Copyright 1992 by the American Psychological Association Inc. 0021-843X/92/S3.00

Journal of Abnormal Psychology 1992, Vol. 101, No. 1,53-60

Span of Apprehension in Schizophrenic Patients as a Rmction of Distractor Masking and Laterally Irene J. Elkins and Rue L. Cromwell University of Kansas

Robert E Asarnow

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Neuropsychiatric Institute University of California at Los Angeles Twenty schizophrenic patients, 10 depressed control patients, and 20 normal control subjects were compared in a forced-choice, target-detection method for assessing the span of apprehension. The detection task required the subject to report which of 2 target letters was presented among 7 other (distractor) letters. Performance accuracy was examined as a function of target location and whether the distractor letters were masked after their presentation. The backward masking of the distractors improved target-detection accuracy of both control groups but reduced accuracy of the schizophrenic group. In addition, schizophrenics performed particularly poorly on targets located in the left half or lower half of the display. These results suggest that response to the masking of distractors may be a new index of attentional shortcoming in schizophrenia. Various theoretical explanations for the target location findings are also discussed.

stein, Asarnow, & Browne, 1986).' Asarnow, Granholm, and Sherman (1991) suggested that the task is a measure of a stable, possibly genetically transmitted deficit. Also, evidence that performance on this task may be predictive of treatment outcomes and response to certain medications is emerging (Asarnow, Marder, Mintz, Van Putten, & Zimmerman, 1988). Although Miller, Chapman, Chapman, and Barnett (1990) found deficits in schizophrenics' span of apprehension by using a full-report procedure, Cash, Neale, and Cromwell (1972) had earlier found no schizophrenic impairment on full report. If schizophrenics exhibit more impairment on the forced-choice, target-detection test than on full report, it could be because of difficulty in disengaging from irrelevant stimuli. Such an interpretation is consistent with Cromwell and Dokecki's (1968) disattention formulation, which suggests that schizophrenics are unable to "disattend," or withdraw attention from, stimuli that they have already processed. On the detection task, the irrelevant stimuli that must be disattended from are the letters ac-

Measures of attention and information processing have shown considerable promise for advancing the understanding of schizophrenia. Among these measures, the forced-choice, target-detection method for assessing the span of apprehension (Estes & Taylor, 1964) seems highly relevant to schizophrenia. In this task, the subject examines a brief visual display of an array of stimuli for either one of two targets (e.g., a Tor an F). Other letters in the display, which serve as "noise" or distractor elements, are to be ignored as the subject reports or guesses which of the two targets occurred. Unlike the full-report method, in which the subject reports as many letters as possible, the target-detection method requires that each letter be analyzed just well enough to be distinguished from the two possible targets. Impairment on this task has been exhibited by actively psychotic schizophrenics (Asarnow & Sherman, 1984; Dobson & Neufeld, 1987; Neale, 1971; Neale, Mclntyre, Fox, & Cromwell, 1969), by schizophrenics in remission (Asarnow & MacCrimmon, 1981; Asarnow, Stefry, MacCrimmon, & Cleghorn, 1978), by adopted-away children of schizophrenic mothers (Asarnow, Stefry, MacCrimmon, & Cleghorn, 1977), and by a subset of the healthy relatives of schizophrenics (Wagener, Hogarty, Gold-

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In one study, span deficits were not specific to schizophrenia; they could be found during manic states as well (Strauss, Bohannon, Stephens, & Parker, 1984). Other recent research has failed to replicate earlier findings of impairment in schizophrenics in remission (Dobson & Neufeld, 1987) and in children at biological risk for schizophrenia (Harvey, Weintraub, & Neale, 1985). However, unlike some earlier studies, all three of these studies involved the use of narrow visual angles (i.e., 2° or 3°) for the stimulus presentations. Asarnow, Stefry, and Waldman (1985) suggested that deficits in schizophrenics may be more apparent with wider visual angles. Recently, Miller, Chapman, Chapman, and Barnett (1990) found that increasing either the visual angle subtended by the display or the number of distractor letters results in increases in the discriminating power of this task (i.e., the sensitivity of the task to differential levels of ability).

This research was supported by a grant from the General Research Fund at the University of Kansas. We thank all the patients and staff at the Colmery O'Neill Veterans Administration Medical Center in Topeka, Kansas, who participated in and assisted us with this project, especially Tom Patterson, Chief of Psychology. We are also indebted to James Juola, Douglas Denney, Ken Sewell, and others at the University of Kansas. Correspondence concerning this article should be addressed to Irene Elkins, Department of Psychology, University of Kansas, Lawrence, Kansas 66045.

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I. ELKINS, R. CROMWELL, AND R. ASARNOW

companying the target that are not reported. This interpretation guided some aspects of our investigation. The disattention formulation would apply primarily if the stimuli in the detection task were scanned serially. According to feature-integration theory (Treisman & Gormican, 1988), if a target does not possess a simple feature that is absent from the distractors in a visual search task, items must be scanned serially. Electrophysiological evidence supporting this conclusion was recently presented by Luck and Hillyard (1990). The featural similarity of the target and the distractor letters used in our study (i.e., all letters had horizontal or vertical line segments) would tend to enhance serial more than parallel processing modes. However, even if some parallel processing is involved, the schizophrenic's deficit on this task could be related to a reduced ability to inhibit distracting information, which has been used to explain the negative priming and dichotic listening results obtained by other investigators (e.g., Beech, Powell, McWilliam, & Claridge, 1989; Spring, Lemon, Weinstein, & Haskell, 1989). The study of backward pattern masking in schizophrenics and in persons at risk for schizophrenia has also been an active area of interest. Backward pattern masking, which involves the presentation of a visual pattern just after a stimulus and in the same location, is thought to limit the amount of processing performed on a stimulus (Saccuzzo, Hirt, & Spencer, 1974). Schizophrenics are particularly susceptible to the degradation of information through backward masking (Saccuzzo et al., 1974). In addition, persons who are hypothetically psychosis prone, particularly those with elevated scores on certain Minnesota Multiphasic Personality Inventory (MMPI) scales, appear to be similarly susceptible to masking effects (Balogh & Merritt, 1985; see review by Balogh & Merritt, 1987). None of these investigators assessed masking that is based solely on information not relevant to the task. We examined this type of masking by using patterns to mask only the distractor letters on the aforementioned target-detection task. If schizophrenics are indeed impaired because their attention is locked onto irrelevant stimuli, and if masking limits the amount of irrelevant processing that can be performed, their deficit on the detection task should be essentially eliminated. Otherwise, the masks might represent additional irrelevant input. In a pilot study involving 15 college undergraduates in an abbreviated version of the task, masking did indeed produce a slight improvement in performance. Therefore, the question was whether this masking would also help eliminate the deficit shown by schizophrenics. The assumption that processing is completely halted at the onset of the mask has been questioned (Schuck & Lee, 1989). Eriksen (1980) contended that when the mask structurally resembles the target and is presented at a high energy level, an integration of the features of the mask and the target occurs. If this happens, the target becomes difficult to identify. An alternative explanation is that a mask could simply interrupt processing of a target through the inhibition of the sustained processing channels by transient channels in the visual system. These transient channels are activated by the onset of the mask (Breitmeyer, 1984). Whether integration or interruption predominates may depend on stimulus parameters. The masking procedure in our study was designed to in-

terrupt the processing of the nonrelevant letters, thereby limiting the amount of processing that could be performed on them. The 60-ms stimulus-onset asynchrony between the letters and masks is within the range (50-100 ms) in which normal subjects are interrupted (Legge, 1978). In addition, the masking stimuli were presented at a lower energy level than were the preceding letter stimuli. Along with masking, target position was also evaluated in this study. Emphasis was placed on laterality (left vs. right field) of target detection. Accordingly, this exploration focused on the possible functional contributions of the right and left hemispheres of the brain. In addition, the distance of the target from central fixation, a component of visual angle, and the vertical location of the target were also investigated. Although attentional deficit in schizophrenia has been a long-standing issue, theories regarding hemispheric dysfunctions of schizophrenics have also proliferated. Most of these theories have focused on schizophrenia as a left-hemisphere temporal-limbic dysfunction of overactivation (Gruzelier, 1984; R. E. Gur, 1978). Other theories posit a right-hemisphere deficit for schizophrenia (Cromwell, 1987; Venables, 1984). These theories agree with previous ones in which left-hemisphere hyperarousal is associated with deficits in higher (or later) processing functions. However, they see this deficit as originating in the right hemisphere. One problem may be the transfer of activities normally assumed by the right hemisphere to the left hemisphere (Venables, 1984). Another problem may be the transfer of faulty information from a dysfunctional right hemisphere to the left (Cromwell, 1987). In either case, it is proposed that the right hemisphere has a relative advantage for preattentional screening, and this screening is not working properly in schizophrenics. Results of psychophysiological and positron emission tomographic (PET) studies tend to support the notion of the predominance of the right hemisphere in attentional functions (R. C. Gur et al., 1983; Heilman & Van den Abell, 1980). We assessed the effect of laterality by presenting the target systematically to the right or the left visual field (RVF or LVF, respectively). With initial fixation at midfield, the RVF and the LVF project entirely to the contralateral hemispheres. Although visual input is initially segregated by this method, interhemispheric transfer may occur in later processing stages because of the greater number of callosal connections beyond the primary visual cortex (Marzi, 1986). This interhemispheric transfer could pose complications in lateral interpretation. However, because the detection task emphasizes earlier, precategorical processes and feature detection, its interpretation in terms of lateral brain allocation is more viable than in some cases. Because earlier split-field studies on this type of masked span task are lacking, and because theory on hemispheric differences in schizophrenia is contradictory, the null hypothesis was predicted for performance as a function of laterality. Method

Subjects Subjects were male inpatients and hospital staff from the Colmery O'Neil Veterans Administration Medical Center in Topeka, Kansas. Twenty schizophrenic inpatients, 10 depressed inpatients (psychiatric

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SPAN OF APPREHENSION IN SCHIZOPHRENICS controls), and 20 hospital staff (normal controls) participated in the study. The groups did not differ in age in years (for schizophrenics, M= 37.0, SD = 5.8; for depressed subjects, M = 39.1, SD = 9.7; and for normal controls, M = 37.1, SD = 10.7) or in years of education (for schizophrenics, M = 13.1, SD = 1.8; for depressed subjects, M = 13.6, SD = 1.6; and for normal controls, M = 14.0, SD = 2.1). Of the schizophrenic subjects, 19 were receiving antipsychotic medication at the time of the study: 6 were taking haloperidol; 4 were taking dibenzoxazepine; 2 each were taking chlorpromazine, fluphenazine decanoate, mesoridazine, and thioridazine hydrochloride; and 1 was taking trifluoperazine hydrochloride. The mean daily dose, expressed as chlorpromazine equivalents (Held, Cromwell, Frank, & Fann, 1970), was 525 mg. Thirteen of the schizophrenic subjects receiving neuroleptic agents were also taking anticholinergic agents (11 were taking benzotropine mesylate, and 2 were taking trihexyphenidyl hydrochloride). Of the depressed subjects, 8 were taking some form of psychoactive medication, either antidepressants (1 each was taking desipramine hydrochloride, trazodone hydrochloride, doxepin hydrochloride, and perphenazine-amitriptyline hydrochloride) or antianxiety agents (2 were taking alprazolam, and 1 was taking diazepam). One depressed subject was taking both antipsychotic (dibenzoxazepine) and anticholinergic medication (benzotropine mesylate). Of the normal control subjects, 7 were taking medication for physical conditions; none were taking psychiatric medication. Candidates for the two inpatient groups were initially screened for a case folder diagnosis of schizophrenia or major depression. Patients with schizoaffective or bipolar diagnoses were not included. Patients with a previous diagnosis of schizophrenia were not considered for the depressed control group, regardless of current diagnosis. A concurrent diagnosis of dysthymia or posttraumatic stress disorder was not used as a basis for exclusion from the depressed group. Patients with a history of organic brain damage, mental retardation, severe substance abuse, or oculomotor problems that may have prevented proper visual scanning of the stimuli (e.g., nystagmus) were excluded. In addition, patients exhibiting facial symptoms or severe extremity symptoms of tardive dyskinesia were excluded as well. Of the 58 patients who met the case folder criteria, 11 schizophrenics and 4 depressed patients declined to participate. Three left the hospital before testing was completed. Patients were then independently diagnosed with the National Institute of Mental Health (NIMH) Diagnostic Interview Schedule (DIS; Robins, Helzer, Croughan, & Ratcliff, 1981), Version III-A, and diagnoses were based on DSM-HI-R criteria (American Psychiatric Association, 1987). Three subjects whose hospital diagnoses were not confirmed were excluded. For the remaining subjects, the interview and the case folder data were used to code indices of current symptoms. Normal control subjects were also interviewed. They were excluded if they had taken psychiatric medications, if they had abused alcohol or drugs, if they had any complicating physical conditions, or if they or a member of their extended family had ever been hospitalized for psychiatric illness. A history of major psychiatric illness in self or family resulted in the exclusion of 2 normal control subjects. One was excluded because of a history of extensive substance abuse. All subjects were administered a 10-item questionnaire (Annett, 1967) to determine handedness. Classification was based on a 90% lateral preference. Of the schizophrenic subjects, 16 were right-handed and 4 were left-handed; of the depressed subjects, 8 were right-handed, 1 was left-handed, and 1 was mixed-handed; and of the normal control subjects, 16 were right-handed and 4 were left-handed. For patients, we recorded the number of years since first hospitalization (for schizophrenics, 11 years; for depressed subjects, 8 years), and the number of admissions (for schizophrenics, 11; for depressed subjects, 7) were recorded. Although the patients in the sample were hospitalized rather chronically, all had been admitted recently (instead of being long-term

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patients), and there were no significant correlations between chronicity and performance on the dependent measures in the study.

Apparatus and Materials The test stimuli were presented in a Scientific Prototype (Model N-1000) three-field tachistoscope. Exposure duration was controlled by a Scientific Prototype lamp-driver and logic module that also triggered a millisecond timer at the onset of a trial. A hood attached to the viewing port kept head position constant and eliminated extraneous light. To maximize the lateralization of stimuli after central fixation, the stimuli were presented at durations (10 ms) too brief for eye movements to occur, in accordance with the standard techniques of R. E. Gur(1978). The 64 stimulus displays consisted of eight-letter arrays placed on 5 X 7 in. (12.7 X 17.8 cm) white index cards. The letters were boldface black and 0.5 in. (1.3 cm) in height. Each array subtended 5.6° of visual angle horizontally and 4.3° vertically. Each array contained one target letter (Tor F) and seven nonrelevant (distractor, noise) letters. Tand F each occurred in half of the arrays. Within an imaginary 4 X 4 matrix, the target letter was assigned to each of the 16 matrix positions an equal number of times. This enabled us to evaluate target position systematically as a function of laterality (left vs. right), verticality (top vs. bottom), and distance (proximal vs. distal) of the target in relation to the fixation point. Targets that occurred in the inner four matrix positions were considered proximal; those occurring in the outer 12 matrix positions were considered distal. Nonrelevant letters were randomly assigned to the remaining positions with the restriction that an equal number of letters (four) were assigned to each side of the card. Nonrelevant letters were chosen randomly without replacement from letters that contain at least one horizontal or vertical line segment. The four respective columns of the imaginary matrix were 1.0° and 2.5° in visual angle to the right or left of the center of each card (i.e., the fixation point). These distances were chosen to minimize the mixing of right- and left-field inputs, which may occur at the center of the foveal region, and to avoid visual acuity problems in the periphery. Another set of cards contained the masks for the seven respective noise letters. Each mask was a superimposed X and O (®) and was slightly larger than the respective letter being masked. In order to prevent the masking stimuli from impeding perception of the target stimulus, they were of slightly lower contrast than the letter stimuli. The fixation field was illuminated at 16.68 Ix/s, the exposure field at 22.53 Ix/s, and the masking field at 19.90 Ix/s.

Procedure Visual acuity was tested with a Snellen eye chart. Six subjects were excluded because of vision worse than 20/40. In addition, 2 others were excluded later during practice trials on the task for lack of peripheral target detection. The tachistoscopic procedure followed the vision screening and the interview. For normal control subjects, all procedures were conducted in a single session. Because of the longer interview required for patients, they underwent the interview and vision screening in one session and the tachistoscopic procedure in another. These procedures were conducted on either the same day or succeeding days. In the tachistoscopie task, subjects were given a standard set of instructions and a series of practice trials. The instructions stressed the importance of fixating on the center. The practice trials included 10 with only a Tor an Fpresent, 12 with eight-letter displays, and 10 with eight-letter displays followed by masking. The practice trials were given to ensure that the subject understood the instructions and was habituated to the task. Additional instructions were given if necessary.

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I. ELKINS, R. CROMWELL, AND R. ASARNOW

Subjects whose performance did not exceed what would be expected by chance on the practice trials were to be excluded from the study. As a result, 2 schizophrenic subjects were excluded when it became apparent that they were too mentally disrupted to comprehend the task and respond to it appropriately; no depressed or normal control subjects were excluded. Each subject was then tested in 128 trials with eight-letter displays. These displays were based on the 64 stimulus cards, each presented in the masked and unmasked conditions. Subjects were tested in 16 blocks of eight trials each. Sequence was randomized under the restriction that equal numbers of masked and unmasked trials and equal numbers of RVF and LVF target presentations occurred within each block. A 30-s rest period was provided between blocks, and a 5-min rest period was provided halfway through the procedure. The task lasted about 1 hr for normal subjects but usually took a few minutes longer for the patients. Each trial began with the experimenter's verbal ready signal. Then the subject fixated on the dot in the center of an illuminated channel (which was always present for light-adapted presentation). After 500 ms, the letters were displayed for 10 ms.2 In half of the trials, the letters were followed by 50 ms of illuminated blank screen, then 100 ms of masking stimuli, and again by the blank illuminated screen. Placed in a second field of the tachistoscope, the seven masking stimuli of each trial were presented in the same matrix positions as the seven nonrelevant letters in the first field. In the other half of the trials, the 10-ms target/distractor presentation was followed by a blank illuminated screen, which terminated the trial. At the end of each trial, masked or unmasked, the subject gave a verbal response as to which target was present. A second experimenter recorded this response. Intertrial interval varied with the individual subject's response rate, but an average of approximately one trial every 10 s was maintained.

Results The data were analyzed in a 3 (groups) X 2 (masking) X 2 (laterality) X 2 (top vs. bottom) X 2 (proximal vs. distal) mixedmodel analysis of variance with group as a between-subjects variable and masking, laterality, verticality, and distance as within-subject (repeated) variables. Percentage of correct responses was the dependent variable. Means and standard deviations for all three groups in each condition are presented in Table I. 3 Significant main effects were found for group, F(2, 47) = 22.35, p < .001; for laterality, F(l, 47) = 25.29, p < .001; for verticality, F(l, 47) = 31.62, p < .001; and for distance, F(l, 47) = 47.71, p < .001. The main effect of the masking variable was not significant, F(l, 47) = 0.10, p > . 10. Because the group main effect involved more than two conditions, it was broken down as follows: The depressed and normal control groups did not differ significantly from each other in overall performance, F(l, 47) = .68, p > .10; therefore, they were combined into a single group. Schizophrenics performed significantly more poorly than the combined control group, F(l, 48) = 47.50, p < .001.4 The other significant main effect findings may be accounted for by the higher percentages of correct performance in the RVF than in the LVF, in the upper than in the lower matrix positions, and in the proximal than in the distal matrix positions. All of the significant interactions involved group: for Group X Masking, F(2, 47) = 7.70, p = .001; for Group X Laterality, F(2,47) = 5.01, p < .02; and for Group X Verticality, F(2, 47) = 4.48, p < .02. None of the other possible two-way or higher order interactions reached acceptable levels (p < .05) of statistical significance.

The Group X Masking interaction is portrayed graphically in Figure 1, which depicts percentage of correct response as a function of the masked versus unmasked condition for the normal, depressed, and schizophrenic groups. As may be seen, the performances of the three groups were more similar to each other in the unmasked condition. In the masked condition, the schizophrenics diverge toward poorer performance, and the normal and depressed groups diverge toward better performance. Breakdown analyses verified that both of the control groups did better in masked than in unmasked trials: for the normal subjects, t(l 9) = -2.36, p < .05; for the depressed subjects, t(9) = —2.64, p < .05. In contrast, the schizophrenics performed significantly worse when the irrelevant stimuli were masked, t(19) = 3.15, p < .01. These analyses confirmed that whereas backward masking of irrelevant stimuli facilitated performances by the normal and depressed subjects, the performances by the schizophrenics were hampered by this procedure. Independent two-tailed t tests of mean difference scores indicated that the differences between performances by the schizophrenics and the combined control groups in the unmasked and masked conditions were significant, £(48) = 4.79, p < .001, but the differences between performances by the depressed and normal controls were not, ;(28) = —0.79, p > . 10. Also, the differences between performances by the schizophrenic and the combined control groups were significant under both masked conditions, F(l, 48) = 52.56, p < .001, and unmasked conditions, F(l, 48) = 20.93, p< .001. This analysis confirmed that the performance by schizophrenics was inferior to that of the other groups, even in the unmasked condition. The Group X Laterality interaction is portrayed graphically in Figure 2; percentage of correct responses is depicted as a function of RVF versus LVF laterality in the schizophrenic, depressed, and normal groups. As may be seen, all groups performed more poorly in the LVF, but the schizophrenics appeared to be especially debilitated there.5 Breakdown analyses indicated that the schizophrenic and 2

The pilot data collected from a sample of undergraduates indicated that a 10-ms duration was optimal for ensuring individual differences in performance, and for normal subjects, this value exceeded the critical stimulus duration for accurate, nonmasked identification. 3 Because of the tendency toward greater variability among the schizophrenics than among the control subjects, tests of homogeneity of variance were performed for all eight conditions so as to check for violations of assumptions. Variability differences were significant only for the proximal condition (Bartlett-Box F= 4.37, p < .05), and therefore no correction procedure was applied for the data analyses. 4 Ad hoc t tests confirmed that statistically significant (p < .05) differences between both the schizophrenic and depressed groups and the schizophrenic and normal groups were present in each analysis in which the results for the combined control group are reported, except for one: The mean scores between performances by the schizophrenic and depressed groups in top and bottom trials were not significantly different, f(28) = 1.71, p > .05. 5 Because the Group X Laterality and Group X Verticality interactions could result from ceiling effects in the control groups, we performed a test for normality, which showed that the data from the combined control groups did not differ significantly from a normal distribution (Kolmogorov-Smirnov z = .488, p > .90). Therefore, there was no need to transform the data for the interaction analyses.

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SPAN OF APPREHENSION IN SCHIZOPHRENICS Table 1

Percentage Correct by Group for Each Condition Depressed control

Schizophrenic Condition Masking condition Masked Unmasked Field position Left Right

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Top Bottom Proximal Distal

M

SD

M

SD

M

SD

68.6 75.1

10.1

85.9 81.2

7.0 6.2

87.2 84.2

6.8 5.8

65.2 78.5 77.1 66.5 78.3 69.7

9.6 7.9 7.8

80.8 86.4 85.5 81.7 89.1 81.7

6.9 7.2 8.2 6.5 7.5 6.5

83.6 87.8 87.8 83.5 91.5 83.7

7.7 7.2 5.2 6.8 5.4 6.3

6.4

10.1 10.9

7.9

depressed groups did significantly better on RVF than on LVF trials: for the schizophrenics, /(19) = —6.78, p < .001; for the depressed subjects, t(9) = -2.34, p < .05. The mean of the normal control group was in the same direction, although it did not attain statistical significance, £(19) = -1.95, p > .05. The mean differences between performances by the schizophrenics and the combined control groups in the RVF and the LVF were significant, £(48) = 3.36, p< .01, but differences between performances by the two control groups were not, £(28) = 0.40, p > .10. Significant differences between schizophrenics and the combined control groups were also present in both RVF condi-

90

Normal control

tions, F(l, 48) = 16.74, p < .001, and LVF conditions, F(l, 48) = 51.72, p O

i o

80

O

—

.10. Significant differences were also present between schizophrenics and pooled controls in top trials, F(l, 48) = 24.64, p < .001, and in bottom trials, F(l, 48) = 44.19, p < .001. Discussion The main concern of this study was whether schizophrenic impairment on the detection task would be eliminated or reduced if the distractor letters were backward masked immediately after they accompanied the relevant (target) letter. The findings clearly indicate that backward masking of the distractor letters not only failed to improve schizophrenic subjects' performance but, instead, made their performance worse. Their percentage of correct responses deteriorated by 6.5% when the distractors were backward masked as opposed to when they were unmasked. Of the 20 schizophrenics in the study, 16 performed more poorly with the masking of distractors. In contrast, the control groups did benefit from the masking. The masking may have helped them disengage attention from the irrelevant letters or may have served as a poststimulus cue for the location of the target letter. Although the gain from masking was significant in both groups, it was only moderate, in comparison with the loss shown by the schizophrenics. Seven of the 10 depressed subjects and 12 of the 20 normal

control subjects performed better with masking. The normal subjects' performance improved from 84% to 87% correct with the masking; the depressed subjects' performance improved from 81 % to 86% correct. It is notable that the deficit shown by schizophrenic patients with distractor masking cannot be attributed to a general effect of psychopathologic disorder; otherwise, the depressed control group would have been similarly impaired. If these results are stable and replicable, a new index of attentional shortcoming in schizophrenia may have been revealed. Theoretically, a major question concerns what phase or function in the processing of visual information accounts for these results. Several studies of iconic integration and decay have suggested that the formation and the decay of iconic memory (sensory storage characteristics) in schizophrenics are essentially identical to those in other subjects (Knight, Sherer, & Shapiro, 1977; Spaulding et al, 1980). Such findings would point to a faulty phase after these processes. One problem with this interpretation is that iconic integration and decay under conditions in which distracting visual stimuli are present have not been investigated. In addition, if schizophrenics are generally lacking in available processing resources (Nuechterlein & Dawson, 1984), the results could reflect an overload of input rather than a fault in the processing sequence. The schizophrenic impairment caused by distractor masking could also result from a hypersensitivity to onset transients (J. G. Flowers, personal communication, February 9, 1989). Several investigators have suggested schizophrenic dysfunctions in transient channel activity (Balogh & Merritt, 1987; Schuck & Lee, 1989), which is activated by onsets and offsets of visual stimuli (Breitmeyer & Ganz, 1976). The onsets of the target and distractor stimuli, and then the onset of the respective masking stimuli, may accumulate, resulting in impaired performance by schizophrenics. This would explain why the performances by schizophrenics become worse but not necessarily why those by normal and depressed control subjects improve. Such opposite effects of the masking on groups are, however, not surprising if there is a masking mechanism unique to schizophrenics as a result of abnormal transient channel activity, as suggested by Schuck and Lee (1989). Varying the spatial and temporal frequencies of the stimuli (e.g., the stimulus-onset asynchrony) may represent one way of examining the differential effects of masking on the schizophrenic and control groups. Some authors have contended that some adaptive process (disattention) is necessary to eliminate attention to immediately prior information in order for a person to be ready to process new information (Cromwell & Dokecki, 1968). Data indicating that this adaptive function is deficient in schizophrenics have been described (Cromwell & Dokecki, 1968; Salzinger, 1971). Masking the irrelevant elements apparently aids in disattention for normal and depressed subjects because they serially scan their sensory storages for the relevant target letter. That the masks did not help the schizophrenics disattend from the nonrelevant letters is clear. If the masks distract attention away from the target letter or if they add new (irrelevant) information from which to be disengaged, the findings of increased impairment of schizophrenics would be compatible with a disattention interpretation. Alternatively, if parallel as well as serial processes are tapped by this task, it is possible that schizophrenics may have difficulty in rejecting masks as irrelevant

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SPAN OF APPREHENSION IN SCHIZOPHRENICS (Knight, 1984) or that masks may act as distractors (Schuck & Lee, 1989). Field position of the target was also investigated, and main effects were found for laterality, vertically, and distance. The tendency for all groups to perform better when the target was located in the RVF supports the usual finding that the left hemisphere is superior in processing verbal (including letter) stimuli (e.g., Worral & Coles, 1976). Subjects also performed better when the target was located in the top half than in the bottom half of the display and when the target was closer to the center (proximal) than when it was more peripheral (distal). A response set or attentional bias could account for some of these main effects. In addition, because visual acuity declines in the periphery, this decline probably accounts for the poorer performance on distal presentations. A more distinctive finding is that the impairment of schizophrenics in this task, in comparison with control subjects, is in the LVF and the bottom half of the display. To the extent that LVF stimuli in this study are dependent on right hemispheric operations, our results would support a theory of right-hemispheric impairment in schizophrenia, which would be compatible with the positions of Venables (1984) and Cromwell (1987), mentioned earlier. If portions of the right hemisphere are primarily responsible for the integration of perceptual input, as suggested by Tucker and Williamson (1984), or for preattentional activity, regardless of the task being performed (Cromwell, 1987), they may possibly have become overloaded among schizophrenics in our task. Measures of span of apprehension, such as the detection task that we used, are designed to estimate the amount of information simultaneously attended to and processed. Brief presentation of such large amounts of information could presumably tax the capacity of a dysfunctional right hemisphere and result in impaired performance. This lateral impairment could also be attributable to subcortical structures involved in information processing, inasmuch as some of these structures, such as the thalamus (Oke & Adams, 1987), appear to be abnormal in schizophrenics. It is also conceivable that schizophrenics may overrespond to an attentional bias for the right side or the top of a display or that they use a method of scanning the stimuli that results in the decay of information from the left or lower portion of the display by the time that portion is searched. However, an analysis of performance by quadrant did not reveal any systematic method of scanning the stimuli. In addition, task parameters such as bilateral stimulation (the presence of letters on both sides of the fixation point) could have enhanced the laterality effects (M. I. Posner, personal communication, December 3, 1987). Both the Group X Laterality and the Group X Vertically interactions may have resulted from psychometric artifact caused by differences in discriminating power between conditions (e.g., RVF and LVF) at the levels of accuracy for which the variance is greatest; see Chapman & Chapman, 1988, for a discussion of this issue. Although it was not feasible within our design to match the conditions for discriminating power, we partially addressed this issue by calculating the true score variance (the product of the reliability and observed score variance) for each condition. Chapman and Chapman (1978) indicated that if two conditions enable experimenters to measure abilities that are equally deficient in a low-scoring group, the condition with the greater true score variance yields the larger difference

between the low-scoring group (i.e., the schizophrenics) and a higher scoring group (i.e., the controls). Because the differences between the schizophrenic and the control groups were largest under the LVF and bottom conditions and because these conditions produced the largest true score variances (118.2 for LVF, 54.4 for RVF, 135.3 for bottom, and 52.2 for top of display), an artifactual interpretation of these interactions cannot be ruled out, and these interactions should be viewed with caution until additional data are available. It is difficult to assess the effect that antipsychotic medication may have had on the schizophrenics' responses to the distractor masking and laterality measures. In a review of the effects of neuroleptic and anticholinergic medication on cognitive functions in schizophrenia, Spohn and Strauss (1989) concluded that therapeutic doses of neuroleptics usually produce some improvement on span-of-apprehension measures and tend to lessen distractibility as well. Therefore, the testing of medicated schizophrenics in this study would more likely contribute to an underestimation than to an exaggeration of their deficits. The effect of anticholinergic agents on these measures is not known. Several important questions are raised by these findings. Does the impairment of schizophrenics that is caused by masking indicate vulnerability to the disorder? Is this impairment genetically transmitted? Is the masking deficit specific to schizophrenia, or is it a by-product of the psychotic state? Does the masking deficit occur only for schizophrenics who take antipsychotic medication, or would unmedicated schizophrenics show the same pattern of impairment? Further research on these questions should be conducted.

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Received October 31,1989 Revision received March 22,1991 Accepted May 17,1991 •