Scopolamine reduces sensitivity to auditory gaps in the rat, suggesting a cholinergic contribution to temporal acuity

Prior research [Caine et al., 1981] suggested that scopolamine, a central cholinergic antagonist, may increase gap thresholds in young human listeners. If confirmed, an effect of scopolamine on gap detection might help to explain why both aged humans and aged laboratory animals have less sensitive temporal acuity on gap detection tests, as they may be presumed to have less effective cholinergic mechanisms. Here we measured the effect of scopolamine on gap detection in rats (n=8) using reflex modification audiometry, which depends on the fact that brief gaps in noise presented immediately prior to a loud noise inhibit the acoustic startle reflex. Scopolamine increased the gap threshold and reduced reflex inhibition produced by gaps that were presented at and beyond about 40 ms prior to the startle reflex, but not at shorter lead times. A peripheral antagonist had no effect at long lead times. These data indicate that central cholinergic mechanisms are involved in relatively high level perceptual processing of gaps. This conclusion is consistent with the hypothesis that temporal acuity may be compromised in the aged listener because of deficits in the efficacy of these central mechanisms.