Department of Psychology
University of Washington
The auditory brainstem of echolocating bats contains a number of different parallel pathways. One pathway that is especially prominent comprises the monaural nuclei of the lateral lemniscus (NLL), a system of cell groups that receive input from the contralateral ear and send dense projections throughout the inferior colliculus (IC), the main midbrain auditory center. One population of neurons in the NLL provide a precisely timed marker for the onset of sound and respond selectively to downward frequency modulated (FM) signals similar to those used by bats during echolocation. These neurons are unresponsive to amplitude modulations (AM). Another population of NLL neurons respond throughout the time a signal is within their response area, and their discharges follow the envelope of AM signals.
Many neurons in the IC are tuned to biologically important parameters of sound such as signal amplitude, signal duration, FM sweep direction, and the rate of periodic FM or AM. Our laboratory has used several different approaches to investigate the mechanisms that create specialized tuning of IC neurons to sound parameters that are important for echolocation. These approaches include neuropharmacological blocking of inhibitory input to IC neurons, in vivo whole-cell patch clamp recording to examine excitatory and inhibitory postsynaptic currents at IC cells, and reversible inactivation of the different cell groups of the NLL while recording responses of IC neurons in tonotopically matched areas. The resulting data show that the selectivity of IC neurons for a number of biologically important parameters of sound is determined by the temporal relationships between at least two convergent inputs. The inputs may be excitatory or inhibitory, with different latencies and different time-courses. The NLL provide inputs that help shape IC neurons' amplitude tuning, duration tuning, response latencies, and tuning to periodic frequency modulations. Using what we know about the temporal dynamics of the excitatory and inhibitory inputs to IC cells, we are now able to construct simple models of the neural circuitry that could produce tuning to each stimulus parameter.
Research supported by NIH grants DC-00607 and DC-00287, and NSF grants IBN-9210299 and IBN-9511362.