Associative learning modifies startle reflexes at the lateral lemniscus.

Acoustic and electrical brain stimulation studies have revealed that the ventral nucleus of the lateral lemniscus is a specific site within the brain stem where a previously conditioned stimulus modulates a simple reflex, the acoustic startle response. Sixty rats were implanted with bilateral electrodes in the ventral cochlear nucleus (VCN), ventral acoustic stria (VAS), dorsal lateral lemniscus (DLL), ventral lateral lemniscus (VLL), or the nucleus reticularis pontis caudalis (RPC). Following recovery all rats were conditioned to be fearful of a light by pairing a light with a shock for 10 trials on each of 2 days. One day later, the rats were placed in cages equipped to measure startle responses. Startle was elicited either acoustically or electrically through the electrodes that had been implanted in various parts of the acoustic startle circuit. Startle was elicited in darkness or during a brief presentation of the ligh previously paired with the shock. In all groups, acoustic startle amplitude was significantly greater in the presence of the light than it was in darkness, which is consistent with previous data showing that fear increases startle. Startle elicited electrically from the VCN, VAS, and VLL was also significantly increased by the light. In contrast, startle elicited electrically in the DLL or the RPC was not affected by the light despite the fact that the same rats in the same test session had elevated acoustic startle amplitude in the presence of light. Thus, it seems that for the first time in a complex vertebrate, a locus has been found within the nervous system (the VLL) where a conditioned stimulus acts to alter neural transmission so as to affect behavior.