Abstract
BackgroundThrough adaptation, animals can function visually under an extremely broad range of light intensities. Light adaptation starts in the retina, through shifts in photoreceptor sensitivity and kinetics plus modulation of visual processing in retinal circuits. Although considerable research has been conducted on retinal adaptation in nocturnal species with rod-dominated retinas, such as the mouse, little is known about how cone-dominated avian retinas adapt to changes in mean light intensity.Methodology/Principal FindingsWe used the optokinetic response to characterize contrast sensitivity (CS) in the chick retina as a function of spatial frequency and temporal frequency at different mean light intensities. We found that: 1) daytime, cone-driven CS was tuned to spatial frequency; 2) nighttime, presumably rod-driven CS was tuned to temporal frequency and spatial frequency; 3) daytime, presumably cone-driven CS at threshold intensity was invariant with temporal and spatial frequency; and 4) daytime photopic CS was invariant with clock time.Conclusion/SignificanceLight- and dark-adaptational changes in CS were investigated comprehensively for the first time in the cone-dominated retina of an avian, diurnal species. The chick retina, like the mouse retina, adapts by using a “day/night” or “cone/rod” switch in tuning preference during changes in lighting conditions. The chick optokinetic response is an attractive model for noninvasive, behavioral studies of adaptation in retinal circuitry in health and disease.
Highlights
Vision functions over a vast range of light intensities, as much as 14 log10 units [1]
Cone-driven Contrast Sensitivity Function Photopic experiments were all performed in the daytime, from clock time 9 am to 3 pm, at 1.98 log cd/m2, contrast sensitivity (CS) was tested under a series of temporal frequencies (TFs) (TF = 0.9, 1.8, 3.6, 4.5, 6.0 cyc/sec)
Contrast sensitivity peaked at an intermediate spatial frequency (SF), ca. 0.5 cyc/deg, where CSmax = 13.262.8 in Bovan chicks (n = 8) and 19.165.2 in Lohmann chicks (n = 8)
Summary
Vision functions over a vast range of light intensities, as much as 14 log units [1]. Animals are able to do this – to see effectively over a range of light intensities, from weak starlight to brilliant sunshine – because visual sensitivity and gain can adjust automatically to ambient light intensity, optimizing visual function under widely varying conditions. This property of vision is called adaptation. Retinal circuitry changes functionally – from high sensitivity and low acuity at low intensity, to low sensitivity and high acuity at high intensity This switching between rod- and cone-driven retinal circuits, and the adjusting of the sensitivity and gain of both photoreceptor responses and post-receptoral circuits, are the main factors that make useful vision possible over such a wide range of light intensities. Considerable research has been conducted on retinal adaptation in nocturnal species with roddominated retinas, such as the mouse, little is known about how cone-dominated avian retinas adapt to changes in mean light intensity
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