It has long been known that damage to the brain can produce a variety of different deficits of visual perception and attention (e.g., neglect, Balint's syndrome, visual agnosia). While the underlying causes of such deficits are often not well understood, recent attempts to apply advances from the study of normal cognitive functioning to the study of patients with brain injury have begun to increase our understanding of the precise nature of the mechanisms impaired by damage to different regions of the brain. During our last funding period, we used this strategy to isolate four distinct mechanisms involved in visual perception and attention which are selectively impaired by damage to different posterior cortical regions. The experiments in the present proposal will permit us to specify more precisely `the functional characteristics of each of these four mechanisms. Specifically, the proposed experiments will examine mechanisms involved in the analysis of hierarchically organized visual stimuli. Hierarchically organized stimuli are ones in which local level objects (e.g. eyes, nose, mouth) collectively form a global level object (e.g., a face). Our previous work has shown that a mechanism involved in local level analysis is affected by left temporal-parietal lesions while a mechanisms involved in global level analysis is affected by right temporal-parietal lesions. A third mechanism, involved in the integration of local and global information, is disrupted by lesions of either the left or the right-temporal-parietal area or by complete commissurotomy. Finally, the ability t~ shift attention between local and global objects is affected by rostral parietal lesions. The proposed experiments will determine what stimulus attributes are important for the functioning of each of these four mechanisms. Specifically, we will determine whether the functioning of a given mechanism is based on the hierarchical structure of the stimulus, on its spatial frequency content, or both. In addition, we will directly examine the relationship between perceptual analysis as it occurs with relatively simple """"""""artificial"""""""" stimuli typically used in the laboratory, and with the more complex """"""""naturalistic"""""""" stimuli encountered in the course of every day activities. This information is essential to the understanding of the perceptual and attentional deficits exhibited by patients with injury to different areas of posterior cortex and will also provide insight into how the normal intact visual stem functions.
