Population vector coding by the giant interneurons of the cockroach

We tested two alternative models of integration among the cockroach giant i nterneurons (GIs) for determining the directions of wind-evoked escape turn s. One model, called steering wheel, pits contralateral GIs against one ano ther; the other, called population vector model, involves a vector computat ion among the GIs. In testing each model theoretically, the population vect or was found to account far better for the actual behavior. Both models cou ld account for the results of previous behavioral-physiological experiments in which spikes had been added to the right GI3 together with wind stimuli from the right side. The two models revealed a critical behavioral-physiol ogical experimental test that we then performed; namely, when delivering wi nd from the right side, adding spikes experimentally to the right GI2 shoul d increase turn size according to the steering wheel model but should decre ase turn size according to the population vector model. The latter result w as obtained. The population vector, but not the steering wheel, model also could account for previous behavioral-physiological experiments in which sp ikes were added experimentally to a GI contralateral to the wind stimuli. T he results support the population vector model as accounting for direction determination among the cockroach GIs.