2-CHANNEL POLARIZATION ANALYZER IN THE SUSTAINING FIBER DIMMING FIBERENSEMBLE OF CRAYFISH VISUAL-SYSTEM

Polarization sensitivity (PS) was examined in two classes of neurons, sustaining fibers and dimming fibers, in the medulla externa (second o ptic neuropile) of the crayfish, Pacifasticus leniusculus. Visual resp onses were recorded intracellularly and extracellularly. The influence of e-vector orientation (theta) was probed in steady-state responses, with brief flashes and with a rotating polarizer. The results indicat e that the entire sustaining fiber population appears to be maximally sensitive to vertically polarized light. Although the evidence is less complete for dimming fibers, they appear to be maximally inhibited by vertically polarized light and excited by horizontally polarized Ligh t. Thus the sustaining fibers and dimming fibers form a two-channel po larization analyzer that captures the main features of the polarizatio n system established in photoreceptors and lamina monopolar cells. The available evidence suggests that this two-channel system has the same characteristics across most or all of the retinula. Lateral inhibitio n in sustaining fibers is differentially sensitive to theta. Inhibitio n is substantial at theta = 90 degrees (horizontal) and essentially ab sent at theta = 0 degrees. The details of the sustaining fiber polariz ation response closely follow features established in more peripheral neurons, including the magnitude of PS, enhanced responsiveness to a c hanging e-vector, and modest directionality to a changing e-vector in similar to 40% of the cells.