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.

Spatiotemporal contrast sensitivity (CS) functions were obtained from four White Carneaux pigeons. The spatial frequency for each session was selected randomly from a group of five spatial frequencies ranging from 0.42 to 1.26 c/deg. Within the session, the temporal frequency varied from 1 to 32 Hz. When plotted as a function of spatial frequency, the CS functions peaked in the range 0.7-1.0 c/deg. When compared to data that had been collected at 0 Hz temporal modulation, the temporally modulated spatial CS functions showed reduced CS, especially at the higher spatial frequencies, and reduced peak spatial frequency. When plotted as a function of temporal frequency, the CS functions were flat up to 8-16 Hz. Above 16 Hz, the curves showed a sharp roll off. When plotted as a three-dimensional, spatiotemporal CS surface, the data had a number of characteristics in common with the three-dimensional spatiotemporal model of CS proposed by Burbeck and Kelly (J. Opt. Soc. Am. 70 (1980) 1121).

Choroidal blood flow (ChBF) in birds is regulated by a neural circuit whose components are the retina, the suprachiasmatic nucleus, the medial division of the Edinger-Westphal nucleus (EWM), the ciliary ganglion, and the choriod. We have previously shown that lesions of EWM appear to result in pathological alterations in the retina. To determine whether EWM lesions also lead to altered visual functions, we have examined the effects of EWM lesions on visual acuity in pigeons. Bilateral lesions of EWM were made electrolytically, and visual acuity for high-contrast, square-wave gratings was determined behaviorally about 1 year later and compared to that of a group of pigeons that had received sham lesions of EW about 1 year prior to acuity testing. Because lesions targeting EWM invariably resulted in damage to the adjoining lateral part of the Edinger-Westphal nucleus (EWL), which controls pupillary constriction and accommodation, two additional control groups were studied. In one such control group, bilateral lesions in the area pretectalis (AP), which innervates the pupillary control part of EWL and thereby controls pupillary constriction, were made and the effects on visual acuity determined about 1 year later. In the second such control group, the effects of acute accommodative and pupillary dysfunction on acuity were studied in pigeons made cycloplegic. The accuracy of all lesions was later confirmed histologically. The mean acuities of birds with AP lesions (9.1+/-1.4 cycles/deg) and sham lesions (7.1+/-1.5 cycles/deg) were not significantly different from normal, based on published normative data on pigeons. In contrast, pigeons with lesions that completely destroyed EW bilaterally showed visual acuity (2.7+/-0.1 cycles/deg) that was well below the acuity of the sham and AP-lesion control groups. The acuity of the cycloplegic pigeons (4.8+/-0.3 cycles/deg) and one pigeon with a nearly complete bilateral EWL but a unilateral EWM lesion (6.4 cycles/deg) indicated that only about half of the loss with a bilateral EW lesion could be attributed to accommodative dysfunction. Thus, bilateral destruction of EWM appears to have led to a loss in visual acuity. This conclusion suggests that disruption of adaptive neural regulation of ChBF may impair visual function. Destruction of EWM was, however, associated with damage to the somatic components of the oculomotor and trochlear nuclei. The possibility cannot be excluded that such damage also contributed to the acuity loss.

Contrast sensitivity (CS) is often used to assess spatial and temporal vision in animals. Conventional behavioral psychophysical techniques are both time and labor intensive, whereas measurement of CS functions by means of the pattern electroretinogram (PERG) is considerably more rapid and efficient. Are the two methods comparable, however? To answer this question, contrast-sensitivity functions were obtained using both the PERG and behavioral psychophysics in the same subjects, which were White Carneaux pigeons. The stimuli, in both methods, were phase-reversing, contrast-modulated sweeps of sinusoidal gratings. The PERG-CS functions were recorded via corneal electrodes and the behavioral data were collected using a modified staircase method that used moderate food deprivation and food reward. The results indicated that the PERG-CS functions had comparable bandwidth and peak spatial frequency to the behavioral CS functions. The PERG-CS functions, however, were lower on average than the behavioral curves by about 54%. The visual acuity of the two methods, as estimated from the high-frequency cutoff of the CS functions, differed by 37%. Both of these values are roughly consistent with the square root of 2 advantage of binocular viewing (behavioral method) over monocular viewing (PERG method). In addition, the peak spatial frequency showed a decrease of 0.125 c/deg with the PERG method and bandwidth was reduced by approximately 0.5 octave. These findings suggest that the PERG is an acceptable alternative to behavioral measurement of CS functions, especially in animal psychophysics, if one takes into account the underestimation of CS by the PERG method and the small changes in peak spatial frequency and bandwidth.