Comparison of auditory responses in the medial geniculate and pontine gray of the big brown bat, Eptesicus fuscus

The inferior colliculus has been well studied for its role of transmitting information from the brainstem to the thalamocortical system. However, it is also the source of a major pathway to the cerebellum, via the pontine gray (PG). We compared auditory responses from single neurons in the medial geniculate body (MGB) and PG of the awake big brown bat. MGB neurons were selective for a variety of stimulus types whereas PG neurons only responded to pure tones or simple FM sweeps. Best frequencies (BF) in MGB ranged from 8 kHz to > 80 kHz. BFs of PG neurons were all above 20 kHz with a high proportion above 60 kHz. The mean response latency was 19 ms for MGB neurons and 11 ms for PG neurons. MGB and PG contained neurons with a variety of discharge patterns but the most striking difference was the proportion of neurons with responses that lasted longer than the stimulus duration (MGB 13%, PG 58%). Both nuclei contained duration-sensitive neurons; the majority of those in MGB were band pass whereas in the PG they were long pass. Over half of the neurons in both nuclei were binaural. Differences between these nuclei are consistent with the idea that the thalamocortical pathway performs integration over time for cognitive analysis, thereby increasing selectivity and lengthening latency, while the colliculo-pontine pathway, which is more concerned with sensory-motor control, provides rapid input and a lasting trace of an auditory event.

► Eptesicus MGB contains a complete representation of frequencies spanning the entire range of social and echolocation frequencies.
► PG appears to be specialized for high frequency processing (no responses to <20 kHz). The mean best frequency of PG neurons (46 kHz) is higher than that of MGB neurons (37 kHz).
► MGB neurons respond to more types of stimuli, including complex stimuli, than PG neurons, and have longer average first spike latencies than neurons in PG.
► A large proportion of neurons in PG have prolonged responses, which have not previously been described in PG.