Visual acuity and spatial contrast sensitivity in tree squirrels

The visual sensitivity of tree squirrels from three different species (two western gray squirrels, Sciurus griseus; three fox squirrels, Sciurus niger; and an eastern gray squirrel, Sciurus carolinensis) was measured for spatial patterns defined by luminance differences. Spatial contrast sensitivity functions were determined in behavioral discrimination experiments in which the stimuli were sinusoidally-modulated gratings. At an average luminance level of 3.4 cd/m(2) these squirrels were maximally sensitive to spatial frequencies of about 0.5 cycles/degree (c/d). These experiments, in conjunction with additional measurements involving discrimination of square-wave grating patterns, also indicate that the finest gratings these squirrels can discriminate average 2.2 c/d (SD = 0.42 c/d). There are no obvious differences in resolution acuity among the three species. Grating acuity also was measured at a much higher luminance level (340 cd/m(2)). The results of this experiment indicate that the spatial acuity of the tree squirrel increases with luminance level, reaching a maximum of 3.9 c/d at 340 cd/m(2).