More than (where the target) meets the eyes: disrupted visuomotor transformations in optic ataxia.

Visually guided reaching entails multiple coordinate frame transformations between retina-centered target location and body-centered limb location. Reaching errors in optic ataxia (OA) may be caused by disruptions to these transformations. Consistent with this proposal, previous studies report that reaching errors in OA depend primarily on the location of a target relative to the patient's gaze regardless of its location relative to the head or body midline. We attempted to replicate this finding by testing KE, a patient with OA following bilateral parietal and left premotor lesions (as well as significant non-specific white matter disease) on a reaching task that varied the orientation of his head and torso while holding the gaze-relative position of the target constant (always foveated). In contrast to previous reports, we observed that rotating the head or body away from the midline led to decreased reaching accuracy. Further analyses showed that multiple visuomotor transformation steps might have been disrupted in KE. These results suggest that gaze-relative target position is not the sole determinant of reaching errors in all OA patients.

Simultanagnosia: effects of semantic category and repetition blindness.

When confronted with two identical stimuli in a very brief period of time subjects often fail to report the second stimulus, a phenomenon termed "repetition blindness". The "type-token" account attributes the phenomenon to a failure to individuate the exemplars. We report a subject, KE, who developed simultanagnosia (the inability to see more than one item in an array) as a consequence of bilateral parietal lobe infarctions. With presentation of two words, pictures or letters for an unlimited time, KE typically reported both stimuli on less than half of trials. Performance was significantly influenced by the semantic relationship between items in the array. He reported both items significantly more frequently if they were semantically related; in contrast, when presented either identical or visually different depictions of the same item, he reported both items on only 2-4% of trials. Performance was not influenced by the visual similarity between the stimuli; he reported visually dissimilar objects less frequently than visually similar but different objects. We suggest that KE's bilateral parietal lesions prevent the binding of preserved object representations to a representation computed by the dorsal visual system. More generally, these data are consistent with the claim that the posterior parietal cortex is crucial for individuating a stimulus by computing its unique spatio-temporal characteristics.

The effect of gaze direction on sound localization in brain-injured and normal adults.

This study examined the effects of eye position on sound localization in normal and brain lesion subjects. On the assumption that cerebral lesions may disrupt the representation of or attention to auditory space in the contralesional hemispace, we predicted that subjects with brain lesions would be less accurate in localizing sounds in the contralesional hemispace. In Experiment 1 we showed that gazing to the midline subjects with brain lesions were indeed impaired in localizing sounds in the contralesional hemispace. On the assumption that spatial attention is deployed at the site to which gaze is directed, we predicted that sound localization would be better on the side to which subjects directed their gaze. In Experiment 2, brain lesion subjects performed significantly better in the contralesional hemispace when they directed gaze to that hemispace. This improvement was accompanied by deterioration of performance in the ipsilesional hemispace. When subjects directed gaze to the ipsilesional hemispace, performance in the contralesional hemispace was further impaired. The effect of gaze was also observed in normal subjects in Experiments 2 and 3, independently of response mode (verbal versus pointing responses). These findings are consistent with the hypothesis that sound location may be mapped in eye-centered coordinates and that directing gaze to one hemispace reduces attentional allocation to the other hemispace.

Bare hands and attention: evidence for a tactile representation of the human body.

If brain lesions impair the allocation of attention to a representation of the body surface and the hand may serve as an attentional focus or "wand", one might expect that somatosensory deficits caused by cerebral lesions would be ameliorated by contact with the ipsilesional hand. To test this prediction, tactile detection tasks were administered to two subjects with right hemisphere lesions. Subject CB's left tactile extinction was investigated in conditions in which the degree of contact between the right and left hands and the spatial relationship between his hands was systematically varied. His left tactile extinction was significantly reduced by touch of the right hand. Similarly, extinction at the left knee was ameliorated by touch of the knee by the right hand; touch of the right foot had no effect. Subject NC's ability to detect a tactile stimulus delivered to the left side was systematically assessed in conditions in which the hands touched and the spatial relationship between the hands was varied. His ability to detect a touch on the left hand improved in conditions in which the left hand was touched by the right hand. This effect was not observed if direct contact between the two hands was prevented by inserting a thin cloth between the hands. For both subjects, placing the right hand in close proximity to the left hand or altering the spatial location of the hands relative to the body did not influence performance. These data demonstrate that the hand may serve as a conduit for attention and provide strong evidence for a distinct representation of the body surface that is at least in part independent of spatial representations.