V1 responses to transparent and nontransparent motions

It is well known that a stimulus composed of two independent sets of random dots moving in opposite directions produces a percept of two overlapping transparent surfaces moving across each other, while a counterphase grating composed of two identical sine wave gratings drifting in opposite directions does not. We recorded from the directionally selective V1 cells of behaving macaque monkeys using these two types of stimuli in order to investigate the physiological basis of transparent motion perception. Previous single-unit recording experiments from our laboratory indicated that many V1 cells respond well to transparent random dot patterns, while MT cells' responses to the same patterns are strongly suppressed in comparison with their preferred direction responses. This observation alone would seem to suggest that V1 activity could better explain transparent motion perception than MT activity. However, one could argue to the contrary based on the psychophysical observation that there is a motion threshold elevation under the transparency condition. We decided to determine the correlation between V1 activity and the transparent motion perception directly by recording from V1 cells using both transparent random dot patterns and nontransparent counterphase gratings. It is found that V1 cells on the average could not reliably tell the two types of patterns apart. Our results further the idea that additional processing beyond V1 is involved in transparent motion analysis.