1. Directional units in the optic nerve of the California ground squirrel (Spermophilus beecheyi) were studied with respect to their response to diffuse light, preferred directions of motion, tuning for preferred direction, the relationship between spatial and directional tuning characteristics, and receptive-field size and areal summating properties. 2. Directional units in the ground squirrel optic nerve are of the 'on-off' type. No purely on or off units were encountered in a sample of 356 directionally selective fibers. 3. The distribution of preferred directions of image motion for 356 units was significantly anisotropic; >50% of the directional units prefer motion in the direction of the superior-nasal visual quadrant. 4. Mean directional bandwidth, measured at half-amplitude response, for 39 units was 88.5°. The distribution of directional bandwidths suggests that two subpopulations of directional units may exist: a broadly tuned (106.4° bandwidth) group preferring image motion in the superior-nasal direction, and a narrowly tuned group (59.9° bandwidth) with a uniform distribution of preferred direction. 5. Tuning for direction of motion and for spatial frequency were significantly positively correlated in a sample of 35 directional units. 6. Area-vs.-response measures for directional units show that they possess excitatory discharge centers with a concentric antagonistic surround, plus a larger suppressive surround activated specifically by moving luminance contours, which may be asymmetric. 7. Critical activation areas for directional units, as measured along orthogonal orientations, were highly positively correlated. This suggests that these receptive fields possess the property of linear spatial summation, not of luminance flux, but of areas of moving luminance contours.
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