Psychophysical thresholds to different wavelengths in light adapted turtles

Abstract

Visual thresholds to different spectral stimuli were determined behaviourally on two species of turtle, Chrysemys picta belli, and Pseudemys scripta elegans, under light adapted conditions. With a background illumination of 0.126 ft. lamberts the spectral sensitivity curve showed a maximum at about 490 nm, and a submaximum at about 650 nm. These results are identical to previous behavioural results obtained under scotopic conditions. Thresholds were also obtained for two selected wavelengths (498 and 615 nm) under different intensities of background illumination. The relative sensitivity to the two spectral stimuli was unchanged, even though the background was varied over 3.2 log units, up to a maximum value of 14 ft. lamberts. Curves relating the percentage correct responses ( Pc) to intensity were determined. The shape of these curves was unaffected by the spectral composition of the stimulus, but Pc increased more rapidly with intensity when high background illuminations were used. The results are considered in relation to previously reported physiological and behavioural dated. It is suggested that the receptors responsible for the maximum at 490 nm show more convergence on to higher nervous elements than those responsible for the 650 nm maximum, but that the later are more numerous.