Relation of brightness to threshold for light-adapted and dark-adapted rods and cones: effects of retinal eccentricity and target size.

'Equal-brightness' functions of retinal eccentricity and target diameter were measured by a matching procedure, and compared with the corresponding threshold functions for four different adaptation conditions: light-adapted cones (LAC), dark-adapted cones (DAC), light-adapted rods (LAR) and dark-adapted rods (DAR). The separation between log brightness matches and log thresholds decreased with eccentricity and increased with target size for all adaptation conditions, but overall separation was substantially greater for the DAR condition than for the other three. A two-channel model of achromatic brightness is proposed to explain the results. The model assumes 'strong' and 'weak' channels, which contribute unequally to brightness. These channels are tentatively identified with tonic and phasic classes of retinal ganglion cells.