Absolute visual sensitivity of the goldfish

Absolute visual threshold was measured in dark-adapted goldfish, using classical conditioning of heart and respiration rates. For threshold detection of a 132[deg] diameter stimulus near the peak of the rod pigment's sensitivity, only 1 quantum was incident for every 2000-4000 rods, and 400-830 photoisomerizations were needed for vision. Spectral sensitivity was also determined at absolute threshold, sing respiration rate conditioning. The shape of the function matches the electrophysiologically determined spectral sensitivity of goldfish retinal ganglion cells, but both curves are relatively more sensitive in the long wavelengths than would be predicted on the basis of the rod pigment alone. A dim 703 nm background depressed sensitivity to a 636 nm test light more than to a 532 nm test, proving that another class of photoreceptors is active at absolute threshold. If the other receptors were the red cones, as seems most likely, then only 1 quantum was incident per 220 red cones at their absolute threshold, and photoisomerizations occurred in each of 164 red cones at threshold.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/22776/1/0000331.pd