Chapter 3 Dynamics of directional selectivity in MT receptive field center and surround

We studied receptive field organization of motion sensitive neurons in macaque middle temporal cortical area (MT) by mapping direction selectivity in space and in time. Stimuli consisted of pseudorandom sequences of single motion steps presented simultaneously at many different receptive field locations. Spatio-temporal receptive field profiles were constructed by cross-correlating stimuli and spikes. The resulting spike-triggered averages revealed center-surround organization. The temporal dynamics of the receptive fields were generally biphasic with increased probability for the preferred direction at short latency (50-70ms) and decreased probability at longer latency (80-100 ms). The response latency of the surround was on average 16 ms longer than that of the receptive field center. Our results show that surround input and biphasic behavior reflect two different mechanisms, both of which make MT cells specifically sensitive to motion contrast in space and time. 53 Introduction Motion detection of a moving pattern is easier when the pattern is surrounded by other patterns moving in the opposite direction. This increased sensitivity to motion contrast or relative motion is generally related to center-surround receptive fields of visual motion sensitive neurons. Motion sensitive neurons with a center-surround receptive field organization such as those in cortical area MT are most responsive when motion is presented to the preferred direction in their receptive field center, and the opposite direction in the surrounding area (Allman et al., 1985). Due to the strong link between relative motion perception and the physiological properties of MT neurons, the investigation of spatial and temporal characteristics of center-surround receptive fields can be important for a better understanding of the spatiotemporal properties of relative motion perception.