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Cognitive Brain Research

Research report

Differential processing of word and color in unilateral spatial neglect

Sharon Morein-Zamira,*, Avishai Henika, Meirav Balasa, Nachum Sorokerb

aDepartment of Behavioral Sciences and Zlotowski Center for Neuroscience, Ben-Gurion University of the Negev, Beer-Sheva, IsraelbLoewenstein Rehabilitation Hospital, Raanana, and Sackler Faculty of Medicine, Tel-Aviv University, Israel

Accepted 25 October 2004

Available online 23 December 2004

Abstract

The aim of the present study was to investigate mechanisms underlying processing of contralesional visual stimuli in brain-damaged

patients with unilateral spatial neglect (USN). Nine right-hemisphere-damaged stroke patients with left-sided neglect and nine controls

performed a reaction-time task involving manual response to a central color patch (target stimulus) flanked to the left or right by a Stroop

stimulus they had to ignore. While the word dimension of the flanker affected patients’ responses considerably and equally when presented to

either side, the color dimension of the flanker had no effect when presented to the left, but had a large effect when presented to the right. Four

of the patients performed a control task requiring same/different judgments between either of the two flanker dimensions (color and word)

and the central target. Their performance indicated that they were able to process color information from the contralesional field, despite their

results in the first experiment. These findings demonstrate a dissociation between how the patients processed different dimensions of the

same stimuli and imply that the extent of processing in the contralesional hemifield depends both on task requirements and on the exact

features of the stimuli. The implications of these results on normal attentional mechanisms is also discussed.

D 2004 Elsevier B.V. All rights reserved.

Theme: Human cognition

Topic: Attention

Keywords: Unilateral spatial neglect; Extinction; Spatial attention; Selective attention; Implicit processing; Stroop; Flanker

1. Introduction

Brain-damaged patients with unilateral spatial neglect

(USN) fail to notice and report the existence of salient stimuli

in the contralesional side of space [17,19]. Although they do

not orient toward these stimuli nor respond to them explicitly,

various experimental procedures have pointed to the

existence of implicit processing of the neglected information.

Volpe et al. [45] first showed that USN patients can perform

correct same/different judgments between bilaterally pre-

sented stimuli while failing to name the contralesional

stimuli. Karnath [22] replicated this finding, as did Berti et

al. [4] who demonstrated processing to the level of

categorical identification with an extinction patient (but see

0926-6410/$ - see front matter D 2004 Elsevier B.V. All rights reserved.

doi:10.1016/j.cogbrainres.2004.10.013

* Corresponding author. Department of Psychology, 2136 West Mall,

University of British Columbia, Vancouver, BC, Canada V6T 1Z4.

E-mail address: smorein@interchange.ubc.ca (S. Morein-Zamir).

Ref. [15] for an alternative explanation of such dissociative

performance). Marshall and Halligan [30] asked USN

patients to select one of two seemingly identical drawings,

which in fact differed in details on the contralesional side.

Patients’ behavior implied that they were unconsciously

taking these details into account (see also Refs. [6,11]).

Studies using the priming task have also found evidence for

implicit processing of contralesional information such as

pictures and words at the categorical level [5,25,31].

Implicit processing of neglected/extinguished informa-

tion was also studied using the flanker task, a paradigm

widely used in studies of selective attention. In this task, a

target stimulus is flanked by either identical stimuli or

stimuli leading to the same response (the congruent

condition), or by different stimuli leading to an opposite

response (the incongruent condition). Participants are asked

to identify the target stimulus while ignoring the task-

irrelevant flankers. It is possible to include an additional

23 (2005) 259–269

S. Morein-Zamir et al. / Cognitive Brain Research 23 (2005) 259–269260

neutral condition where the flankers are dissimilar to the

target but do not lead to any response. Typically, responding

to the congruent condition is faster than responding to the

neutral condition and this in turn, is faster than responding

to the incongruent condition [14,33,41]. The flanker effect

has been shown to include a component of interference due

to physical dissimilarity and a component of interference

due to response competition processes [14]. Several studies

have utilized the spatial arrangement of the flanker display

to investigate processing of contralesional flanking stimuli

in USN. Audet et al. [1] presented flanking letters to the

contralesional side of target letters. While no effect was

found in one neglect patient, another exhibited interference

when the flanker was presented before the target and he was

asked to take the flanker into account. Di Pellegrino and De

Renzi [10] also found an interference effect in a patient with

extinction (a related disability where patients demonstrate

attentional deficits to contralesional stimuli only in the

presence of additional, competing stimuli). The patient’s

reaction times (RT) to ipsilesional stimuli were longer when

incongruent versus congruent stimuli were presented to the

contralesional field. Cohen et al. [8] presented a color target

flanked to the left or right by a congruent or incongruent

color patch and found a comparable flanker effect for both

sides. In contrast, in a study by Ro et al. [38] using similar

stimuli with a slightly different procedure, two patients with

USN displayed only an ipsilesional effect.

A second task prevalent in the investigation of selective

attention is the Stroop task. In this task, one is asked to name

the color of the ink in which color words appear [39] (for

review, see Ref. [29]). The words can either be congruent

with the ink color (RED in red ink) or incongruent (GREEN

in red ink); in both cases, the correct response is red. The

Stroop effect is thought to indicate the automaticity of word

processing, or at least the lesser attentional demands needed

for the word to be processed. It is also thought to be due, in

part, to response selection mechanisms [29]. The effect is

quite robust and can even be seen with spatial separation

between the target color and the word [12,16,32]. Since the

Stroop effect does not typically involve the use of selective

spatial attention, it has not been widely used in USN research.

Berti and Rizzolatti [3] reported a patient with severe neglect

dyslexia (where patients fail to read the contralesional side of

words [13]), who still demonstrated the Stroop effect,

suggesting the possibility that the entire word was processed

implicitly. Additional evidence in USN patients has pointed

to the robustness of implicit word processing. Despite many

patients neglecting the contralesional side of specific objects

[23], only a few display neglect dyslexia [27]. Furthermore,

in many patients with neglect dyslexia, implicit partial

processing of the entire word can be deduced from the

existence of the word length effect; patients neglect half of the

word irrespective of its length, implying that they have

information about the word length [7,26].

Despite the fact that many studies have demonstrated

implicit processing of neglected and extinguished stimuli,

several reservations may be raised. First, the results

concerning the implicit processing of color stimuli in the

contralesional field are inconclusive. While Cohen et al. [8]

found that patient performance indicated equivalent pro-

cessing of color stimuli in both the left and right visual

fields, this was not the case with patient performance in the

study of Ro et al. [38]. Second, implicit processing has been

found primarily when conditions were favorable for the

processing of contralesional information. For example, such

processing may have occurred on account of prolonged

duration of presentation [1], or the display of salient and

non-complex stimuli [8]. Baylis et al. [2] used multi-

attribute stimuli in an extinction paradigm and found

implicit processing of irrelevant attributes in the neglected

field. However, it is unclear whether USN patients

implicitly process contralesional information if presented

as multidimensional, complex stimuli displayed for dura-

tions comparable with those used with normal participants.

Additionally, the various paradigms used to demonstrate

implicit processing of contralesional information vary

significantly. In regular matching tasks [4,45], processing

of contralesionally presented information is explicitly

demanded, while in tasks using the flanker procedure, the

subject is typically instructed to ignore the flanker (see Ref.

[26] for a related discussion). One may ask whether the

implicit processing in both types of tasks is comparable.

Finally, the stimuli presented in the contralesional field often

require different degrees of visual processing ranging from

simple color processing, to form perception in letters and

semantic processing in words. It is unclear whether different

dimensions of the stimuli are processed to the same degree

when presented in the contralesional hemifield.

The current study aimed to address the above issues by

a procedure that combined both the flanker and Stroop

tasks. This procedure was first used by Henik et al. [20].

They presented normal subjects with a central color patch

as a target, flanked with a Stroop stimulus on one of the

sides. Both flanker dimensions (word and color) influ-

enced manual response latencies to the central color

patch, without interacting [20]. Here, we used the Stroop-

flanker task to further examine the processing of complex

stimuli in the contralesional field of patients. The

procedure offers an opportunity for a direct and straight-

forward comparison of different stimulus dimensions

within the same display under conditions of constrained

cognitive processing. Using two distinct tasks with

identical stimuli but different task demands, we examined

whether patients suffering from contralesional attentional

deficits are likely to display differential processing of both

the word and color dimensions of ipsilesional and contrale-

sional Stroop-flanker stimuli. In the first task participants

were asked to identify the central color patch and ignore

any flanking stimuli [8,10,20,38]. In the second task,

participants were asked to perform same/different judg-

ments of the central color patch with one of the flanker

dimensions [4,22,45].

S. Morein-Zamir et al. / Cognitive Brain Research 23 (2005) 259–269 261

2. Methods

2.1. Participants

Nine right-handed first-episode stroke patients partici-

pated in the study. The average age was 56F 16.2 years and

the educational level was 12.8 F 2.8 years of formal

schooling. All patients had infarctions (six ischemic, three

hemorrhagic) confined to the territory of the right middle

cerebral artery. Visual fields were preserved in eight patients

who nevertheless demonstrated contralesional extinction

under conditions of bilateral simultaneous visual stimula-

tion. One patient had a left lower quadrantanopsia. Soon

after the onset of stroke, all of the patients manifested left-

sided neglect in the standardized bBehavioral Inattention

TestQ [18]. Individual demographic, clinical and lesion data

of the patients are presented in Table 1. Nine (six men, three

women) age and education matched right-handed healthy

individuals served as controls. Like the patients, all had

normal, or corrected-to-normal, visual acuity and no color

blindness. The average age of controls was 64 F 6.9 years

and the educational level was 13 F 2.4 years. Age and

education level were not significantly different between the

two groups. Four of the patients (RY, SZ, YB, SY) and five

of the controls also participated in Experiment 2.

2.2. Experiment 1—flanker task

2.2.1. Stimuli and apparatus

A central square patch of color (red or green), 18 by 18,served as a target stimulus. A Stroop color word (dREDT,dGREENT or dXXXXT) in color (red, green or blue) was

used as a flanker that appeared simultaneously to the left or

right of the target. The color words were in Hebrew; all

words consisted of four letters and were 2.38 by 0.98. Thecentral color patch and the flanking word were horizontally

aligned and the distance between the center of the color

patch and the nearest edge of the flanking word was 1.28.The two dimensions of the flanker (color and word) were

Table 1

Summary of demographic, clinical and lesion data for the patient group

Patient Age/sex Education (years) TAO (weeks) HP H

RY 55/M 10 15 ++ ++

SZ 62/F 8 10 ++ FYB 52/M 12 20 ++ ++

SY 75/M 14 6 ++ ++

HA 84/M 14 10 ++ +

HS 34/F 16 14 ++ +/

ZA 58/F 17 15 ++ FAM 45/M 12 11 + +/

OO 39/F 12 6 ++ ++

TAO = time after the onset of stroke, HP = hemiplegia, HSL = hemi sensory loss, V

moderate impairment, [++] = severe impairment, e = extinction upon bilateral simu

Inattention Test (a standardized test battery for neglect in the visual modality, ma

Lesion data: I = ischemic infarction; H = hemorrhagic infarction; F, SM, T, P, O

capsular-putaminal, intrahemispheric white matter.

manipulated orthogonally to yield nine flanker variations.

The flankers were combined with 1 of 2 central patches (red

or green) and could appear on either side of the target, so

that 36 possible variations resulted. Each session consisted

of 3 blocks of 180 trials and opened with a practice block

containing all 36 variations. In each experimental block, all

36 variations were presented 5 times in random order. A 14-

in. SVGA monitor was used. Manual response latencies

were recorded by a key-activated relay, interfaced to the

computer.

2.2.2. Task and procedure

Participants were asked to respond manually, as soon as

possible, upon the appearance of the central color patch

(target stimulus), while ignoring the flanking Stroop

stimulus. Gaze direction was monitored during practice

and at the beginning of a block and corrected if needed. The

experiment was conducted in a dimly illuminated room.

After ascertaining that the instructions were understood, the

practice block was initiated and the experimenter ensured

that the task was performed correctly. After completion of

the practice block, the participant performed three exper-

imental blocks. After each block, the participant was

encouraged to rest before pressing one of the keys to

initiate the next block. Response mapping was counter-

balanced so that, while half the participants were instructed

to press the top key when the target was red and the bottom

key when the target was green, the other half responded in

the opposite order. In the second session, the mapping of

responses was reversed for each participant.

Each trial consisted of a central light-gray fixation dot

presented for 500 ms, after which the central target and

flanker appeared simultaneously. The stimuli remained on

screen until the participants responded, and after 1500 ms

the next trial began. The computer automatically discarded

response latencies above 7000 ms and error trials were not

repeated.

Three independent variables were manipulated orthogo-

nally: color, word and flanker side. Color defined the

SL VFD BIT score Lesion Lesion location

�/e 113 H CP, IHWM

/e �/e 103 I F, SM, T, P, CP, IHWM

�/e 39 H CP, IHWM

�/e 61 I SM, T, P, O, CP, IHWM

�/e 110 I F, SM, T, P

e �/e 125 I F, SM, T, P, CP, IHWM

/e �/e 111 I F, SM, T, P, CP, IHWM

e �/e 122 I F, SM, T, P, CP, IHWM

LLQA 38 H CP, IHWM

FD = visual field defect, [�] = no impairment, [F] = mild impairment, [+] =

ltaneous stimulation, LLQA= left lower-quadrant anopsia, BIT = Behavioral

ximal score: 146, cut-off for normality: 130 [16]).

, CP, IHWM = frontal, sensory-motor cortex, temporal, parietal, occipital,

S. Morein-Zamir et al. / Cognitive Brain Research 23 (2005) 259–269262

congruency of the flanker color with that of the central

patch, and could be incongruent, neutral or congruent. Word

defined the congruency of the flanking word with the color

of the central color patch, and could also be incongruent,

neutral or congruent. Finally, the flanker appeared either to

the left or right of the target patch. As all patients exhibited

left-hemifield neglect, a flanker appearing in to right of the

central color patch was in the ipsilesional field, while a

flanker appearing to the left was in the contralesional field.

The dependent variables were response latency, measured

with millisecond accuracy and response accuracy.

Fig. 1. Results of Experiment 1: Mean response times in the flanker task as a functi

(A) The effect of flanker color and (B) the effect of flanker word.

2.3. Experiment 2—similarity judgement task

2.3.1. Stimuli and apparatus

The stimuli and apparatus were identical to those of

Experiment 1 with the exception that flankers contained no

neutral color (blue) or word (XXX), but only incongruent

and congruent values (red and green colors; dREDT and

dGREENT words). The additional variable of task was

introduced. There was a color comparison task and a word

comparison task. There was one block of 240 trials for each

task and a practice block of 32 trials preceded each task.

on of flanker congruency and flanker side, for the patient and control groups.

S. Morein-Zamir et al. / Cognitive Brain Research 23 (2005) 259–269 263

2.3.2. Task and procedure

Here the aim was to examine whether a dissociation

between contralesional color and word processing would be

apparent in a matching task. Participants were required to

perform similarity judgments using the same type of stimuli

employed in Experiment 1. Similarity judgments are

indirect in the sense that they do not require specific overt

responses to the contralesional stimulus [27]. However, in

contrast to the previous flanker experiment, processing of

contralesional information is part of the task requirements.

Two types of tasks were incorporated in Experiment 2.

The color comparison task was to judge whether the color of

the flanking Stroop stimulus was the same as the color of the

central color target or different from it. The word

comparison task was to judge whether the flanker word

corresponded to the color of the central target or not. Thus,

in each task one dimension of the flanker stimulus had to be

processed in order to arrive at the correct answer, while the

other was irrelevant. It has previously been demonstrated

that when attention is shifted to the contralesional side,

processing of stimuli on this side improves [23,34]. Hence,

it was of interest to examine: (a) if the relevant flanker

dimension, presented in the contralesional visual field,

would be processed and (b) if the irrelevant dimension

would also be processed.

Participants performed the color and word comparison

tasks in two separate blocks in each of two sessions (task

order was counterbalanced between sessions). In the color

comparison task, participants responded to the question: Are

the colors of the flanker and the target the same or different?

In the word comparison task, they responded to the

question: Are the flanker word and the target indicating

the same or a different color? This meant that in each task,

the two flanker attributes has a different status. As opposed

to Experiment 1, processing one of the flanker attributes was

part of the task demands. Hence, when the relevant flanker

attribute was congruent with the central color patch,

participants should respond dsameT and when the relevant

flanker attribute was incongruent, participants should

respond ddifferentT. As in Experiment 1, the irrelevant

flanker attribute has no direct bearings on task requirements.

Half of the participants in each group used the top key for

dsameT and the bottom key for ddifferentT, while, for the

other half, the order was reversed.

1 The patient group was also analyzed separately and the same results

obtained. There were significant effects of side [ F(1,8) = 32.7, p b 0.01],

flanker color [ F(2,16) = 8.8, p b 0.01] and flanker word [ F(2,16) = 4.3, p b

0.05]. The interaction between color and side was significant [ F(2,16) = 6,

p b 0.01], with a large congruency effect on the right [ F(1,8) = 15.8, p b

0.01] but not on the left [ F b 1], while flanker word did not interact

significantly with any other factor.

3. Results

3.1. Experiment 1—flanker task

3.1.1. Response latencies

The medians for each of the conditions were computed

and used in an analysis of variance (ANOVA) with group

(patients, controls) as a between-participant variable and

color (incongruent, neutral, congruent), word (incongruent,

neutral, congruent) and flanker side (left, right) as within-

participant variables. Patients were much slower (1174 ms)

than controls (592 ms) [F(1,16) = 37.9, p b 0.001]. When

the flanker appeared to the left, RTs were faster (850 ms)

than when it appeared to the right (917 ms) [F(1,16) =

36.3, p b 0.001]. Four two-way interactions were signifi-

cant including group and color [F(2,32) = 3.4, p b 0.05],

color and side [F(2,32) = 7.6, p b 0.01], group and word

[F(2,32) = 4.3, p b 0.05], and group and side [F(1,16) =

28.5, p b 0.001]. All interactions involving group indicated

larger effects for the patients as compared to the controls

(see below). A separate analysis of patient data indicated a

highly significant effect of flanker side with responses to

central targets with contralesional flankers being signifi-

cantly faster (1111 ms) than responses with ipsilesional

flankers (1237 ms) [F(1,8) = 32.7, p b 0.001].

3.1.1.1. Influence of flanker color. A flanker color effect

was found with 915, 876 and 858 ms for incongruent, neutral

and congruent colors, respectively [F(2,32) = 14.3, p b

0.001]. Of major interest was the three-way interaction of

group, color and flanker side, shown in Fig. 1A [F(2,32) =

4.1, p b 0.05]. Comparisons for the patient group indicated

that, while flanker color had a marked effect when the

flanker was presented on the right (the ipsilesional field)

[F(1,16) = 29.4, p b 0.001], it did not influence responses

when presented to the left of the target (i.e., in the

contralesional field) ( p N 0.25). The RTs for incongruent,

neutral and congruent colors were 1331, 1208 and 1174 ms,

respectively, when the flanker appeared on the right, and

1111, 1125 and 1096 ms, respectively, when the flanker

appeared on the left. The ipsilesional flanker color effect

consisted of significant interference [F(1,16) = 38.2, p b

0.001] and non-significant facilitation. An analysis of the

healthy controls indicated a marginally significant congru-

ency effect for flanker color [F(1,16) = 3.8, p b 0.07], with

means of 610, 586 and 581 ms for incongruent, neutral and

congruent colors, respectively. This flanker color effect

consisted of marginally significant interference [F(1,16) =

3.8, p b 0.07] and non-significant faciliaton.1

3.1.1.2. Influence of flanker word. In the analysis a flanker

word effect was found with 914 ms for incongruent words,

867 ms for neutral strings and 868 ms for congruent words

[F(2,32) = 3.99, p b 0.05]. An examination of the influence

of the word dimension of the flanker (as in a spatially

separated Stroop stimulus) revealed a different pattern of

results from that found with the flanker color. Neither the

two-way interaction between word and flanker side ( p N

0.1), nor the three-way interaction of group, word and

Fig. 2. Results of Experiment 2A: Mean response times in the color comparison task for patients and controls, as a function of flanker color and flanker side.

Flanker color in the color comparison task was task relevant although no explicit response to it was required.

S. Morein-Zamir et al. / Cognitive Brain Research 23 (2005) 259–269264

flanker side ( p N 0.1) were significant. Fig. 1B presents the

relevant data, in order to illustrate the contrasting pattern of

results of the two flanker dimensions. A closer examination

of the patient data revealed the same pattern of results. RTs

for incongruent, neutral and congruent words were 1334,

1180 and 1197 ms, respectively, when the flanker appeared

on the right, and 1141, 1094 and 1097 ms, respectively,

when the flanker appeared on the left. The means of the

controls were 591, 596 and 589 ms for incongruent, neutral

and congruent words, respectively, and did not demonstrate

a significant congruency effect.2

3.1.2. Accuracy data

Mean error rate for controls was low (1.3%) precluding

analysis. Analysis of patient error data indicated only a

significant effect for flanker side [F(1,8) = 8.47, p b 0.05].

The patients responded less accurately to targets when the

flanker appeared to the right of the target (14.6% error) than

to its left (12.5% error).

3.2. Experiment 2—similarity judgement tasks

3.2.1. Response latencies

The medians for each condition were computed and used

in an ANOVA where group (patients, controls) was a

between-participant variable and task (color comparison,

2 Based on previous findings in the literature, we did not expect a non-

significant word effect. However, we suspect it arose due to the large

variability in the control group. In support of this, unpublished data from

the second authorTs lab using different samples of older controls do

demonstrate significant word and color flanker effects. Thus, we are

cautious in over interpreting this finding.

word comparison), word (congruent/same as the color patch,

incongruent/different from the color patch), color (congru-

ent/same as the color patch, incongruent/different from the

color patch) and flanker side (left, right) were within-

participant variables. Flanker attributes were considered to

be dsameT or ddifferentT when they were part of the task

demands and considered congruent or incongruent when

they were task irrelevant. All main effects but flanker side

were significant. Controls responded faster than patients

(837 vs. 1933 ms, respectively) [F(1,7) = 15.5, p b 0.01].

Responses in the color comparison task were faster (1138

ms) than in the word comparison task (1631 ms) [F(1,7) =

18.9, p b 0.01]. Responding to incongruent/different words

was longer (1447 ms) than to congruent/same words (1322

ms) [F(1,7) = 14.4, p b 0.01] as was found for incongruent/

different and congruent/same colors (1440 vs. 1330 ms,

respectively) [F(1,7) = 17.88, p b 0.01].

There were notable differences in performance between

the two tasks, which appeared to have different require-

ments and task entered into higher order interactions.

Consequently, the data from each task was analyzed

separately.

3.2.1.1. Color comparison task. The analysis indicated a

main effect of group [F(1,7) = 9.4, p b 0.05] where controls

responded faster than patients (607 vs. 1670 ms, respec-

tively). There was also a marginally significant effect for

word [F(1,7) = 4.7, p b 0.067] with response to incongruent

words being slower than to congruent words (1187 vs. 1089

ms, respectively), and a marginally significant interaction of

group and word [F(1,7) = 3.79, MSE = 35,975, p b 0.1],

with the effect of word being considerably larger for patients

S. Morein-Zamir et al. / Cognitive Brain Research 23 (2005) 259–269 265

(185 ms) than for controls (10 ms). There was a significant

two-way interaction between color and side [F(1,7) = 7.1,

p b 0.05] and a marginally significant three-way interaction

between group, color and side [F(1,7) = 4.6, p b 0.07]. As

seen in Fig. 2, this interaction was due to a significant

difference [F(1,7) = 5.6, p b 0.05] between left and right

flankers for the dsameT color condition in the patient group

alone. No other comparisons for this interaction reached

significance.

3.2.1.2. Word comparison task. The analysis revealed a

pattern of results somewhat different from that of the color

comparison task. Main effects for group [F(1,7) = 20.9, p b

0.01] and color [F(1,7) = 64, p b 0.001] were significant.

The mean RT for controls was 1066 ms and for patients

2196 ms. Mean RT for incongruent flanker colors was

longer than that for congruent flanker colors (1736 ms vs.

1526 ms, respectively). The interaction between group and

color was significant [F(1,7) = 6.1, p b 0.05], as was the

interaction between color and word [F(1,7) = 21.2, p b

0.01]. Finally, the three-way interaction between group,

color and word was also significant [F(1,7) = 5.9, p b 0.05].

As seen in Fig. 3, this was due to a larger color congruency

effect for dsameT words in patients [F(1,7) = 58.5, p b

0.001] as compared with controls [F(1,7) = 11.9, p b 0.01].

All effects involving flanker side did not reach significance.

3.2.2. Accuracy data

Low error rates for controls (1.9%) precluded their

analysis. Mean error rates for the patient group were 11.1 F9.7% and 12.0 F 6.4% for the color and word comparison

tasks, respectively. Analysis of the errors in each task

Fig. 3. Results of Experiment 2B: Mean response times in the word comparison ta

Flanker word in the word comparison task was task relevant although no explici

different factors than those presented in Fig. 2 and are therefore not analogous to

revealed only main effects for the irrelevant dimension. In

the color comparison task, there was an effect for word

[F(1,7) = 14.5, p b 0.05] and in the word comparison task

there was an effect for color [F(1,7) = 16.2, p b 0.05]. The

means were 13% vs. 9.2% errors for incongruent and

congruent flanker word, in the color comparison task, and

17% vs. 7% for incongruent and congruent flanker color, in

the word comparison task.

4. Discussion

4.1. Contralesional processing in the flanker task

In Experiment 1, patients’ performance exhibited a large

effect of flanker side: RTs were longer with flankers

presented to the right (ipsilesional) visual field as compared

to the left field. This finding is compatible with response

patterns of USN patients, indicating contralesional disad-

vantage in data processing. More importantly, while the

flanker word dimension affected response to the central

target equally from both sides, the flanker color dimension

had a striking effect when presented to the right but no

effect when presented to the left. Both flanker dimensions

influenced patient performance significantly more than that

of controls. Finally, word and color dimensions did not

interact, in accordance with the findings of Henik et al. [20]

in normal participants.

The dissociation between flanker word and color

dimensions in the same display points to several interesting

conclusions. Word processing is sufficient to influence

responses to a central target whether presented to the

sk for patients and controls, as a function of flanker word and flanker color.

t response to it was required. Note that interaction portrayed here involves

each other.

3 The influence of the irrelevant flanker color in the word comparison

task is larger and more robust than the flanker word effect in the color

comparison task. However, RTs to the latter task are also much slower,

suggesting that the larger congruency effect may be due to the difference in

RTs. When accounting for overall latencies in each task, the color

congruency effect is still larger and more robust than the corresponding

word congruency effect suggesting that overall latencies are not sufficient

to explain the data. This finding can be interpreted as additional evidence in

support of the weak translation models as described in detail later in the

text.

S. Morein-Zamir et al. / Cognitive Brain Research 23 (2005) 259–269266

contralesional or ipsilesional side. This is consistent with the

conclusions from priming experiments, which suggest

comparable levels of implicit word processing in both

visual fields [25,31]. In contrast, flanker color dimension

affected responses to the central target only when the flanker

was on the right, indicating that color processing in the

contralesional visual field is not sufficient to influence

performance. The dissociation between word and color

dimensions, when presented simultaneously as part of the

same object, is consistent with word processing being

automatic and requiring less attention to influence responses

[39]. This also provides insight to the low frequency of

neglect dyslexia among USN patients [9,27].

The dissociation between word and color dimensions can

be explained within the framework of the perceptual load

model [28]. Lavie and Tsal [28] proposed that task require-

ments determine the extent of processing of irrelevant

information. If perceptual load is high, only features

necessary for task performance will be processed. If the load

is low, not all capacity is directed towards task completion

and spare attentional capacity will automatically process

irrelevant information. In Experiment 1, the spare (but

severely limited) attentional resources in the left visual field

are sufficient only for activation of the word representation.

The spare attentional resources are not sufficient for com-

parable activation of the color representation. Hence, the

combination of two dimensions in a flanker creates a load

where only the word, due to its inherent characteristics,

influences responses to the target. The demonstration of

comparable effects of flanker color across sides by Cohen et

al. [8] would reflect the smaller perceptual load and

attentional demand in their experiment (a pre-cue was used;

the flanker was unidimensional and larger than the central

target). Ro et al. [38], employing a similar procedure with a

very brief flanker exposure (thereby increasing the perceptual

load), found only an ipsilesional flanker effect in two USN

patients, similar to the present findings.

The current findings also demonstrated that flanker color

effects on the right and flanker word effects on both sides

were larger in patients compared to controls. This is

consistent with a ceiling effect leading to reduced influence

of the flanker attributes in the controls. Nevertheless, even

when the proportion of the effects in relation to the overall RT

is considered, these effects are still significantly larger for the

patient group. The perceptual load hypothesis [28] appears

inconsistent with this finding, as it would predict a smaller

flanker effect for patients due to their limited attentional

capacity. Kinsbourne’s [23] borienting biasQ hypothesis of

USN can explain this seeming inconsistency as it proposes

that reduced right on left hemisphere inhibition in a context of

normal inter-hemispheric reciprocal inhibition leads to an

exaggeration of the normal attentional bias towards the right

hemifield. Moreover, disinhibition of the left hemisphere

would lead to increased, or faster, information processing

targeted primarily to this hemisphere, for example, verbal

information. Hence, flanker information processed primarily

by the left hemisphere would receive higher activation, as in

the case of colors presented on the right hemifield or words

presented on either side, in turn leading to larger interference

effects. In any case, the present results demonstrate: (a)

different levels of processing of different flanker attributes on

the contralesional side and (b) a magnified and consistent

influence of all flanker attributes on the ipsilesional side.

4.2. Contralesional processing in the similarity-judgement

task

The results of Experiment 2 point to several issues. First,

all patients performed the color and word comparison tasks

with above-chance accuracy. Second, the effect of flanker

side was not significant. Third, flanker word and color

dimensions were symmetrical in that each affected perform-

ance when it was the irrelevant flanker dimension.

Furthermore, the congruency effects of the irrelevant flanker

dimensions were larger for patients than for controls.

Finally, there were some asymmetries, as indicated by the

higher-level interactions. In the color comparison task,

where flanker word should be ignored, color interacted

marginally with flanker side and group (see Fig. 2). In the

word comparison task, where the flanker color should be

ignored, word interacted with group and color (see Fig. 3).

In Experiment 2, as in previous studies [16,45], patients

could perform a similarity-judgement task when one of the

stimuli was presented in the contralesional field. Patients

succeeded in matching identical attributes (i.e., color and

color), and different attributes referring to a common

concept (i.e., word and color) [4,31]. The above-chance

accuracy of the patients in the color comparison task makes

deficient color processing an unlikely explanation for the

finding in Experiment 1, where contralesional flanker color

did not influence processing of the central target.

Some commonalties exist between the color comparison

and word comparison tasks. The irrelevant flanker dimen-

sion influenced performance in both tasks (although the

effect of flanker word in the color comparison task was

marginal),3 and had a greater influence on the performance

of patients compared to controls. Yet, there are also

differences between the two tasks. Notably, only in the

word comparison task did flanker word and color interact.

Different task demands can account for these results. In the

color comparison task, two colors were compared while

flanker word was ignored. As there was no need to label the

S. Morein-Zamir et al. / Cognitive Brain Research 23 (2005) 259–269 267

identity of the target and flanker colors, perceptual process-

ing was sufficient. In the word comparison task, a color and

word were compared, while flanker color was ignored. To

perform this task, participants had to use a common concept

comparing the perceptual input of the central color and the

semantic input of the flanker word. Thus, the interaction

between group, color and word in the word comparison task

would result from factors operating at the concept level. The

interaction between the flanker word and color dimensions

results from the need to directly compare a word and color,

which refer to a common concept (in this case color). The

results are similar for both patients and controls and the

higher level interaction is due to a difference in the

magnitude of the interference effect. In fact, the interaction

between color and word is due to the faster RTs in the

conditions where the word and color dimensions of the

flanker are both congruent with the target color.4

Previously, Treisman and Fearnley [42] employed a card

sorting task where participants matched either the color or

word of a Stroop stimulus with either colored x’s or color

words printed in black. They found that matching within

attributes with interference across attributes (matching color

to color, with interference from word) was faster than

matching across attributes with interference within attribute

(matching word to color, with interference from color). This

is analogous to the faster performance in the current

experiment on the color comparison task compared to the

word comparison task. Later studies have replicated this

finding and have suggested that it supports translational

models [44]. Such models claim that words have no special

attentional status and that the Stroop effect is a consequence

of task requirements, specifically, the need for translation

between a color stimulus and a verbal response. They

suggest that stimulus attributes (i.e., word and color) are

processed in parallel, with no cross-talk between the

systems, unless translation from one system to another is

required. In the present experiment, the need for translation

between color and word dimensions would account for the

interaction between them in the word comparison task,

providing additional support for the translation model.

Interestingly, this translation mechanism operated in a

similar manner in both patients and controls.

4.3. Effects of task requirements on contralesional

processing

In both experiments, processing of contralesional stimuli

was demonstrated. While the large difference in RTs of

4 The faster RTs when both flanker attributes are the same as the color

patch could also be accounted by a response bias in this unique situation, as

flanker color is also the same as flanker word (we thank an anonymous

reviewer for pointing this out). However, such an account would never-

theless necessitate processing of the task-irrelevant flanker attribute.

Likewise, a response bias account would still have to address the fact that

an interaction between word and color was present only in the word

comparison task, thus referring to additional mechanisms at play.

patients and controls prevailed in both experiments, the

differential influence of flanker side on patients’ responses

in the flanker task no longer existed in the similarity-

judgement tasks. Furthermore, the dissociation between

color and word processing in the contralesional field noted

in the flanker task, was not apparent in the similarity-

judgement task, although differences between color and

word were still noted. Finally, flanker word and color

interacted in a similarity-judgment task, unlike the additive

effects between them observed in the flanker task, here and

in previous studies [20].

Current USN theorizing offers an explanation for the

effect of flanker side in Experiment 1 and its absence in

Experiment 2. In the flanker task, an effect for flanker side is

predicted as attention may have been dcapturedT involun-

tarily by the right flanker [23]. Slowed RTs in the presence

of a right task-irrelevant flanker are believed to reflect

difficulty in ddisengagementT of attention from the flanker,

prior to relocation of attention towards the central target

(Posner et al. [34,35]). Also, Humphreys and Riddoch’s [21]

explanation for the mobilization of spatial attention in USN

would predict that patients naturally oriented themselves to

the rightmost part of the display, and then reoriented

attention in order to complete the task [24]. The lack of a

clear effect of flanker side in the similarity-judgement tasks

is also consistent with these accounts. As similarity-judg-

ment tasks require processing of two stimuli, attention must

shift between a more ipsilesional stimulus and a more

contralesional one. This is true regardless of the hemifield in

which the flanker appears (a leftward ddisengagementTdifficulty will be manifested both during comparison

between the right flanker and the central stimulus, as well

as during comparison between the central stimulus and the

left flanker).

What can be learned from the demonstration of contrale-

sional processing of color in the similarity-judgement task

as opposed to the flanker task? While in the flanker task,

both flanker attributes were in no way relevant to the task, in

the similarity-judgement task one of the flanker attributes

was relevant to the completion of the task, although

participants did not respond to it directly. Thus, when

flanker color was irrelevant in the word comparison task, its

processing benefited from the processing of the flanker

word dimension. As noted, bilateral processing of the

flanker word dimension was evident both when the flanker

as a whole was irrelevant (Experiment 1) and when only the

flanker word dimension was irrelevant (Experiment 2, color

comparison task), in accord with the notion of word

processing requiring less attention.

The literature reports examples where neglect patients

benefit from allocating attention to the contralesional

hemifield [34–36,45] in accordance with our findings on

effects of flanker color. Yet, there have also been studies

reporting no need for USN patients to allocate attention to

the contralesional hemifield in order to process information

quite efficiently there [31], as with our findings concerning

S. Morein-Zamir et al. / Cognitive Brain Research 23 (2005) 259–269268

the effect of the flanker word dimension. Hence, it is evident

that the flanker and similarity-judgement tasks lead to

different strategies for allocating attention to the contrale-

sional stimulus, even though neither required a direct

response to contralesional information and the visual display

in both was identical. Thus, the extent of stimulus

processing in the neglected field is heavily influenced by

the task-induced strategies for allocating attention. More-

over, although both matching tasks and flanker tasks have

both been used to demonstrate implicit processing of

information in the neglected field, the present results reveal

some notable differences between them. In keeping with

Humphreys and Riddoch’s [21] account of USN, the

comparison between the two experiments suggest that the

most important factors in allocating attention are task

requirements and stimulus characteristics.5

4.4. Implications for normal cognition

The present findings have several implications concern-

ing cognitive theory. According to feature integration, as

suggested by Treisman and Gelade [43], color (being a

feature) is processed automatically without attention. Yet,

this is inconsistent with the finding that word, and not color,

influenced responses when presented to the contralesional

side of patients. Task requirements in the flanker paradigm

differ considerably from those employed by feature detec-

tion tasks and are more similar to those employed in the

Stroop and reverse-Stroop effects where generally word

processing influences color processing but not vice versa

[29]. As noted previously, feature detection, much like the

feature matching (as in the color comparison task), can be

performed at the perceptual level. In feature detection tasks

one identifies the presence of, for example, a red target

amongst green distracters, and no activation of the concepts

of dgreenT and dredT is required after initial encoding of

instructions. Once a perceptual representation is established,

5 An alternative interpretation for the results of Experiment 1 suggests

that the dissociation between color and word resulted exclusively from task

demands. In particular, it has previously been demonstrated that features

along the same dimension as the relevant target are more easily

extinguished [2]. Hence, in Experiment 1, this would entail flanker color

being more easily extinguished than flanker word. However, we believe this

unlikely. For one, stimulus similarity cannot easily explain the results of

Experiment 2 where better performance was observed for dsameT responsescompared to ddifferentT responses. Furthermore, the original finding was not

demonstrated on words, which seem to have a unique attentional status

[3,26,27,29]. In addition, the effect of stimulus similarity was previously

found to be unchanged by whether the stimuli were perceptually or

semantically similar [37]. In the present case, the word and color both

demonstrated the same semantic meaning, suggesting that the similarity

effect should hold for both. Finally, further studies have demonstrated that

the effect of stimulus similarity occurs when both stimuli are task relevant

and require a response [46], which was not the case in Experiment 1.

Nevertheless, stimulus similarity should be examined in future research to

ascertain whether word processing in the contralesional field is vulnerable

when word processing occurs elsewhere in the visual field (we thank two

anonymous reviewers for their ideas on this matter).

no further reference to color names or concepts is needed. In

contrast, the flanker task required the constant maintenance

of the concepts of dgreenT and dredT in working memory and

linking them to a particular response. This concept main-

tenance is also necessary in the word comparison task,

explaining the relative dissipation of the asymmetry

between color and word in Experiment 2.

The present findings provide support for modified trans-

lational models. Strong translational models would predict

neither a flanker word effect in Experiment 1, nor a flanker

word effect in the color comparison task. As found previously

[20,40], the influence of the flanker word on manual

responses to a color target poses a problem for strong

translational models. Weak translational models are consis-

tent with the findings as they assume automatic processing of

words, such that words influence performance even when

they are task irrelevant and no cross-talk between systems is

necessary. Consequently, task requirements appear to play a

significant role in determining performance, but inherent

characteristics of word processing such as its attentional

demands are also important. The additive effects between

flanker attributes found in Experiment 1 and the interaction

between them in the word comparison task also support

translational models [42,44]. When translation between

attributes is not required, additive effects between flanker

attributes are expected. However, when translation between

attributes is required, an interaction is expected.

In conclusion, the use of tasks involving processing of

contralesional multidimensional stimuli (flanker task and

similarity-judgment) implied that the extent of implicit or

indirect processing depended upon task requirements, the

nature of the stimuli and the strategic allocation of attention.

Thus, conclusions concerning the extent of information

processing available to neglect patients in their contrale-

sional hemifield must take into consideration all these

factors. The use of both color and word as flanker attributes

indicated that only when no explicit translation between the

two attributes was required, did they have additive effects.

Furthermore, the results were interpreted as consistent with

words being processed automatically even when attentional

resources are restricted or unavailable.

Acknowledgments

This research was supported by the Israel Science

Foundation, grant no. 859/01. The authors wish to thank

three anonymous reviewers for their helpful comments.

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