The paradoxical conditioning effect of the human pupil might be evaluative in nature

Conditioning the human pupillary response with variations in brightness (brightness vs. darkness) as an unconditioned stimulus (UCS) has yielded contradictory findings (see Voigt, 1968). In the current study, we conditioned Landolt Rings with bright and dark monitor backgrounds (UCS). Before conditioning, the Landolt Rings were rated equally neutral and moderately arousing and resulted in comparable pupillary changes. After conditioning, the Landolt Ring conditioned with a bright background was rated more arousing and more negative and resulted in a larger pupillary dilation than the Landolt Ring conditioned with a dark background. Hence, although it is well known that pupil size decreases with increasing brightness, we found larger pupil sizes for the stimulus conditioned with a bright monitor background. The paradoxical pupil effects are discussed with regard to the contribution of the evaluative change (evaluative conditioning) in the formation of the conditioned pupillary reaction. Further, the possibility of a conditioned compensatory reaction is discussed.