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stroop Paper

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Schizophrenia Research 20 (1996) 57 68

SCHIZOPHRENIA

RESEARCH

Stroop interference and facilitation in the cerebral hemispheres in

schizophrenia.

Mary L. Phillips *, Peter W.R. Woodruff, Anthony S. David

Department of Psychological Medicine, Kings”” College Hospital and Institute of Psychiatry, 103, Denmark Hill, London,

SE5 8AZ, UK

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Received 20 January 1995; accepted 25 September 1995

Abstract

A divided visual field presentation of the Stroop colour-word test was used to study interhemispheric (trans-

callosal) function in schizophrenic and normal subjects. The modified test used a colour strip paired with either an

incongruent, neutral or congruent colour word. Interference and facilitation were defined as the difference between

the mean reaction times for incongruent stimuli and neutral stimuli and that between neutral stimuli and congruent

stimuli, respectively. The stimuli were presented tachistoscopically with the two components either separated across

the midline centrally, up or down from the midpoint (bilateral presentation), or presented to a single visual half-field

(unilateral presentation). The inclusion of the ‘up’ and ‘down’ bilateral conditions controlled for the reaction time

advantage for central compared with unilateral stimulus presentations noted in an earlier study (David, 1993a). The

difference in reaction time to an incongruent compared with a congruent colour-word pairing, the Combined Stroop

Effect (CSE), was not affected significantly by stimulus presentation position (bilateral vs. unilateral) in either the

control or schizophrenic group, unlike in the earlier study of David. For controls, interference was significantly greater

than lhcilitation for both bilaterally- and unilaterally-presented stimuli. Schizophrenics had a significantly smaller

interference effect for bilaterally-presented stimuli, indicating reduced interhemispheric interference in this group. For

the control group, there was no significant difference between right and left CSEs, interference being significantly

greater than facilitation on both visual half-field presentations. For schizophrenics, the CSE for left-presented stimuli

was significantly greater than that for right-presented stimuli. In addition, left-field facilitation was significantly greater

than right-field. Reduced Stroop facilitation with right-field presentations in schizophrenic subjects may reflect

increased vulnerability of colour-naming by word-reading in the left hemisphere – evidence for the non-unitary nature

of attentional processes and disintegration of these in schizophrenia.

Keywords.””

Schizophrenia; Stroop; Interhemispheric; Interference; Facilitation

* Corresponding author. Tel: 0171-740-5078. Fax: 0171-740-5129

0920-9964/96/$15.00 © 1996 Elsevier Science B.V. All rights reserved

SSDI

0920-9964 ( 95 ) 00088-7

58

Mary L. Phillips et al./Schizophrenia Research 20 (1996) 57 68

Introduction

The corpus callosum has been the subject of

much interest and investigation in relation to

schizophrenia, with various studies providing evi-

dence of structural abnormalities (Rosenthal and

Bigelow, 1972: Nasrallah et al., 1986; Raine et al.,

1990; Woodruff et al., 1993,1995; David, 1994).

The notion of abnormal interhemispheric callosal

function in schizophrenia has led to a large number

of neuropsychological investigations in recent

years (Merriam and Gardner, 1987: David, 1987;

for a review, see Coger and Serafetinides, 1990),

with some results suggestive of an impairment in

trans-callosal transmission in schizophrenia. This

has been linked to dysfunction in forebrain

and brainstem interhemispheric processes (Doty,

1989). Studies employing electrophysiological

techniques including sensory evoked potentials and

interhemispheric EEG coherence have, on the

other hand, demonstrated the presence of increased

interhemispheric transfer in schizophrenic subjects

(Gulman et al., 1982; Merrin et al., 1989), particu-

larly in subjects with early onset of the illness

and negative symptomatology (Coger and

Serafetinides, 1990).

The colour-word test of John Ridley Stroop

(Stroop Test) has been used extensively in neuro-

psychological investigation of a variety of subject

groups (MacCleod, 1991 ). In the classical Stroop

Test, the subject is presented with a colour-word

written in either the same-colour ink as the colour-

word (congruent condition) or in a different colour

(incongruent condition). The subject is asked to

name either the colour word or the colour ink in

which the word is written, with the response to

the incongruent condition being slower or more

error-prone than that to the congruent condition.

The difference in reaction time (RT) in the incon-

gruent condition compared with the congruent

condition is referred to as the Combined Stroop

Effect (CSE). The task is thus a useful test of

selective attention.

A version of the colour-word test has been used

in order to study interhemispheric (trans-callosal)

function in various subject groups (David,

1992, 1993a). The modified test uses a colour strip

paired with either an incongruent or congruent

colour word and presented tachistoscopically with

the two components either separated across the

midline (central presentation) or presented to a

single visual half-field (lateral presentation)

(David, 1993a). The CSE has been calculated in

previous studies employing this modified test, with

the results demonstrating the presence in schizo-

phrenics of a larger CSE centrally compared with

normal controls or with subjects with affective

disorder (David, 1993a). This was interpreted by

David as evidence either for increased interhemi-

spheric interference for incongruent stimuli or

increased interhemispheric facilitation for congru-

ent stimuli for schizophrenics compared with

controls.

This current investigation attempted to investi-

gate whether it is interference or facilitation or

both which are increased trans-callosally in schizo-

phrenia. The difficulties associated with accurate

measurement of interference and facilitation have

been highlighted by Lindsay and Jacoby (1994),

who demonstrated the independence of the two

processes. The current study, by incorporating a

neutral condition in addition to the interfering

(incongruent) and facilitating (congruent) condi-

tions provided a baseline against which approxim-

ations of the two processes can be measured. Both

parameters may then be compared across different

subject groups.

The previous study of David (1993a) had

demonstrated a bilateral RT advantage for the

Stroop stimuli (see also Ludwig et al., 1993). Such

a finding may have confounded the CSE results in

David’s study. The current study therefore con-

trolled for this by incorporating two further central

stimulus presentation positions with the stimulus

presented in the vertical axis above and below the

midpoint – referred hereafter as the ‘up’ and

‘down’ positions.

The effect of illness subtype on performance in

the Stroop task has recently been investigated by

Carter et al. (1993). Using a non-divided field

presentation technique, they demonstrated that

patients with the paranoid subtype showed normal

amounts of facilitation and increased interference

whereas the undifferentiated subtype showed

increased facilitation.

Mary L. Phillips et al./Schizophrenia Research 20 (1996) 57 68

59

The aims of the current study were therefore the

following:

to attempt to replicate the findings of David

(1993a), in which schizophrenic subjects demon-

strated a greater CSE in the central compared with

the unilateral presentation positions.

to examine the CSE when the bilateral condition

included stimuli presented above and below the

midpoint (the ‘up’ and ‘down’ positions).

to measure separately interference and facilita-

tion in the unilateral and bilateral conditions, in

both schizophrenic and normal subjects.

to demonstrate an effect of illness subtype on

performance in the divided visual field version of

the Stroop test.

Reading Test (NART) – an estimate of premorbid

IQ (Nelson et al., 1978).

Subject details are summarised in Table 1. The

mean BPRS score for the schizophrenic subjects

was 15.11 (±SD 11.05). The groups were well-

matched for age (p=0.43). The mean IQ of the

schizophrenics was somewhat lower at 107 (__ SD

10.02) compared with 113 (±SD 9.25) of the

controls (p=0.02). Schizophrenic subjects were

taking a variety of medications: neuroleptics (22),

lithium (4), antidepressants (3), anticholinergics

(11) and benzodiazepines (2). No normal controls

were taking medication, except one normal control

subject who was prescribed a benzodiazepine.

2.2. Apparatus and stimuli

2. Method

2.1. Subjects

All subjects in the study were recruited as part

of a larger neuroimaging study restricted to males.

Twenty-seven patients with a diagnosis of schizo-

phrenia (DSM-III-R criteria based on symptoms

at interview and in the case notes; APA, 1987) at

various stages of illness: acute-on-chronic (n = 1),

chronic (n=15), in remission (n=ll); and with

different subtypes: paranoid (n=9L undifferenti-

ated (n=2), residual (n=5), remission (n–ll))

were recruited from the in- and outpatient depart-

ments of the Maudsley and Bethlem Royal

Hospitals, London. All were right-handed (Annett

criteria: Annett, 1970). Overall psychopathology

was rated on the modified Brief Psychiatric Rating

Scale (range: 0-216; Bech et al., 1986; Bech, 1993).

The presence and number of first rank symptoms

(FRS) was also noted (0-3 with the value label

‘4’ designated for three or more FRS), defined

according to Wing et al. (1974). The presence and

type of auditory hallucinations was also noted. In

addition, 31 male, right-handed normal control

subjects were recruited from local job centres and

from staff at the Institute of Psychiatry. These

were matched for age with the schizophrenic

subjects.

All subjects completed the National Adult

This consisted of a Kodak S-AV 2050 projector

with shutter attachment, controlled by a tachisto-

scope timer panel. Slides (stimuli) were projected

on to a perspex screen with a central fixation

point. Subjects sat with their chin resting on a

support 50 cm from the screen and wore a set of

headphones and microphone attached to a voice

key. The verbal response of each subject to each

stimulus activated the voice key, enabling the

reaction time to be recorded and entered directly

into a personal computer for analysis. The maxi-

mum time allowed for a response was 2500 ms,

thereafter an error being recorded for the stimulus.

A Polaroid filter was used to reduce the glare from

the projector beam.

All stimuli consisted of a vertical colour strip,

approx. 1 ° in width and 3 ° in height, coupled with

a word written vertically in upper case letters

(black Helvetica medium type) and with the same

dimensions. The word and colour strip were always

presented alongside each other and separated by

1.4 °.

The word and colour strip were presented either

centrally – colour strip and word equidistant from

the fixation spot; laterally (right or left visual half-

field) – the medial edge of each part of the stimulus

3 ° lateral (right or left) to the fixation spot; or

centrally – vertically above or below the fixation

spot (inferior edge of the stimulus 3 ° above or

superior edge 3 ° below the fixation spot) (see

Fig. 1 ).

60

Mary L. Phillips et al./Schizophrenia Research 20 (1996) 57-68

Table 1

Comparision of schizophrenic and normal control subjects

Schizophrenics Normal controls t P

Total No. 27 31

Age (years) 32.89 (1.28) 31.35 (1.43) 0.79 0.43

IQ (NART) 107.00 (1.93) 113.06 (1.67) 2.40 0.02

Errors (%) 6.70 (1.17) 2.78 (0.48) 3.10 0.004

Overall mean reaction time (ms) 890.15 (40.61) 725.95 (20.34) 3.62 0.001

The numbers in parentheses refer to the standard error of the mean.

bl

71

o

R

Fig. 1. Examples of divided field Stroop stimuli. The word and

colour strip were presented either centrally (‘central’) colour

strip and word equidistant from the fixation spot, laterally (right or left visual half-field – ‘right’ or ‘left’) the medial edge of

the stimulus 3 ° lateral (right or left) to the fixation spot, or

centrally but vertically above (‘up’) or below (‘down’) the fixa-

tion spot (inferior edge of the stimulus 3 degrees above or supe-

rior edge 3 ° below the fixation spot, respectively). In all five

presentation positions, the word and colour strip were always

separated by 1.4 °. The figure demonstrates examples of bilater- ally presented stimuli (‘central’ and ‘up’) and of a unilaterally

presented stimulius (‘right’). The colour strip was red, blue or

green. The word was either colour-congruent, (‘RED’, ‘BLUE’ or ‘GREEN’, respectively), colour-incongruent (colour word

different from the colour of the colour strip) or neutral (‘DOG’,

‘COOK’ or ‘CHAIR’).

The colour strip was red, blue or green. This

was paired with either the congruent colour word,

an incongruent colour word or with a ‘neutral’

word. Neutral words were matched in terms of

number of letters and approximate frequency of

occurrence in written English. These control words

also had different initial letters from those of the

colour words to avoid cueing. They were: DOG,

COOK, CHAIR.

In total, 180 slides were shown, 36 in each of

the five presentation positions (central, right lat-

eral, left lateral, up and down). Of each block of

36 slides, 12 were congruent, 12 were incongruent

and 12 neutral in pairings. Of each 12 slides, 6

had the colour strip on the right and 6 had the

colour strip on the left.

Prior to testing, all subjects demonstrated their

ability to read all the words and name all the

colours in the slides. 20 practice slide were shown

before the testing procedure. The slides were pre-

sented in pseudo-random order with respect to

stimulus position and congruity at approx. 1 every

2.5. s.

2.3. Procedure

Subjects were instructed to look at the central

fixation spot at all times. A warning tone sounded

over the headphones 500 ms before the presenta-

tion of each slide. Each slide remained for 120 ms.

Subjects were asked to name the colour (as

opposed to the word) as quickly and as accurately

as possible.

3. Results

The data were analysed with the use of multivari-

ate analyses of variance with diagnosis as the

between subject factor, position (5 levels) and

congruity (3 levels) as within subject factors.

Univariate analyses of variance, two-tailed

unpaired t-tests and matched pairs t-tests were

used for post-hoc exploratory analyses as

appropriate.

3.1. Effects of diagnosis, stimulus position and

congruity on reaction time

Reaction times (RT) for the different stimulus

conditions are shown in Table 2. Diagnosis exerted

a significant effect, with total mean RT for the

Mary L. Phillips et al./Schizophrenia Research 20 (1996) 57-68

61

Table 2

Reaction times (ms) for different stimulus conditions

Schizophrenics Controls Mean difference

Position

All central 876.26 (39.09) 712.60 (20.08) **

All up 904.55 (43.47) 740.40 (23.49) *

All down 879.83 (40.14) 715.50 (21.36) **

All right 881.09 (39.37) 736.07 (21.21) * All left 880.47 (43.91) 725.90 (22.87) *

All bilateral 886.88 (39.81) 722.83 (20.01) **

All unilateral 880.78 (41.13) 730.99 (21.46) *

Congruity

All incongruent 933.40 (41.38) 788.46 (23.75) *

All congruent 838.93 (37.84) 680.15 (19.79) **

All neutral 884.13 (44.35) 708.78 (19.08) **

Right and Left half-field presentation positions

Right incongruent 908.76 (34.86) 796.48 (22.56) *

Right congruent 865.70 (43.62) 691.39 (24.25) **

Right neutral 863.62 (43.30) 716.11 (22.68) *

Left incongruent 961.96 (50.25) 790.98 (29.26) *

Left congruent 803.67 (35.61) 675.62 (22.96) * Left neutral 880.61 (56.32) 707.30 (20.10) *

The numbers in parentheses refer to the standard error of the mean.

*P

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