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1 Neurobiology, Northeastern Ohio Universities College of Medicine, Rootstown, OH, USA; Neuroscience Program, School of Biomedical Sciences, Kent State University, Kent, OH, USA
2 Neurobiology, Northeastern Ohio Universities College of Medicine, Rootstown, OH, USA
* To whom correspondence should be addressed. E-mail: jjw{at}neoucom.edu.
In the cochlear nucleus (CN) of awake mustached bats, single and two-tone stimuli were used to examine how responses in major CN subdivisions contribute to spectro-temporal integrative features in the inferior colliculus (IC). Across CN subdivisions, the proportional representation of frequencies differed. A striking result was the substantial number of units tuned to frequencies below 23 kHz. Across frequency bands, temporal response patterns, latency, and spontaneous discharge differed. For example, the 23-30 kHz representation, which analyzes the fundamental of the sonar call, had an unusually high proportion of units with onset responses (39%) and low spontaneous rates (53%). Units tuned to 58-59 kHz, corresponding to the sharply tuned cochlear resonance, had slightly but significantly longer latencies than other bands. In units tuned to frequencies above 30 kHz, 31% displayed a secondary excitatory peak, usually between 10 and 22 kHz. The secondary peak may originate in cochlear mechanisms for some units, but in others it may result from convergent input onto CN neurons. In 20% of units tested with two-tone stimuli, suppression of BF responses was tuned at least an octave below BF. These properties may underlie similar IC responses. However, other forms of spectral interaction present in IC were absent in CN: we found no facilitatory combination-sensitive interactions, and very few combination-sensitive inhibitory interactions of the dominant IC type, in which inhibition was tuned to 23-30 kHz. Such interactions arise above CN. Distinct forms of spectral integration thus originate at different levels of the ascending auditory pathway.
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