Talk abstract:
Brainstem Circuits for Analyzing Temporal
Patterns
of Sound in Echolocating Bats
Ellen Covey
Department of Psychology
University of Washington
ecovey@u.washington.edu
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.
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1998-1999
Mathematics in Biology
