consecutive blocks of 50 trials each, separated by a 2-minute interval. In this part, an RSVP sequence was embedded with either one target stimulus (single-target trial) or two target stimuli (dual-target trial). For the latter, the relative location of the first and second targets varied among one of nine possible T1-T2 lag conditions (see Stimuli and RSVP Sequences above). Hence, there was a total of ten different trial types. Within each trial type, the location of Tl within a given sequence also varied among five possible locations (see Stimuli and RSVP Sequences). Trial types and Tl locations were fully counterbalanced (5 X 10) with 50 trials per block, presented according to a pseudorandom order. The second part of the experimental session was a single-target identification task designed to assess RSVP tar-


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the first part of the experimental session. Following these sections are descriptions of the results of the second part of the experimental sessions, which explored target identification rate in single-target RSVP streams. Additional correlative analyses and the analysis of the cooperativeness scores are presented in the last two sections.

Figure 1. Performance on the dual-target identification task. The probability of T2 (second target) identification given thatTI (first target) was successfully identified, denoted as Pr(T2/T1), was expressed as a function of T1-T2 lag. The marked reduction in Pr(T2/T1) between Lag 2 and Lags 5-6, relative to the Lag 1 condition, constitutes the attentional blink effect. Error bars represent twice the standard error of measurement of the associated mean values. 1.0

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Part 1, AB Effect. The AB effect was evaluated by calculating the conditional probability, Pr(T2/Tl), which referred to the rate of T2 identification on trials in which Tl was correctly identified (see Chun and Potter 1995). The dependent measure Pr(T2/Tl) is expressed as a function of lag length between Tl and T2 (T1-T2 lag) in figure 1. As illustrated, performance at Lag 1 was highly comparable between groups. The AB effect was seen as a marked reduction of Pr(T2/Tl) relative to the Lag 1 condition, which began from Lag 2. This effect decayed gradually as lag length increased and eventually returned to a level that was comparable to performance at Tl, giving rise to a typical U-shaped curve, which was apparent in healthy controls as well as in the schizophrenia group. Graphically, the patients appeared to exhibit a more pro-

nounced AB effect, which was accompanied by an overall reduction in performance relative to controls. These impressions were confirmed by a 2 X 9 (group X lag) split-plot ANOVA on the dependent measure, Pr(T2/Tl), which yielded a significant effect of lag (F[8, 352] = 34.39, p < 0.001) and of group (F[l, 44] = 12.89, p < 0.001). There was no evidence for a significant interaction (F[8, 352] = 1.60, p = 0.125), which undermined the visual impression that the AB effect seen in schizophrenia patients was different in form from that in controls. However, the asymptotic performance in the final two lag conditions (Lags 10 and 12) might have inflated possible type II error. Hence, a restricted analysis was performed excluding data from the last two lags. With the last two lags excluded, the analysis would consist of a range of lag length that matches that adopted by previous reports, which were sufficient to capture the entire development of the AB effect (e.g., Chun and Potter 1995). This restricted analysis again failed to yield a significant interaction (F[6, 264] = 1.87, p = 0.086), but a significant interaction in the quadratic trend of the repeated measure was obtained (F[l, 44] = 6.13, p < 0.05). The fact that the expression of the AB effect was best described by the quadratic trend sug-

Attentional Blink in an RS VP Paradigm


gested tentatively that the expression of the AB effect might indeed be different from group to group. This possibility was therefore further explored with additional analyses. Because of the existence of baseline differences between groups, it was difficult to compare the expression of AB as such. As a solution, a more informative measure—a suppression ratio (Estes and Skinner 1941)—was adopted. It was calculated using the expression below to index the degree of performance reduction relative to the level of performance on single-target identification (Tl alone) trials within the first part of the experimental session. The value of the suppression ratio is bounded by unity (asymptotically) and zero, with 0.5 representing "no change." The lower the value of the suppression ratio, the greater is the reduction in Pr(T2/Tl) at a given lag relative to the single-target identification rate. Pi 0.5) = 0. Hence, the use of a one-



the two longest periods, when asymptotic performance was evident in both groups. A 2 X 8 (group X period) split-plot ANOVA on RSVP target identification accuracy revealed a significant effect of group (F[ 1,42] = 12.18, p< 0.005) and of period (F[7, 294] = 123.61, p < 0.001) but not of their interaction (F < 1.0, ns).

Part 2, Single-Target Identification. As suggested by the analyses for part 1 (above), the schizophrenia group consistently exhibited an overall reduction in the accuracy of target identification within RSVP sequences. This finding was further supported by the data collected in the second part of the experimental session, which involved a systematic evaluation of target identification rate using single-target RSVP streams under various rates (frequencies) of presentation (i.e., I/period). Two schizophrenia patients did not complete this task because of fatigue, so the schizophrenia group size was reduced to 22 in this analysis. As depicted in figure 4, there was a steady increase in accuracy as a function of period (i.e., as presentation rate slowed down) in both groups. The schizophrenia group was always performing more poorly than controls, even at

Correlative Analysis. Given that a negative relationship is suggested to exist between the accuracy of target identification in RSVP stream and the magnitude of AB (Seiffert and Di Lollo 1997), correlation analyses were carried out to evaluate whether such a relationship was present in our data (figure 5). To this end, an index for the individual's magnitude of AB was calculated with a slight modification of the method described by Seiffert and Di Lollo (1997; see legend of their figure 6 on p. 1071). This index represents the summative score obtained by taking the difference between 0.5 and the suppression ratio on the dual-target trials from Lag 2 onward, then summing the values, for each subject. This index was correlated with either (1) the individual's average Tl identification rate on the same dual-target trials from Lag 2 onward, or (2) overall per-

Figure 4. Accuracy, expressed as probability, of target identification in the single-target identification task in the second part of the experimental session.This is depicted as a function of period, defined as the length of time that each stimulus remained on the display monitor. Error bars represent twice the standard error of measurement of the associated mean values. Q.

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1 condition, at which both groups exhibited a clear reduction, which gave rise to a significant effect of trial type (F[9, 396] = 10.35, p < 0.001). Post hoc comparisons revealed that the accuracy at Lag 1 was significantly lower than for all other trial types (min q10 = 8.51, df = 396, p < 0.05). There was no significant interaction.


V. Cheung et al.

Figure 5. Scatter plots illustrating the relationship between an index of the magnitude of AB (as defined in the Results section) and either (a) the identification rate of theT1 in dual-target streams averaged from Lag 2 onward or (b) the overall performance on the single-target task in the second part of the experiment. The straight lines (denoted as "C" and "S") correspond to the fitted linear regression line for the healthy control group (n = 22) and the schizophrenia group (figure 5a: n = 24; figure 5b: n = 22), respectively.The arrow (-») points to the outlying control subject's value, rejection of which would reduce the significance of the correlation within the control group to a nonsignificant level (see Results).

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