A model of lateralization and asymmetries in cortical maps

Svetlana Levitan, Ioana Stoica, James A. Reggia Depts of Computer Science and Neurology, Inst. Adv. Comp. Studies University of Maryland, College Park, MD 20742 USA Email: [email protected], [email protected] Abstract Cortical sensory maps in mammalian brain exhibit various kinds of asymmetry and lateralization, the causes and mechanisms of which are not well understood and are a subject of controversy. This study uses a neural network model of left and right cortical regions connected via a corpus callosum to examine possible factors involved in the development of map lateralization and asymmetries. Competitive distribution of activation and competitive learning rules are used. The effects of varying callosal connection strength (both excitatory and inhibitory) in the symmetric case and in the presence of some model asymmetries are studied along with the e ects of varying training input overlap. The results may provide some insight into how underlying cortical asymmetries and callosal in uences can interact to cause hemispheric map asymmetries. Introduction Topographic cortical maps are regions of the cerebral cortex that represent some aspect of the environment (e.g., visual space, body surface, etc.) in a topologypreserving fashion. It has recently been demonstrated electrophysiologically that topographic maps in primary sensory and motor cortex exhibit a rich range of patterns of individual lateralization and asymmetry. For example, in one recent study with more than 100 animals, it was found that somatosensory, auditory and visual map asymmetries are almost always present in an individual animal's primary sensory cortex, even in the context of coarse bilateral stimuli [2]. These map asymmetries were classi ed as qualitative (complete lateralization), quantitative (partial lateralization), topographic (no lateralization but asymmetric spatial distributions or mosaic patterns), and quantitative-plustopographic (lateralization and asymmetric spatial distribution). The dependence of asymmetries on callosal connectivity was shown by the fact that callosal sectioning generally caused a partial or complete loss of map asymmetries. The causes of these map asymmetries are not known. This study uses a neural network model of left and right cortical regions connected via a simulated corpus callosum to examine possible factors involved in the development of map lateralization and asymmetries.