Cooperative synchronized assemblies and orientation discrimination

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Abstract
There is no clear link between the broadly-tuned responses of single neurons and the fine behavioral capabilities of orientation discrimination. We have examined whether the joint activity of synchronized cells with similar preferred orientations would support finer discrimination than found in the rate code of single cells. We recorded from small populations of cells with a multi-electrode array in Area 17 of three cats (22, 25, and 23 cells) that were paralyzed and anesthetized with Propofol and N2O. Analysis of joint firing provides a substantial advantage (i.e., cooperation) in fine angle discrimination (4 degrees). This advantage increases from 50–300% as the population of an assembly is increased from 2–6 cells. The improvement of discrimination from cooperation accelerates with respect to the number of cells, yielding efficient encoding of orientation with either fewer cells or in less time than from independent coding. The advantage provided by the joint activity of 6 cells grows from 75–540% as the orientation discrimination task is reduced from 10 to 2 degrees, which suggests that cooperation could potentially be a mechanism that supports higher acuity. There is almost no significant difference between the responses to 2-degree variations in individual responses making it unreasonable to achieve this level of discrimination by simply examining large populations of cells independently. The cooperation is measured at a temporal resolution of 3–6 ms and incorporates another 3–6 ms of discharge history, which corresponds to the time scale of the orientation-dependent synchrony we find among the cellular assemblies. The results provide quantitative evidence that supports a role for synchrony in visual perception.

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