The independence of the photopic receptor systems underlying visual thresholds in a teleost

Abstract

The photopic spectral sensitivity curve of the rudd shows three maxima, at about 620 nm, 510 nm, and at short wavelengths, which presumably reflect the activity of three separate receptor systems. In the present experiments spectral sensitivity curves (called background sensitivity curves) were obtained behaviourally by finding the intensity of different monochromatic lights necessary to reduce the probability of detecting a stimulus to a given (threshold) level. Three stimuli were chosen, whose wavelengths coincided with the maxima of the photopic sensitivity curve. Each of the three background sensitivity curves obtained agreed with one of the maxima of the photopic spectral sensitivity curve, but showed no sign of either of the other two maxima. The procedure was therefore successful in isolating the rudd's three receptor systems. The results indicate that the receptor systems act independently in their response to both the stimulus (excitation) and the background (adaptation). The independence during adaptation shows that there are separate adaptation pools for each receptor system. Possible physiological mechanisms underlying this independence are considered.