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Biomedical subjects

R Rafal

Publications and source records attributed to R Rafal.

16 recordsLinked to original sources
PubMed2001

Ipsilesional biases in saccades but not perception after lesions of the human inferior parietal lobule.

We examined the effects of chronic unilateral lesions to either the inferior parietal lobe, or to the dorsolateral prefrontal cortex including the frontal eye fields (FEFs), upon human visual perception and saccades in temporal-order-judgment (TOJ) tasks. Two visual events were presented on each trial, one in each hemifield at various stimulus onset asynchronies (SOAs). In the saccade task, patients moved their eyes to whichever stimulus attracted gaze first. In the perceptual-manual task, they pressed a button to indicate which stimulus was perceived first. Frontal patients showed appropriate TOJs for visual targets in both tasks. Parietal patients showed appropriate TOJs in the perceptual-manual but not the saccade task; their saccades tended to be ipsilesional unless the contralesional target led substantially. This reveals a bias in saccade choice after parietal damage that cannot be attributed to deficient visual perception. These results challenge previous claims that only anterior lesions produce motoric spatial biases in humans. However, they are in accord with recent neurophysiological evidence for parietal involvement in saccade generation, and also with suggestions that visuomotor transformations in the parietal lobe serving direct spatial motor responses can dissociate from conscious perception as indicated by indirect arbitrary responses.

Adult↗
PubMed2000

Control of eye movement reflexes.

We investigated the effect of strategic suppression of reflexive eye movements on external control over fixation using a fixation offset paradigm. A visual signal at fixation facilitates the fixation reflex and inhibits eye movements. Certain preparatory states render the fixation reflex less reactive to visual stimulation at fixation, as evidenced by a reduction in the fixation offset effect (FOE). For example, past studies have suggested that the reduced FOE during anti-saccade tasks results from the requirement to inhibit reflexive eye movements. We tested whether suppressing reflexive saccades reduces external control over ocular fixation using a go-nogo saccade paradigm. During each trial, one of two targets appeared in the periphery. Participants were instructed to saccade to one target (go), but when the other target appeared they either had to maintain fixation (nogo) or move their eyes in the direction opposite the target (anti). When nogo trials were admixed with target-directed saccades a large FOE was observed compared to when target-directed saccades occurred alone (experiment 1); however, when anti-saccades were mixed with target-directed saccades, a small FOE was observed for both types of eye movements (experiment 2). We conclude that suppressing reflexive eye movements does not reduce external control over fixation. Further research is necessary to elucidate which other component of preparing to make an anti-saccade diminishes the FOE.

Adolescent↗
PubMed1999

Localization of the human frontal eye fields and motor hand area with transcranial magnetic stimulation and magnetic resonance imaging.

We localized the neuroanatomical correlates for control of saccadic eye movements and for finger movements using a combined transcranial magnetic stimulation (TMS) and magnetic resonance imaging (MRI) approach. Two participants underwent TMS while performing an endogenous saccade task. The motor hand area was localized by TMS and the region anterior to it was mapped to identify the borders of a region where TMS produced delays in generating contralateral saccades. MRI scans were then obtained with fiducial markers placed over the motor hand area and 2 cm anterior to it, the common cortical region that produced saccadic delays in these two subjects. It was also shown that the structural anatomy of the hand area, physiologically defined by visible contractions of the contralateral hand following TMS, corresponded to the knob-like structure recently reported [18, 19]. These results demonstrate that TMS can be a precise, non-invasive tool for neuroanatomical mapping of cortical structures when combined with structural images of the brain.

Adult↗
PubMed1999

Interactions between color and word processing in a flanker task.

To examine interactions between color and word attributes, participants responded, either manually or vocally, to a central target (color patch or word) flanked by a Stroop stimulus. Color and word attributes of the flanker affected both vocal and manual responding to color patches. Color and word flankers also affected manual responding to word targets, but only word flanker affected vocal responding to word targets. These results are not consistent with models (e.g., translational models) of Stroop tasks, which posit that interactions between colors and words occur only when vocal responding is required.

Adult↗
PubMed1999

Memory lost and regained following bilateral hippocampal damage.

We present a longitudinal neuropsychological study (31 examinations over a period of 18 months) of patient DE DF demonstrated bilateral atrophy of the hippocampal formation and globus pallidus resulting from carbon monoxide poisoning. Eighteen months after the event, the volume of the hippocampal formation was reduced by 42% on the left side and 28% on the right. The patient initially presented with a severe global amnesia. Then, he showed a gradual, yet selective recovery of episodic memory function. Verbal free recall and spatial memory performance remained reduced, whereas immediate word recall and recognition memory, as well as picture learning and memory, improved to levels at the lower range of normal performance. Interestingly, nonspatial associative learning was never much impaired and recovered completely by the end of testing. These data are taken as evidence that the human hippocampal formation does not equally support different forms of episodic memory.

Adult↗
PubMed1998

Effects of parietal lesions in humans on color and location priming.

To determine whether the parietal lobes contribute tot he selection of nonspatial features known to be processed in the ventral stream, the current study examined the effect of chronic unilateral parietal lobe lesions in humans on color and location priming. Patients and normal controls performed a go/no-go color discrimination task in which either the same color and different color pairs of stimuli (prime and probe) were projected sequentially either in the same hemifield or in opposite hemifields. Control subjects and patients both showed independent effects of color and location priming. In the patients, primes in either field produced color priming for target probes in the ipsilesional field but not for probes in the contralesional field. This observation implicates the parietal cortex in processing activated codes of stimulus attributes not only for spatial information but also for visual features processed in the ventral visual pathways.

Aged↗
PubMed1997

Object-based facilitation and inhibition from visual orienting in the human split-brain.

Object-based attention was examined in 2 split-brain patients. A precued object could move within a visual field or cross the midline to the opposite field. Normal individuals show an inhibition in detecting signals in the cued object whether it moves within or between fields. Both patients showed this effect when the cued object moved within a visual field. When it crossed the midline into the opposite visual field, however, detection was faster in the cued box. These results reveal both facilitatory and inhibitory effects on attention that are object based and may last for several hundred milliseconds. However, the inhibition requires an intact corpus callosum for interhemispheric transfer, whereas the facilitation is transferred subcortically.

Adult↗
PubMed1996

Response channel activation and the lateral prefrontal cortex.

The role of lateral prefrontal cortex in transducing perception into action was studied in 10 patients with chronic, unilateral lesions. They identified colors in the center of a visual display, while a flanking, distractor color was presented simultaneously in either the ipsilesional or contralesional field. The flanker could be either the same color as the target, or incompatible with the correct response. The effects of compatible and incompatible flankers on reaction time (RT) served as a measure of response channel activation by the flanker. Flankers in the contralesional field influenced RT less than did those in the ipsilesional field. These results suggest that the lateral prefrontal cortex is involved in maintaining stimulus-response channels.

Adult↗
PubMed1995

Increased sensitivity to ipsilateral cutaneous stimuli following transcranial magnetic stimulation of the parietal lobe.

Transcranial magnetic stimulation of the sensorimotor cortex results in decreased sensitivity of threshold electrical stimuli to fingers of the contralateral hand. It has been suggested that one factor contributing to neglect contralateral to a unilateral parietal lesion is a release of the normal hemisphere from reciprocal interhemispheric inhibition by the damaged hemisphere. Consistent with this account, the current study demonstrated that transcranial magnetic stimulation over the parietal cortex results in increased sensitivity to cutaneous stimulation ipsilateral to the stimulation. The likely mechanism is a transcranial magnetic stimulation-induced transient dysfunction of the ipsilateral parietal cortex that then results in disinhibition of the contralateral parietal cortex.

Adolescent↗
PubMed1994

Effects of inhibition of return on voluntary and visually guided saccades.

Four experiments examined the effects of inhibition of return on endogenously generated and visually guided saccades. In Experiments 1-3, subjects responded to a peripheral target by making either a prosaccade (toward the target) or an antisaccade (toward the field opposite the target). Prior to the appearance of the target, one of the two equiprobable target locations was activated by presenting a peripheral precue (Experiments 1 and 2), or by executing an endogenous saccade in response to a central precue (Experiment 3). In Experiment 1, the eyes remained fixed when the cue appeared; in Experiment 2 subjects made a saccade to the peripheral cue, and returned their eyes to the centre before the target appeared. In both experiments, saccade latencies were longer for targets appearing at the precued location for both prosaccade and antisaccade tasks. In Experiment 3, saccade latencies were longer for targets appearing at the precued location only in the prosaccade task; no effect of the precue was obtained in the antisaccade task. These results suggest that endogenously generated saccades activate both an inhibition of target detection and a motor alternation bias. Experiment 4 showed that inhibition of return generated by a peripheral precue increased the latency for a subsequent endogenous saccade (from a central, arrow target) toward the precued location. Inhibition of return may affect perceptual processing and also produce a motor alternation bias dependent upon whether it is activated exogenously or endogenously.

Attention↗
PubMed1993

Cognitive neuropsychology is more than single-case studies.

S. M. Sokol et al. (1991) claim that "The Cognitive Neuropsychology Approach" (p. 355) is limited to the single-case study design. The present article takes issue with this claim. Contrary to the beliefs of Sokol et al., we argue (a) that cognitive modularity is best studied by group design, (b) that the possibility of neural reorganization in patients should be tested through converging evidence from different populations using various methods, and (c) that cognitive neuropsychology can benefit from being a part of cognitive neuroscience where both neural and cognitive mechanisms underlying normal cognition are relevant.

Bias↗
PubMed1990

Extrageniculate vision in hemianopic humans: saccade inhibition by signals in the blind field.

The functional competence of extrageniculate visual pathways in hemianopic humans was demonstrated by showing that distractor signals in the blind half of the visual field could inhibit saccades toward targets in the intact visual field. This inhibitory effect of unseen distractors in patients occurred only when distractors were presented in the temporal half of the visual field, was specific to oculomotor responses, and did not occur in normal subjects. These results show that a peripheral visual signal activates retinotectal pathways to prime the oculomotor system and that these pathways can mediate orienting behavior in hemianopic humans.

Analysis of Variance↗