Wavelength discrimination by the goldfish near absolute visual threshold

Abstract

Goldfish were classically conditioned to discriminate between 532 nm and 636 nm lights while light adapted, and then tested for their ability to continue the discrimination as the intensities were reduced. In all tests, the two stimuli were balanced for effectiveness at absolute threshold. We find that goldfish can discriminate between these wavelengths about 1 log unit above absolute threshold. In contrast, fish who were trained to discriminate between 532 nm and 441 nm stopped discriminating 2–3 log units above absolute threshold. There are two conclusions from these results. First, some aspect of the neural signals generated by the rod and red cone mechanisms must be different, despite electrophysiological evidence that suggests they are the same. Second, the goldfish has color vision near rod threshold.