Behavioural Processes, 7 (1982) 361- 315 zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLK Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands 367 zyxwvutsr VISUAL ACUITY AND SPATIAL CONTRAST SENSITIVITY IN TREE SQUIRRELS GERALD H. JACOBS, DAVID G. BIRCH* and BARBARA BLA KESLEE Department of Psychology, University of California, Santa Barbara, CA 93106 (U.S.A.) *Present address: Berman-Gund Laboratory for the Study of Retinal Degenerations, Massachusetts Eye and Ear Infirmary, Boston, MA 02114 (U.S.A.) (Accepted 8 June 1982) A BSTRA CT Jacobs, G.H., Birch, D.G. and Blakeslee, B., 1982. Visual acuity and spatial contrast sensitivity in tree squirrels. Behav. Processes, 7 : 367- 37 5. 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 ex- periments in which the stimuli were sinusoidally-modulated gratings. At an average luminance level of 3.4 cd/ m2 these squirrels were maximally sensitive to spatial frequen- cies 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/ ml). The results of this experiment indicate that the spatial acuity of the tree squirrel increases with lumi- nance level, reaching a maximum of 3.9 c/ d at 340 cd/ m2. INTRODUCTION Tree squirrels are diurnal, arboreal rodents. Their retinas are unusual among those of most mammals in that they contain more cones than rods; only about 40% of all tree-squirrel photoreceptors can be classified as rods (Cohen, 1964; West and Dowling, 1975; Yolton, 1975). It has been verified in both electrophysiological (Arden and Tansley, 1955; Gouras, 1964; Green and Dowling, 1975) and behavioral (Arden and Silver, 1962; Silver, 1966; Dippner and Armington, 1971; Jacobs, 1974) experiments that these two classes of photoreceptors provide the structural basis for functionally separable scotopic and photopic capacities. In addition to having a unique mixture of rods and cones, tree squirrel retinas are also notable for their lack of great regional variations. Thus, al- 03766357/ 82/ 0000-0000/ $02.75 0 1982 Elsevier Scientific Publishing Company