The goal of this research is to understand how visual information is processed in the lateral geniculate nucleus (LGN) and its target, the primary visual cortex. Emphasis is on mechanisms that alter cells response timings and on how these timings are used to create cortical cells that encode the direction of object motion. In cat cortex, neurons display different timing delays across their receptive fields; this spatiotemporal (S-T) receptive-field structure helps to make the cells direction selective (DS). The LGN creates most of these timing delays; in particular, it generates long delays that are necessary for cortical cells to be DS at low rates of motion. Work in the cat has revealed basic mechanisms of motion analysis, but the extent to which they are universal, and operate in other species, is not known. We will address this issue by examining neural mechanisms of direction selectivity in the brain of the monkey.
Specific aims are as follows: (1) We will determine how DS in cortical cells varies with the rate of stimulus motion, to characterize the operating range of underlying mechanisms. (2) S-T receptive-field structure and direction selectivity will be compared in single cells to determine the contribution of S-T structure, and additional mechanisms, to DS. (3) Responses in different positions of these receptive fields will be measured to reveal the range of timing delays present in the fields. (4) To investigate the potential sources of these timings we will measure the timings of nondirection-selective cortical cells that are likely inputs to the DS cells. (5) To determine the potential contribution of thalamic inputs to the cortical responses we will measure response timings and other properties of cells in the LGN and compare them to the cortical timings. (6) In cortex, we will examine laminar relationships among properties such as DS tuning, S-T structure, and response timing to gain insights into potential connectional relationships and mechanisms that underlie direction selectivity. Throughout, we will determine whether long delays exist among cells, if so, at what stage in the visual pathway the delays are introduced. These studies will provide new, critical insights into temporal processing mechanisms in primates. More broadly, understanding how the brain uses timing information is important because, in humans, deficits in temporal signal processing are linked to some cognitive disorders such as dyslexia.
