Passive avoidance learning produces focal elevation of bursting activity in the chick brain: amnesia abolishes the increase

Presentation of a bright bead to day-old chicks (Gallus domesticus: Ross 1 Chunky Chicks) elicits spontaneous pecking. If the bead is coated with an aversive substance (e.g., methylanthranilate), they will avoid similar beads subsequently; if it is coated with water, they peck avidly on re-presentation. Formation of a memory for this one-trial passive avoidance task is unaffected by subconvulsive transcranial electroshock when applied 10 min after training in 60% of birds, whereas "immediate" post-training electroshock renders 63% of chicks amnesic. Memory formation and retention is associated with a large bilateral enhancement in trained over control chicks (320 and 350% in left and right hemispheres, respectively; p less than 0.001) of a particular spontaneous multi-unit activity firing pattern, that is, short-duration (15-40 ms) bursts of large-amplitude (greater than or equal to 200 microV, 450 microV max p-p), high-frequency (400-450 Hz) spiking in anesthetized chicks. This effect is observed in data lumped from 1-13 h after training and is restricted to the intermediate medial hyperstriatum ventrale. When chicks are rendered amnesic by electroshock immediately following training, there is a complete abolition of this increase in burst firing; in those chicks where this treatment fails to elicit amnesia, the increase in bursting is still observed. In birds in which the shock is delayed and memory formation occurs, the increase in bursting activity is maintained; however, if the delayed shock produces apparent amnesia, then the increase is once again abolished. The electroshock had no effect on bursting per se in untrained chicks. There was no significant difference in tonic spiking between the chicks. A marked increase in the occurrence of bursting epochs in the IMHV of anesthetized chicks following passive avoidance training is therefore closely associated with memory formation, but not with the nonspecific concomitants of the training procedure.