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Biomedical subjects

R D Rafal

Publications and source records attributed to R D Rafal.

At least 19 recordsLinked to original sources
PubMed1996

Perception of geometric illusions in hemispatial neglect.

A patient with left hemispatial neglect, while completely unaware of features on the left side of figures, nevertheless perceived geometric illusions dependent on these features. Bisection errors were amplified not only by increasing line length, but also by perceived line length induced by these illusions. Bisection of Judd illusion figures was as much influenced by neglected features on the left as by perceived features on the right. These observations demonstrate that geometric illusions are generated through preattentive processes. They also suggest that in visual neglect there may be preattentive processing of location as well as shape information.

Brain↗
PubMed1994

Axis-based neglect of visual shapes.

A new test of "object-centered" visual neglect uses equilateral triangles, with ambiguous principal axes that can be manipulated by context. Three left neglect patients detected gaps in such triangles. The location of the gap relative to the biased principal axis was varied, while maintaining the same egocentric locus. More gaps were missed on the left of the axis. This supports Driver and Halligan's (Cognit. Neuropsychol. 8, 475-496, 1991) claim that neglect can apply to the contralesional side of a shape's principal axis, while avoiding serious flaws in their method. The relation between axis-based neglect and other cases of object-centred neglect is discussed.

Attention↗
PubMed1994

Reading of letters and words in a patient with Balint's syndrome.

A patient with bilateral parietal damage, and Balint's syndrome, named visual letters. These were presented individually or within four-letter strings. Solitary letters were identified very accurately. In the case of strings, more letters were correctly reported for words than for nonwords, and more for pronounceable than for unpronounceable nonwords. When required to read words as a whole, performance was better than predicted by letter-reports. These results extend the object-based limitation apparent in Balint's syndrome to the case of reading. The component letters of a string benefit when they form a familiar global object, rather than requiring representation as multiple separate objects. The patient occasionally made homophonic errors when listing the letters in a visual word. This suggests an attempt to bypass visual simultanagnosia by treating the string as a single object, deriving a holistic phonological code for it, and then decomposing this into component letters via spelling rules.

Agnosia↗
PubMed1994

Neglect.

Neglect is a disorder of orienting in which patients are unaware of objects in their contralesional visual field. Yet their pre-attentive vision is still able to parse the scene to segregate figure from ground, group objects, and to define their primary axis. Therefore, it appears that perceptual processing may be intact up to the level of semantic classification, and that neglect only acts at the level of selection for action and access to awareness. Several mechanisms contribute to neglect, including disinhibited orienting to the ipsilesional field, a deranged representation of space, and deficits in disengaging attention, oculomotor corollary discharge, and representation of contralesional movement trajectories. Recent studies have begun to identify the neural substrates involved in these mechanisms.

Attention↗
PubMed1994

Shifting visual attention between objects and locations: evidence from normal and parietal lesion subjects.

Space- and object-based attention components were examined in neurologically normal and parietal-lesion subjects, who detected a luminance change at 1 of 4 ends of 2 outline rectangles. One rectangle end was precued (75% valid); on invalid-cue trials, the target appeared at the other end of the cued rectangle or at 1 end of the uncued rectangle. For normals, the cost for invalid cues was greater for targets in the uncued rectangle, indicating an object-based component. Both right- and left-hemisphere patients showed costs that were greater for contralesional targets. For right-hemisphere patients, the object cost was equivalent for contralesional and ipsilesional targets, indicating a spatial deficit, whereas the object cost for left-hemisphere patients was larger for contralesional targets, indicating an object deficit.

Adult↗
PubMed1993

Disinhibition of automatic word reading in Parkinson's disease.

The Stroop phenomenon was studied in patients with early and late onset Parkinson's disease (PD) and in normal controls to examine the effect of this disease on generating and controlling automatic mental processes. In the Stroop task subjects are presented with color or neutral words in various colors and asked to ignore the word and name its color as fast as possible. We examined the facilitory and interference effects of the irrelevant word upon naming the color by using computerized version of the Stroop test. Vocal reaction times of both early and late onset PD patients presented an augmented facilitory effect. In addition, error data of the late onset PD patients showed an enlarged interference effect. These effects are related to an impairment in the ability to control automatic-reflexive processes. The augmented facilitory effect is manifest early in the course of PD suggesting that the basal ganglia act to inhibit some automatic cognitive processes. The origin of the interference effect in late onset PD patients is less clear. It may reflect more severe basal ganglia dysfunction; or a decline in cortical inhibitory processes.

Adult↗
PubMed1992

Preserved figure-ground segregation and symmetry perception in visual neglect.

A central controversy in current research on visual attention is whether figures are segregated from their background preattentively, or whether attention is first directed to unstructured regions of the image. Here we present neurological evidence for the former view from studies of a brain-injured patient with visual neglect. His attentional impairment arises after normal segmentation of the image into figures and background has taken place. Our results indicate that information which is neglected and unavailable to higher levels of visual processing can nevertheless be processed by earlier stages in the visual system concerned with segmentation.

Aged↗
PubMed1992

Bimodal extinction without cross-modal extinction.

Three patients with unilateral neurological injury were clinically examined. All showed consistent unilateral extinction in the tactile and visual modalities on simultaneous intramodal stimulation. There was virtually no evidence for cross-modal extinction, however, so that contralateral stimulation of one modality would have extinguished perception of ipsilateral stimuli in the other modality. It is concluded that the attentional system controlling the encoding of tactile and visual stimuli is not unified across the two sensory domains.

Adult↗
PubMed1989

Saccade preparation inhibits reorienting to recently attended locations.

We measured manual reaction time in normal human subjects to confirm that an eccentric visual signal has a biphasic effect on covert attention and eye movements. First, it summons attention and biases a saccade toward the signal; a subsequent inhibition of return then slows responses to signals at that location. A temporal hemifield dominance for inhibition of return was shown; this finding converges with observations in neurologic patients to suggest that it is mediated by midbrain pathways. Endogenous orienting of attention, from a central arrow cue, did not activate inhibition of return, whereas endogenous saccade preparation did so as effectively as an exogenous signal, even when no saccade was made. Inhibition of return is activated by midbrain oculomotor pathways and may function as a location "tagging" mechanism to optimize efficiency of visual search.

Adult↗
PubMed1989

The role of cerebellar structures in the execution of serial movements.

Thirteen patients with bilateral cerebellar disease and 12 patients with unilateral cerebellar disease were instructed to execute movement sequences in response to a simple reaction signal. Each to-be-executed sequence consisted either of a single, two, or three keypress components. Evidence for cerebellar involvement in the execution of programmed responses was sought in the pattern of response onset times and interkeypress times. Patients with mild bilateral cerebellar dysfunction or mild unilateral dysfunction, and neurologically unimpaired subjects showed increases in response onset time as sequence length increased from L = 1 to L = 3. In contrast to this, there were negligible or no effects of sequence length on response onset time in patients with moderate bilateral cerebellar dysfunction and in patients with moderate unilateral cerebellar dysfunction who responded with the hand ipsilateral to the lesion. Furthermore, cerebellar dysfunction was associated with significantly slower interkeypress reaction times. These results support the hypothesis that the translation of a programmed sequence of responses into action involves cerebellar structures which schedule a sequence of ordered responses before onset of movement.

Adult↗
PubMed1989

Preparation of manual movements in hemiparkinsonism.

Twenty patients with asymmetric Parkinson's disease were studied in a reaction time (RT) experiment in which the performance of the more affected ("bad") hand was compared with performance of the less affected ("good") hand. Simple RT and choice RT were tested in separate blocks, and the benefit afforded by advance information in the simple RT condition (choice RT minus simple RT) served as a measure of motor preparation. RT was longer in the "bad" hand in both the simple RT and choice RT conditions. There was no difference in the effect of advance information between the two hands. It is concluded that slowness in RT movement initiation in Parkinson's disease is not due to a deficiency in motor preparation, and that intact basal ganglia function is not required for this stage of motor programming.

Adult↗
PubMed1988

Orienting of visual attention in progressive supranuclear palsy.

Orienting of visual attention was studied in 8 patients with progressive supranuclear palsy (PSP) and 8 parkinsonian control subjects. While maintaining fixation on the centre of a visual display, subjects made simple reaction time (RT) key press responses on detecting visual targets which appeared above, below, to the left or right, equidistant from fixation. On each trial the target was preceded by a preparatory cue, either a peripheral luminance change or a central arrow, to summon attention to one of the four locations. The orienting of attention was measured as a facilitation in detection RT at the cued location. For the parkinsonian controls, this facilitation was equal for horizontal and vertical directions, whereas for both types of cues, PSP patients were slower moving attention in the vertical than in the horizontal plane. Midbrain retinotectal pathways are important not only for controlling eye movements, but also for orienting attention.

Aged↗
PubMed1987

How do the parietal lobes direct covert attention?

In cases of unilateral parietal damage patients have trouble with stimuli in the visual hemifield on the side opposite the lesion (contralesional). This deficit is clear when they are first cued to attend to a location on the side of the lesion (ipsilesional) and then given a target on the opposite side of fixation. Our first experiments indicated that these patients do worse when cued to a location in either field and then given a target in the contralesional as compared to the ipsilesional direction. The results of this experiment can be accounted for by a representational or a directional view. A second experiment seeks to discriminate between the two. The results tend to favor the directional view.

Attention↗
PubMed1987

Deficits in human visual spatial attention following thalamic lesions.

There has been speculation concerning the role that thalamic nuclei play in directing attention to locations in visual space [Crick, F. (1984) Proc. Natl. Acad. Sci. USA 81, 4586-4590]. We measured covert shifts of visual attention in three patients with unilateral thalamic hemorrhages shortly after the lesion and after a 6-month recovery period. The experiment measured reaction time to targets that occurred at locations to which attention had been cued (valid trials) or at a currently unattended location (invalid trials). Although the patients showed no deficits in visual fields with perimetry and no neglect in the 6-month follow-up, we found slow reaction times for targets on the side contralateral to the lesion whether or not attention had been cued to that location. Deficits have also been found in this task with cortical and midbrain lesions, but the patterns of performance are quite different. The results with thalamic patients suggest they have a specific deficit in the ability to use attention to improve the efficiency of processing visual targets contralateral to the lesion (engage operation). This finding is in accord with hypotheses of a thalamic link between cortical visual attention and pattern recognition systems proposed by Crick.

Aged↗
PubMed1987

Programming and execution of sequential movements in Parkinson's disease.

In separate blocks of a simple reaction time (RT) task, eight Parkinsonian and eight control subjects executed finger press sequences with one (index finger), two (index finger-ring) or three (index finger-ring-middle) components. Programming was inferred from the increase latency to initiate the first component as a function of the length of the entire sequence; and from the systematic decrease in inter-response latencies for the second and third components. Overall RT was slower in the Parkinsonians but the programming effects were comparable in the two groups. Intact basal ganglia function appears not to be necessary for programming sequential finger movements, or retrieving subprograms for execution.

Adult↗
PubMed1987

Limb dystonia in progressive supranuclear palsy.

Progressive limb dystonia contributed to disability in 8 of 30 patients with progressive supranuclear palsy (PSP). In five, it was present when the patients were on no medication. In four, it had been present before the distinctive ophthalmoplegia permitted a correct diagnosis. The severity of limb dystonia did not correlate with the severity of either ophthalmoplegia or neck dystonia. The importance of dystonia in the pathophysiology of PSP is emphasized, with regard to both the branchial dystonia that underlies several cardinal features of the disease, and to the frequent occurrence of limb dystonia as an early sign.

Aged↗
PubMed1986

Seizures triggered by blinking in a non-photosensitive epileptic.

An epileptic girl with Lennox-Gastaut syndrome had seizures triggered specifically by blinking, but not by other eye movements or by photic stimulation. Electrographic and clinical seizures were most reliably precipitated by repetitive blinking produced voluntarily on command, by reflex blinking on corneal stimulation, or by psychogenic triggers of blinking such as social stress or cognitive effort.

Adolescent↗