Biomedical subjects
Robert Rafal
Publications and source records attributed to Robert Rafal.
A dissociation between visual and auditory hemi-inattention: Evidence from temporal order judgements.
Patients with right hemisphere brain lesions often suffer from deficits in spatial attention that can be manifested in different sensory modalities. It has recently been claimed that a relationship (i.e., association) could exist between symptoms of hemi-inattention in different modalities, based on correlations between the results of visual and auditory clinical tests of neglect or extinction. However, it should be noted that the visual and auditory tasks varied greatly both in response type and level of sensitivity. Here, we have examined cross-modal associations in spatial attention deficits using a temporal order judgment task (TOJ) in which patients were required to identify which of two visual or auditory objects had appeared first. When compared to age and education matched control participants, the patients needed, on average, the contralesional stimulus to lead the ipsilesional stimulus to achieve the point of subjective simultaneity (PSS). No association between the degree of visual and auditory hemi-inattention was observed amongst the patients, suggesting that there is a certain degree of independence between the mechanisms subserving spatial attention across sensory modalities.
Selection for action and selection for awareness: evidence from hemispatial neglect.
In bedside testing of patients with hemispatial neglect, we have found that extinction for contralesional stimuli is less when the contralesional and ipsilesional items are different on the dimension to be reported relative to when they are the same. Importantly, a study that investigated this observation found that similarity on visual features that are not necessary for response does not impact the amount of extinction. These findings suggest that response requirements may determine what stimuli will and what stimuli will not gain access to awareness. In a related study, we found that extinction of contralesional stimuli was not determined by perceptual similarity of the ipsilesional and contralesional items but by whether they shared the same semantics (e.g., ONE + 1 ) or the same response (e.g., ONE = WON). Here, we report a single case study in which extinction was determined by whether the competing items shared the same response, regardless of whether they shared or differed in their visual features or semantics. When asked to read the item in each field, there was equivalent extinction in the conditions (ONE + ONE) and (ONE + 1) but less extinction in the condition (ONE + TWO). By contrast, when asked to count the number of characters in each field, there was more extinction in the condition (ONE + TWO) than (ONE + 1). When asked to categorize each item as either a word or digit, the degree of extinction was determined both by whether the items shared the same semantics and by whether they required the same response. The results are consistent with a biased competition model in which competition for selection is resolved flexibly depending on response requirements. Furthermore, the data provide evidence that unattended stimuli are processed to the level of response.
Feedback of brain-imaging findings: effect on impaired awareness and mood in acquired brain injury.
PRIMARY OBJECTIVE: To investigate the utility of feedback of brain-imaging findings as an intervention for improving impaired awareness, depression and anxiety levels. RESEARCH DESIGN AND METHODS: A prospective within-subjects repeated measures design was used with an intervention sample of 17 adults with acquired brain injury. Baseline and post-intervention measures included self-report and questionnaire-based self-awareness instruments and self-report questionnaires for depression and anxiety. Participants also completed a range of neuropsychological tests. INTERVENTION: The intervention consisted of a session where a Consultant Neurologist explained the findings of brain scans in terms of pathological findings and possible neurobehavioural outcome. RESULTS: Scores on measures of unawareness and two of three mood measures decreased significantly following the intervention. This improvement was maintained at 2-week follow-up. CONCLUSION: Individuals with an acquired brain injury may benefit from a feedback procedure where the findings of brain scans are presented.
Visual restoration in cortical blindness: insights from natural and TMS-induced blindsight.
Unilateral damage to visual cortex of the parietal or occipital lobe can cause the patient to be unaware of contralesional visual information due to either hemispatial neglect or hemianopia. It is now known that both neglect and hemianopia result from the disruption of a dynamic interaction between cortical visual pathways and more phylogenetically primitive visual pathways to the midbrain. We consider the therapeutic implications of these cortical-subcortical interactions in the rehabilitation of hemianopia. We start with the pheonmenon of "blindsight", in which patients with hemianopia can be shown, by implicit measures of visual detection or discrimination, to process visual information without conscious awareness. Some variants of blindsight have been postulated to recruit subcortical processes, while others may reflect compensatory optimisation of processing of spared visual cortex. Both mechanisms may offer opportunities for innovative strategies for rehabilitation of visual field defects. We relate the neural mechanisms that have been proposed to underlie blindsight to those that have been suggested to underlie the recovery of visual function after rehabilitation. It is suggested that the similarity and overlap of the neural processes supporting blindsight and recovery of visual function might provide insights for effective rehabilitation strategies for restoring visual functions.
Implicitly evoked actions modulate visual selection: evidence from parietal extinction.
It has been proposed that the mental representation of a graspable object involves not only a description of its visual properties but also encodings of the motor programs to act upon it. Thus, observing a handle automatically primes the motor programs responsible for reaching and grasping it. Here, we provide neurological evidence that such action-related object features can bias visual selection. Two patients with visual extinction after right-parietal injury detected cups with left- or right-oriented handles, briefly displayed in either or both visual fields. People with this disorder have deficient awareness for stimuli toward the contralesional, left side of space, especially when competing stimuli appear further to the right. This contralesional extinction was significantly reduced when cups had handles affording a left-hand grasp, even though no hand response was required. No effect was found when handles were replaced with patches equated for position, size, and mean luminance. These data suggest that action-related information may be correctly extracted by the visual system, even though they are unavailable for conscious report. It is proposed that an object affordance for grasping modulates attentional selection by activating specific motor schema that, in turn, enhance the competitive strength of that object representation.
Competition between endogenous and exogenous orienting of visual attention.
The relation between reflexive and voluntary orienting of visual attention was investigated with 4 experiments: a simple detection task, a localization task, a saccade toward the target task, and a target identification task in which discrimination difficulty was manipulated. Endogenous and exogenous orienting cues were presented in each trial and their validity was manipulated orthogonally to examine whether attention mechanisms are mediated by separate systems and whether they have additive and independent effects on visual detection and discrimination. The results showed that each orienting mechanism developed its typical and independent effect in every case except for the difficult identification task. A theoretical framework for understanding the relationship between endogenous and exogenous orienting of attention is proposed, tested, and confirmed.
Inhibition of return generated by voluntary saccades is independent of attentional momentum.
Summoning attention to a peripheral location, either by a peripheral cue with the eyes fixed or when a voluntary saccade is made to it and gaze is then returned to the centre, delays detection of subsequent targets at that location compared to a location in the opposite visual field. It has been proposed that oculomotor activation generates this inhibition of return (IOR). This account presupposes that the asymmetry in detection results from inhibition at the cued location rather than facilitation at the uncued location. This has been confirmed for exogenously generated IOR. However, it has not, heretofore, been confirmed for "IOR" generated by voluntary saccades. The current study investigated whether the asymmetry in target detection, elicited either by a peripheral flash or by an eye movement generated in response to a central arrowhead, reflects facilitation at the opposite location due to the path of attentional momentum. Reaction times at the cued location were slower than reaction times at the opposite or perpendicular locations, which did not differ. Opposite facilitation due to attentional momentum requires that opposite be faster than perpendicular, which was not obtained. The results were the same whether IOR was generated by an exogenous cue or by a saccade executed endogenously to a central arrow.
Parietal lobe lesions disrupt saccadic remapping of inhibitory location tagging.
Maintaining a coherent percept of the visual scene while eye position continuously changes requires that saccades be accompanied by remapping of the visual environment. We studied saccadic remapping in patients with unilateral lesions in the intraparietal sulcus and healthy controls, using inhibition of return (IOR)-an inhibitory tag that enables efficient visual search. In healthy controls, IOR was found at both retinal and environmental locations of the cue, indicating that the inhibitory tag had been remapped into environmental coordinates. In contrast, right parietal patients demonstrated IOR only at the retinal location of the cue, indicating that the intraparietal sulcus is involved in remapping of the environment after eye movements to afford a stable, environmentally based reference frame. Note that patients did not show environmental IOR in either visual field. These results also suggest that this region may be the neural substrate for encoding inhibitory spatial tags in an environmentally based reference frame.
Contributions of the human pulvinar to linking vision and action.
In 3 patients with unilateral pulvinar lesions, we tested the pulvinar's role in selective attention processing. Each patient completed four variants of a flanker interference task in which they reported the color of a square of a specified size while ignoring an irrelevant flanker that appeared either contralesionally or ipsilesionally to the target. The main finding was that when target location was not known and target and flanker were associated with competing responses, reaction times to contralesional targets were longer than those to ipsilesional targets. Our findings suggest that pulvinar damage produces a contralesional deficit in response competition.
Visual detection is gated by attending for action: evidence from hemispatial neglect.
We report observations in patients with visual extinction demonstrating that detection of visual events is gated by attention at the level of processing at which a stimulus is selected for action. In one experiment, three patients reported the identity of numerical words and digits presented either in the ipsilesional field, the contralesional field, or both fields. On the critical bilateral trials, extinction was greater when the competing items shared the same meaning and response, regardless of whether the items were visually different (e.g., ONE + 1), or identical (e.g., 1 + 1). A fourth patient was tested in a second experiment in which the competing items on bilateral trials were either different (e.g., ONE + TWO), identical (e.g., ONE + ONE) or homophones that were visually and semantically different but shared the same response (e.g., ONE + WON). Homophones and identical items caused similar extinction with less extinction occurring on different item trials.
A temporal/nasal asymmetry for blindsight in a localisation task: evidence for extrageniculate mediation.
Some patients with hemianopia due to striate cortex lesions show above chance ability in reporting visual stimuli presented in the blind visual field, a phenomenon commonly known as blindsight. Here we report a patient with a dense right hemianopia whose blindsight shows a temporal/nasal asymmetry. MP was tested in a two-alternative forced-choice localisation task, with either the right eye or the left eye patched in separate blocks. When targets appeared in the contralesional temporal hemifield, MP's localisation performance was extremely accurate, whilst she performed at chance with targets in the contralesional nasal hemifield. This is the first demonstration of a temporal/nasal asymmetry for blindsight in a forced-choice paradigm, and is consistent with blindsight in MP's hemianopic field being mediated by a subcortical, extrageniculate route.
Attending to the thalamus: inhibition of return and nasal-temporal asymmetry in the pulvinar.
Inhibition of return (IOR) is a mechanism whereby the attentional system favors novel locations by inhibiting already scanned ones. An important question is what the neural structures are involved. Recently, we studied a patient with damage to the superior colliculus (SC) and concluded that the SC generates IOR. However, it is possible that IOR is generated beyond the colliculus, for example, by the pulvinar. In this paper we tested three patients with unilateral damage to the pulvinar and demonstrated that the pulvinar is not necessary for IOR generation, providing additional support to the suggestion that the SC generates IOR. In addition, since we used monocular presentation, we were able to furnish behavioral evidence for nasal-temporal asymmetrical representation of visual input in the pulvinar.
Deficits in spatial coding and feature binding following damage to spatiotopic maps in the human pulvinar.
We report a patient with unilateral damage to the rostral part of the pulvinar who was impaired in localizing stimuli in the inferior visual field contralateral to the lesion and who made errors in the binding of shape and color in that quadrant. The findings demonstrate the importance of the pulvinar in spatial coding and provide support for the function of the thalamus in binding of features. They also provide evidence for a homology between the visual field maps of the inferior and lateral subdivisions of the pulvinar in monkeys and in humans, such that the inferior visual field is represented in the rostral part of the nucleus.