Spatial resolution across the macaque retina

Grating acuity was measured as a function of eccentricity from the fovea in two macaques. A vertical-horizontal orientation discrimination was used to determine acuity, and the retinal locus of the test grating was controlled by training them to fixate a spot placed at various distances from the stimulus. Their head was fixed in place and fixation was monitored with a scleral search coil. The acuity of monkeys across the retina was similar to that previously measured in human subjects, reaching a peak of about 38 c/deg at the fovea, and decreasing about 10-fold by 30 deg eccentricity. Acuity was slightly higher in the temporal than in the nasal visual field. The shape of the acuity-eccentricity function suggested a dependence on cone density near the fovea, and on the density of P ganglion cells at eccentricities beyond 10 deg. Existing physiological data suggest the possibility that macaque acuity may also be limited in part by spatial averaging across the receptive field of retinal ganglion cells.