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Qing Shi, Yves Sauve, William K. Stell; Rod-Mediated Visual Function in the Cone-Dominated Retina of the Chick. Invest. Ophthalmol. Vis. Sci. 2012;53(14):2451.
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In the vertebrate retina, vision at low light levels is mediated by rods, which are intrinsically more sensitive than cones. Dark-adaptation and post-receptoral mechanisms further extend the sensitivity of rod systems by summation, increasing gain and reducing noise. Optokinetic responses (OKR) in rod-rich mammals, such as mouse, reveal pure rod function at I (mean luminance) = 10-6 to 10-4 cd/m2, mixed rod-cone function at I = 10-4 to 10-2 cd/m2, and pure cone function at I > 10-2 cd/m2 (Umino et al. 2006 J Neurosci). In the chicken retina, rods comprise only 12-25% of all photoreceptors, and rod-dependent ERGs require testing at night (Schaeffel et al. 1991 Vis Neurosci; Manglapus et al. 1998 J Neurosci). In this study, we tested whether the rod-mediated OKR in chick differs from that in rod-dominated mammals.
White Leghorn cockerels, on a 12:12 hr light cycle (on @ 07:00), were tested at P5-P14. Contrast thresholds for OKR at various spatial frequencies (SF) and velocities (V) were measured using OptoMotryR. Unattenuated mean luminance, I = 55 cd/m2 (1.74 log cd/m2), was reduced in 0.9 log steps with ND filters (Lee). Flash ERGs (Diagnosys LLCR) were recorded from anesthetized animals. OKR and ERG were obtained in day (10AM-6PM) and night (9PM-3AM) after 2 hr dark-adaptation.
OKR: (1) In chick, as in mouse (Umino et al.), peak CS (CSmax) shifted downward and to lower SF as mean I was reduced. CSmax of the threshold OKR at night was lower (8.1±1.92) and occurred at lower SF (0.32 c/d) than CSmax of the OKR at full luminance in daytime (22.4±4.07 @ 0.50 c/d). (2) In daytime, the lowest I that elicited an OKR (Ithr) was -1.86 log cd/m2, but at night, Ithr = -2.76 log cd/m2.ERG: Flash ERG of dark-adapted chicks confirmed that the range of purely rod-driven activity extends <1 log I below the cone-system threshold.
(1) In dark-adapted chicks, a dim-light nighttime OKR can be detected at mean luminance ~1 log unit below the threshold for a dim-light daytime OKR. Rod-mediated function expands the range of vision at low luminance very little in the cone-dominated chick retina, compared to the rod-rich mouse or human retina.(2) Threshold luminance for the chick OKR lies within the cone-only, photopic range of the mouse OKR. This suggests that the operating range of luminances for cone-mediated vision may be substantially higher in chick than in mouse or human.
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