Key words: ambient light-bisensor system -human -visual system -light field -photic orientation -residual vision Non-patterned photic stimulation of the eyes of human subjects was obtained by homogeneous illumination of the closed eyes. The subjects perceived a difference between the stimulation intensity of the left and of the right eye as a field of view consisting of a bright and a dark half, adjoined across an indeterminate vertical midline. It is not clear which parts of the visual system mediate the residual perceptual functions that survive this stimulatory reduction of man's eyes to a photic bisensor system. It is argued that animals with side-set eyes might use this perceptual ability for photic orientation in an anisotropic light field.Elaborate schemes for classifying orientational responses have been developed on the basis of the type of stimulus, the type of response, and the arrangement of the relevant receptors. For tropotaxis the organism must be provided with two sense organs (sensors, see SchOne, ref. 22) which are so arranged that they are not equally strongly stimulated in most of the positions that the organism can adopt. According to the tropism theory of Loeb [ 16] the central nervous system strikes a balance between the inputs from the two sense organs, for instance the eyes. Loeb's idea that an organism therefore moves straight towards or straight away from a light source was adopted by K0hn [14], Fraenkel and Gunn [6], and Jander: basotaxis (in ref. 13). It is doubtful, however, whether basic directions or positions actually exist (see ref. 22, pp. 68-69). From a cybernetic point of view it seems more likely that an animal selects any appropriate position or direction on the basis of a predetermined difference between the stimulation intensities of the two sense organs (bisensor system, ref. 22). The fact that an organism in which one eye is put out of action keeps turning towards one side (circus or manege movements) would result from the organism's inability to establish the ratio of stimulation intensities which corresponds to a selected orientation (open-loop condition of the control system). 0166-4328/82/0000-0000/$02.75
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