Visual contrast thresholds of the goldfish ( Carassius auratus)

Abstract

Contrast thresholds of the goldfish eye were determined by the method of constant stimuli using a yes-no conditioned cardiac-response. Computerized probit analysis was used to treat the data. Thresholds were determined for varying temperatures, target sizes, fish sizes, target positions and adaptive radiance levels. Positive contrast, circular targets were used throughout. Both target and adaptive radiance were monochromatic light. Threshold was found to decrease as fish size increased. This change was attributed to the density of cones which decline more slowly as the eye grows than is required to compensate for the calculated expansion of image area in the larger eye. Threshold was found to be temperature dependent, being lowest at 20°C. Threshold was found to vary with the position of the target image on the retina in general agreement with cone topography. Binocular thresholds were lower than monocular thresholds. The relation between target size and contrast threshold was studied at four adaptive levels from a photopic level of 85 μW/cm 2/ω to a near scotopic level of 0.07 μW/cm 2/ω. The data were examined to determine critical visual angle and the region of complete summation. Single-point visual acuity for the goldfish was interpolated from the data. The use of contrast threshold data in underwater visibility problems is discussed. No evidence was found to indicate a special contrast-enhancement mechanism in fishes.