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S. Nymark, C. Haldin, A.-C. Aho, A. Koskelainen, K. Donner; Temporal Integration in Dark-Adapted Toads at the Behavioral "Output" and Rod "Input" Levels of Vision. Invest. Ophthalmol. Vis. Sci. 2009;50(13):3056.
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© ARVO (1962-2015); The Authors (2016-present)
The visual sensitivity of a toad on a cool night is about 10 times higher than that of humans. Our psychophysical experiments on toads snapping at worm dummies have indicated that the capacity for temporal integration is a key variable explaining a considerable part of the variation in absolute visual sensitivity. In the present work we wanted to investigate whether the response kinetics of the rod photoreceptors can explain temporal integration at the behavioral level.
Rod photoresponses to brief flashes of 544 nm light were studied by ERG recordings across the isolated aspartate-treated toad (Bufo bufo) retinas at two temperatures, 15 and 25 oC. From these experiments rod integration time (ti) was estimated and compared with ti values of behavioral experiments.
Rod photoresponses decelerate with cooling so that there is an extension in the time scale of the entire response. Determining rod integration time from these measurements gave ti values of 4.3 ± 0.4 s at 15 oC and 1.0 ± 0.1 s at 25 oC. From behavioral experiments we had obtained ti values of 4.4 ± 1.0 s at 15 oC and < 0.9 s at 25 oC.
Our data shows a strong correlation between the integration times of rods and behavior. Particularly, a "warm" toad was unable to integrate signals in time over more than the ca. 1 s allowed by rod photoresponses, although its visual sensitivity would have been greatly improved by longer integration. The results suggest that the integration time of dark-adapted vision is limited by rod phototransduction at the input to the visual system.
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