Temporal Characteristics of Zebrafish Cone Types and Their Role in Separate Visual Processing

Chromatic and luminance information are processed separately by the visual systems of most higher vertebrates via anatomically separate pathways. Research on primates suggests that the luminance mechanism transmits information about stimulus luminance, movement, and flicker while chromatic mechanisms signal color and detailed information. In order to signal rapid movement, the luminance channel transmits information more rapidly than the chromatic mechanism. While some lower vertebrates such as the zebrafish (Danio rerio) do not demonstrate anatomical separation of processing, it is believed that separate processing is a basic requirement of vertebrate vision. The spectral sensitivity functions (relative sensitivity to different wavelengths of light) of the separate mechanisms were determined in zebrafish using heterochromatic flicker photometry (HFP). The spectral sensitivity function of the luminance channel was determined using a flicker rate of 16 Hz which assessed its rapid response rate while the chromatic channel was stimulated with 4.6 Hz flicker. The electrical responses of the visual system to the HFP stimuli were measured by the electroretinogram. The spectral sensitivity functions of the two channels were modeled by a nonlinear regression equation to assess the relative contributions of each cone type to the spectral sensitivity functions of the two channels. In addition, the temporal resolution of the four cone types was assessed to determine if the temporal response rates of the cone types are responsible for the determination of the channels to which the cones contribute. The spectral sensitivity functions determined by HFP showed no significant difference between the two temporal rates suggesting that zebrafish do not have separate channels for the processing of color and luminance information. In addition, the cone contribution modeling showed no opponency characteristic of chromatic processing. It therefore appears that the zebrafish does not process color information through chromatic opponent channels characteristic of vertebrate color vision. However, problems with the use of HFP to determine the spectral sensitivity function of the luminance channel are addressed. Finally, at the highest temporal rates, it appears that the ultraviolet sensitive cone type processes visual information faster than the middle and long wavelength sensitive cone types. It appears that temporal response rates of the cone types do not determine their relative contribution to separation of processing. This conclusion is supported only if the shortcomings of HFP cited in text are unfounded