DEVELOPMENT OF RESPONSE TIMING IN THE VISUAL SYSTEM

DESCRIPTION (adapted from abstract): When neurons respond to stimuli is at least as important as how much they respond. However, response timing has been largely neglected by vision neuroscientists. Motion processing is a key problem for the brain, and depends on timing information. Recent work has suggested that the cat visual system processes motion by creating a novel cell type in the lateral geniculate nucleus, lagged cells. Lagged cells differ from nonlagged cells by the timing of their responses. Lagged and nonlagged cells project to visual cortex, where the timing difference between them may be used to create direction-selective responses. Direction-selective cells seem a key substrate in motion processing. For instance, raising cats in an environment illuminated only by a strobe light causes a specific loss of direction-selective cells, and causes a specific behavioral deficit in motion processing. The studies that have generated the results stated above relied on testing responses of adult cats. Strobe-rearing is a manipulation performed during a critical period early in a kitten's life, however. Young kittens appear to have many direction-selective cortical cells, but these cells differ in response timing from those seen in adults. Can we observe the formation of mature direction-selective cells in kitten visual cortex, and does this maturation correspond to what one would expect from the development of lagged and nonlagged cells in the lateral geniculate nucleus? Furthermore, can we follow the destruction of direction selectivity induced by strobe-rearing, and see whether this destruction comes about because of effects on the lagged and nonlagged inputs to cortex? These questions will be addressed by a study of the postnatal development of response timing in the lateral geniculate nucleus and visual cortex of cats. The goal is to learn when lagged and nonlagged cells arise as distinct cell groups, and more generally how geniculate response timing matures. These results will be compared to a parallel study of the development of cortical response timing. The details of the disruption of this normal development by strobe-rearing will be examined by recording from kittens with brief experience in the strobe environment. This work will help to show how the visual system deals with time, as well as how thalamic inputs influence cortical cells. In the long term, these results could suggest how the brain develops strategies for temporal processing, and this would have major implications for both basic and clinical research, for instance in treatment of learning disabilities.