Spatial and temporal integration of motion signals in area MT
The motion of images on the retina is initially analyzed by direction-and orientation-selective neurons with small receptive fields. Such neurons encode the motions of local features, which often differ from the motion of the object to which they belong. To perceive global motion, the feature motions must be integrated. A candidate site for this integration is cortical area MT, where cells with large receptive fields combine overlapping motion signals to compute the motion of a coherent pattern. We asked whether the entire MT receptive field is the unit of integration of different motions. We measured the responses of MT cells to overlapping drifting gratings and compared them with the responses to the same gratings separated in the receptive field. Cells that were selective for the direction of motion of the whole pattern when the gratings were overlapping lost this selectivity when the gratings were separated, and became selective only for the direction of motion of the individual components. Our results suggest that the computations underlying pattern direction are done on a spatial scale smaller than the whole receptive field.
When presented with bar textures moving obliquely to their orientation cells in MT change their direction selectivity over time. Their initial response is dominated by motion orthogonal to the bars; over time their direction preference shifts toward the direction of terminator motion. The Fourier spectra of bar textures have an interesting distribution of component contrasts. Components parallel to the bars have the highest contrast, while those at other orientations have lower contrasts. We wondered if the changes in direction selectivity were due to a well-known effect of contrast on visual response latency. We filtered bar textures down to their four fundamental component gratings, maintaining the contrast ratios among the components. We used these stimuli to examine the time evolution of directional selectivity in MT neurons. MT cells changed their direction selectivity over time for the reduced stimuli in precisely the same way they did for unfiltered bar textures. We conclude that the temporal change in direction selectivity is due to the delayed processing of the low contrast components.