Visual perception is mediated by the activity of neuronal populations. In the primary visual cortex (V1), this activity can be imaged using voltage sensitive dyes (VSD), which provides high temporal resolution. Studies of VSD activity indicate that steady stimuli result in activity that spreads to a large region of cortex. However, preliminary results using periodic, rapidly changing stimuli indicate that the activity is highly localized. The proposed study investigates the dynamics of the spread of activity in anesthetized, paralyzed cats. (1) First, the spread of activity will be characterized in terms of its dependence on stimulus temporal frequency. (2) The spread of activity could be due to the mechanisms that cause V1 receptive fields to appear larger at low contrast. This is tested by systematically varying stimulus contrast. (3) The spread of activity might be due to lateral connections in cortex, which are anatomically anisotropic; this is tested by studying the isotropy of the spread. (4) The spread of activity could be influenced by contrast-dependent interactions between collinear stimuli. This is tested by examining the contrast and orientation dependence of the interactions between nearby stimuli. (5) The spread of activity could vary through time because of signal delays in lateral connections. Changes in the preference for spatial position through time are tested using a noise stimulus designed to examine dynamics. By elucidating the conditions that lead to a spread of activity in visual cortex, this study will contribute to the understanding of pathologies such as epilepsy. ? ? ?
