Long-range processing in dorsal-occipital cortex

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DESCRIPTION: (provided by applicant) The organization and functions of many regions of the human cortex are beginning to be understood, particularly in sensory processing areas such as the occipital lobe. This proposal focuses on the least-known part of occipital cortex, its dorsolateral segment. The occipital, temporal and parietal areas surrounding this region are well studied, yet its properties are largely obscure. For example, the dorsolateral segment shows little or no retinotopic mapping in fMRI (functional Magnetic Resonance Imaging) studies that reveal up to 9 medial and ventral retinotopic areas in the occipital cortex. The few hints as to the function of this dorsolateral area suggest that it may be involved in the processing of the configurational connections required for object processing of various types. These processes include pattern self-similarity, symmetry and Gestalt relationships. This interesting viewpoint suggests that such configurational connections are processed, neither in a known retinotopic area nor in the predominant object-responsive area in the ventral parts of occipitotemporal cortex. Symmetries of various types, in particular, constitute a configurational property based on mathematical operators that seem to appeal directly to perception. A key motivation for understanding the functions of lateral occipital cortex (LOC) is that it is subject to local damage by virtue of its blood supply from the posterior cerebral artery (PCA). One particular deficit that we have identified in patients with infarctions of branches of this artery is a specific loss in symmetry perception of otherwise random patterns (but minimal visual field losses), implying a high-level cortical locus for the processing of pattern symmetries. This knowledge of the functions of LOC will allow the development of tests for particular functional losses and improved management of such patients. The proposal is devoted to four projects. First we will construct a battery of stimuli known to activate parts of LOC based on fMRI studies. Next we will focus on the role of symmetry in object processing, which we have found to be localized principally to the middle occipital gyrus (on the LOC). Further, the cortical fMRI sensitivities in symmetry processing will be compared across various classes of natural and artificial objects to determine whether the key aspects of the long-range processing mechanisms for symmetry are based on symmetry in the visual imageor generalized to object symmetry from any viewing angle, or are specific to particular objects such as faces. The third fMRI project will be to develop probe stimuli to determine stimulus sensitivities for other cortical regions adjacent to the putative area for symmetry processing whose functions remain mysterious. These probe stimuli will involve long-range spatial interactions of various visual modalities and configurational effects in the processing of objects and faces. The final project will introduce a clinical component, quantifying symmetry-blindness in neurological patients with relevant lesions, together with comparison studies in matched controls.