The Pars Intercerebralis Neurone PI(2)5 of Locusts: Convergent Processing of Inputs Reporting Head Movements and Deviations from Straight Flight

The pars intercerebralis neurone PI(2)5 of Locusta migratoria is a descending large-field deviation detector (DN), projecting from the brain to the fused abdominal ganglia. It responds with bursts of action potentials (1) to specific movements of an artificial horizon, simulating deviations from course (preferred deviations are roll and yaw to the side of the axon, and pitch-up), and (2) to proprioceptive input from neck receptors signalling head movements in either direction. During compensatory head movements both inputs act together so that the deficit in visual information is balanced by proprioceptive input. The number of spikes elicited by roll deviations depends on the sector of the visual vield initially covered by the horizon (sector specificity), increasing the more the horizon is oriented in the visually preferred direction. Hence, sector specificity involves information about the absolute orientation with respect to the horizon. The number of spikes depends further on the roll position of the head with respect to the prothorax. The response is amplified when the head is oriented in the visually preferred direction, and is attenuated when the head is oriented in the visually antipreferred direction. Visual and proprioceptive modulatory effects balance each other so that sector specificity relates to the body (i.e. the aerodynamic organs) even when head and body are rotated against each other. During flight, the excitability of PI(2)5 is further influenced by tonic inhibition from frontal wind (no directionality), which is overriden by tonic excitation of central origin.