Effects of task relevance and attention on P3 in schizophrenic patients

Research.

Schizophrema

4

(1991) 1 I-2 I

I1

Elsevier

SCHIZO

00145

Effects of task relevance

Christian 1Department

of Psychiatry.

Level, New Haven, Diego,

La Jolla,

San Diego,

Grillon’,

CA 5’2093,

U.S.A.,

ti.S.A..

Ameli’,

Yale Clniwrsity School

CT 06510-3223.

CA 92123,

Rezvan

and attention patients*

Eric Courchesne3g4

of Medicine,

Genetic

Epidemiology

and David L. Braff’ Research

Unit, 40 Temple

Street,

Lower

U.S.A., 2Deparmmenr qf Pqxhiatry, (T-004) School qf Medicine, University of Cali/brnia San 3Neurop.s~cholog,v Research Laboratory, Children’k Hospilal Research Center, 8001 Frost Street,

and 4Department

(Received

on P3 in schizophrenic

19 February

qf Neurosciences, School ofMedicine. CA Y2OY3, U.S.A.

1990. revised received

22 June

Liniversity

1990, accepted

of California,

San Diego,

La Jolla,

25 June 1990)

The P3 component of the event-related potentials (ERPs) to auditory task-relevant and task-irrelevant stimuli in ‘Attend Auditory’ (i.e., reaction time task) and ‘Attend Visual’ (i.e., when the auditory stimuli were being ignored) conditions was investigated in 13 RDC/DSM-III diagnosed schizophrenic patients. ERPs were recorded from Fz, Cz, and Pz. Compared to controls, schizophrenics had a significantly smaller P3 in the Attend Auditory than in the Attend Visual condition and to the task-relevant than to the taskirrelevant stimuli. Furthermore, the patients’ P3 response to the most salient task-irrelevant stimuli in the Attend visual condition was normal. The results are discussed as suggesting that schizophrenics either allocate relatively more resources to task-irrelevant than task-relevant stimuli or that they fail to habituate to task-irrelevant stimuli. Kc>? words.

P3; Task relevance;

Attention;

(Schizophrenia)

INTRODUCTION

The hypothesis that deficits in attention and information processing are central to schizophrenics’ psychopathology is supported by a variety of studies (Callaway, 1970; Shagass, 1976; Nuechterlein and Dawson, 1984; Braff, 1985). Event-related potentials (ERPs) have been one of the most productive avenues of research in this respect. In particular, the study of the P3 component has received the most attention (Roth and Cannon, Correspondence /o: C. Grillon. Department of Psychiatry. Yale University School of Medicine, Genetic Epidemiology Research Unit, 40 Temple Street, Lower Level, New Haven. CT 06510-3223. U.S.A. (Tel.: (203) 785 7215).

*The study was performed at Children’s Hospital Research Center, San Diego. Research supported by NARSAD Fellowship Extension Award to C. Grillon, by NIMH Grant I-ROIMH36840 to E. Courchesne and by NJMH Grant MH42228 to D. Braff.

0920.9964/91/$03.50

?.

I99 I Elsevier Science Publishers

1972; Levit et al., 1973; Shagass et al., 1978; Roth et al., 1981; Baribeau-Braun et al., 1983; Brecher and Begleiter, 1983; Duncan-Johnson et al., 1984; Pfefferbaum et al., 1984, 1989; Duncan, 1988). The characteristics of P3 are well-known under many conditions (Sutton et al., 1965, 1967). For example, when infrequent target stimuli are randomly inserted into a sequence of frequent stimuli, P3 is elicited whenever the subject detects the target stimuli (see review: Pritchard, 198 1). P3 can also be recorded to task-irrelevant stimuli if these stimuli are rare and intrusive enough to attract the subject’s attention (Roth, 1973; Courchesne et al., 1975; Squires et al., 1975; Courchesne, 1978; Roth et al., 1984; Grillon et al., 1990b). In schizophrenics, the P3 to task-relevant stimuli is reduced compared to normal controls (Roth and Cannon, 1972; Baribeau-Braun et al., 1983; Brecher and Begleiter, 1983; Duncan-Johnson et al., 1984; Pfefferbaum et al., 1984, 1989; Duncan, 1988) or compared to subjects with other psychiatric disorders

B.V. (Biomedical

Division)

12

(Levit et al.. 1973: Shagass et al.. 1978; Roth et al., 1981). The P3 reduction in schizophrenics is a robust phenomenon which has been reported in various experimental paradigms (Baribeau-Braun et al., 1983; Pfefferbaum et al., 1984, 1989; Grillon et al.. 1990a), and is not due to a failure to evaluate event probabilities (Duncan-Johnson et al., 1984) a lack of motivation (Brecher and Begleiter, 1983) or antipsychotic treatments (Duncan et al., 1987). The nature of P3 to task-irrelevant stimuli in schizophrenics is not as well documented and the results are controversial. Both normal (Scrimali et al.. 1988) and abnormal (Roth et al.. 1980: Pfefferbaum et al., 1989) P3 to task-irrelevant stimuli have been reported in schizophrenics. Scrimali et al. (1988) have investigated schizophrenics under two conditions: passively listening to click stimuli presented every 5545 s or actively detecting taskrelevant stimuli. They reported that P3 was normal in the passive condition and abnormal in the active condition. Roth et al. (1980) and Pfefferbaum et al. (1989) have reported that in a passive situation. loud (100&105 dB) stimuli elicited smaller P3s in schizophrenics than in controls. Recently, we have shown that P3 to task-irrelevant stimuli was less reduced than P3 to task-relevant stimuli in schizophrenics (Grillon et al., 1990a). Taken together, these results suggest that task-relevance might effect P3 reduction in schizophrenics. ohman et al. (1986) who utilized the skin conductance response (SCR) also reported that task relevance influenced psychophysiological responses differently in controls and schizophrenics. They showed that the SCR to task-relevant and task-irrelevant stimuli was not as differentiated in schizophrenics as in controls. They also demonstrated that the SCR to visual distracters habituated less in schizophrenics than in controls when both groups were instructed to pay attention to an auditory task. ohman et al. (1986) concluded that schizophrenics orient less to task-relevant stimuli and more to task-irrelevant stimuli. compared to controls. The notion of a differential processing of taskrelevant and task-irrelevant stimuli in schizophrenics is consistent with their inability to filter or ‘gate’ irrelevant information (Braff et al., 1978: Adler et al., 1982) and their increased distractibility (Oltmanns and Neale, 1975; Pogue-Geile and Oltmanns, 1980). In the present study, we investigated the effects of the task relevance and the direction of

attention on P3 in schizophrenics. In one series of experiments. subjects were asked to actively pay attention to rare auditory task-relevant stimuli presented along frequent and rare auditory taskirrelevant stimuli. The rare task-irrelevant stimuli were of two types and were chosen because of their distracting effect (Grillon et al.. 1990b). In other series, the same auditory stimuli were presented when subjects had to perform a visual task. Thus, auditory ERPs were recorded to task-relevant and task-irrelevant stimuli when ‘attended’ and when ‘ignored’.

METHOD

i 1) Patients. The schizophrenics were subjects who were involved in a series of experiments some of which are reported elsewhere (Grillon et al.. 1990a). The diagnosis of schizophrenia was based on review of the records, individual interviews, and the administration of the Schedule for Affective Disorders and Schizophrenia Lifetime version (SADS-L) (Spitzer and Endicott. 1975). All patients were diagnosed schizophrenic according to the DSM-III and Research Diagnostic Criteria (RDC) (Spitzer et al., 1978). 22 patients participated in this experiment. Six patients were excluded due to excessive eye/muscle artifacts or lack of cooperation. Data from three patients were lost due to equipment malfunction. Thus. the data analysis was based on I3 patients (mean age 30.2k6.5 years). There were three females and ten males. 11 schizophrenics were paranoid and two were nonparanoid by the Research Diagnostic Criteria. All but one of the schizophrenics were taking antipsychotic medication at the time of the recording. The average daily dosage of neuroleptic was 956 mg (range IOO4000 mg) chlorpromazine equivalent. (1) Controls. I3 adult subjects (age 28.9f6.7 years) chosen from a pool of volunteers participated in this study. They were chosen to match the schizophrenics for gender and age. No subject had any psychiatric illness. history of drug abuse, seizure disorder, head injury, or hearing problems. (B) Prowdurc There were four types of stimuli: (I) frequent or standard 900 Hz tones (duration 50 ms), (2) rare

1600 Hz tones (duration 50 ms), and (3) two types of rare ‘deviant’ sounds. The deviant sounds were either ‘familiar’ (700 Hz tones) or ‘novel’ (consisting of a collection of buzzes, filtered noises, and other unusual, computer-generated sounds). The deviant novel sounds were all different from one another. In an earlier experiment, they were shown to be more distracting and to elicit a larger P3 than the familiar stimuli in normal subjects (Grillon et al., 1990b). The duration of each deviant sound was 100 ms. These stimuli were delivered to the subjects in three different series (Table 1). In the ‘Basic’ series, the frequent standard 900 Hz tones (P=O.85) and the rare 1600 Hz tones (P= 0.15) were delivered to the subjects. The ‘Simple’ series was composed of the frequent standard 900 Hz tones (P=O.70), the rare 1600 Hz tones (P= 0.15), and the rare familiar 700 Hz tones (P= 0.15). The ‘Complex’ series consisted of frequent 900 Hz tones (P=O.70), the rare 1600 Hz tones (P= 0.15) and the rare novel tones (P= 0.15). Thus, the Simple and the Complex series differed only in the type of rare deviant stimuli used along with the standard 900 Hz tones and the rare 1600 Hz tones. Each series was presented under two experimental conditions: (1) an ‘Attend Auditory’ condition in which the subjects had to press a button as quickly as possible upon detection of the rare 1600 Hz tones and (2) an ‘Attend Visual’ condition in which subjects were involved in a visual task while the above auditory stimuli were presented. In the Attend Auditory condition both speed and accuracy were stressed and the subjects were not told that rare deviant stimuli would be presented. In the Attend Visual condition, subjects were asked to press a button as quickly as possible upon detection of the rare red flashes presented on a screen among frequent blue flashes. Subjects were

TABLE Description

qf the type and probability

qf

stimuli

used in each

Each series was presented in an Attend and an Ignore tion. The 700 Hz tone is also called ‘familiar’ stimulus

condi-

Standard

Signal

Deviant

900 Hz (0.85) 900 Hz (0.70) 900 Hz (0.70)

1,600 Hz (0.15) 1,600Hz(0.15) 1.600 Hz (0.15)

700Hz(0.15) ‘Novels’ (0.15)

conditions

Basic Simple Complex

(C)

ERP

recording

ERPs were recorded with non-polarizable electrodes placed at Fz, Cz and Pz according to the lo--20 system. Two electrodes were also placed above and below the right eye to detect eye artifacts, i.e., one was located just below the infraorbital ridge of the eye and the other was located above the eyebrow. All electrodes were referenced to the right mastoid. The ground electrode was located on the left mastoid. The impedance of each electrode was below 5 k0. ERPs were amplified using band-pass settings at 0.15 and 100 Hz and digitized at a rate of 195 Hz for 1200 ms with 200 ms prestimulus baseline. All trials with eyeblinks, eye movements, or excessive muscle artifact as well as miss and false alarm trials were automatically detected by the computer and excluded from averaging. Only subjects whose ERPs to the deviant or signal stimuli were constituted of at least 30 trials were included in the study. This resulted in the exclusion of a few subjects from analysis (see method section, A, 1). (D)

1

series

Stimuli

also told that auditory stimuli would be simultaneously presented and that they should not pay attention to them. In each condition 150 stimuli were randomly presented with a 1 s ISI between stimuli. Each condition was presented three times in a random order. Therefore, there were 60 trials for each deviant or signal stimuli per condition. During the entire recording period, the subjects had their eyes open and sat in a comfortable reclining chair in a dimly lit room. Practice trials in the Attend Auditory condition with the 900 Hz and 1600 Hz tones were administered to all the subjects prior to the recording to ensure that they understood the task.

Data analysis

In the present article, we will report the results of the Simple and the Complex series in the Attend Auditory and Attend Visual conditions. A comparison between the P3 changes from the Basic to the Complex series has been reported elsewhere (Grillon et al., 1990a). ERPs were separately averaged for each condition and each type of stimulus. The P3 component was identified as the largest positive peak between 270 and 550 ms. P3 amplitude was measured baseline-to-peak with 200 ms of the average pre-event

14

EEG tracing serving as the baseline. Analyses of variance with repeated measures (ANOVAs) were performed to analyze waveform characteristics utilizing statistical packages (Freund et al., 1986; Wilkinson, 1986). Reduced degrees of freedom (df) were used to test significance in order to minimize type I error.

RESULTS

The grand average ERP waveforms for the Simple and the Complex series are displayed in Figs. 1 and 2, respectively. In the Attend Auditory condition, the rare 1600 Hz signal tones, the rare deviant

SIMPLE SERIES ATTEND

Patients

AUDITORY

ATTEND

Controls

VlSUAL

LoE

p t

F,

_

_ 7

Cz v Pz

_ t-

5PVL

FAMILIAR

1OOms

Fig. 1. Grand average event-related potentials (ERPs) elicited by the signal and the non-signal (familiar) stimuli in the control and in the schizophrenic groups at the lower eye (LoE), Fz, Cz, and Pz electrode sites in the Simple series, The arrows show the time of stimulus delivery. The LoE electrode is displayed to check for any possible contribution of eye artifacts to the recording at other electrode sites. Note the parietal distribution of the P3 to the signal stimuli and the central distribution of the P3 to the familiar stimuli.

15

COMPLEX

ATTEND

SERIES Controls

-

Patients

AUDITORY

pz + ATTEND

SIGNAL

-

NOVEL

VISUAL

5&VC

’

NOVEL

100 ms

Fig. 2. Grand

average

event-related

potentials

(ERPs)

in the Complex series. The task-irrelevant

familiar tones, and the rare deviant novel tones elicited large P3s. In the Attend Visual condition, the deviant novel tones evoked a large P3. The deviant familiar tones elicited a small P3 and the signal stimuli did not produce an identifiable P3. Separate four-way analyses of variance (ANOVAs) in the controls and the patients were utilized to describe P3 responses in each group. The four factors were: Condition (Attend Auditory, Attend Visual), Series (Simple, Complex), Stimulus Type

stimuli are the novel stimuli.

(signal, deviant), and Electrode Site (Fz, Cz, Pz). Group comparisons were performed with a fiveway ANOVA with the above four factors plus Group (controls, patients) as the fifth factor. (Z) Description

of the P3 responses

in the control

group As shown in Figs. 1 and 2, and Table 2, the P3 responses were influenced by the direction of attention, by the task relevance of the stimuli, and by

P3 amplitude Patients

Simple series

Complex series

Signal Attend Signal Ignore Famili. Attend Famili. Ignore Signal Attend Signal Ignore Novel Attend Novel Ignore

7.5

cs

PZ

F;

CZ

Il.6

16.6

5.7

6.8

(4.0) 2.4

(5.3) 2.0

(6.4) 2.5

(4.2) 2.7

(4.2) 2.9

(2.1) 8.3

(2.2) 11.7

(1.4) II.7

(1.8) 7.3

(1.4) x.3

(4.9) 3.7 (4.2)

(4.6) 4.0

(4.2) 4.1

(3.X) 3.8

(3.5)

(2.9)

(4.1) 3.4 (2.7)

4.7 (2.9) 2.3

7.7

I I.3

14.3

3.5

(4.6) I.8 (3.0) 12.6

(5.1) 2.3 (3.1) 17.7

(5.2) 2.4

(2.6) 2.0

(2.3) 16.4

(1.6) 11.3

(4.0) 4.3

(4.7) 5.7

(4.1) 5.2

(2.1)

(2.0)

(2.2)

the nature of the deviant stimuli. Thus, (1) P3 was larger and earlier in the Attend Auditory than in the Attend Visual condition (Condition: F(1,12) = 96.2, P
(2.9)

P:

7.6 (3.3) 2. I (1.2) 6.6 (2.2) 3.1 (2.5)

(5.4) 5.7

(1.5) 13.8 (3.9) 7.1

6.2 (2.0) 1.6 (1.3) I I.5 (2.8) 4.7

(3.4)

(2.9)

(1.9)

and earlier in the Complex series (when elicited by the deviant novel stimuli), compared to the Simple series (when elicited by the deviant familiar). However, P3 to the signal stimuli did not differ in these two series. These results were statistically confirmed by a Series x Stimulus Type interaction (F(1,12)=22.3, P10.0005 and F(1,12)=5.0. P~0.04, for the amplitude and latency measures, respectively). (II) Group comparisons Like the controls, the patients had (1) a larger and earlier P3 in the Attend Auditory, compared to the Attend Visual condition (Condition: F( 1,12) = 87.9, P
17

globally reduced (F( 1,24) = 8.7, PI 0.006). However. this reduction was not uniform across conditions (Group x Condition: F( 1,24) = 18.6, PI 0.0001) and stimulus types (Group x Stimulus Type: F( I .24) = 8. I, PI 0.009). Separate statistical analyses for each condition indicated that the schizophrenics’ P3 was reduced in the Attend Auditory (Group: F( 1,24) = 14.6, P10.0008) but was normal in the Attend Visual condition (Group: F( 1,24) = 0.2, PI 0.7). Furthermore, in the Attend Auditory condition, the schizophrenics’ P3 was more reduced when elicited by the signal stimuli than by the deviant stimuli (Group x Stimulus Type: F( 1,24) = 5.7, PI 0.02). One of the main findings of this study was the absence of a statistical difference between the P3 of the two groups in the Attend Visual condition, when the auditory stimuli had to be ignored. It is possible that this result merely reflects the fact that there was no P3 to signal stimuli (Figs. 1 and 2) and that P3 to familiar stimuli was small (Fig. 1) in that condition in both groups. In order to verify the validity of our findings, we performed an analysis in the Attend Visual condition in which the P3 to the deviant auditory stimuli was compared in the two groups in a three-way ANOVA (Group x Series x Electrode). The Group main effect was not significant, but there was a Group x interaction. Series Since separate two-way ANOVAs (Group x Electrodes for the deviant faSIMPLE

IT------

,00/_____

_ _ -_-_-_--

miliar and the deviant novel stimuli) did not reveal any significant Group main effect or Group interaction effect, the above Group x Series interaction indicates that the P3 to the deviant novel stimuli was slightly increased in the patients, compared to the controls, whereas the reverse was true for the P3 to the deviant familiar stimuli (Fig. 3 and Table 2). The main findings of the study are summarized in Fig. 3 where the schizophrenic group’s P3 is expressed as percent of the control group’s P3. This figure shows that in the schizophrenics, P3 was reduced in the Attend Auditory, but not in the Attend Visual condition. It also shows that in the Attend Auditory condition, the schizophrenics’ P3 was more abnormal when elicited by the signal than by the deviant stimuli. There were no P3 latency differences between the control and patient groups (i.e., none of the Group main effect or Group interaction effects were significant for the latency data). (III)

Reaction time

The schizophrenic patients had a significantly longer reaction time (RT) than the control subjects. In the Simple series, the control group’s mean RT was 422 ms (SD k 73) and the patient group’s mean RT was 5 19 msec (SD f 80). In the Complex series the RT was 423 ms (SDf 68) and 526 (SDf74) for the control and the patient groups, respectively. A two-way ANOVA with Group and

SERIES

___._

_-______

COMPLEX

SERIES

roo~_____~___.._.~___ _____ _

_

60

0

I 0

0 Attend

Audi

tory:

n

flttend

fluditory:

1

flttend

Visual:

Signal Non-Slgnal Non-Signal

Fz

c.2

R

Stim Stim Stim

Fig. 3. Percent P3 response differences between the control and schizophrenic group. The P3 response of the patient group is expressed as percent of the control group. Her;ce. 100% is the response in the control subjects. Since the signal stimuli did not elicit a clear P3 response in the Attend Visual condition, the signal P3 measures in that condition are not displayed.

18

TABLE Means

3 and standard

deviations

qf

the latency of P3 respon.w P3 latenq Controls

Patients

FZ

Simple series

Signal Attend Signal Ignore Famili. Attend Famili. Ignore

Complex series

Signal Attend Signal Ignore Novel Attend Novel Ignore

351

PL

FZ

Ci

P-_

331 (22) 401

346

367

325

342

(104) 467

(15) 398

(82) 472

(41) 355

(38) 386

(165) 345

(174) 343

(154) 350

(183) 340

(156) 337

(153) 35x

(31) 344

(17) 344

(14) 345

(17) 362

(34) 355

(36) 412

(35) 330

(35) 340

(37) 355

(91) 364

(95) 373

(152) 373

(25) 456

(27) 345 (130)

(30) 366

(46) 435

(46) 405

(44) 413

312

(106) 328

(172) 317

(171) 308

(164) 316

(25) 336

(28) 330

(18) 338

(29) 312

(29) 300

(31) 309

(21)

(25)

(27)

(26)

(34)

(30)

(178) 313

Series as the two factors confirmed the significant group difference in RT (F( l,24)= 12.4, P10.002). The Group x Series interaction effect did not reach significance. (IV) Correlations Two types of Pearson correlations were performed in the patient group: between daily dosage of drug treatment in chlorpromazine equivalent and P3 amplitude, and between RT and the latency of P3 to signal stimuli. None of the correlations reached significance.

DISCUSSION

The main result of this study was that the magnitude of the P3 reduction to auditory stimuli in schizophrenics was larger when the stimuli were attended (Attend Auditory condition) than when they were ignored (Attend Visual condition) and was larger for task-relevant compared to taskirrelevant stimuli (in the Attend Auditory condition). This suggests that P3 reduction in schizophrenics is dependent upon both task relevance and the direction of attention. Consistent with the

literature, the schizophrenics’ P3 to task-relevant (or signal) stimuli was reduced when the stimuli were attended (Roth and Cannon, 1972; BaribeauBraun et al., 1983; Brecher and Begleiter, 1983; Duncan-Johnson et al., 1984; Pfefferbaum et al., 1984, 1989; Duncan, 1988). The magnitude of this reduction was the same in the Complex and in the Simple conditions. Thus, the amplitude of P3 to the task-relevant stimuli was not differentially influenced by the presence of novel or familiar stimuli, suggesting that distraction to these stimuli did not affect the characteristics of the P3 to taskrelevant stimuli. The P3 to the deviant task-irrelevant stimuli in the Attend Auditory condition was also reduced in the schizophrenics, but to a lesser extent than the P3 to the task-relevant stimuli. This result suggests that schizophrenics are deficient in processing all stimuli whether task-relevant or not, as long as they require effortful processing. It should be noted that the notion of task relevance is ambiguous. There is not a clear distinction between taskrelevant and task-irrelevant stimuli in the Attend Auditory condition. Although the deviant stimuli were task-irrelevant, they had signal values since they had to be cognitively processed in order to be recognized as non-signal.

19

The schizophrenics’ P3 to the auditory stimuli was normal in the Attend Visual condition. In fact, P3 to the novel stimuli was normal, whereas P3 to the familiar stimuli was somewhat reduced. The few studies which have attempted to assess P3 to unattended stimuli in schizophrenics have provided conflicting results. Our result suggests that intrusive stimuli elicit a normal P3 in schizophrenics. However, Roth et al. (I 980) and Pfefferbaum et al. (1989) utilized loud (loo&105 dB) stimuli (which might be thought as being intrusive) in a passive situation and found reduced P3s in schizophrenics. It is possible that the P3 elicited by loud noises reflects a cognitive response different from the response associated with novel stimuli. Loud stimuli elicit a defensive response-the startle response-whereas novel stimuli evoke an orienting response (Squires et al., 1975). Another important factor that could explain such conflicting results is the instruction differences. In the present study, subjects were instructed to ignore the auditory stimuli and to actively attend to stimuli in the visual modality, whereas in the above studies, subjects were asked to passively listen. The question of the effects of antipsychotic medication on brain waves in general and P3 in particular could be raised. P3 amplitude has frequently been shown to be insensitive to antipsychotic medication (Pass et al., 1980; Roth et al., 1981; Pfefferbaum et al., 1989). We did not find a significant correlation between the daily dosage of antipsychotic medication and P3 in the schizophrenics of our study. Recently, Duncan et al. (1987) studied the P3 to task-relevant stimuli in schizophrenics off and on antipsychotics and recorded their clinical improvement using the Brief Psychiatric Rating Scale (Overall and Gorham, 1962). They found that the visual P3 was normalized only in patients showing clinical improvement. In contrast, the auditory P3 was not changed by treatment, whether or not improvement occurred. Duncan et al. (1987) raised the possibility that the auditory P3 abnormality could be a trait marker of the vulnerability for schizophrenia. It is not yet known whether P3 to task-irrelevant stimuli is similarly unaffected by antipsychotic medication. Nonetheless, since antipsychotic drugs reduce distraction (Oltmanns et al., 1978) and improve attentional functions (Spohn et al., 1977; Braff and Saccuzzo, 1982), it is possible that the difference in

P3 to task-relevant and task-irrelevant stimuli in schizophrenics was in fact minimized by antipsychotic treatment. The increased P3 abnormality with increased task relevance confirms that schizophrenics suffer from difficulties in handling task-relevant stimuli (Oltmanns, 1978; ijhman et al., 1986) and suggests impairments in ‘controlled’ or effortful processes. The schizophrenics’ P3 to novel stimuli was normal in the Attend Visual condition. Since P3 elicited in this condition may be considered reflective of ‘automatic’ processes (Roth et al., 1984) our findings support the view that in schizophrenics automatic processes are less affected than controlled or effortful processes (Oltmanns, 1978; Callaway and Naghdi, 1982). The schizophrenics in our study demonstrated larger P3 reduction to task-relevant than to taskirrelevant stimuli. This result could be interpreted as indicating that schizophrenics allocated relatively more resources to task-irrelevant than to task-relevant stimuli. It also could be suggested that the schizophrenics failed to habituate to taskirrelevant stimuli. P3 to task-irrelevant stimuli usually shows rapid habituation (Courchesne, 1978; Knight, 1984). It is possible that in the control group the P3 to the task-irrelevant stimuli was initially large, but small at the end, whereas in the schizophrenic group it was initially relatively small and remained so throughout. Due to the technique of averaging ERP responses across many successive trials, these two patterns of responses could result in similar P3 responses in both groups. Indeed, a number of studies have indicated habituation impairment to task-irrelevant stimuli in schizophrenics (Roth and Cannon, 1972; Geyer and Braff, 1982; ijhman et al., 1986). In summary, the schizophrenics in our study demonstrated impairment in processing task-relevant stimuli at the brain electrophysiological level. This impairment could be either due to excessive allocation of resources to task-irrelevant information or to an impairment in habituation to such information.

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