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L C Robertson

Publications and source records attributed to L C Robertson.

At least 19 recordsLinked to original sources
PubMed1992

Attentional asymmetry in schizophrenia: controlled and automatic processes.

Two versions of Posner's covert orienting task were administered to 14 drug-free schizophrenic patients and 12 normal controls. In the schizophrenic subjects, automatic orienting to exogenous cues in the right visual field was impaired. However, this lateralizing general deficit was not present when the schizophrenics were able to direct attention effortfully in the second version of the task using endogenous cues. These findings support the hypothesis that there is a deficit in left hemispheric mechanisms mediating visual spatial attention in schizophrenia. However, when schizophrenics are given the opportunity to use an attentional strategy they are able to partially overcome this lateralized processing deficit.

Attention↗
PubMed1992

Abnormal processing of irrelevant information in chronic schizophrenia: selective enhancement of Stroop facilitation.

Thirteen medication-free chronic schizophrenic patients and 11 normal control subjects were administered a trial by trial version of the Stroop Color Naming Task which evaluated separately the processes of interference and facilitation. There was no difference between the groups in the amount of interference to naming the colors of color-incongruent words. However, patients with schizophrenia showed significantly greater speed in naming the colors of color-congruent words when compared with control subjects. Thus, facilitation on the Stroop Task appears to be selectively enhanced in schizophrenia. Similar findings have been recently observed in patients with Parkinson's disease. This result may indicate a selective disruption of an automatic inhibitory process in this patient group and is consistent with the hypothesis that a deficit in mesocortical dopamine projections to the frontal cortex underlies some of the cognitive deficits of chronic schizophrenia.

Adult↗
PubMed1992

Performance of patients with early HIV-1 infection on the Stroop Task.

HIV-seropositive and seronegative control subjects performed a standard paper-and-pencil version and an experimental reaction time version of the Stroop Color-Word Naming Task. Results indicated that both symptomatic and asymptomatic HIV-positive subjects showed an exaggerated Stroop effect compared to controls, but this increase was only apparent on the RT version of the task. Analysis of components of the effect indicated that HIV-positive subjects showed increased inhibition compared to controls but normal facilitation. These results suggest that HIV-related cognitive slowing has an attentional component, most likely involving controlled processes. In addition, these results emphasize the utility and sensitivity of RT measures in the study of early HIV-1 infection.

AIDS Dementia Complex↗
PubMed1992

Decision-making speed in HIV-1 infection: a preliminary report.

OBJECTIVES: To test the efficacy of reaction time measures derived from cognitive psychology as measures of subclinical cognitive slowing in individuals with HIV-1 infection. DESIGN: Cognitive slowing is the primary deficit in AIDS dementia. One measure of cognitive slowing is decision-making speed, the discrepancy between simple and choice reaction times, which represents an index of central information processing time. METHODS: Nineteen HIV-seropositive and 13 control subjects performed a detection and decision task in a reaction time procedure. All subjects were administered measures of simple and choice reaction time, a control measure of rate of verbal encoding, and measures of psychological distress. RESULTS: Compared with controls, both symptomatic and asymptomatic HIV-seropositive subjects had significantly longer decision times, but performed the control task of rate of encoding normally. Simple reaction times correlated significantly with depression scores, but choice reaction times were unrelated to psychological distress. CONCLUSIONS: The results support mental slowing as the initial cognitive disturbance of HIV-1 infection and emphasize potential use of reaction times as markers of central nervous system involvement in HIV-1 infection.

Adult↗
PubMed1992

Spatial attention in HIV-1 infection: a preliminary report.

In a preliminary study, 30 nondemented human immunodeficiency virus (HIV-1) seropositive subjects without acquired immunodeficiency syndrome and 14 seronegative controls performed a reaction time measure of spatial attention. Compared with controls, seropositive asymptomatic subjects showed normal facilitation of reaction time at short cue-target intervals when attention was precued, but symptomatic subjects were impaired. However, asymptomatic subjects showed no evidence of normal inhibition of attention at the cued location at longer cue-target intervals, suggesting possible subtler spatial attentional deficits in this group. Cognitive slowing in HIV-1 infection may have an attentional component, with possible involvement of both automatic and controlled processes.

AIDS Dementia Complex↗
PubMed1991

Neuropsychological contributions to theories of part/whole organization.

Recent evidence from neuropsychology has proven useful in defining neural and cognitive subsystems involved in certain cognitive processes such as spatial attention (Posner, Walker, Friedrich, & Rafal, 1984) and imagery (Farah, 1988; Kosslyn, 1986). In this article we discuss neuropsychological evidence that suggests a modular system is responsible for the organization of parts and wholes by the visual system. We review both normal and neuropsychological evidence that suggest that hierarchical organization of objects within objects involves the coordination of at least four separate subsystems, one that emphasizes the global properties of a figure (association with right posterior superior temporal-parietal regions), one that emphasizes the local properties of a figure (associated with left posterior superior temporal-parietal regions), one that controls the distribution of attentional resources to these subsystems (associated with right or left lateral parietal lobe), and one that interconnects global and local properties and relies on the integrity of posterior temporal-temporal pathways.

Attention↗
PubMed1991

Normal global-local analysis in patients with dorsolateral frontal lobe lesions.

The role of prefrontal cortex in the analysis of global and local levels of a visual stimulus was assessed by measuring reaction time to identify a target at one level or the other. Unlike patients with temporal-parietal lesions (STG), there were no global-local performance deficits in right or left prefrontal groups (RFL or LFL). Reallocation of attention to global and local levels was measured by examining changes in performance when the probability of a target appearing at one level or the other varied. While patients with lateral parietal lesions (IPL) have been shown to have deficits in these conditions, both RFL and LFL showed normal changes in performance. In sum, the performance of prefrontal groups differed from both IPL and STG groups but not from normal controls. These results strengthen previous arguments that posterior association cortex is crucial in responding to global and local levels of a pattern.

Adult↗
PubMed1990

Component mechanisms underlying the processing of hierarchically organized patterns: inferences from patients with unilateral cortical lesions.

Subjects identified target letters that occurred randomly at the local or global level in a divided attention task. The visual angle of the stimuli was varied. Neurologically intact controls showed a reaction time advantage for local targets which increased as visual angle increased. Patients with lesions centered in the posterior superior temporal gyrus (STG) showed a larger local advantage than controls if the lesion was on the right and a global advantage if the lesion was on the left. STG patients were no more influenced by visual angle than were controls. Control subjects also showed the usual interference of global distractors on responding to local targets. STG patients showed little evidence of interference. Control patients with lesions centered in the rostral inferior parietal lobe performed normally. The findings suggest that several component mechanisms are involved in the processing of hierarchical levels of structure, each linked to specific anatomical regions.

Attention↗
PubMed1990

The effect of visual angle on global and local reaction times depends on the set of visual angles presented.

It has been shown that there is a transition from a global to a local advantage in reaction time as visual angle increases (Kinchla & Wolfe, 1979), and it has been assumed that this transition reflects lower level (e.g., retinal) processes. In three experiments, we examined whether higher level (e.g., attentional) processes play a role in this transition. In each experiment, subjects received a different stimulus set in each of two blocks of trials. In Experiment 1, stimuli subtending 1.5 degrees, 3 degrees, 4.5 degrees, or 6 degrees of visual angle vertically (small-stimuli set) were randomly presented in one block, while the other block consisted of random presentations of 3 degrees, 6 degrees, 9 degrees, or 12 degrees stimuli (large-stimuli set). The subjects' task was to identify targets that appeared randomly at either the local or the global level. It was found that the transition from a global to a local reaction-time advantage took place at a larger visual angle for the large-stimuli set than for the small-stimuli set. The same effects of stimulus set were found in Experiment 2, in which the small-stimuli set included 1.5 degrees, 3 degrees, or 6 degrees stimuli while the large-stimuli set included 3 degrees, 6 degrees, or 9 degrees stimuli. In Experiment 3, eye position was monitored to rule out the possibility that subjects adopted different fixation strategies depending on which stimulus set was being presented. The findings suggest that attention plays a major role in determining the relative speed of processing of local-and global-level information.

Aged↗
PubMed1989

Attention and interference in the processing of global and local information: effects of unilateral temporal-parietal junction lesions.

The processing of hierarchical stimuli was examined in patients with lesions in the temporal-parietal junction. In separate blocks of trials, subjects were instructed to identify the letter at the local or the global level of a hierarchical stimulus. Consistent with previous findings, reaction times for controls were faster in the globally than in the locally directed condition and reaction times to the local level were longer when the letters at the two levels were different (e.g. local "S"s forming a global "H") than when they were the same (e.g. local "S"s forming a global "S"). In other words, controls exhibited interference when locally directed. Patients with lesions centered in the rostral inferior parietal lobe (IPL) showed interference effects similar to controls. In contrast, patients with lesions centered in the posterior superior temporal gyrus and adjacent caudal inferior parietal lobe (STG) showed no interference. The data suggest that the posterior superior temporal plane and adjacent caudal inferior parietal lobe plays an important role in the integration of and/or attention to local and global level information.

Adult↗
PubMed1989

Judging the reflection of misoriented patterns in the right and left visual fields.

The reported study demonstrates that performance asymmetries between normal or reflected letters presented in the right and left visual field favors the right visual field when stimulus patterns are blocked and rotated 90 degrees clockwise and favors the left visual field when they are blocked and rotated 90 degrees counterclockwise. When stimuli are rotated, the orientations of the patterns define a coordinate system that is independent of upright. It is suggested that our results are due to subjects sampling the different areas in the visual field relative to this coordinate system. The right visual field was given priority when the patterns were oriented so their "tops" were pointing rightward and the left visual field was given priority when the patterns were oriented so their tops were pointing leftward. Neither differences in the stimulus image on the retina in the two rotated conditions, eye movements, nor direct access to one hemisphere or the other could account for these effects.

Adult↗
PubMed1989

Effects of chronic alcoholism on perception of hierarchical visual stimuli.

Visuospatial processing in chronic alcoholism was investigated by asking subjects to make similarity judgements of hierarchically constructed visual stimuli. Comparison figures were similar to a standard figure at the global or local level. Alcoholics were less influenced by the global patterns in their similarity judgements than were controls. On the WAIS-R Block Design subtest, alcoholics were also more likely than controls to distort the outer configuration of the design. Results indicate that alcoholism affects global processing on both experimental visuoperception tasks and on clinical measures of visuospatial ability. Implications for models of alcoholic dysfunction are discussed.

Adult↗
PubMed1989

Do response time advantage and interference reflect the order of processing of global- and local-level information?

Navon's (1977) global precedence hypothesis was based primarily on the joint occurrence of two effects: a response time (RT) advantage for identifying global targets, and interference by global distractors on responding to local targets. Although the hypothesis has been questioned on the basis of experiments in which it has been shown that a local RT advantage and local interference can occur, it is still frequently assumed that these two effects are a valid measure of the order in which local and global levels of structure are processed. In the present experiment, this assumption was examined. Subjects identified target letters that occurred randomly at the global or local level in a divided-attention task. The visual angle subtended by the stimulus pattern was varied, a manipulation known to affect the relative speed of response to local- or global-level information. Local targets were identified faster than global targets at the larger visual angles, but there was no difference in RT at the smallest visual angle. Despite this change in RT advantage, the interference effect did not change as a function of the visual angle of the stimulus pattern. Moreover, global distractors interfered with responding to local targets but local targets had no effect on responding to global targets, which is exactly the opposite of the finding one would expect if RT advantage and interference reflected order of processing. These findings are not consistent with the assumption that RT advantage and interference reflect order of processing in a simple way.

Aged↗
PubMed1988

The role of perceptual reference frames in visual field asymmetries.

The direct access model of hemispheric asymmetry was tested in a letter reflection (normal of reflected) judgement task. In the baseline condition letters were presented in their upright orientation, and reaction times were faster to letters presented in the right visual field than to those presented in the left visual field. In two other conditions the slides, and thus the letters, were rotated +90 degrees clockwise (Rotated +90) from upright or -90 degrees counterclockwise (Rotated -90). This resulted in the letters in both rotated conditions being shown in the upper or lower visual field through the sagittal plane but rotated 90 degrees. Despite this fact a "right field" advantage was again found when the right field was defined relative to the orientation of the tops of the rotated letters (lower visual field in the Rotated +90 condition and upper visual field in the Rotated -90 condition). These results demonstrate that the internal representation of locations in space is more important in predicting visual field asymmetries, at least in the present task, than the field of stimulus presentation relative to the fovea.

Adult↗
PubMed1988

Effects of lesions of temporal-parietal junction on perceptual and attentional processing in humans.

When stimuli with larger forms (global) containing smaller forms (local) are presented to subjects with large lesions in the right hemisphere, they are more likely to miss the global form than the local form, whereas subjects with large lesions in the left are more likely to miss the local than the global form. The present study tested whether the global/local impairment in subjects with posterior lesions was due to deficits in controlled attentional processes, passive perceptual processes, or both. Attentional control was examined by measuring reaction time changes when the probability of a target appearing at either the global or local level was varied. Patients with unilateral right or left lesions centered in temporal-parietal regions and age-matched controls served as subjects. Because neurophysiological and neuropsychological evidence have implicated temporal regions in visual discrimination and inferior parietal regions in the allocation of attention to locations in the visual field, patients with left hemisphere lesions were further subdivided into those with lesions centered in the superior temporal gyrus (LSTG) or rostral inferior parietal lobule (LIPL). Patients with right hemisphere injury could not be analogously subdivided. The results revealed that the LSTG group was able to control the allocation of attention to global and local levels normally, while the LIPL group was not. In contrast, the LSTG group showed a strong baseline reaction time advantage toward global targets, while normals and the LIPL group showed no advantage toward one level or the other. Finally, the perceptual component was affected differentially by lesions in the right hemisphere and LSTG, with lesions in the left favoring global targets and lesions in the right favoring local targets. These findings indicate that the hemispheric global/local asymmetry is due to a perceptual mechanism with a critical anatomical locus centered in the STG.

Adult↗
PubMed1987

Reference frames in mental rotation.

Four experiments are reported that investigate whether images or reference frames are transformed during a mental rotation task. In all experiments a display of four identical letters (P1) was presented at either +90 degrees or -90 degrees from upright, and subjects had to decide whether the letters were normal or mirror-image reflections. A single letter (P2) was then presented 100 ms later in a variable orientation with the same task instructions. Reaction times to P2 were assessed to determine whether an image of P2 was rotated to upright or whether an internal reference frame was rotated into congruence with P2 from the orientation of P1. The results as a whole suggest that transformations of P2 can be initiated either relative to upright or relative to the orientation of P1. They further indicate that the probability of using each reference orientation can be changed by procedural variations. The findings are most parsimoniously interpreted as suggesting that mental rotation involves the transformation of reference frames rather than the transformation of template-like representations.

Attention↗
PubMed1987

Effect of acute alcohol on attention and the processing of hierarchical patterns.

Subjects received 100-msec exposures of hierarchical patterns consisting of a large "global" letter composed of several smaller "local" letters. Subjects were to respond whether a target letter was present at the global or at the local level. In the Neutral Bias condition target letters occurred randomly and equally often at the global and local levels. In the Global Bias condition, 79% of the targets occurred at the global and 21% at the local level. In the Local Bias condition, 79% of the targets occurred at the local and 21% at the global level. Subjects indicated by pressing a key which of two target letters had appeared on that trial. Subjects received either a placebo drink, a drink containing 0.7 ml of Alcohol per kg body weight, or a drink containing 1 ml of Alcohol per kg body weight. Placebo subjects were better at detecting letters at the global than at the local level in the Global Bias condition, better at local than global in the Local Bias condition, and about equally good at both levels in the Neutral Bias condition. Alcohol had no effect on detection of letters at the local level in any of the bias conditions but reduced detection of letters at the global level except when subjects were biased globally. The data suggest that alcohol does not affect attentional capacity. Instead, alcohol seems to affect either how attentional capacity is allocated or the global processing mechanism itself.

Adult↗