Journal of Neurophysiology, Vol 52, Issue 6 1200-1212, Copyright © 1984 by APS
Directional filter characteristics of optic nerve fibers in California ground squirrel (Spermophilus beecheyi)
M. E. McCourt and G. H. Jacobs
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.
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. The distribution of preferred directions of
image motion for 356 units was significantly anisotropic; greater than 50%
of the directional units prefer motion in the direction of the
superior-nasal visual quadrant. Mean directional bandwidth, measured at
half-amplitude response, for 39 units was 88.5 degrees. The distribution of
directional bandwidths suggests that two subpopulations of directional
units may exist a broadly tuned (106.4 degrees bandwidth) group preferring
image motion in the superior-nasal direction, and a narrowly tuned group
(59.9 degrees bandwidth) with a uniform distribution of preferred
direction. Tuning for direction of motion and for spatial frequency were
significantly positively correlated in a sample of 35 directional units.
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. 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.
