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Spectral response properties of optic nerve fibers in the ground squirrel spermophilus beecheyi

, : Spectral response properties of optic nerve fibers in the ground squirrel spermophilus beecheyi. Journal of Neurophysiology (Bethesda) 45(5): 891-902

The spectral-response properties of optic-nerve fibers were examined in anesthetized California ground squirrels. As defined by use of stimuli that were large relative to the spatial extent of the receptive-field centers, this optic nerve contains 3 types of units that give spectrally non-opponent responses, while 2 other types of units show evidence for spectral opponency. Of the non-opponent group, units were found to show either direction-selective responses, center-surround antagonism with both regions having the same spectral sensitivity (contrast-sensitive units), or center-surround antagonism with different spectral sensitivity in the 2 regions (BG/G units). The 2 classes of units showing spectral opponency give either an increase in firing rate for stimuli whose wavelength content is < 500 nm and a decrease in firing rate for longer test wavelengths (B/G units), or they show exactly the reverse response pattern (G/B units). In 963 units, about 75% were of the non-opponent varieties. The spectral sensitivities of the retinal mechanisms providing inputs to the optic nerve were determined for each of the various response classes by recording their spectral responses both with and without concurrent chromatic adaptation. Under conditions of light adaptation, 2 spectral mechanisms were isolated. After correction for preretinal filtering, these 2 mechanisms are shown to have sensitivity functions that correspond closely to those predicted by wavelength-dependent visual pigment nomograms with peak sensitivity at 440 and 525 nm. The 525 nm mechanism provides the sole input to the contrast-sensitive and direction-selective units, while the BG/G, B/G and G/B units receive inputs from both of these mechanisms. In addition to the 440 and 525 nm mechanisms, which presumably reflect the operation of cones, a 3rd mechanism having peak sensitivity at 500 nm provides input to some units under conditions of dark adaptation. This 3rd mechanism apparently represents the contribution of the small number of rods (< 10% of all photoreceptors) found in this retina.

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