Dr. George Pollak

2- We use Mexican free-tailed bats in our experiments. These are highly social mammals that employ a rich repertoire of spectrally and temporally complex signals for both echolocation and social communication. The communication calls convey information for a wide variety of social interactions, including mother-infant interactions, courtship, agonistic encounters, and territoriality.

3- Our studies focus on the inferior colliculus, the midbrain auditory nucleus. The inferior colliculus processes and integrates almost all ascending information from lower centers, and it provides almost all ascending auditory information to higher regions in the thalamus and cortex. It is thus the nexus of the auditory system.

4- Inferior colliculus neurons respond to species-specific calls with selectively and diversity. Inferior colliculus neurons are highly selective in that they respond only to some species-specific signals but not to others, even though the signals they fail to respond to have energy that encroaches upon the their excitatory response regions. They are diverse because each neuron responds to a different set of signals, where some neurons respond only to 1-2 calls, others to 3-4 calls and yet others respond to most or all the calls we present.

5- Response selectivity and diversity are shaped by inhibition. The reason that inferior collicular neurons respond to only to some species-specific signals and not others is due to the inhibitory innervation they receive from lower nuclei. When drugs that block inhibition are applied to IC cells, they become unselective and respond to almost all the species-specific signals we present.

6- Each call is encoded uniquely by populations of neurons in the inferior colliculus, due to response selectivity and diversity. Before inhibition is blocked, even calls that have similar acoustic features evoke different responses among the population of the inferior colliculus. Because they are selective, many neurons fail to respond to some signals but respond to others, thereby creating differences in the population responses to calls. Blocking inhibition greatly reduces selectivity and allows almost all neurons to respond to each call.

7- The nuclei of the lateral lemniscus provide most of the inhibitory projections to the inferior colliculus. The vast majority of inhibitory projections come from a group called the nuclei of the lateral lemniscus. The dorsal nucleus of the lateral lemniscus (DNLL) sends a strong input that utilizes GABA exclusively as the inhibitory transmitter. The projection from the columnar portion of the ventral nucleus of the lateral lemniscus (VNLLc) uses glycine as its inhibitory transmitter. Finally, the projections from the multipolar portion of the ventral nucleus (VNLLm) use both GABA and glycine.