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E.C. Solessio, D. Scheraga, G. Engbretson, B. Knox, R. Barlow; Circadian Rhythms in Visual Sensitivity of Xenopus laevis Tadpoles . Invest. Ophthalmol. Vis. Sci. 2003;44(13):4165.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose:Visual sensitivity of Xenopus tadpoles is higher during the day than at night. We investigated the mechanisms underlying this circadian modulation of visual sensitivity. Methods:We measured visual sensitivity of Xenopus tadpoles by observing their optomotor response to a rotating pattern of vertical black and white alternating bars of constant contrast. We varied the mean level and wavelength of illumination, and the spatiotemporal properties of the gratings. Results:During daytime hours, tadpoles (stage ~48) exhibit maximal visual sensitivity at 520 nm. Sensitivity decreased with increasing temporal frequency of the periodic stimulus along two distinct phases. The two phases appear to be generated by two visual mechanisms: (1) A "green-sensitive" mechanism that operates in the low temporal frequency range (<1.2 Hz) characteristic of rods, and (2) a less sensitive mechanism that operates in the high frequency (>1.2Hz) characteristic of cones. Changing spatial frequency of the bar patterns does not change sensitivity. Only the more sensitive "green-sensitive" mechanism exhibited a circadian rhythm. During the night, the ‘green' sensitive mechanism is sensitive to temporal frequencies below 0.35 Hz. Corresponding changes in the time course of the ERG b-wave suggest that the circadian shift originates in the retina. Conclusions:Two visual mechanisms contribute to the visual sensitivity of Xenopus tadpoles. A circadian oscillator modulates the sensitivity to temporal frequencies, enhancing signal integration and detection at night at the expense of temporal resolution.
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