Hemispatial neglect: its effects on visual perception and visually guided grasping.

Hemispatial neglect is a neurological disorder characterized by a failure to represent information appearing in the hemispace contralateral to a brain lesion. In addition to the perceptual consequences of hemispatial neglect, several authors have reported that hemispatial neglect impairs visually guided movements. Others have reported that the extent of the impairment depends on the type of visually guided task. Finally, in some cases, neglect has been shown to impair visual perception without affecting visuomotor control in relation to the very same stimuli. While neglect patients may be able to successfully pick up an object they have difficulty perceiving in its entirety, it does not mean that they are picking up the object in the same way that a neurologically intact individual would. In the current study, patients with hemispatial neglect were presented with irregularly shaped objects, directly in front of them, that lacked clear symmetry and required an analysis of their entire contour in order to calculate stable grasp points. In a perceptual discrimination task, the neglect patients had difficulty distinguishing one object from another on the basis of their shape. In a grasping task, the neglect patients showed more variance in the position of their grasp on the target objects than their control subjects, with an overall shift to the relative right side of the presented objects. The perceptual and visuomotor deficits seen in patients with hemispatial neglect deficits may be the result of an inability to form good structural representations of the entire object for use in visual perception and visuomotor control.

Oculographic analysis of word reading in hemispatial neglect.

Neglect dyslexia is a disorder in which individuals misread text appearing on the contralateral side of space following an acquired lesion, usually to the right parietal lobe. This disorder is generally attributed to an impairment in representing spatial information. To determine whether the spatial representations underlying reading differ from those mediating other forms of visual behavior, we investigated the co-occurrence of neglect dyslexia with that of neglect, which manifests on tasks such as line bisection or line cancellation. We also examined the correlation between neglect dyslexia, when present, and eye movements in order to characterize the neglect dyslexia disorder further. Whereas there is no clear relationship between the reading disorder and other symptoms of visuospatial neglect, suggesting segregated spatial representations, there is a direct correspondence between the oculomotor performance of patients with neglect dyslexia and their reading behavior. This latter result suggests that the reading deficit may well arise from the failure to register and perceive the contralesional information.

The effects of rotation and inversion on face processing in prosopagnosia.

The current study investigated the sensitivity of face recognition to two changes of the stimulus, a rotation in depth and an inversion, by comparing the performance of two prosopagnosic patients, RN and CR, with non-neurological control subjects on a face-matching task. The control subjects showed an effect of depth rotation, with errors and reaction times increasing systematically with rotation angle, and the traditional inversion effect, with errors and reaction times increasing under inverted conditions. In contrast, RN showed no effect of rotation or inversion on his error data but did show a less sensitively graded effect of rotation and the traditional inversion effect on reaction times. CR did not show a graded effect of rotation on his errors or reaction times. Although CR showed the traditional inversion effect on his error data, he displayed an inversion superiority effect on his reaction time data, which supports the claim that the damaged holistic processing systems continue to dominate face processing in prosopagnosia even though they are malfunctioning. These results suggest that the damage that occurs to the ventral temporal cortex in prosopagnosia may have forced the patients to rely on sources of information that are not dependent on the view of the face and, moreover, cannot be adapted to deal with rotated faces under both upright and inverted conditions.