Differences in the optical properties of vertebrate photoreceptor classes leading to axial polarization sensitivity.

Polarization microspectrophotometry recordings were made to investigate possible differences in the way different spectral classes of photoreceptors from coho salmon (Oncorhynchus kisutch) absorb linearly polarized light. The results strongly suggest that rods and cones absorb transversely illuminating polarized light differently. Cones were found to exhibit a tilted optical geometry in which the maximum absorbance occurred when the E-vector was at a small angle to the transverse axis of the outer segment. Solutions to Maxwell's equations were deduced to investigate the effect of this tilt under conditions of axial illumination. Calculations show an approximate 10% difference in the absorbance of orthogonal polarizations, suggesting the possibility of axial dichroism in the cones of this species.