Psychiatry Research, 49:183-196 Elsevier

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Visual Attention in Obsessive-Compulsive Disorder

Elliot Nelson, Terrence S. Early, and John W. Haller

Received July 6, 1992; revised version received March 31, 1993; accepted May 20, 1993.

Abstract. Performance on neuropsychological tasks was compared in 15 subjects with obsessive-compulsive disorder (OCD) and 15 age- and sex-matched psychi- atrically screened control subjects. The tasks chosen’are known from studies in other patient groups to demonstrate lateralized abnormalities of visual and limited capacity attentional impairment. The Posner task performance of the OCD group demonstrated decreased inhibition of return for left visual field targets and no inhibition of return for right visual field targets. The OCD group’s spatial-linguistic conflict task responses were significantly slowed in the conflict condition, as predicted. The results are discussed in relation to the phenome- nology of OCD and prior neuropsychological evaluations.

Key Words. Anxiety disorder, neuropsychological deficits, cognition, perception, reaction time, depressed mood.

Human attentional processing has been the object of considerable investigation within the increasingly overlapping fields of cognitive neuroscience and neuro- imaging. Posner and Petersen (1990) reviewed much of this work as support for their hypothesis regarding the functional anatomy of human attention. They suggest that attention may be viewed as being composed of two distinct, but interacting systems: (1) a posterior attention system including the parietal cortex, thalamus, and superior colliculus involved in orienting to sensory stimuli (the automatic processing of exogenous stimuli) and (2) an anterior attention system including the midfrontal anterior cingulate gyrus and supplementary motor area involved in semantic process- ing and target detection (the more effortful, nonautomatic processing of stimuli based upon endogenous information). They propose that the anterior attention system is dominant in more demanding tasks but “can pass control to the posterior system when it is not occupied with processing other material.“Their hypothesis has been strengthened by subsequent work (Henderson, 1991; Nakagawa, 1991; Carter et al., 1992).

Obsessive-compulsive disorder (OCD) is an illness characterized by recurrent intru- sive thoughts and repetitive intentional behaviors. OCD sufferers have long been observed to allocate undue consideration to normally ignored exogenous (environ- mental) detail and endogenous (cognitive) associations. While these phenomenologi-

Elliot Nelson, M.D., is Instructor of Psychiatry, Washington University School of Medicine, St. Louis, MO. Terrence S. Early, M.D., is Associate Professor of Psychiatry, University of Texas Medical Branch at Galveston, TX. John W. Haller, Ph.D., is Research Assistant Professor of Psychiatry, Department of Psychiatry, Washington University School of Medicine, St. Louis, MO. (Reprint requests to Dr. E. Nelson, Dept. of Psychiatry, Washington University School of Medicine, 4940 Audubon Ave., St. Louis, MO 63110, USA.)

01651781/93/%06.00 @ 1993 Elsevier Scientific Publishers Ireland Ltd

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cal features are consistent with attentional impairment, no specific attentional abnormality has been characterized.

Prior attentional studies of those with OCD have generally involved fairly non- specific subtests given within large test batteries. Flor-Henry et al. (1979) reported significantly poorer Wechsler Adult Intelligence Scale digit span performance in individuals with OCD, but Insel et al. (1983), Malloy (1987), and Boone et al. (1991) failed to replicate this finding. Zielinski et al. (1991) recently reported no difference between individuals with OCD and control subjects on a version of the continuous performance task. All neuropsychological investigations of OCD have reported some type of abnormal performance and, because all cognitive tasks have an at- tentional aspect, this issue is by no means settled.

Brown and Marsden’s (1987) discussion of this question in relation to Parkinson’s disease exemplifies the importance of delineating attentional abnormalities within the characterization of cognitive functioning in a patient population. They state, “All valid cognitive testing depends upon adequate alertness, attention, and vigilance.” They proclaim the need to study “... sustained and directed attention tasks in which the motor components are minimal and controlled experimentally.” Their point is made all the more applicable when one considers the increased rate of OCD oc- currence in individuals with postencephalitic Parkinson’s disease (von Economo, 1931), the presumptive involvement of the basal ganglia in both disorders (Javitch et al., 1984; Baxter et al., 1988; Laplane et al., 1989; Swedo et al., 1989; Weilburg et al., 1989), and recent suggestions of the resemblance of features of obsessional slowness to Parkinson’s disease (Lees, 1989). Similar cognitive deficits have been reported in the two illnesses, including abnormalities of visuospatial functioning (Behar et al., 1984; Brown and Marsden, 1985; Della Sala et al., 1985; Cox et al., 1989; Martinot et al., 1990; Boone et al., 1991; Zielinski et al., 1991) and ability to shift cognitive set (Behar et al., 1984; Canavan et al., 1984; Brown and Marsden, 1985~; Malloy, 1987; Cox et al., 1989; Head et al., 1989; Boone et al., 1991; Hymus et al., 1991; Zielinski et al., 1991).

The current study was undertaken to compare performance on two attentional tasks of a group of individuals with OCD with a psychiatrically screened control group matched for age and sex. Well-studied attentional tasks (for which the involved anatomy is fairly well known) were chosen to facilitate the characterization of attentional abnormalities suggested by the phenomenology of the illness. It was hypothesized that those with OCD would demonstrate particular difficulty on the spatial-linguistic conflict task because it measures “higher level”(anterior) attention- al processing in the presence of conflicting environmental cues. Individuals with OCD had previously been reported to have deficits in their ability to switch cognitive set (Behar et al., 1984; Cox et al., 1989; Head et al., 1989; Hymas et al., 1991), and clinically, these patients have notorious difficulties with decision-making. Perfor- mance on the less demanding Posner task was expected to be unimpaired, reflecting sparing of the “lower level” (posterior) component of visuospatial attention.

Methods

Subjects. Sixteen subjects with OCD were recruited from the patient population of the

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Washington University Medical Center. Exclusion criteria consisting of left handedness, history of severe head injury, and late onset (age > 50 years) led to the exclusion of one subject (left handed with onset of OCD at age 74). An unstructured diagnostic interview was performed by one of the authors (E.N.) only to confirm the DSM-III-R diagnosis of OCD (American Psychiatric Association, 1987) with no systematic assessment of comorbid diagnoses. Of the 15 subjects, eight were men and seven women with an overall mean age of 38.7 years. The group included two never-medicated, six previously medicated, and seven currently medicated individuals whose present psychotropic medication consisted only of antidepressants (5 patients receiving fluoxetine, 1 receiving clomipramine, and 1 receiving imipramine) and/ or anxiolytics (4 taking benzodiazepines and 1 taking buspirone [the lone exception being an individual also taking trifluoperazine, 4 mg daily]). A Beck Depression Inventory (BDI; Beck et al., 1961) was given to 11 of the 15 subjects resulting in a mean score of 17.6 with a standard deviation of 3.0.

Control subjects were selected from people who were employed within the medical center or had previously participated as controls in local epidemiologic or positron emission tomography research. Control subjects were screened with the DSM-M-R Checklist (Hudziak et al., submitted) for the presence of Axis I diagnoses. Exclusion criteria consisting of the presence of any Axis I diagnosis, left handedness, or history of head injury resulting in clear loss of consciousness or any subsequent neurologic deficit led to the exclusion of one control subject (history of a motor vehicle accident resulting in a head injury requiring extensive rehabilitation). Control subjects were matched to conform to the patient population in age and sex, and thus the control group included eight males and seven females with a mean age of 38.9 years. The control subjects were also given the BDI and found to have a mean score of 2.5 with a standard deviation of 3.6.

Attentional Measures. Posner task. This task (Posner et al., 1982) is used to gauge the subjects’ ability to orient

attention in response to peripheral visual cues. Subjects are asked to respond to a simple target (an asterisk) appearing within one of two continuously visible boxes (one present in the right and the other in the left visual field) with the reaction time recorded on each trial serving as the dependent variable. The target is preceded on 60% of trials by a valid cue consisting of an increase in luminance of the box in which the target will then appear. The remainder of trials are evenly divided between the target appearing following an invalid cue to the opposite visual field and the target appearing without a cue. Reaction time differences between these three conditions may be used to define three simple operations of visual attention: (1) disengaging attention from its current focus, (2) moving it to a new location, and (3) engaging it at a new location. Work by Posner and colleagues has supported involvement of the parietal lobe in disengaging attention (Posner et al., 1984) the superior colliculus in moving attention (Posner et al., 1982) and the thalamus in engaging attention at a new location (Rafal and Posner, 1987). Recent studies that have examined the Posner task in groups of psychiatric patients have found distinctive patterns of aberrant performance in individuals with schizophrenia (Posner et al., 1988) and attention deficit disorder (Swanson et al., 1991).

Spatial-linguistic conflict task. This task is designed to examine the effects of conflict between a spatial and a linguistic cue. Prior studies by Posner and colleagues (Sandson et al., 1989) in stroke patients had found a significant effect for laterality of lesion with left-sided damage tending to be associated with impaired responses to linguistic cues while right-sided damage impaired responses to spatial cues. Significant impairment has also previously been reported in schizophrenic subjects on this task (Posner et al., 1988).

Procedure. Subjects were seated in front of a Compaq 286 portable computer at a distance of 18 to 24 inches and administered the Posner task utilizing the Micro Experiment Laboratory (MEL) software in a manner identical to that described by Swanson et al. (1991). Subjects were instructed to keep their gaze fixed on a plus sign (fixation cross) at the center of

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the screen. Two boxes (squares) were displayed at 5 degrees of visual display from the fixation cross and these three elements remained illuminated throughout the series of trials. The target consisted of an asterisk presented inside one of the boxes. The subjects kept their right index finger on the response bar throughout the task and responded by pressing it upon seeing the target. The asterisk disappeared with each response or after 3000 msec if no response occurred. After a lOOO-msec intertrial interval, a new trial would begin.

A total of 240 trials was given over 5 blocks of 48 trials, each with brief rest periods between blocks. On 40 of these trials, no cue was presented. The remainder of the trials had the presentation of a cue consisting of a second border appearing around one of the squares giving the impression of the square brightening. Subjects were told that they might see one of the boxes brighten, but should ignore this and “... respond only to the star [asterisk].” The cue heralded the appearance of a target on the same side on 160 of the trials (valid cue) and occurred on the opposite side on 40 trials (invalid cue). Also, the cue-to-target interval (delay) was 100 msec during half of the trials and 800 msec during the other half. Finally, in half of the trials, the target appeared on the right and in the other half on the left. Fig. 1 presents a schematic illustration of the procedure. Reaction times were recorded on each trial and mean reaction times were calculated for each subject for each of the 12 conditions determined by combinations of the independent variables: cue, visual field, and delay.

Fig. 1. Schematic illustration of the Posner task display

VALID cl + IElI

CUE TRIAL

cl + El TARGET

INVALID TRIAL

0 CUE

+

+

cl

El TARGET

Subjects were allowed a brief rest after completion of the Posner task. They then proceeded with the second experiment which used the same Compaq 286 portable computer and MEL software under similar conditions. Responses on this task were made by the subjects using two buttons placed so that the right index finger rested on the left button and the right middle finger rested on the right button throughout the experiment.

Subjects performed four blocks, each consisting of 96 trials. Each block was preceded by the subject’s having been instructed to respond only to either the arrow (pointing to the right or the left) or the word (“right” or “left”) by depressing the appropriate key. Each block contained 32 trials of the stimulus alone, 32 trials of stimuli together in which they were in agreement, and 32 trials in which the stimuli were together and disagreed. Each trial began with a central fixation cross (plus sign) followed 0.5 second later by the stimulus. Trials were

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terminated after 4000 msec if the subject failed to respond. Intertrial interval remained fixed at 1000 msec. Fig. 2 presents the displays used in the procedure.

Fig. 2. Schematic illustration of the spatial-linguistic conflict task display

ALONE COMPATIBLE INCOMPATIBLE

ATTEND ARROW - Right

ATTEND WORD Right Right

Left

R;ght

Results

Posner Task. A four-way repeated measures analysis of variance (ANOVA) with Greenhouse-Geiser correction was performed using one between-subject factor (group) and three within-subject factors (cue, delay, and visual field). Significant main effects were found for cue (F = 18.81; df = 2, 56; p < 0.0001) and delay (F = 43.28; df = I, 28; p < O.OOOl), but not for visual field (F = 0.88; df = 1, 28; p > 0.35) or group (F= 2.43; df= 1,28; p > 0.1). Additional scrutiny revealed that the subjects’ mean reaction times (RT) were shorter in the valid when compared with either the invalid (RTI - RTv = +19 msec) or the no cue (RTN - RTv = +22 msec) condition, and at the 800 msec versus the 100 msec delay (RTIuo - RTsuu = +39 msec) averaged across other factors (see Figs. 3 and 4).

Fig. 3. Mean reaction time for 15 individuals with obsessive-compulsive disorder (OCD) and 15 control subjects at the lOO-msec cue-to-target interval of the Posner task

460 -

-Left ---Right

360 - I , I

No Cue Valid Invalid

CUE CONDITION

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Fig. 4. Mean reaction time for 15 individuals with obsessive-compulsive disorder (OCD) and 15 control subjects at the 800-msec cue-to-target interval of the Posner task

420-

320

-Left

---Right

I I I I

No Cue Valid Invalid

CUE CONDITION

Table 1. Mean reaction time for the Posner task experimental conditions

Condition Reaction time X group

Delay Target Control OCD (msec) Cue VF (msec) (msec)

100 v L 371 397 100 V R 374 399 100 I L 424 442 100 I R 406 449 100 N L 391 441 100 N R 402 426 800 V L 351 379 800 V R 347 397 800 I L 335 371 800 I R 331 412 800 N L 374 383 800 N R 385 385

Note. L = left. R = right. V = valid. I = invalid. N = null. VF = visual field

A significant cue X delay (F = 26.42; df = 2, 56; p < 0.0001) interaction was observed which presumably reflected both the disappearance of the advantageous effects of valid versus invalid cuing at the longer delay (RTvtoo - RT~luo = -45 msec vs. RTvsou - RTrsuo = f7 msec), as well as the relatively greater advantage of any cuing versus the no cue condition at the longer delay (RTcloo - RT~tou = -7 msec vs. RTcsuu - RTp,suo = -16 msec). The three-way interaction cue X delay X group was also significant (F = 9.59; df = 2, 56; p < 0.0005) and probably is explained

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primarily by two between-group differences at the 800-msec delay. First, averaged across visual fields, individuals with OCD continued to show an advantage of valid cuing (RTvsou - RTIsuc = -3 msec) while control subjects displayed a distinct disadvantage (RTvsuu - RTIsoc = +16 msec). Second, those with OCD had faster RTs with no cue versus cuing (RT ~800 - RTcsou = -5 msec), while control subjects were much slower when not cued (RT~scu - RTcsou = +39 msec).

A significant cue X group interaction (F = 4.70; df = 2, 56; p < 0.025) averaged across other factors was noted. Again, this seems to stem from a greater effect of valid versus invalid cuing in individuals with OCD (RTv - RTr = -25 msec) versus controls (RTv - RTr = -13 msec) and from a smaller effect of any cuing versus no cuing in the patient group (RTc - RTN = -3 msec) versus the control group (RTc - RTN = -21 msec). A further three-way cue X field X group interaction was also significant (F = 6.19; df = 2,56; p < 0.01). This apparently reflects the presence of a component of laterality in the above intergroup differences: (1) The valid - invalid cue difference in individuals with OCD was -18 msec with targets on the left and -32 msec with targets on the right, while in control subjects the values were -18 msec and -8 msec, respectively. (2) The effect of any cue versus no cue in individuals with OCD was -15 msec for left field targets and +9 msec for right field targets, while these values for control subjects were -12 msec and -29 msec, respectively.

A significant delay X field interaction (F = 4.96; df = 1, 28; p < 0.05) was also observed. Subjects had slightly faster RTs for right field targets at the lOO-msec delay (RT~lou - RT~leu = -2 msec) and slower right target times at the 800-msec delay (RT~suu - RT~scc = i-l 1 msec). A trend toward a three-way interaction of delay X field X group was noted (F= 3.31; df = 1, 28; p < 0.1). This trend seems to result completely from the OCD group’s demonstration of a lateralized slowing of response at the 800-msec delay to right field targets (RT~suu - RT~suu = +20 msec) versus little difference in control subjects (RT~suu - RT~soc = l kl msec). No other interactions were significant.

A post hoc four-way repeated measures ANOVA with Greenhouse-Geisser correction was performed on the OCD group’s data with one between-subject factor (current medication status) and three within-subject factors (cue, delay, and visual field). No significant main effect was found for medication status (F = 0.23; df = 1, 13; p > 0.6), nor was it involved in any significant interactions.

Separate post hoc four-way repeated measures ANOVAs with Greenhouse- Geisser corrections were performed to examine the effects of depressive symptoms on RT in each group. It was felt that problems with colinearity necessitated separate analyses. The OCD group included only the 11 individuals who completed the BDI, and the control group included all 15 control subjects (all control subjects completed the BDI). One between-subject factor (BDI score) was employed in each analysis as well as three within-subject factors (cue, delay, and visual field).

The control group ANOVA demonstrated no significant main effect for BDI score (F = 1.83; df = 1, 13; p > 0.15) nor was it involved in any significant interactions. Main effects for cue and delay remained significant as did their interaction. No other significant effects or interactions were noted.

The OCD group ANOVA had no significant main effects whatsoever. The only significant interactions were cue X visual field (F = 3.97; df = 2, 18; p < O.OS), cue X

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visual field X delay (F= 5.72; df = 2, 18; p < 0.025), and cue X visual field X delay X BDI score (F= 5.00; df = 2, 18;~ < 0.05).

Spatial-Linguistic Conflict Task. A three-way repeated measures ANOVA with Greenhouse-Geisser correction was performed using one between-subject factor (group) and two within-subject factors: attend (arrow or word) and condition (conflict, nonconflict, or alone). Significant main effects were found for condition (F = 69.57; df = 2, 56;~ < 0.0001) and attend (F= 50.53; df = 1, 28;~ < O.OOOl), but only a trend was noted for group (F = 4.11; df = 1, 28; p < 0.06). The significant main effect for condition appeared to result from slowing of responses in the conflict versus the other two conditions (see Fig. 5). The significant main effect for attend was clearly due to quicker responses to the arrow than the word. The trend toward a primary group effect is clearly due to the OCD group’s overall slower RTs.

Fig. 5. Mean reaction time and error rate (in parentheses) for 15 individuals with obsessive-compulsive disorder (OCD) and 15 control subjects in the spatial-linguistic conflict task

I Attended’ word

I I I I I

Stimulus 1 arrow, , word arrow, , word arrow

CONFLICT NON-CONFLICT ALONE

CONDITION

A significant condition X attend interaction (F = 13.51; df = 2, 56; p < 0.0001) was also found. This presumably represented the differential effects on RT between the nonconflict and conflict conditions (RTc - RTNc) when the individual was to attend to the arrow (+13.4 msec) or the word (l-53.4 msec). Similarly, a difference was also seen in the effects on RT between the nonconflict and alone conditions (RTA - RTNc) when the individual was asked to. respond to the arrow (-16.7 msec) or the word (f6.0 msec).

A significant condition X group interaction (F = 4.27; df = 2, 56; p < 0.025) was also seen. This appeared to reflect greater slowing in the conflict versus the nonconflict condition (RTc - RTNc) in the OCD group ($40.7 msec) than the control group (l-26.1 msec). The three-way condition X attend X group interaction was not significant (F= 1.15; df= 2, 56;~ > 0.30).

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A post hoc three-way repeated measures ANOVA with Greenhouse-Geiser correction was again performed on the OCD group’s data with one between-subject factor (current medication status) and two within-subject factors (condition and attend). Significant main effects were again found for condition (F = 37.28; df = 2, 26; p < 0.0001) and attend (F = 20.75; df = 1, 13; p < 0.0005), but not for medication status (F = 1.14; df = 1, 13; p > 0.30). The condition X attend interaction remained significant (F = 6.60; df = 2, 26; p < 0.01). The attend X medication status interaction showed a trend toward significance (F = 3.47; df= 1, 13; p < 0.1) which seemed to relate to the currently medicated group’s responses being slowed to the word versus the arrow. No other interactions including

medication status approached significance. Again, separate post hoc three-way repeated measures ANOVAs with Green-

house-Geisser corrections (to avoid colinearity) were performed to examine the effects of depressive symptoms (BDI score). Data from the 11 individuals with OCD who completed the BDI were used along with those of the entire control group. One between-subject factor was used (BDI score) as well as two within-subject factors (condition and attend). No significant main effect was found for BDI score among control subjects (F= 0.89; df = 1, 13;~ > 0.35) or individuals with OCD (F= 0.08; df = 1,9; p > 0.75) nor was it involved in a significant interaction in either analysis.

Conclusions

The control group’s Posner task performance at the lOO-msec delay was very similar to that of control subjects in other reports using this task (Posner et al., 1984, 1988). The performance of individuals with OCD at the lOO-msec delay resembled that of the control subjects with the exception of a relatively global RT slowing (Fig. 3).

The 800-msec cue-to-target interval data (Fig. 4) require a more detailed explanation to be understood. The control group’s performance pattern differed from that seen previously on this version of the task (Posner et al., 1988; Swanson et al., 1991) in that valid cues seemingly slowed RT. This cost of a valid cue at a larger RT has been termed inhibition of return and is felt to represent an attentional bias toward novel information (Posner et al., 1985). The OCD group evidenced less inhibition of return for left visual field targets and no inhibition of return for right field targets,

The control group also showed considerably more slowing in the absence of any cue than did the OCD group. The OCD group’s faster responses in the no-cue condition may have been the result of greater anxiety yielding greater vigilance (Jonides and Mack, 1984).

The control group’s performance on the spatial-linguistic conflict task did resemble previously reported control data (Posner et al., 1988). The OCD group’s RTs tended to be slower overall, with this effect accentuated to statistical significance primarily in the conflict condition. This confirms the hypothesis that OCD patients would have greater difficulty with conflicting cues.

The degree of depressive symptoms as measured by the BDI scores did not significantly interact with RT on the spatial-linguistic conflict task in either group. No significant interaction between BDI score and RT parameters was seen in control

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subjects on the Posner task, but a significant four-way interaction was seen in the OCD group. The smaller number of individuals in the OCD group who completed the BDI (n = II), as well as the possibility that a higher BDI score may be a reflection of more symptomatic OCD, must be taken into account when attempting to understand this interaction. The medication status (currently medicated or unmedicated) of individuals within the OCD group was not a significant main effect nor was it involved in a significant interaction with another factor in either task.

Discussion

The spatial-linguistic conflict task findings, relatively global slowing in those with OCD accentuated to significance in the conflict condition, will be discussed first. The overall slowing seen in OCD is consistent with the recent work of Christenson et al. (1992) which included planned and post hoc analyses to determine the degree to which slowing of task performance might account for any significant findings they obtained. They summarized this aspect of their results as follows: “From performance on timed and untimed measures of the same constructs, it appears that OCD patients score more poorly than controls when speed is a factor.”

The finding that this overall slowing is further increased by conflicting cues is fairly consistent with deficits seen by some investigators (Behar et al., 1984; Cox et al., 1989; Head et al., 1989; Hymas et al., 1991) in the ability of individuals with OCD to switch cognitive set. The lack of a significant attend X group interaction is somewhat inconsistent with prior suggestions of generalized hemispheric dysfunc- tion (Flor-Henry et al., 1979; Insel et al., 1983; Behar et al., 1984; Boone et al., 1991; Zielinski et al., 1991).

The Posner task performance of the control group resembled that reported previously (Posner et al., 1988; Swanson et al., 1991) at the lOO-msec, but not the 800-msec cue-to-target interval. The recent work of other investigators may help to explain this discrepancy.

Henderson (1991) has recently suggested that the Posner task may involve both exogenous and endogenous cuing (and thus both posterior and anterior attentional systems). Carter et al. (1992) expressed similar concerns, proposing the bias toward valid cuing (80% of cued trials) as the source of endogenous cuing. They addressed this specific question by examining the performance of a control group and a group of individuals with schizophrenia on two somewhat altered versions of the Posner task designed to discriminate between its automatic (exogenously cued) and controlled (endogenously cued) aspects. The version that emphasized exogenous cuing was very similar to the Posner task reported here except for two factors: (1) no bias toward valid versus invalid or “neutral” cuing was incorporated and (2) the “neutral” cue condition involved both boxes brightening rather than neither. The control subjects were found to demonstrate inhibition of return at the 800-msec cue-to-target interval, and their performance was very similar to that we report at the 100-msec cue-to-target interval.

Why then did our control group perform on the “standard” version of the Posner task in a manner similar to that observed in the control subjects of Carter et al. on their version developed to emphasize exogenous cuing? Several factors may have

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tended to accentuate exogenous and diminish endogenous cuing in our study. First, the task was administered without a practice session, giving the subjects less opportunity to generate rules with which to derive endogenous cues. Second, the subjects were explicitly instructed to discount potential cues and respond only to the target. Third, a bias toward very compliant control subjects may have been introduced by the use of individuals who either had served as control subjects in other studies or were employed within the medical center (or both).

If one accepts that the above elements of task administration, as well as the nature of the control group used, tended to emphasize exogenous cuing, then what would explain the differences observed between the OCD group and the control group? The OCD group clearly seemed to evidence inhibition of return, albeit less than control subjects for left field targets. No evidence of inhibition of return was seen in their performance for right field targets. This would suggest that some individuals with OCD may have a lateralized deficit of inhibition of return, suggesting perhaps a lateralized lesion of the brainstem or retinotectal pathway (Posner et al., 1985; Rafal et al., 1989).

Inhibition of return was initially felt to be a means of favoring novel, rather than previously attended, environmental (exogenous) cues (Posner et al., 1985). Posner et al. (1982) also considered the possibility that a similar inhibition might be observed to occur with “previously attended lexical items.” More recently, Rafal et al. (1989) suggested that it may also serve a broader function “...to coordinate the oculomotor system’s responses to exogenous and endogenous information.”

A better understanding of a seemingly related aspect of the anterior attentional system may be found in a 1989 review by Neill. He reviews the nonautomatic inhibitory processes which the human attentional system uses in the selective inhibition of distracting stimuli, a process which he terms “the suppression of cognitive noise.” He uses the phrase, the “distractor suppression effect,” to refer to the finding across several paradigms that a response is slowed to a target that has just served as a distractor on the preceding trial. He includes a review of evidence suggesting that this effect is not due to “an automatic inhibitory mechanism” but is instead dependent on context (endogenous information) in determining whether inhibition or facilitation occurs. This processing based upon endogenous informa- tion would thus seem to be based in the anterior attentional system.

Enright and Beech (1990) examined the performance of individuals with OCD on a Stroop-based information-processing task known to demonstrate “negative priming,” an example of the “distractor suppression effect” in control subjects (Beech et al., 1989~). They found that subjects with OCD evidenced decreased negative priming compared with individuals with other anxiety disorders. The authors observed (quite understandably) that the performance of their OCD group resembled that of schizophrenic patients in their prior study using this task (Beech et al., 1989b) while those with other anxiety disorders showed negative priming of an extent comparable to that they had seen previously in normal control subjects (Beech et al., 1989a). They then proceeded to suggest that this similarity of performance, as well as higher scores in the OCD group versus control subjects on a questionnaire designed to rate schizotypy, may reflect a misclassification of OCD as an anxiety disorder rather than a member of “ . ..the schizophrenic constellation of disorders....”

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Our results would seem to favor another interpretation of their data. First, the performance of the OCD group in our study was quite different from that seen previously (Posner et al., 1988) in schizophrenic subjects on the Posner task (the schizophrenic subjects’ response pattern resembled that of control subjects at 800 msec [except for overall slowing] while demonstrating particular difficulty with invalidly cued right field targets at 100 msec). Second, a negative priming deficit in individuals with OCD would represent a failure in the function of the anterior attentional system quite comparable to an inhibition of return deficit in the posterior attentional system. These “malfunctions” are observed to occur at comparable interstimulus intervals. If one accepts the broader function which Rafal et al. (1989) ascribe to inhibition of return, coordinating the oculomotor system’s responses to exogenous and endogenous information, could a deficit result in a breakdown of the gating mechanism allowing normally ignored, exogenous stimuli to occupy endogenous resources? As mentioned previously, a breakdown of this type has long been postulated to occur in OCD.

Our preliminary finding of a lateralized impairment of inhibition of return in individuals with OCD will clearly require replication in a nondepressed group of patients in whom both OCD and depressive symptoms are quantified. If replicated, our results may prove pertinent to the interpretation of reports of visuospatial task performance deficits in individuals with OCD reported by several groups (Behar et al., 1984; Cox et al., 1989; Martinot et al., 1990; Boone et al., 1991; Hymus et al., 1991; Zielinski et al., 1991) in addition to suggesting further study using tasks specifically designed to demonstrate abnormalities of inhibition of return.

Acknowledgments. We are grateful to James Swanson, Ph.D., for his assistance in incorporating these tasks into the MEL software and his help with the analysis of our data. We also thank Michael Posner, Ph.D., for his guidance in the use of the tasks which he developed and for his thoughtful review. We deeply appreciated the statistical assistance of Edward Spitznagel, Ph.D., and the secretarial support of Alice Stephenson, Sylvia Sirkin, and Carolyn Miles. Finally, we thank the reviewers of the initially submitted version of this article whose suggestions helped us to correct a problem in the original statistical analyses.

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