Determination of Chromatic and Luminance Channels in the Zebrafish Visual System Using an Increment Threshold Technique
It is well established that the primate visual system possesses anatomically separate chromatic and luminance channels; these pathways are responsible for the visual coding of color and brightness information, respectively. In lower vertebrates, such as the zebrafish (Danio rerio), there does not appear to be any anatomical separation of cell types mediating color and luminance information. However, several studies suggest that these vertebrates may be able to discriminate these attributes of a visual stimulus. The purpose of this study was to determine whether the zebrafish visual system possesses chromatic and luminance channels. In addition, the contribution of each cone photoreceptor type to the chromatic and luminance channels was determined. An increment threshold procedure was used to elicit electroretinogram (ERG) b-wave responses to monochromatic light under two different intensities of white background adaptation. From the ERG b-wave responses, a spectral sensitivity function was derived under low and high levels of white background. By examining spectral sensitivity functions under these two levels of white background adaptation, it was possible to determine if two functional channels exist for color and luminance in the zebrafish visual system. The results of this study suggest that zebrafish possess a chromatic channel, but do not seem to have a luminance channel. The low level of white background adaptation yielded a spectral sensitivity function that was similar to the spectral sensitivity function of the chromatic channel in the primate. Alternatively, the high level of white background produced a spectral sensitivity function which appears to represent neither a chromatic nor a luminance channel; rather, the high level of white background only suppressed the cone contributions to the spectral sensitivity function. In addition, the ultraviolet cones were found to contribute substantially to spectral sensitivity functions derived under both the high and low white background adaptation levels. In conclusion, the results of the present study suggest that anatomical separation may be required for separation of function. Furthermore, the finding that zebrafish possess only a chromatic channel, as well as substantial ultraviolet sensitivity, may be a reflection of the type of environment in which they are normally found.