Seasonal change of contrast detection in the toad'sBufo bufo (L.) visual system
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- Ewert, J.P. & Siefert, G. J. Comp. Physiol. (1974) 94: 177. doi:10.1007/BF00611865
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During the summer months, toad's prey-catching activity released by white squares moving in front of a black background was much more readily triggered than with black objects of the same size moving before a white background (Fig. 1A). In the fall and winter months, this preference was reversed: black in front of white then attracted relatively more than white in front of black (Fig. 1B).
The response of retinal class II ganglion cells to corresponding objects moved through the center of their receptive fields showed similar changes in the black/white sensitivity. It was influenced by the following factors: (i) the location of the receptive field; (ii) the angular size of the stimulus; (iii) the time of year.
During the summer, neurons with receptive fields in the lower visual field were more strongly activated by square white objects (1–4° in size) than by black ones, provided the value of the stimulus background contrast remained constant (Fig. 2C). Conversely, with stimuli of 4–16°, neurons with receptive fields in the upper visual field were more strongly activated by black than by white (Fig. 2A).
During the fall and winter months, this relationship is partially reversed (Fig. 2; see also Fig. 7). At this time the response of class II neurons with ventral receptive fields was maximal to black objects (4–16° in size). Neurons with dorsal receptive fields, however, were activated maximal by white patterns (1–4° in size).
The cause for this seasonal dependency of contrast-detection of a stimulus could be based on efferent influence of the “on”- and “off”-zones in the receptive field. The question of biological significance admits only speculation at this time. Possibly these phenomena are involved in determining “search patterns” for the capture of prey.