Fig. 3. Experimental paradigm to test an animal’s ability to detect the angle of polarization (e-vector orientation, ) independently of radiant intensity (I) and to determine the animal’s sensitivity to different values of . In the first discrimination paradigm, the animal is trained to an unpolarized stimulus, which it later has to discriminate from linearly polarized stimuli (₁-₅) that are varied in intensity (abscissa). The resulting family of response curves (choice frequency versus logI; one for each value of ) allows one to define combinations of /I that are discriminated equally well, e.g. by 75% responses, from the unpolarized training stimulus. Hence, the animal perceives them as equally bright. If, in a second training and discrimination paradigm, the animals are able to discriminate these /I combinations from each other, this ability must be due exclusively to the stimulus differences in . In addition, the family of response/logI characteristics allows one to compute the animal’s sensitivity to various values of . This is because the sensitivity to is proportional to the reciprocal of the intensity values that elicit equal responses for all values of (see orange arrowheads and black dotted lines in the upper part of the graph). [Actually the response/logI functions have been taken from an analogous study on colour vision in fish. The latter data can be restored by replacing the unpolarized stimulus by an uncoloured (grey) stimulus, and the ₁-₅ values by ₁=461nm, ₂=555nm, ₃=434nm, ₄=599nm and ₅=719nm (Neumeyer, 1986).] Note that the rationale behind the experimental paradigm described for detecting different values of is strictly valid only for a stationary (rather than scanning) detector system. Such restrictions are not necessary in tests on colour vision.

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