Characteristics of rod driven off-responses in cat ganglion cells

The off-response of dark adapted cat ganglion cells shows a tripartite response-intensity function in the optic nerve response (ONR) as well as in extracellular recordings of single cells. While responses increase when stimuli of low or high intensities are increased, the rod driven off-response shows a strong decrease (dip) for intermediate intensities before the cone part of the function starts to rise. In contrast, on-responses increase monotonically or stay at a maximum. The dip in the response-intensity function of the off-response has a constant shape with test lights of increasing as well as of decreasing irradiance. The action spectrum of the descending part of the function peaks at 500 nm, indicating that a rod driven mechanism is responsible for the response reduction reflected by the dip. Changing the stimulus diameter from 24 deg to a 1 deg test field centred on a ganglion cell's receptive field has minimal effect on the response reduction. This points to a temporal rather than a spatial mechanism being responsible for the dip.