Behavioural studies of monocular hyperacuity in barn owls (vernier acuity)

Humans can determine the relative positional difference of spatially non-aligned features with a precision that corresponds to only a fraction of the eye’s resolving power. This ability, a hyperacuity phenomenon, is called vernier acuity. Psychophysical measures of vernier thresholds yield values down to 1-5 seconds of arc, which is compared to thresholds derived from tasks that are physically limited by foveal cone spacing, such as two-point or grating acuity, about 6-30 fold lower (better). So far vernier thresholds have not been obtained in an avian species.

The barn owl is a highly specialized nocturnal predator with exceptional preying skills. Besides their superior sound-localization capabilities, also the visual system in this bird shows anatomical, functional and physiological specializations. The barn owl has high-quality frontally oriented eyes that create an unusual large binocular field of view compared to other birds. The barn owl has coupled accommodation in both eyes, and an enlarged visual Wulst with a high degree of binocular interaction and selectivity for binocular disparity. It has been shown that owls possess stereopsis and use disparity as a depth cue with hyperacute precision. The question asked here is whether the barn owl displays hyperacuity in a vernier task. This was tested behaviourally with two kinds of stimuli under binocular and monocular viewing conditions.

Our data show that barn owls can discriminate vernier stimuli below 1 arcmin displacement angle. Based on grating acuity estimation our findings indicate that vernier acuity is a hyperacute percept in this species. The lowest threshold (0.58 arcmin) is 6 fold lower (better) than the assumed grating acuity. Statistical analysis of different viewing conditions indicate that binocular viewing outperforms monocular viewing by some 30-50%. Thus binocular summation seems to play a role in vernier discrimination by owls. Performance is similarly affected as in humans by the choice of stimulus configuration. Bar stimuli yielded lower values than grating stimuli, an effect referred to as crowding in human subjects.