Center-surround interactions in the middle temporal visual area of the owlmonkey

Microelectrode recording and 2-deoxyglucose (2dg) labeling were used to inv estigate center-surround interactions in the middle temporal visual area (M T) of the owl monkey. These techniques revealed columnar groups of neurons whose receptive fields had opposite types of center-surround interaction wi th respect to moving visual stimuli. In one type of column, neurons respond ed well to objects such as a single bar or spot but poorly to large texture d stimuli such as random dots. This was often due to the fact that the rece ptive fields had antagonistic surrounds: surround motion in the same direct ion as that preferred by the center suppressed responses, thus rendering th ese neurons unresponsive to wide-field motion. In the second set of complem entary, interdigitated columns, neuronal receptive fields had reinforcing s urrounds and responded optimally to wide-field motion. This functional orga nization could not be accounted for by systematic differences in binocular disparity. Within both column types, neurons whose receptive fields exhibit ed center-surround interactions were found less frequently in the input lay ers compared with the other layers. Additional tests were done on single un its to examine the nature of the center-surround interactions. The directio n tuning of the surround was broader than that of the center, and the prefe rred direction, with respect to that of the center, tended to be either in the same or opposite direction and only rarely in orthogonal directions. Su rround motion at various velocities modulated the overall responsiveness to centrally placed moving stimuli, but it did not produce shifts in the peak s of the center's tuning curves for either direction or speed. In layers 3B and 5 of the local motion processing columns, a number of neurons responde d only to local motion contrast but did so over a region of the visual fiel d that was much larger than the optimal stimulus size. The central feature of this receptive field type was the generalization of surround antagonism over retinotopic space-a property similar to other "complex" receptive fiel ds described previously. The columnar organization of different types of ce nter-surround interactions may reflect the initial segregation of visual mo tion information into wide-field and local motion contrast systems that ser ve complementary functions in visual motion processing. Such segregation ap pears to occur at later stages of the macaque motion processing stream, in the medial superior temporal area (MST), and has also been described in inv ertebrate visual systems where it appears to be involved in the important f unction of distinguishing background motion from object motion.