Zerebral bedingte Sehstörungen

Abstract

The present work concentrates on different aspects of disturbed visual information processing. The patients examined have either a cerebral lesion or a psychiatric disease (schizophrenia). Patients and groups of patients are distinguished based on supposed site of disruption on an early or else a late visual processing stage.The second chapter considers disturbances of early vision. It is shown 1) that a lesion of extrastriate cortex is sufficient to cause a visual field defect and 2) that processing of specified stimulus features within the remaining visual field of the affected quadrant is reduced in a patient with a circumscribed occipital lesion, as revealed by both functional imaging and psychophysics. These results suggest that essential differences exist between species (e.g. human and macaque) already on early visual processing stages.Moreover, in a group study it can be shown that unilateral occipital lesions of both hemispheres always cause a visual exploration deficit. This deficit manifests itself as a reduced processing speed of all patients in an attention demanding visual search task. The results furthermore demonstrate that the performance is worse for the contralesional attentional window, if the size of this window is scaled according to task demands.In the third chapter patients with cerebral lesions causing a late visual processing deficit are described. For the first time a substantial reduction of size constancy is shown in a patient (HE) with a unilateral lesion of right occipito-parietal cortex. HE is not able to adjust the veridical size of two stimuli, if these stimuli are presented at different distances. Especially under monocular viewing conditions, the estimated size of a stimulus is based mainly on retinal image size. This strengthens the hypothesis that the right parietal cortex, which is involved in processing of spatial information, must be involved in the representation of object size, too. Managing precise (grasping) movements requires knowledge of true object size. Impairment of size constancy after occipito-parietal lesion is consistent with the idea that dorsal information processing is responsible for visually guided actions.The second part of the third chapter concentrates on visual exploration performance within a group of patients with right middle cerebral artery stroke. Due to systematic variation of target position in a parallel and in a serial visual search task, the group can be splitted according to systematic schemes of target detection, which in turn can be linked to damage of specific cortical areas. In patients showing an egocentric serial search deficit, the premotor cortex and probably the frontal eye field are damaged. In contrast, the centre of lesions in patients with an allocentric serial search deficit is located in the ventro-medial cortex near the parahippocampal gyrus. These results confirm the theoretical distinction between ventral and dorsal processing, with the first representing detailed object representations within an allocentric reference frame and the latter mediating visual guided actions within an egocentric reference system.Two aspects of the well-known backward masking deficit in schizophrenic patients are investigated in the last chapter. By means of psychophysical measurements it is shown in the first part that the deficit is neither due to prolonged visible persistence nor to a general slowing down of information processing. The difficulty for the patient appears to lie rather in the processing of stimuli with high temporal vicinity when they are additionally presented close together. In the second part it is shown that schizophrenic patients can perform as well as healthy controls in a backward masking task with additionally presented collinear contextual elements, when the stimulus configuration is individually adjusted. This suggests that in schizophrenic patients excitatory but not inhibitory neural connections are modified.