It is commonly believed that the complexity of visual stimuli represented by individual neurons increases towards higher cortical areas. However, even in early visual areas an individual neuron's response is influenced by stimuli presented outside its classical receptive field. Thus, it has been proven difficult to characterize the coding of complex stimuli at the level of single neurons. We therefore investigated population responses using optical imaging in cat area 18 to complex stimuli, plaids. Plaid stimuli are composed of two superimposed gratings moving in different directions. They may be perceived as either two separate surfaces or as a global pattern moving in intermediate direction to the components' direction of motion. We found that in addition to activity maps representing the individual components' motion, plaid stimuli produced activity distributions matching the predictions from a pattern-motion model in central area 18. Thereby, relative component- and pattern-like modulations followed the degree of psychophysical pattern bias in the stimulus. Thus, our results strongly indicate that area 18 exhibits a substantial response to pattern-motion signals at the population level suggesting the presence of intrinsic or extrinsic mechanisms that allow for integration of motion responses from far outside the classical receptive field.
Schmidt, K. E., Castel-Branco, M., Goebel, R. W., Payne, B. R., Lomber, S. G., & Galuske, R. A. W. (2006). Pattern motion selectivity in population responses of area 18. European Journal of Neuroscience, 24, 2363-2374. https://doi.org/10.1111/j.1460-9568.2006.05112.x