Understanding the relationship between neuronal morphology and the function of visual system neurons is an important step in understanding the neural mechanisms of visual function. We propose three experiments using intracellular (horseradish peroxidase) labeling of physiologically identified neurons to explore this relationship in the tree shrew. This animal has well-developed geniculostriate and extrastriate visual pathways, both capable of mediating form vision. Thus it provides a excellent model to investigate the relationship between the receptive-field properties of neurons and the form vision mediated by these properties in the two separate afferent pathways. 1) In the extrastriate pathway, we will determine, at the level of the superior colliculus, whether the five functional cell classes identified by Albano et al. (1978) are related to the distinct morphological classes of cells seen in Golgi studies. Additionally, we will determine whether any or all of these physiologically-identified cell classes have locally ramifying axon-like processes which may contribute to circuitry within the superior colliculus. 2) In the geniculostriate pathway, at the level of the LGN, we will determine whether W-, X- and T-like cells have the same morphological characteristics as W-, X- and Y-cells in cat. Anatomical and physiological studies in tree shrew suggest, alternatively, that morphological characteristics may be more closely related to whether the cells are ON-center of OFF-center. 3) In the striate cortex we will determine whether parallel ON and OFF channels are present from the retina through the striate cortex in tree shrew by examining whether neurons responsive to light ON and light OFF are segregated into different subregions of layer IV. In addition, we will examine the destination of the locally projecting axons of these cells to determine whether ON and OFF pathways continue beyond the first synapse in cortex.
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