Processing of polarized light by squid photoreceptors

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Abstract
Behavioural tests1–4 have demonstrated that cephalopods can discriminate light polarized in different planes, and the receptors have been localized by electrophysiological studies of the eye5–10. Discrimination of the plane of polarization is a consequence of both the structure of the microvilli in the outer segments of the photoreceptors11 and the orientation of the photosensitive chromophore on these membranes2,12,13. However, between the depolarizing receptor response resulting from photoreception and the behaviour of the animal, nothing is known about neuronal processing of polarized light by cephalopods. Here we show that some squid photoreceptors discriminate the plane of polarization within the spike train, and that any particular plane is seen as a variable intensity. Given the well known orthogonal orientation of microvilli in outer segments of adjacent photoreceptors and the physiological preference for one of two mutually perpendicular planes of polarization by single photoreceptors, we conclude that cephalopod vision is based on two complementary views of the world, each determined by the transformation of polarization-sensitive receptors into complementary intensity scales. A visual system based on this transformation would lead to enhanced contrast underwater and visualization of object details obscured by confounding highlights.