Theory and Measurement of Visual Mechanisms X. Modifications 0]~ the Flicker Response Contour, and the Significance of the Avian Pecten by W. J. Crozier and Ernst Wolf

The retina of the zebra finch Taeniopygia castinotis (Gould) contains visual ceils histologically of one general type only (cones), although some of these are differentiated by oil droplets; some are "double" cones; the retina has a well developed fovea. In keeping with the subsequently observed simplex character of the primary photoreceptor population, the flicker response contour (F vs. log I), with white light and light-time cycle fraction tL = 0.50, was found to be a single symmetrical probability integral (Crozier and Woff, 1940-41 b). This was confirmed by a number of additional experiments with colored lights (Crozier and Wolf, 1941-42 d). The eye of Taeniopygia contains, however, a fairly large and complex pigmented pecten. The position of this organ (cf. Krause, 1922, Fig. 88) is such that it, or rather the presence of its serrated shadow on the retina, may well be expected to have an effect upon the sensory influence of moving images, as suggested by Menner (1938), although in a recent review of this matter Wails (1942, pp. 367, 520, 648) concludes it to be unlikely that the pecten casts a shadow outside its own base or elsewhere "where it would do any good." No specific signs of an influence of the pecten could be detected in our measurements with t~ = 0.50. The symmetry of the F log I contour was not disturbed, and subsequent series of observations at tL = 0.10 with lights from different spectral regions have confirmed this (Crozier and Wolf, 1941-42 d). The very low intensity level at which the F -log [,~ curve is located, by comparison with that for other vertebrates, is sufficiently accounted for by the bird's higher temperature (Crozier and Wolf, 1940-41 b), and requires no assumption that the presence of the pecten is responsible. We were led to believe that study of the modification of the F -log I curve by systematically changing the light-time cycle fraction (tz), employing the methods already used with various animals (Crozier, Wolf, and ZermhnWolf, 1937-38 d, e; Crozier and Wolf, 1940, 1939-40 b, 1940-41 d), might reveal a distinctly unusual situation. The comparative morphology of the pecten, and the complexity of its shadow on the background of the eye in diverse birds of different habits, strongly suggests (Menner, 1938) that the effect of the pecten might be to increase the sensory action of small moving images. Con287