Wavelength discrimination by the goldfish near absolute visual threshold.

Goldfish were classically conditioned to discriminate between 532 nm and 635 nm lights while light adapted, and then tested for their ability to continue the dis~mjnation 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 I log unit above absolute threshold. In contrast, fish who were trained to discriminate between 532 nm and WI 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. Kr!: Words-goldfish; rod-cone interactions; color discrimination; respiratory conditioning; scotopic &on: retina.