Modulating wavelength discrimination in goldfish with ethambutol and stimulus intensity

Wavelength discrimination in goldfish was measured behaviourally. Both acute application of ethambutol injected into the eye and chronic application by feeding the animals daily 25 mg ethambutol for 1 month had the same effect on wavelength discrimination in the range of 560-640 nm. This means that: (1) electrophysiological experiments, in which drug application is primarily acute, reflect the same disturbance as behavioural experiments, in which drug application is chronic; and that (2) the origin of the color vision defect must be retinal. Furthermore reduction in stimulus intensity by 2 log units caused, in control fish, a similar disturbance in wavelength discrimination as induced by ethambutol, whereas an increase of stimulus intensity by 2 log units abolished in ethambutol-fed fish the discrimination disturbance. These results indicate that ethambutol shifts the threshold for wavelength discrimination without changing the absolute sensitivity of the cone systems.