CRITICAL PARAMETERS OF THE SPIKE TRAINS IN A CELL ASSEMBLY - CODING OF TURN DIRECTION BY THE GIANT INTERNEURONS OF THE COCKROACH

Cockroaches (Periplaneta americana) respond to air displacement produc ed by an approaching predator by turning and running away. A set of 4 bilateral pairs of ventral giant interneurons is important in determin ing turn direction. Wind from a given side is known to produce more sp ikes, an earlier onset of the spike trains, and different fine tempora l patterning, in the ipsilateral vs the contralateral set of these int erneurons. Here we investigate which of these spike train parameters t he cockroach actually uses to determine the direction it will turn. We delivered controlled wind puffs from the right front, together with i ntracellular injection of spike trains in a left ventral giant interne uron, under conditions where the animal could make normally directed t urning movements of the legs and body. In trials where our stimuli cau sed the left side to give both the first spike and more total spikes t han the right, but where our injected spike train included none of the normal fine temporal patterning, 92% of the evoked turns were to the right-opposite of normal (Figs. 4-6). In trials where the left side ga ve the first spike, but the right side gave more spikes, 100% of the t urns were to the left-the normal direction (Figs. 8, 9). Comparable re sults were obtained when each of the left giant interneurons 1, 2 or 3 were electrically stimulated, and when either weak or stronger wind p uffs were used. Stimulating a left giant interneuron electrically in t he absence of a wind puff evoked an escape-like turn on 9% of the tria ls, and these were all to the right (Fig. 9). These results indicate t hat fine temporal patterning in the spike trains is not necessary, and information about which side gives the first spike is not sufficient, to determine turn direction. Rather, the key parameter appears to be relative numbers of action potentials in the left vs the right group o f cells. These conclusions were supported by similar experiments in wh ich extracellular stimulation of several left giant interneurons was p aired with right wind (Figs. 11, 12).