Detection and analyzing visual motion is an important task for the visual system. In the past half a century, research on motion information processing has been focused primarily on the dorsal visual areas (e.g. area MT/V5). Other visual areas (e.g. V2, V4) are virtually unexplored in this aspect, despite the fact that the motion-sensitive neurons in these areas are significantly present and form functional domains. In this study, we combined optical imaging with single-cell recordings to specifically record from direction-selective neurons in the second largest visual area, V2, in macaque monkeys, in order to have a full picture of how motion information is analyzed in the brain.
We found that motion-sensitive neurons in area V2 have characteristic features that are different from those in the well-studied motion areas like area MT. Particularly, these neurons have small receptive field and strong surround suppression, which make them sensitive to “motion contrast”, a key information delineating object boundaries. These features suggest that area V2 detects visual objects based on motion information.
In the primate visual system, direction-selective (DS)neurons are critical for visual motion perception. While DS neurons in the dorsal visual pathway have been well characterized, the response properties of DS neurons in other major visual areas are largely unexplored. Recent optical imaging studies in monkey visual cortex area 2 (V2) revealed clusters of DS neurons. This imaging method facilitates targeted recordings from these neurons. Using optical imaging and single-cell recording, we characterized detailed response properties of DS neurons in macaque V2. Compared with DS neurons in the dorsal areas (e.g., middle temporal area [MT]), V2 DS neurons have a smaller receptive field and a stronger antagonistic surround. They do not code speed or plaid motion but are sensitive to motion contrast. Our results suggest that V2 DS neurons play an important role in figure-ground segregation. The clusters of V2 DS neurons are likely specialized functional systems for detecting motion contrast.
Macaque, V2, direction selectivity, RF surround, motion contrast