Abstract
Birds have excellent spatial acuity and colour vision compared to other vertebrates while spatial contrast sensitivity is relatively poor for unknown reasons. Contrast sensitivity describes the detection of gratings of varying spatial frequency. It is unclear whether bird brightness discrimination between large uniform fields is poor as well. Here we show that budgerigars (Melopsittacus undulatus) need a Michelson contrast of 0.09 to discriminate between large spatially separated achromatic fields in bright light conditions. This is similar to the peak contrast sensitivity of 10.2 (0.098 Michelson contrast) for achromatic grating stimuli established in earlier studies. The brightness discrimination threshold described in Weber fractions is 0.18, which is modest compared to other vertebrates.
Highlights
Among vertebrates, birds have relatively large eyes [1], excellent optical quality [2,3], sophisticated colour vision [4] and high spatial acuity [5]
Within birds, brightness discrimination has only been described in pigeons (Columba livia; [10]) and it is unclear how discrimination of large separate fields relates to the contrast sensitivity function (CSF) and spatial acuity
We test brightness discrimination in budgerigars (Melopsittacus undulatus) and compare our results to the CSF that was measured in similar light conditions [7] as well as to brightness discrimination thresholds in pigeons and other vertebrates
Summary
Birds have relatively large eyes [1], excellent optical quality [2,3], sophisticated colour vision [4] and high spatial acuity [5]. The CSF approach ignores another fundamental category of visual stimuli that is encountered in natural scenes; large (low spatial frequency) uniform fields. This information can instead be obtained by measuring brightness discrimination thresholds. Within birds, brightness discrimination has only been described in pigeons (Columba livia; [10]) and it is unclear how discrimination of large separate fields relates to the CSF and spatial acuity. We test brightness discrimination in budgerigars (Melopsittacus undulatus) and compare our results to the CSF that was measured in similar light conditions [7] as well as to brightness discrimination thresholds in pigeons and other vertebrates
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