Tone-induced changes in excitability of abducens motoneurons and of the reflex path of nictitating membrane response in rabbit (Oryctolagus cuniculus).

Excitability of the reflex nicitating membrane (NM) response to air puff and of the abducens motoneurons (final common path) to direct electrical stimulation (measured as amplitude of the evoked NM response) in rabbit was tested at various times after onset and offset of a 350-msec tone. Excitability to air puff showed a substantial increase during tone on a gradual decrease following tone off, in agreement with Ison and Leonard. Excitability of motoneurons showed a similar marked increase during tone on a transient decrease immediately following tone off, and then a gradual decrease. It is suggested that these excitability changes, which parallel closely the interstimulus-interval conditionability function reported by Gormezano for the same preparation, may provide an independent measure of the "molar stimulus trace". Excitability of abducens motoneurons tested in the intertrial intervals during subsequent tone-air puff conditioning showed no consistent changes. Surprisingly, animals given tone-abducens nucleus shock testing developed conditioned responses. In subsequent conditioning, using standard tone-air puff training, they showed 85% savings in acquisition relative to nonstimulated controls.