<p>Neurons throughout the auditory pathway respond selectively to the frequency and amplitude of sound. In the auditory midbrain there exists a class of neurons that are also selective to the duration of sound. These duration-tuned neurons (DTNs) provide a potential neural mechanism underlying temporal processing in the central nervous system. Temporal processing is necessary for human speech, discriminating species-specific acoustic signals as well as echolocation. This dissertation aims to explore the role and underlying mechanisms of DTNs through single-unit in vivo electrophysiological recordings in the auditory midbrain of the big brown bat. The durations that DTNs are selective to in echolocating and non-echolocating species are first compared to the durations of each species vocalizations. This comparison reveals that the durations DTNs respond best to correlates to the durations of echolocation calls in echolocating species and to species-specific communication calls in non-echolocating species. The ability of DTNs in the bat to respond to stimulus parameters thought to be important for echolocation processing, such as pairs of pulses and binaural sound localization cues, are subsequently tested. The responses of DTNs to a paired tone spike suppressing paradigm presented monaurally and binaurally are also compared to characterize the role each ear plays in recruiting inhibition known to be involved in duration tuning. The results show that DTNs are able to respond to pairs of pulses at a timescale relevant to bat echolocation, and a majority also responded selectively to binaural sound localizing cues. Nearly half (48%) of DTNs did not show spike suppression to an ipsilaterally presented suppressing tone. When ipsilaterally evoked spike suppression occurred, the effect was significantly smaller than the suppression evoked by a contralateral suppressing tone. These findings provide evidence that DTNs may play a role in echolocation in bats as DTNs are able to respond to the outgoing pulse and returning echoes and localize the echo source and that the neural mechanism underlying duration tuning is monaural in nature.</p> / Doctor of Philosophy (PhD)

Duration tuning in the mammalian inferior colliculus (IC) is created by the interaction of excitatory and inhibitory synaptic inputs. We used extracellular recording and paired-tone stimulation to measure the strength and time-course of the contralateral inhibition and offset-evoked excitation underlying duration-tuned neurons (DTNs) in the IC of the awake bat. The onset time of a short, best duration (BD), excitatory probe tone was varied relative to the onset of a longer-duration, non-excitatory (NE) suppressor tone. Spikes evoked by the roving BD tone were suppressed or facilitated when the stationary NE tone was varied in frequency or amplitude. When the NE tone frequency was presented away from the cell’s best excitatory frequency (BEF) or at lower SPLs, the onset of inhibition was relatively constant whereas the offset and duration of inhibition decreased. Excitatory and inhibitory frequency response areas were measured and best inhibitory frequencies matched best excitatory frequencies; however, inhibitory bandwidths were broader than excitatory bandwidths. Excitatory rate-level and inhibitory suppression-level functions were also measured and the dynamic ranges and inflection points were similar, which is hypothesized to play a role in the level tolerance of responses measured from DTNs. We compared the latency of offset-locked facilitation to the onset or offset of inhibition as a function of frequency and amplitude; we found that the facilitation was more related to the onset of inhibition. Moreover, facilitation typically preceded the offset of inhibition – suggesting that it is a separate excitatory input to DTNs and not a rebound from inhibition. We conclude that DTNs receive inputs that generate and preserve temporal selectivity. / Dissertation / Doctor of Philosophy (PhD)