Midget ganglion cell anatomy and center structure: More complex than it looks.

It is usually assumed that the center of the midget ganglion cell in the primate retina derives input from a single cone in and near the fovea, providing a 'private line to the brain'. Thus, the physiological center should have the dimensions of a single cone sampling aperture. Centers measured physiologically are much larger, partly due to optical blur. With retinal eccentricity, at 10 degrees eccentricity cones are larger but optics do not change much. Yet physiological centers have much increased in size and are still much larger than expected from the anatomy. As retinal eccentricity increases further, convergence of retinal bipolars begins and this convergence becomes more pronounced at greater eccentricities. With convergence from 2 to 3 midget bipolars onto a ganglion cell, markedly anisotropic centers would be expected physiologically, but have not been reported. Dendritic tree anisotropies would also be expected to cause center irregularities at even greater eccentricities. These discrepancies between anatomy and physiology have received little attention but from a functional perspective would seem critical for understanding processing of information in the midget ganglion cell system. The 'private line' hypothesis may have to reformulated to take these inconsistencies into account.