A role for terminators in motion processing by macaque MT neurons?

At low speeds and contrasts, bar textures moving obliquely to their orientation change their apparent direction. Initially they seem to move at right angles to the bar orientation, then over ∼200 ms they change and appear to lock to the true direction of motion defined by the bars' ends ("terminator motion"). Responses in macaque MT neurons behave analogously - initially they are dominated by motion orthogonal to the bars; over time they come to respond to the direction of terminator motion. These changes in perception and neuronal selectivity are thought to be due to a transition between two motion signals, a rapid one related to orientation and a slower one related to terminators. The Fourier spectra of bar textures have an interesting distribution of component contrasts for different orientations. 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 processing: lower contrast targets are processed more slowly than high contrast ones. We filtered bar textures down to their four fundamental component gratings, maintaining the contrast ratios among the components. The resulting patterns have no obvious terminators but their spectral structure retains the essential structure of bar texture spectra. We used these stimuli to examine the time evolution of directional selectivity in MT neurons recorded in anesthetized, paralyzed macaques. Despite the absence of terminators, 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 motion of terminators is not responsible for the change, and propose that the late transition to terminator motion sensitivity is due simply to the delayed processing of the low contrast components that carry information about the true direction of motion.