Spatial characteristics of cooperative interactions in the striate cortex.
In a complex visual scene, different objects with specific features are presented in combinations, which raises the question of how the visual system processes and represents structure in the huge amount of visual information that it receives every moment. Besides independent response rate modulation, which has been traditionally considered to be the primary coding mechanism in the visual system, correlated neural responses in the form of synchronized neural firing have been proposed as providing a versatile means of encoding. In this dissertation, the role of neural correlation in visual perception was explored by analyzing synchronized responses in cat primary visual cortex. We first tested the neural response to collinear and cocircular contours and found that the strength of synchrony between cells is not only affected by the receptive field properties but also determined by the effectiveness of the visual stimulus in driving cells. Synchrony was found to be more reliable for detecting cocircular contours than the independent firing rate, suggesting that contour integration through neural synchrony could start as early as in the lowest level of visual cortex. We then explored the relationship between spike timing synchronization and coherent frequency oscillation. Strong correlation between cross-correlation analysis and coherence analysis suggests synchrony and coherence are internally related, though these two estimates reflect neural connectivity from different perspectives. By systematically perturbing the timing accuracy in neural responses, we also discovered that the temporal structures of spike trains are important in maintaining neural correlation. We last studied how the synchronized neural response modulates with the change of spatial integrity in visual stimulation. We found that the general association between neural spike trains depends strongly on spatial integrity, with coherence in the gamma band showing greater sensitivity to the change of spatial structure than other frequency bands. Temporal integrity, and not spatial integrity, generates synchrony; spatial integrity however is critical in triggering subsequent gamma band synchronization.