Visual information decoding deficits in schizophrenia

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Psychiatry Research, 44:203-216 Elsevier

Visual Information

Decoding Deficits in Schizophrenia

Kenneth M. Weiss, Heather A. Chapman, Grover C. Gilmore

Milton

E. Strauss, and

Received March 4, 1992; revised version received October 15, 1992; accepted November 15, 1992.

Abstract.

Schizophrenic

and control subjects were tested on two-flash fusion

(TFF) and visual backward masking (VBM) tasks in a repeated measures design. Each subject was tested in a single session. Both tasks used the same equipment and stimuli. There was no difference between the groups in their ability to detect the presence of two separate stimuli in the TFF task. Schizophrenic subjects did require longer interstimulus intervals (ISI) than control subjects to accurately report one of the two targets in the VBM task. Analysis of individual targets reveals that the VBM deficit is a function of the similarity of the target and mask. The more feature detail discrimination necessary, the longer an ISI is required in VBM. The data are interpreted as supporting the conclusion that since the groups did not differ in their performance of the TFF task, which would also have been affected by a sensory abnormality, the deficit in VBM must be explained by reference to a higher level of information processing. The VBM deficit is a failure to decode the target stimulus, and is not simply a function of abnormalities due to an overactive transient channel system.

Key Words. Vision, processing, attention.

backward

masking,

two-flash

fusion

task,

information

One of the characteristics of schizophrenia is a disturbance in cognitive functions. Schizophrenic subjects perform more poorly than control subjects on tasks from the Wechsler Adult Intelligence Scale (WAIS) such as digit recall, symbol-digit task, and others (Payne, 1973). More recently, investigators (Spaulding and Cole, 1984; Rund and Land@, 1990) have focused on cognitive information processing as an important area in schizophrenia. In addition to cognitive dysfunction, however, individuals with schizophrenia may have sensory input abnormalities. As long ago as 1896, Kraepelin recognized that schizophrenic subjects had deficits in their ability to perceive very brief stimuli (Diefendorf, 1923). The performance tasks used by psychologists in studying sensory input and information processing deficits in

Kenneth M. Weiss, Ph.D., is Staff Psychologist at the Department of Veterans Affairs Medical Center, Cleveland, OH, and Adjunct Assistant Professor, Department of Psychology, Case Western Reserve University, Cleveland, OH. Heather A. Chapman, M.A., is Research Assistant, Department of Veterans Affairs Medical Center, Cleveland, OH, and a graduate student at the Department of Psychology, Kent State University, Kent, OH. Milton E. Strauss, Ph.D. is Professor, and Director of Clinical Training, Department of Psychology, Case Western Reserve University, Cleveland, OH, and Research Associate, Department of Veterans Affairs Medical Center, Cleveland, OH. Grover C. Gilmore, Ph.D., is Associate Professor, Department of Psychology, Case Western Reserve University, Cleveland, OH. (Reprint requests to Dr. K.M. Weiss, Psychology Service, 116B(B), DVA Medical Center, 10000 Brecksville Rd., Cleveland, OH 44141, USA.) 0165-1781/92/$05.00

@ 1992 Elsevier Scientific

Publishers

Ireland

Ltd

204 schizophrenia are complex. Although the paradigms used are apparently simple, they may be subject to influences on input, central, and/or output stages (Venables, 1964). This raises the question of whether a task that purports to measure a cognitive information processing dysfunction in schizophrenia may be confounded by a coexisting sensory input abnormality. Since 1974, numerous studies using the backward masking task have found a deficit in schizophrenia that is widely interpreted as demonstrating a cognitive dysfunction (Saccuzzo et al., 1974). The backward masking paradigm consists of a visual stimulus containing information (i.e., the target) that is briefly presented, followed by a blank interval, then followed by a second stimulus (the mask). The time between the two stimuli is varied to determine the minimum time required to correctly report the target stimulus. It has been reported numerous times that schizophrenic subjects require longer interstimulus intervals than control subjects to obtain the same rate of accuracy. The visual backward masking (VBM) performance of schizophrenic subjects has been interpreted as cognitive slowing in the processing of visual information (Saccuzzo et al., 1974; Braff and Saccuzzo, 1981; Saccuzzo and Braff, 1981; Saccuzzo and Shubert, 1981). In this conceptualization of visual information processing, there is a very brief (< 500 msec) precategorical sensory filter of high capacity and fast decay. Visual information is formed, stored, and finally transferred to higher cortical centers. Saccuzzo et al. (1974) argued that schizophrenic subjects have a deficit in the third component of this model. Subsequently, they suggested a two-factor deficit of input and higher level information processing (Braff and Saccuzzo, 1981). Others (Turvey, 1973; Muise et al., 1991) have also proposed two-factor models of VBM. More recently, attempts to account for poorer performance by schizophrenic subjects on VBM have emphasized an early sensory deficit, with little attention being paid to a two-factor model that incorporates both early sensory functions and higher cortical information processing. It is not yet clear that early sensory dysfunction can adequately account for VBM deficits in schizophrenic subjects. Sensory deficits in schizophrenic subjects have been regularly demonstrated. This raises the question whether the poorer performance of schizophrenic subjects on this task is a function of a sensory dysfunction rather than an information processing failure. Recently, Schuck and Lee (1989), and Schwartz et al. (1989) have posited explanations of VBM deficits in schizophrenic subjects based on the assumption that abnormalities in the neural physiological channels in the visual system retard the formation of visual information. Specifically, the cells referred to as “transient” (Legge, 1978), “Y cells” (Enroth-Cugell and Robson, 1966), or “magnocellular” (Livingstone and Hubel, 1987) are proposed to account for the VBM deficits.’ The transient cells of the visual system are believed to respond optimally to low spatial frequency (global) components, abrupt onset or offset, or movement. Transient channel cells are characterized as having very short firing latencies, and brief I. The scientific literature in vision has now generally adopted the convention of referring to these pathways as “magnocellular”(M-cells) and “parvocellular”(P-cells) (Bassi and Luhmkuhle, 1990), but we shall use the older “transient” and “sustained” terminology for the sake of clarity in referring to the existing psychological literature.

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durations. The sustained channel responds predominantly to high spatial frequency (detail) components, stationary or slowly moving targets. The cells of this system exhibit longer response latencies and firing durations (Breitmeyer and Ganz, 1976). Schwartz et al. (1989) compared four groups on a two-pulse detection task using high and low spatial frequency stimuli. The subjects were required to report which of two presentations had an interstimulus interval (ISI). The grating (dark and light bars) was presented and followed by a 180” phase shift. For one presentation there was no IS1 and for the other there was an ISI. A 79% IS1 detection threshold was used as the measure of visible persistence. Presented with stimuli in the retinal periphery, schizophrenic subjects, on the average, required 150-200 msec to detect the presence of an ISI, whereas normal and depressed control subjects required only about 50 msec. With stimuli presented in the fovea, an area marked by a preponderance of sustained cells, the performance of schizophrenic subjects was actually superior by an average of about 22 msec to their performance in the periphery where there are mainly transient cells. Depressed and normal control subjects showed a similar pattern of superior performance when stimuli were presented to the fovea. Schwartz et al. (1989) suggested that schizophrenic deficits on visual information processing tasks such as backward masking may be attributed to a dysfunctional transient system. Schuck and Lee (1989) have also proposed that a transient channel abnormality accounts for VBM deficits in schizophrenic subjects, on the basis of a theoretical analysis and model of visual information processing. In this model, schizophrenic subjects have transient discharges of abnormally high amplitude in response to the onset of the mask which leads to interference with the sustained channel response necessary for feature analysis of the target. Presumably, this hypothesizes that the system of transient cells produces a greater potential through temporal or spatial summation (Stevens, 1979), since single cells all ‘have the same duration and magnitude throughout the nervous system (Curtis et al., 1972). While these theoretical analyses are heuristic, they have been subject to only limited empirical testing, so it is uncertain if they adequately account for the impaired backward masking in schizophrenia. The development of a transient channel abnormality hypothesis of backward masking deficits has been based on the pattern of performance of schizophrenic patients on tasks other than VBM that are theoretically relevant to processes involved in visual masking. Testing hypotheses about mechanisms responsible for performance deficits by studying performance across a range of theoretically relevant tasks is an important approach to validating theoretical models of schizophrenic deficits (Knight, 1984). As may be noted from the foregoing discussion, the findings in support of this hypothesis are mixed. This is not unusual in schizophrenia research. The incomplete agreement reflects, at least in part, the difficulties encountered in integrating the findings from diverse patient samples, various experimental designs, and different laboratory procedures. The extrapolation of findings from studies of the perception of single letters viewed in a tachistoscope to performance with spatial frequency gradients presented on an oscilloscope is difficult. In addition, the studies have compared normal subjects, “chronic” schizophrenic subjects, “nonparanoid” schizophrenic subjects, “paranoid” (as defined by DSM-III, DSM-III-R, or the Maine

206 scale) schizophrenic subjects, schizoaffective subjects, and schizotypal subjects, among other variously defined patient groups. These differences in procedures, equipment, stimuli, and diagnostic systems are not trivial, and may account for the difficulty in integrating the body of data on visual information processing deficits in schizophrenia. One objective of this study was to obtain comparable data on two related tasks using the same equipment and same stimuli in a single session. The main question addressed was whether there is a visual information decoding deficit independent of, or in addition to, any sensory deficits that may be attributed to visual channel pathways. Two paradigms were used: two-flash fusion (TFF) and visual backward masking (VBM). The TFF procedure presents two stimuli in series, separated by a blank ISI. The TFF threshold is the shortest IS1 at which the subject reports two stimuli separated by an interval rather than a single stimulus. TFF and VBM procedures can be similarly conceptualized. Each presents two very short duration visual stimuli, separated by an interval that is varied to determine a threshold. In the case of TFF, the two stimuli are usually not meaningful (e.g., simple flashes of light or a pattern), while in VBM one of two letters (e.g., A or T) is presented as the first stimulus (target) and a field of other letters (e.g., X or W) is presented as the second, or mask stimulus. Meaningful stimuli can be used for the TFF as well as VBM procedures, and the instructions given to the subject determine whether TFF or VBM thresholds are being measured: the VBM paradigm asks the subject to identify the first stimulus, and so is a task in which the subject has to decode the content of the stimulus. TFF, on the other hand, does not involve decoding the content of the stimulus. By using the same stimuli that are used in VBM, we are able to compare the TFF performance with that of VBM. This study directly compares TFF and VBM performance using the same apparatus, stimuli, and a repeated measures design to determine the relative deficits of schizophrenic subjects on two tasks that should both be vulnerable to disruption by transient channel dysfunctions. Many researchers have been faced with difficulty when designing a protocol to measure schizophrenic subjects’ performance due to problems of attention, physiological arousal, motivation, and cognitive capacities. Because of the inherent difficulties of this clinical population, the TFF and VBM tasks were designed to be administered in a brief period of time (usually 5 minutes). Because the reliability and validity for such a brief performance measure would be questionable, a reliability test was built into each task. Methods Subjects. Two groups of subjects were used: 18 schizophrenic subjects and 13 nonpsychiatric control subjects. All schizophrenic subjects were inpatients on the Research Ward of the Department of Veterans Affairs Medical Center, Brecksville Division, Cleveland, Ohio. The patients met DSM-III-R criteria for schizophrenia (American Psychiatric Association, 1987) as determined by the consensus of clinicians and trained research interviewers using the Schedule for Affective Disorders and Schizophrenia-Lifetime version (SADS-L; Endicott and Spitzer, 1978). The control subjects were gathered from a variety of sources, and included nine inpatients from the Veterans Addiction Recovery Center. The other control subjects were residents in the VA Domiciliary. None of the control subjects had an axis I DSM-III-R

207 diagnosis other than substance dependence or abuse. These patients had all been free of medication, drug, and alcohol for a minimum of 21 days, as verified by urinalysis. The inclusion of veteran subjects with a history of drug and alcohol abuse in our control group probably provides a better match to our schizophrenic subjects than many other groups. These control subjects have a similar educational and socioeconomic background, and since chronic mentally ill persons have a high incidence of drug and alcohol abuse, it is probably a common factor across both groups. Furthermore, unlike most studies with “normal” control groups, the present design enabled us to verify that these control subjects were drug and alcohol free for at least 21 days at the time of testing. All subjects were male, had normal or corrected to normal vision (20/25) as assessed by a Snellen visual acuity chart, and were between the ages of 23 and 44. Mean ages for schizophrenic and control groups were 35.5 (SD = 5.5) and 32.1 (SD = 5.6), respectively, and were not reliably different by t test. Subtype diagnoses in the schizophrenic group consisted of 16 undifferentiated type (4 of whom were not receiving medication), 1 paranoid type, and 1 residual type (both of whom were medicated). The scores on the Brief Psychiatric Rating Scale (BPRS; Overall and Gorham, 1962) of the schizophrenic subjects ranged from 18 to 62 (mean = 38.3, SD = 11.1). Procedure. Each subject was tested on two-flash fusion (TFF), critical stimulus duration, and visual backward masking (VBM) during a single session. Testing order remained constant for all subjects. The same equipment was used for both tasks. Subjects were seated in front of the test apparatus, which was housed in an anechoic chamber. The inside of the apparatus was painted matte black, with ambient light inside the box being supplied by a 40-watt candelabra incandescent bulb mounted below the viewing hood and out of the subject’s direct sight. The viewing distance was 71 cm. The display, composed of a 6.86 mm high and 4.32 mm wide 5 X 7 matrix of red (660 nm) light-emitting diodes (Dialight 745-0007) was covered by a neutral density filter (Roscolux #60) to prevent the subject from seeing the individual light-emitting diodes when they were not illuminated. Subjects could fixate on the blank display, which clearly stood out from the background. The visual angle of the display was 0.35’ horizontally and 0.55O vertically. The display was controlled by a Metrabyte PDMA-16 interface card and a microcomputer, housed in an adjacent room. Except for the subjects’ verbal responses, which were entered into the computer by a technician, all aspects of the procedures were under computer control. All subjects were tested to determine the critical stimulus duration to ensure that the targets were of sufficient duration to be reported accurately in the absence of a mask. This was done by simply presenting the target without following it by the mask. The “6” and “8” stimuli were each presented six times at 10, 50, 90, 130, 170, 210, and 250psec in a random order of both stimuli and durations. All subjects were able to report accurately (> 80%) the “6” and “8” targets well under 0.5 msec. There were also 12 dummy trials of a tone followed by no stimulus to assess false reporting of a stimulus. None of the subjects responded to the catch trials. The criterion used to determine this unmasked target detection threshold was five out of six trials correct (83%). The schizophrenic group means for “6” and “8,” respectively, were 132 psec (SD = 48.5) and 130psec (SD = 52.9). The respective means for the control subjects were I 14psec (SD = 55.5) and I 18 ,usec (SD = 35.4). There were no statistically significant interactions or differences between groups or stimuli. These data were gathered simply to ensure that our targets were of adequate duration for a good percept to be formed for each group. These are very brief durations as compared with reports of tachistoscopic displays. Our displays provided much faster rise and fall times. The rise time is the duration during which the leading edge of a pulse is increasing from 10% to 90% of its maximum amplitude. Tachistoscopes usually offer a minimum duration exposure of 1 msec (Gerbrands Corporation, 1984) and use special fluorescent lamps with a rise and fall time of about 20 psec. The use of light-emitting diodes in vision research is increasing because of their advantages (Nygaard and Frumkes, 1982). Our equipment uses light-emitting diodes with rise times of about 90 nsec. With the use of our computer-controlled interface with its own 10 MHz clock, it is possible to drive the display with great accuracy. Our display also differs from typical tachistoscopes by presenting red characters on a black background, rather than black letters on a white field. The luminous

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intensity of the light-emitting diodes is 100 pcd per unit. It should be noted that there is a great deal of similarity between the targets and stimuli that we used (Fig. 1). Within the 5 X 7 LED matrix, the “6” shares all but 17 light-emitting diodes that form the mask “B.” The target “8” shares all but light-emitting diodes in the “B.” Because the “8” shares more light-emitting diodes mask than the “6,” it may be considered more similar.

masking 4 of the 3 of the with the

Fig. 1. Characters used in backward masking tasks for targets and mask l l l

mm

.

l

m-m

8 m

0 l

l mm B

c2mm l

l l l

0 0

~3 l l 6

l

c: m l

mm

m

;:3 8 l 0 l

l l

l

m

L-; l I

l

l l

8

The dotted lines indicate the elements of the mask (“B”) that were not illuminated on the targets

TFF. Three different pairs of stimuli were used to determine TFF. Each stimulus pair was tested separately. One set of stimuli was a pair of the number “1” consisting of seven lightemitting diodes in a vertical column. The second set consisted of a “6”followed by a “B,“and the third set was an “8” followed by a “B” (see Fig. 1). Trials were preceded by an auditory warning signal 1 second before the presentation of the first stimulus. Each stimulus was 10 msec in duration. Subjects were instructed to report if they saw (1) one unflickering stimulus or (2) two separate stimuli. The initial IS1 was set to 20 msec, which was increased by 2 msec following each report of a single stimulus, and decreased by 2 msec following each report of two stimuli until the ISI was > 30 ms. Above an ISI of 30 msec, the increment or decrement value was 4 msec. The threshold was defined as the ISI value presented at a minimum of five trials with 80% correct responses. To determine that subjects were reliably reporting their perceptions and not responding with “false positives,” on every fifth trial the ISI was set to 0. These 0 ISI catch trials were used to double check the subjects’ responses, avoiding results that were recorded precipitously. Responses on these trials had no effect on the ISI of subsequent trials. If the subject failed to respond correctly to at least 75% of the 0 IS1 trials, the task was repeated. Although this was calculated on the overall trials, rather than at each ISI, it provides more power to detect response perseveration than most other similar studies. Four schizophrenic subjects failed to meet this criterion on the first testing and needed to have the procedure repeated. Two required only one repeated testing, one required two repeated testings, and one required three repeated testings. VBM. VBM thresholds were determined individually for each of two targets, “6” and “8.” The order of determination for “6” or “8” was randomized, but subjects were not told that the threshold for only one target was being measured. The masking stimulus was always “B” and all stimuli were 10 msec in duration. The use of a suprathreshold target is common in VBM research with schizophrenic subjects, the purpose of which is to ensure that one is assessing the effect of the mask and not a poorly formed percept. The initial ISI was 20 msec and the incremental step size was 5 msec. If a subject made an error on a trial that presented the stimulus being tested, then the ISI was increased by 5 msec. Errors on trials that presented the stimulus not being tested had no effect on the ISI of future trials. All trials were preceded by an auditory warning signal 1 second before onset. Determination of the target stimulus was randomized by the computer. When the threshold of one target was measured, trials of the other target had no effect on the IS1 of subsequent trials. To discourage response perseveration and prevent the subject from reaching threshold spuriously, if the subject reported the same target on three consecutive trials, the target on the succeeding trial was changed to the other target.

209 The threshold was defined as the lowest ISI value at which the subject answered seven trials correctly without an error. This is similar to the procedure used by Saccuzzo and his colleagues (e.g., Saccuzzo and Miller, 1977) in many of their studies of VBM and schizophrenia. Subjects were instructed that they would see either a “6” or “8” before the “B” and they should report which it was. They were not told any other details. Subjects were unable to detect any target at the shortest ISIS and typically reported “just a ‘B’.” After the first target threshold was determined, the procedure for the second target threshold was begun after a 3- or 4minute break. The second VBM determination was explained as a “repeat” since it appeared identical to the subject. Again, due to the inherent difficulty in reliably measuring schizophrenic subjects’ responses, when criterion was met, the program initiated a block of 20 trials to verify the result. Subjects were unaware of when the reliability test trials began. Randomly, 10 trials were presented with no target (only the mask), and 10 with the IS1 set to the value at which they met the criterion. Of these 10 target trials, five were with each of the two targets. Correct responses to trials without a target were that they were unable to report a target. A minimum of 75% correct responses on these stimuli on the last 20 trials was required for the subjects’ data to be considered valid; otherwise the procedure was repeated. Only two subjects, both schizophrenic, failed this criterion on the first testing. One subject failed when being tested with the “6-B” and one with the “8-B” stimulus pair. On the second testing, they met criterion. These two subjects were not the same as those who failed the TFF criterion.

Results To examine the specific effects of the groups and two stimuli in common (“6” and “S”), a group X task (TFF or VBM) X stimulus (“6” or “8”) analysis of variance (ANOVA) with repeated measures was computed for the group means. All main effects and interactions were significant (p’s < 0.001-0.05) in this analysis (Table 1). Since the interpretation of main effects and first order interactions is constrained by the higher order interaction, an ANOVA was computed separately for the TFF and VBM tasks to determine the source(s) of the interaction. As previously noted, there were three stimulus conditions in the TFF task. In the ANOVA for TFF, all three were included. The two-way ANOVA was computed between groups and within the three stimulus pairs (“l-l,” “6-B,” and “8-B”). Only the stimulus effect was significant (F = 17.1 I; df = 2, 58; p < 0.0001). The group (p > 0.26) and interaction (p > 0.80) effects were not significant. Fig. 2 illustrates the mean thresholds for each stimulus for each group. The TFF threshold varies with each stimulus pair for both groups. It appears that as the similarity of the two stimuli increases, so does the IS1 required to detect two separate stimulus events. The

Table 1. Task, stimulus, and diagnostic group effects Factor

(CL,

Diagnosis

29)

P

5.64

0.02

Task

25.89

0.00

Stimulus

37.70

0.00

4.52

0.04

Diagnosis

X task

Diagnosis

X stimulus

3.93

0.05

Task X stimulus

13.22

0.00

Diagnosis

11.89

0.00

X task X stimulus

210

Fig. 2. Means and standard deviations of the thresholds of the schizophrenic and control groups on the two-flash fusion tasks with each stimulus pair

SCHIZOPHRENICS N=18

’

CONTROLS N=13

threshold was significantly higher for “8”(mean = 55.1, SD = 20.7) than that for “6” (mean = 43.3, SD = 14.8) (F= 9.86; df = 1,29;p < 0.005) (Fig. 2). Most important, TFF thresholds of schizophrenic subjects were not different from those of the control group, indicating that on this task there was no evidence for longer visible persistence in the schizophrenic subjects. The differences between “6” and “8” were comparable in the schizophrenic (12 msec) and control patients (13 msec). For the VBM task, the differences between groups (F= 6.34; df = 1,29;p < 0.02) stimulus ( F = 54.45; df = 1,29; p < O.OOOl), and the interaction (F = 13.26; df = 1, 29; p < 0.001) were all significant. To identify the source of the variance, groups were compared on each stimulus separately by Newman-Keuls tests. The only significant difference was between schizophrenic subjects and control subjects on the “8-B” (p = 0.05) condition. As Fig. 3 shows, the threshold for the target stimulus “8” among schizophrenic subjects was substantially higher (mean = 100, SD = 41.1) than for control subjects (mean = 62.7, SD = 15.1) and higher than for “6,“for either schizophrenic subjects (mean = 59.2, SD = 27.2) or control subjects (mean = 48.8, SD = 9.8). Unlike the TFF task, in the VBM task, the target stimulus “8” required higher thresholds for the schizophrenic group than for the control group. The difference between the two stimuli in backward masking was substantially greater for the schizophrenic group (41 msec) than for control subjects (14 msec). This difference between the thresholds on the two stimulus pairs is illustrated in Fig. 4, which also shows that the differences between stimuli for TFF were the same for schizophrenic subjects and control subjects. There were no significant correlations between any of the tasks and BPRS ratings, indicating that severity of symptoms does not appear to be an important influence. This is similar to previous reports that more disturbed patients do not perform more poorly on VBM (Braff and Saccuzzo, 1982; Braff et al., 1991). Previous reports also indicate that medication is not a significant variable (Braff and Saccuzzo, 1982).

211

Fig. 3. Means and standard deviations of the thresholds of the schizophrenic and control groups on the backward masking tasks with each target-mask stimulus pair

SCHIZOPHRENICS N=le

CONTROLS N=13

Ficr.4. Differences between the mean thresholds on TFF and VBM with each stimulus pair 45-n

TWO-FLASH FUSION

E

SCHIZOPHRENICS m

CONTAOLS 1

Discussion Two factors led us to conclude that the transient system explanation was inadequate to explain our data. The first is that we found no differences between groups in the TFF task. This is especially important because we matched the stimuli in the TFF task to be the same as in the VBM task. Therefore, any neural activity of the visual pathways would be the same in both TFF and VBM. We measured performance on the TFF task, which is presumed to rely on transient channel function because it is composed of two abrupt onset stimuli (Livingstone et al., 1991). If VBM deficits

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were due to transient channel abnormalities, we would expect to find deficits on the TFF task as well. We have not found evidence of a TFF difference between schizophrenic subjects and control subjects. The failure to find such a difference supports the conclusion that the sensory mechanism required for our TFF task is intact. The overactivity of the transient system, which presumably inhibits sustained channel activity, would impair the schizophrenic subjects’ TFF performance relative to that of control subjects. Second, when the task was VBM, we found differences between groups only with one of the two targets. Transient activity would not differ substantially between these two target conditions and therefore seems unlikely to account for the poorer VBM performance. If we rule out a transient channel explanation, we must either assume a dysfunctional sustained system, or posit a deficit at a level beyond sensory input. While we must caution against ruling out a sustained channel deficit without direct empirical evidence, so long as there is no evidence of such a deficit in other reports of schizophrenic subjects, we believe it is also premature to rule out the deficit at another level of information processing. We believe that our results are more reasonably attributed to a postsensory deficit, supporting the hypothesis that there is a second contributing factor to VBM deficits in schizophrenia. Since we have failed to find a transient system abnormality, we should consider what evidence there is for a sustained system abnormality to account for the VBM deficit. The transient system is most sensitive to low spatial frequencies, has a high temporal resolution, and responds to quickly moving targets and to stimulus onsets and offsets. The sustained system is most sensitive to high spatial frequency, has a long response persistence and low temporal resolution, and responds in a sustained fashion to stationary or slowly moving targets (Williams et al., 1991). Our two tasks used the same stimuli with the same abrupt onset and offset, thereby eliminating the possibility that differences in performance could be due to either spatial frequency or abruptness of presentation. Place and Gilmore (1980), and Wells and Leventhal (1984) have reported a global processing deficit in schizophrenic subjects that actually permits schizophrenic subjects to process detail more accurately. This implies that the sustained channels are not substantially impaired in schizophrenic subjects. Royer (1984) demonstrated that schizophrenic subjects are significantly deficient in detecting low spatial frequency, but are able to detect high spatial frequency. Merritt and Balogh (1990) compared VBM performance with high and low spatial frequency masks. They found that schizotypic subjects only differed from normal subjects when the mask was of low spatial frequency, a finding that supports the notion of a transient channel abnormality but not a sustained channel deficit in this population. Our study, which found no difference between groups in the TFF tasks, is consistent with Haber and Standing’s (1968) finding that subjects could report seeing a clear test stimulus but could not identify it in a backward masking procedure. Breitmeyer and Ganz (1976) explain this phenomenon by noting that form recognition requires the sustained channels that are disrupted by the onset of the transient channels triggered by the mask. This interpretation explains why a longer ISI is required for VBM, in which the subject must identify the target, than for TFF, in which the subject merely needs to determine the presence of one or two stimuli.

213 Performance by schizophrenic subjects on our TFF task is not deficient, indicating either that the transient channel response is intact or that this task does not rely upon it. Since the VBM task requires more form recognition of details, information presumably carried by the sustained channel, one could argue that it is this channel that is impaired in schizophrenic subjects. The differential deficit of schizophrenic subjects could either be attributed to less sustained channel activity or to the inhibition of sustained channel activity by increased transient channel activity, as proposed by Schuck and Lee (1989) and Schwartz (1990). A factor that may account for the poorer performance of schizophrenic subjects on VBM in our study when the target is “8” is the degree of similarity between targets and the mask, “B” (see Fig. 1). The “8” is more similar to the “B” than to the “6” in terms of light-emitting diodes in common. The subjects required more time to identify the “8” than the “6” when each was followed by the “B.” The schizophrenic deficit appeared principally when the decoding between the target and mask was more difficult rather than when detection of two separate stimuli (or flickers) was required. This implicates a “higher level”cognitive/ perceptual rather than a sensory deficit. The data for the individual targets in both the TFF and VBM tasks demonstrate that the backward masking deficit principally occurred when the target was “8.” In all other comparisons between groups for specific stimulus targets, there were no statistically significant differences. The target “8” is masked at shorter ISIS than is the target “6.” We believe that this is the first demonstration of differential masking performance as a function of different target stimuli with schizophrenic subjects. The relationship between target set and mask with normal subjects has been examined by Muise et al. (1991). They varied the similarity of the target set, keeping the mask constant, and found that when the target set was more similar, it required more time for subjects to escape the mask. In the light of previous demonstrations of transient system dysfunction in schizophrenic subjects (Schwartz et al., 1989), it may seem more surprising that schizophrenic subjects show no difference on TFF than that they show a deficit on VBM. One possible explanation for this result is that we used stimuli of a different color from those used in other studies. Williams et al. (1991) compared the effect of different wavelength masks with a white on black target. They found red (longer wavelength) produced less masking than green (shorter wavelength), and therefore concluded that the response magnitude and speed of the transient channel were lowest for the red masks. Other work has reported that red light inhibits activity in the transient pathway of the monkey (Dreher et al., 1976; Kruger, 1977; Schiller and Malpell, 1978). This may explain why we were able to obtain TFF thresholds in schizophrenic subjects that did not differ from those in control subjects. Furthermore, it suggests that our red stimuli may have inhibited transient activity, and thus may have allowed an examination of the cognitive information processing that was less confounded by transient abnormalities than would otherwise have been the case. Such a conclusion is consistent with both views that schizophrenic subjects have a transient channel (perceptual) abnormality and an information processing (cognitive) dysfunction, i.e., a two-factor deficit.

214 It is difficult to reconcile all the reports of visual sensory system abnormalities in schizophrenic subjects. These TFF data demonstrate that there is not necessarily a difference between schizophrenic subjects and control subjects in transient system function. Certainly transient system abnormalities have been demonstrated in schizophrenic subjects. Our data demonstrate that one cannot necessarily extrapolate the constructs used to understand one task (TFF) to account for another (VBM). The VBM task relies on both transient and sustained systems, while the TFF task relies more heavily on the transient channel. Another possible explanation for the longer VBM thresholds for the target “8” is a bias for saying “6” when uncertain. This response bias would yield shorter ISIS for “6” than “8.” To measure response bias, one would need to present the targets at suprathreshold IS1 values; otherwise the target stimulus which can be detected at the lower ISI is more likely to be detected and reported on perceptual grounds alone. Our method precludes analysis of response bias, because IS1 values that are long enough for subjects to detect the “6” are not long enough for them to detect the “8.” Other methods that specifically address bias, such as signal detection analysis, might answer this question, but testing requires much greater time. An objective of our study was to use a method that was practical in a clinical setting, with subjects whose motivation and attention were likely to be impaired. We did not examine the relationship between symptom clusters (e.g., positive vs. negative symptoms) and performance on our various tasks. Green and Walker (1984, 1986), Braff (1989), and Weiner et al. (1990), have reported a greater deficit among patients with negative symptoms than among those with positive symptoms, although both groups differed from control subjects. Since we are proposing that there are both sensory abnormalities in peripheral visual sensory processing and a more central information processing dysfunction, it would be of interest to explore if the groups of schizophrenic subjects categorized by symptom clusters would differ based on this distinction. Braff and Saccuzzo (1981) based on data from good and poor prognosis schizophrenic patients, proposed that there are deficits both in the earliest stages of stimulus identification and in subsequent levels of information analysis. Our data support the idea that VBM deficits in schizophrenia are still best explained by a two-factor model rather than by a single sensory input dysfunction. Acknowledgment. This material is based on work supported by the Office of Research and Development, Medical Research Service, Department of Veterans Affairs, and by MH-41684, Clinical Research Center for the Study of the Major Psychoses, H.Y. Meltzer, M.D., Principal Investigator.

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