Fundamentals of spatial vision in the barn owl (Tyto alba pratincola) : ocular aberrations, grating acuity, contrast sensitivity, and vernier acuity

摘要

Optical quality in barn owl eyes was measured with a standard Tscherning-type wavefront aberrometer. The results revealed that barn owl eyes have excellent optical quality. Higher-order aberrations were about 3-fold lower than has been reported in human eyes with similar methods and under comparable conditions. The spatial contrast sensitivity function (CSF) of the barn owl was measured in several psychophysical experiments with an orientation discrimination task. As in humans and virtually any other animal, the CSF of the barn owl rendered the typical band-limited inverted u-shaped function, with attenuated sensitivities for high and low spatial frequencies. Maximal sensitivity of about 20 was at a spatial frequency of about 1 cyc/deg. With a similar discrimination task, rating acuity was measured in a separate behavioral experiment. The results, 3.7 cyc/deg, are in good agreement with the high-frequency cut-off of the CSF, and put the barn owl on the very low end of the acuity spectrum compared to other owls and birds. The barn owl displays discrimination thresholds in psychophysical vernier acuity experiments that are substantially lower than the resolution limit derived from the high-frequency cut-off of the behavioral and electrophysiological CSF. Moreover, the thresholds were also lower than the behaviorally measured grating acuity, and than theoretical values derived from an anatomical study of the bird’s retina. It is concluded, that vernier acuity is a hyperacute percept in the barn owl. Vernier acuity thresholds of the barn owl seem to be similarly influenced by the choice of stimulus configuration and viewing conditions as it is the case in human observers. Specifically, vernier thresholds were worse in a crowded stimulus situation, and better under binocular viewing conditions. The latter is referred to as binocular summation in human subjects.