Representation of species-specific vocalizations in the inferior colliculus of the guinea pig.

The responses of individual neurons to 4 typical guinea pig vocalization calls (purr, chutter, chirp, and whistle) were recorded in the inferior colliculus (IC) of anesthetized guinea pigs. All calls elicited a response in about 80% of units. Unit selectivity for individual calls was low, given that a majority of neurons (55% of 124 units) responded to all vocalizations and only a small portion of neurons (3%) responded to only one call or did not respond to any of the calls (3%). In 15% of units, the response to one call was > or =25% stronger than the response to any other sound (tone, noise, and other calls); these neurons were selective for chirp or whistle, and no unit preferred chutter or purr. Neuronal activity provided information about the spectrotemporal patterns of the calls. Peristimulus time histograms (PSTHs) reflected the energy of the near-characteristic frequency band, and the population PSTH reliably matched the sound envelope for calls characterized by one or more short impulses (chirp, purr, and chutter) but did not exactly fit the envelope for whistle--a slow-modulated and relatively long call. Calculations based on firing rates indicated the approximate positions of the main spectral peaks but did not always reflect their relative magnitude. The time-reversed version of whistle elicited on average a weaker response than did the natural whistle (by 24%), but there were neurons with a significantly stronger response to the natural ("forward-selective," 30%) as well as to the time-reversed whistle ("reverse-selective," 15%). This study does not prove the existence of units selectively responding to animal calls, but it provides evidence for the encoding of the spectrotemporal acoustic patterns of vocalizations by IC units.