Increment thresholds of the three spectral mechanisms in the retina of the California ground squirrel (Spermophilus beecheyi).

Increment threshold (IT) functions were obtained for the electroretinogram (ERG) and for single units in the optic nerve of the California ground squirrel. The three spectral mechanisms providing input to ganglion cells in this retina were isolated and their increment thresholds to large field, long-duration stimuli were examined. Mean IT functions for the 519 nm mechanism (irrespective of unit class), the 500 nm mechanism, and the ERG all showed shallow log-log slopes between 0.52 and 0.63. The slope of the mean IT function for the 440 nm mechanism depended on the spectral character of the adapting light. When the dominant wavelength of this light was close to the peak sensitivity of the 440 nm mechanism, the mean IT function was steep (0.92), but when the dominant wavelength of the adapting light was at the cross-point of the opponent cells, the slope of the function was shallow (0.49). The difference in IT slope under these two conditions may be attributed to an additional sensitivity loss occuring at a spectrally-opponent site.