1. The spatial response properties of single optic nerve fibers in the California ground squirrel (Spermophilus beecheyi) were investigated. Results are reported for each of several response classes. 2. Resolution limits determined for 165 spectrally nonopponent optic nerve fibers ranged from 0.1 to 4.2 cycles/degree (c/d), with a mean resolution limit of 1.49 c/d. Directionally selective units possessed the highest resolution limits (mean = 1.91 c/d). Sustained and transient units possessed mean resolution limits of 1.44 and 1.09 c/d, respectively. 3. The correlation between response and sensitivity measures of spatial tuning was examined for 29 units. Optimal spatial frequency and spatial frequency bandwidths estimates derived from the two measures were highly correlated. 4. As measured from response functions, optimal spatial frequencies of 97 spectrally nonopponent units ranged from 0.1 to 2.2 c/d (mean = 0.44 c/d). Directionally selective units possessed the highest optimal spatial frequencies (mean = 0.60 c/d). Mean optimal spatial frequencies for sustained and transient units were 0.32 and 0.27 c/d, respectively. Mean half-amplitude response bandwidth for 96 optic nerve fibers was 2.80 octaves. Directionally selective units were narrowly tuned, possessing a mean half-amplitude bandwidth of 1.99 octaves. Sustained and transient units had mean half-amplitude response bandwidths of 3.56 and 2.80 octaves, respectively. 5. The response bandwidths of directionally selective optic nerve fibers were highly negatively correlated with optimal spatial frequency; no significant correlation between these measures existed for sustained or transient units. 6. Peak contrast sensitivity in 88 optic nerve fibers ranged from 3.0 to 55.0. Mean contrast sensitivity was 16.7 and did not differ between fiber response classes. Peak contrast sensitivity was not correlated with optimal spatial frequency. 7. Poststimulus time histograms of unit responses to sinusoidal luminance modulation revealed that some sustained units modulate their discharge rates around spontaneous levels and show predominant Fourier amplitude at the fundamental frequency. Transient units possess low levels of spontaneous discharge and respond abruptly to the passage of grating half-cycles, resulting in a characteristically dispersed spectrum of Fourier amplitude. Directionally selective units respond to sinusoidal luminance gratings with an elevation of maintained firing rate and possess moderate Fourier amplitudes corresponding to the fundamental and second harmonic frequencies. Sustained and transient ground squirrel optic nerve fibers are similar to X- and Y-cells, respectively, found in the retinas of other species. 8. The proportion of units with resolution limits equal to the behavioral acuity of the ground squirrel is 3-4%, similar to proportions of 'high-resolution' units reported in the subcortical visual pathways of a variety of other species. Concentric units demonstrating high spatial resolution are estimated to possess receptive-field center diameters compatible with the sizes of the smallest dendritic fields of bipolar and ganglion cells seen in anatomical studies of the ground squirrel retina. 9. The high spatial resolution, narrow spatial frequency tuning, and strong relationship between optimal spatial frequency and response bandwidth for directionally selective units is discussed in view of their putative projection exclusively to tectal regions, and in light of the dependence of pattern vision in sciurids on the integrity of extrageniculate pathways.
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