Movement-sensitive, polarization-sensitive, and light-sensitive neurons of the medulla and accessory medulla of the locust, Schistocerca gregaria.

Several lines of evidence suggest that the accessory medulla of orthopteroid insects is implicated in the control of circadian rhythms. To investigate the role of this brain area in more detail, anatomical and physiological properties of accessory-medulla neurons of the locust were studied by intracellular recordings combined with Lucifer dye injections. The responses of these neurons to visual stimuli were compared with visual responses of adjacent tangential neurons of the medulla. Principal neurons of the accessory medulla showed weak tonic excitations to stationary light stimuli, but they were not sensitive to movement stimuli or to different e-vector orientations of polarized light. These neurons connected the accessory medulla to the lamina, the anterior medulla, and to several areas in the midbrain including the superior protocerebrum and the posterior optic tubercle. A second class of neurons had tangential arborizations in the medulla, a few sidebranches in the accessory medulla, and projections to the lamina or to the contralateral optic lobe. Several of these neurons were sensitive to polarized light. Finally, a third class of neurons had tangential arborizations in the medulla and axonal projections to the lobula and to the lateral protocerebrum. These neurons showed phasic responses to light and nondirectional selective responses to motion stimuli. The results show that neurons of the accessory medulla receive photic input and support an involvement of this neuropil in circadian timekeeping functions. The possible role of the accessory medulla in polarization vision is discussed.