Visual motion integration by neurons in the middle temporal area of a New World monkey, the marmoset

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Non-technical summary

The machinery of motion vision is highly conserved across New World and Old World monkeys, according to our study of the marmoset visual cortex. The marmoset is a New World primate, part of a lineage that diverged from Old World monkeys some 30–40 million years ago. A small part of the cerebral cortex, area MT, can be identified anatomically in both New and Old World primates. In the macaque, an Old World primate, this area is thought to be important in analysing the motion of complex patterns. Here we quantified the capacity of neurons in area MT of marmosets to extract motion from complex patterns. We find the responses of neurons in area MT of marmosets to be indistinguishable from those in macaques, suggesting that the functional role of this small area of the visual cortex is highly conserved over evolution.

The middle temporal area (MT/V5) is an anatomically distinct region of primate visual cortex that is specialized for the processing of image motion. It is generally thought that some neurons in area MT are capable of signalling the motion of complex patterns, but this has only been established in the macaque monkey. We made extracellular recordings from single units in area MT of anaesthetized marmosets, a New World monkey. We show through quantitative analyses that some neurons (35 of 185; 19%) are capable of signalling pattern motion (‘pattern cells’). Across several dimensions, the visual response of pattern cells in marmosets is indistinguishable from that of pattern cells in macaques. Other neurons respond to the motion of oriented contours in a pattern (‘component cells’) or show intermediate properties. In addition, we encountered a subset of neurons (22 of 185; 12%) insensitive to sinusoidal gratings but very responsive to plaids and other two-dimensional patterns and otherwise indistinguishable from pattern cells. We compared the response of each cell class to drifting gratings and dot fields. In pattern cells, directional selectivity was similar for gratings and dot fields; in component cells, directional selectivity was weaker for dot fields than gratings. Pattern cells were more likely to have stronger suppressive surrounds, prefer lower spatial frequencies and prefer higher speeds than component cells. We conclude that pattern motion sensitivity is a feature of some neurons in area MT of both New and Old World monkeys, suggesting that this functional property is an important stage in motion analysis and is likely to be conserved in humans.

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