MULTINEURON RESPONSE DYNAMICS IN CAT VISUAL CORTEX DURING THE PRESENTATION OF TIME-VARYING NATURAL SCENES
Charles M. Gray, Jonathan L. Baker, Shih-Cheng Yen
Center for Computational Biology, Montana State University, Bozeman, USA

The most common paradigm for studying sensory processing in the visual system has been to examine the activity of single neurons in response to artificial stimuli such as bars and gratings. While this approach has been successful, little has been learned about how groups of neurons jointly respond to natural scenes. To address this issue, we have recorded spike activity from small groups of 3-10 well isolated, single units in the striate cortex of anesthetized cats and analyzed their responses to the repeated presentation of time-varying natural scenes (movies). We were interested in answering the following questions: 1) What are the properties of cortical neuronal responses to time-varying natural images? Can the responses be characterized as sparse or dense? What is the distribution of response durations and firing rates across cells? 2) How often do the responses of simultaneously recorded neurons overlap in time and what are the properties of the joint activity? The results revealed a number of interesting properties not predictable from the responses of single neurons to artificial stimuli. 1) Neuronal responses in cat striate cortex to movies are sparse and brief. Significant responses occur approximately 10% of the time, and typically last less than 200 ms. 2) Nearby cells exhibit a high degree of heterogeneity in their responses to natural scenes. Joint responses occur with a wide distribution of probabilities and have an average value of ~0.5. Periods of joint activity displayed a broad range of spike-count correlations across trials (ranging from .2 to .8) that could not be accounted for by the stimulus alone. This finding suggests that responses to natural scenes involve dynamic patterns of neuronal interaction that are selective to the properties of the image. Thus, periods of joint activity could reflect independent responses to the stimulus as well as interactions within the cortical network that vary dynamically with the stimulus.