A CELLULAR BASIS FOR POLARIZED-LIGHT VISION IN RAINBOW-TROUT

Polarized light sensitivity was examined in single units of the rainbo w trout (Oncorhynchus mykiss) torus semicircularis, a sub-tectal visua l area with a high degree of ultraviolet sensitivity. First, chromatic ally isolated torus units with inputs from each of the four cone mecha nisms found in the trout visual system were separately examined for e- vector sensitivity. UV ON-response units showed polarization sensitivi ty for vertically (0 degrees and 180 degrees) polarized stimuli, while ON-response units of the short, middle and long cone mechanisms were not polarization sensitive. No OFF-response units of the UV or short c one mechanism were observed, but OFF-response units of the middle and long cone mechanisms show polarization sensitivity for horizontally (9 0 degrees) polarized stimuli. Second, e-vector sensitivity was observe d in color-coded units which received inputs from more than one cone m echanism and showed different sign responses (ON or OFF) at different points of the spectral sensitivity curve. Biphasic units which had ON input from the UV cone mechanism and OFF inputs from the middle and lo ng cone mechanisms showed polarization opponency. This opponency was o bserved with a 380 nm stimulus when the threshold sensitivities of the alpha-band absorption peak of the UV mechanism and the beta-band abso rption peak of the middle and long cone mechanisms were equal. We beli eve that biphasic torus units provide a possible cellular basis for po larized light vision in rainbow trout.