Uniform range of conduction times from the lateral amygdala to distributed perirhinal sites

Much data indicate that the perirhinal (PRH) cortex plays a critical role in declarative memory and that the amygdala facilitates this process under emotionally arousing conditions. However, assuming that the amygdala does so by promoting Hebbian interactions in the PRH cortex is hard to reconcile with the fact that variable distances separate amygdala neurons from their PRH projection sites. Indeed, to achieve a synchronized activation of distributed PRH sites, amygdala axons should display a uniform range of conduction times, irrespective of distance to target. To determine if amygdala axons meet this condition, we measured the antidromic response latencies of lateral amygdala (LA) neurons to electrical stimuli delivered at various rostrocaudal levels of the PRH cortex in cats anesthetized with isoflurane. Although large variations in antidromic response latencies were observed, they were unrelated to the distance between the PRH stimulation sites and LA neurons. To determine whether this result was an artifact due to current spread, two control experiments were performed. First, we examined the antidromic response latency of intrinsic PRH neurons. Although we used the same methods as in the first experiment, the antidromic response latency of PRH neurons to electrical stimuli applied in the PRH cortex increased linearly with the distance between the stimulating and recording sites. Second, we measured the antidromic response latency of PRH neurons projecting to the LA. In this pathway, we also found a statistically significant correlation between conduction times and distance to target. Thus these results support the intriguing possibility that the conduction velocity and/or trajectory of LA axons are adjusted to compensate for variations in distance between the LA and distinct rostrocaudal PRH sites. We hypothesize that because of their uniform range of conduction times to the PRH cortex, LA neurons can generate short time windows of depolarization facilitating Hebbian associations between coincident, but spatially distributed, activity patterns in the PRH cortex. In this context, the temporal scatter of conduction times in the LA to PRH pathway is conceived as a mechanism used to lengthen the period of depolarization to compensate for conduction delays within intrinsic PRH pathways. In part, this mechanism might explain how the amygdala promotes memory storage in emotionally arousing conditions.