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C. Haldin, A.–C. Aho, K. Donner, A. Koskelainen; Temporal summation: blessing or curse for the hunting toad . Invest. Ophthalmol. Vis. Sci. 2004;45(13):1357.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose: To determine to what extent the visual summation time of toads hunting for moving prey at different temperatures in very dim light reflects the time scale of rod photoresponses, and to study benefits and costs from extending temporal summation with concomitant slowing down of vision. Methods: The time scale of toad vision was modulated by changing temperature (natural for poikilothermic animals, e.g. toads). The effect of temperature on rod photoresponses was studied by ERG recording across the isolated toad retina. The rod integration times thus obtained were compared with summation times measured behaviourally at 15 and 25°C. The latter measurements were based on the prey–catching behaviour: worm–dummies were moved at different velocities (allowing different degrees of temporal summation) and the limit where worm velocity ceased to affect the visual threshold was determined. Threshold in each condition was the light intensity where 50% of the toads still snapped at the moving worm dummy. For comparison, the effect of temperature on the speed of running woodlice (a common prey item of toads) was also determined. Results: When temperature decreased from 25°C to 15°C the time to peak of the photoreceptor response to weak stimuli more than doubled, from 0.8 to 2.2 seconds. In the behavioural experiments, slow worm–dummies were detected in much dimmer light than faster worms at 15 °C, but not at 25 °C, indicating prolonged summation times at low temperature. The summation times derived from such data were ca. 0.9 s at 25 °C and 2.9 s at 15 °C. The improvement in absolute sensitivity to slow targets at low temperature was accompanied by an increasing displacement of the snap towards and finally behind the rear end of the worm. Real woodlice also slowed down with cooling, but somewhat less than toad vision. Conclusions: The deceleration of rod photoresponses with decreasing temperature essentially explains the increase in behavioural summation times. This was coupled to slower vision, evident as a loss in the accuracy of snapping for moving targets. Thus temporal summation as well as the speed of vision near absolute threshold appeared to be limited by the same primary mechanism, i.e., phototransduction in retinal rods. At least with woodlice that do not slow down with cooling to the same extent, the cost (decreased accuracy of snapping) finally outweighs the benefit (increased sensitivity) for the hunting toad.
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