Spatial contrast sensitivity of birds

Abstract

Contrast sensitivity (CS) is the ability of the observer to discriminate between adjacent stimuli on the basis of their differences in relative luminosity (contrast) rather than their absolute luminances. Prior to this study, birds had been thought to have low contrast detection thresholds relative to mammals and fishes. This was a surprising phenomenon because birds had been traditionally attributed with superior vision. In addition, the low CS of birds could not be explained by retinal or optical factors, or secondary stimulus characteristics. Unfortunately, avian contrast sensitivity functions (CSFs) were sparse in the literature, so it was unknown whether low contrast sensitivity was a general phenomenon in birds. This study measured CS in six species of birds The quail and pigeon data obtained in this experiment fit well with existing CS data for these species. The kestrel data were not similar to kestrel data in the literature; however the data in the literature were collected from a single subject. All of the birds studied had contrast sensitivities that were consistent with their retinal or optical morphologies relative to other birds (in species for which such data exists) and seem well suited for their natural environments. In addition, all of these birds exhibited low CS relative to humans and most mammals, which suggests that low CS is a general phenomenon of birds.Explanations for this avian low CS phenomenon include a possible trade-off between contrast mechanisms and UV mechanisms in cone systems, and lateral inhibitory mechanisms that are perhaps categorically different from mammals. Lateral inhibition affects contrast gain, and has been shown to differ according to ganglion cell types, which in turn will differ in vertebrate species.

Vision on the high seas: spatial resolution and optical sensitivity in two procellariiform seabirds with different foraging strategies

“…By way of comparison, humans can detect contrast differences as low as 0.6% (De Valois and Morgan, 1974), but spatial resolution decreases more than eleven times as contrast goes down from 100% to 0.6%. In birds, resolution decreases more than six times as contrast goes down from 100% to 7-14% depending on the species (Reymond and Wolfe, 1981;Ghim and Hodos, 2006;Harmening et al, 2009;Jarvis et al, 2009;Lind et al, 2012). If we assume a similar decrease in spatial resolution for our study species as in other birds, then Leach's storm petrels would be able to detect lowest contrast with a spatial resolution of 1.1 cycles deg −1 , whereas northern fulmars would do so with a resolution of 7.2 cycles deg −1 .…”

“…barn owl (Tyto alba), wedge-tailed eagle, chicken, pigeon (Columba livia), parrots], birds have been shown to have lower contrast sensitivity compared with humans and some other mammals (summary in Lind et al, 2012). The minimum contrast birds can detect ranges from 7 to 14% (Reymond and Wolfe, 1981;Ghim and Hodos, 2006;Harmening et al, 2009;Jarvis et al, 2009;Lind et al, 2012). By way of comparison, humans can detect contrast differences as low as 0.6% (De Valois and Morgan, 1974), but spatial resolution decreases more than eleven times as contrast goes down from 100% to 0.6%.…”

Bird colour vision: behavioural thresholds reveal receptor noise

“…The Weber fraction for human LWS cone photoreceptors has been suggested to be 0.018 (Wyszecki and Stiles, 2000), which is much smaller than the Weber fraction for bird LWS cones (0.06 for chickens and 0.1 for Leiothrix and budgerigars). This difference could explain why we found no obvious difference in colour discrimination threshold between humans and birds and may also help to explain why birds generally have lower contrast sensitivity than humans (Ghim and Hodos, 2006;Gover et al, 2009;Harmening et al, 2009;Lind et al, 2012).…”

Comparative neurophysiology of spatial luminance contrast sensitivity.

“…Harmening et al (2009) found a contrast sensitivity peak of 1-2 cpd and visual acuity of 3-4 cpd. Ghim and Hodos (2006) used pattern electroretinography to compare the CSF of several bird species, including falcons (Falco sparvarius), owls (Tyto alba), European starlings (Sturnus vulgaris), quails (Coturnix coturnix japonica), red-bellied woodpeckers (Melanerpes carolinus), and pigeons (Columbia livia). They found that these birds had a band-pass CSF that peaked at 3 cpd (falcon), 1-2 cpd (pigeon, starling, and owl), 0.8-1 cpd (quail), and 0.5-0.7 cpd (woodpecker).…”