Polarization analysis in the crayfish visual system

It is proposed that polarization sensitivity at the most peripheral stages of the crayfish visual system (lamina ganglionaris and medulla externa) is used to enhance contrast and thus may contribute to motion detection in low contrast environments. The four classes of visual interneurons that exhibi t polarization sensitivity (lamina monopolar cells, tangential cells, susta ining fibers and dimming fibers) are not sensitive exclusively to polarized light but also respond to unpolarized contrast stimuli. Furthermore, many of these cells and the sustaining fibers in particular exhibit a greater di fferential e-vector responsiveness to a changing e-vector than to e-vector variations among steady-state stimuli. While all four cell types respond mo destly to light flashes at an e-vector of 90 degrees to the preferred orien tation, the dynamic response to a changing e-vector is small or absent at t his orientation. Because the sustaining fibers exhibit polarization sensiti vity, and they provide afferent input to a subset of optomotor neurons, the latter were also tested for polarization sensitivity. The optomotor neuron s involved in compensatory reflexes for body pitch were differentially sens itive to the e-vector angle of a flash of light, with maximum responses for e-vectors near the vertical. The motor neurons also exhibited a maximum re sponse near the vertical e-vector to a continuously rotating polarizer. Two scenarios are described in which the sensitivity to a changing e-vector ca n produce motion responses in the absence of intensity contrast.