In the mustached bat's inferior colliculus (IC), combination-sensitive neurons display time-sensitive facilitatory interactions between inputs tuned to distinct spectral elements in sonar or social vocalizations. Here we compare roles of ionotropic receptors to glutamate (iGluRs), glycine (GlyRs), and GABA (GABA A Rs) in facilitatory combination-sensitive interactions. Facilitatory responses to 36 single IC neurons were recorded before, during, and after local application of antagonists to these receptors. The NMDA receptor antagonist CPP [(Ϯ)-3-(2-carboxypiperazin-4-yl)-propyl-1-phosphonic acid], alone (n ϭ 14) or combined with AMPA receptor antagonist NBQX (n ϭ 22), significantly reduced or eliminated responses to best frequency (BF) sounds across a broad range of sound levels, but did not eliminate combination-sensitive facilitation. In a subset of neurons, GABA A R blockers bicuculline or gabazine were applied in addition to iGluR blockers. GABA A R blockers did not "uncover" residual iGluR-mediated excitation, and only rarely eliminated facilitation. In nearly all neurons for which the GlyR antagonist strychnine was applied in addition to iGluR blockade (22 of 23 neurons, with or without GABA A R blockade), facilitatory interactions were eliminated. Thus, neither glutamate nor GABA neurotransmission are required for facilitatory combination-sensitive interactions in IC. Instead, facilitation may depend entirely on glycinergic inputs that are presumed to be inhibitory. We propose that glycinergic inputs tuned to two distinct spectral elements in vocal signals each activate postinhibitory rebound excitation. When rebound excitations from two spectral elements coincide, the neuron discharges. Excitation from glutamatergic inputs, tuned to the BF of the neuron, is superimposed onto this facilitatory interaction, presumably mediating responses to a broader range of acoustic signals.

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Peterson DC, Voytenko S, Gans D, Galazyuk A, Wenstrup J. Intracellular recordings from combination-sensitive neurons in the inferior colliculus. J Neurophysiol 100: 629-645, 2008. First published May 21, 2008 doi:10.1152/jn.90390.2008. In vertebrate auditory systems, specialized combination-sensitive neurons analyze complex vocal signals by integrating information across multiple frequency bands. We studied combinationsensitive interactions in neurons of the inferior colliculus (IC) of awake mustached bats, using intracellular somatic recording with sharp electrodes. Facilitated combinatorial neurons are coincidence detectors, showing maximum facilitation when excitation from low-and high-frequency stimuli coincide. Previous work showed that facilitatory interactions originate in the IC, require both low and high frequency-tuned glycinergic inputs, and are independent of glutamatergic inputs. These results suggest that glycinergic inputs evoke facilitation through either postinhibitory rebound or direct depolarizing mechanisms. However, in 35 of 36 facilitated neurons, we observed no evidence of low frequency-evoked transient hyperpolarization or depolarization that was closely related to response facilitation. Furthermore, we observed no evidence of shunting inhibition that might conceal inhibitory inputs. Since these facilitatory interactions originate in IC neurons, the results suggest that inputs underlying facilitation are electrically segregated from the soma. We also recorded inhibitory combinatorial interactions, in which low frequency sounds suppress responses to higher frequency signals. In 43% of 118 neurons, we observed low frequency-evoked hyperpolarizations associated with combinatorial inhibition. For these neurons, we conclude that low frequency-tuned inhibitory inputs terminate on neurons primarily excited by high-frequency signals; these inhibitory inputs may create or enhance inhibitory combinatorial interactions. In the remainder of inhibited combinatorial neurons (57%), we observed no evidence of low frequency-evoked hyperpolarizations, consistent with observations that inhibitory combinatorial responses may originate in lateral lemniscal nuclei.

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 spectrotemporal 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 <23 kHz. Across frequency bands, temporal response patterns, latency, and spontaneous discharge differed. For example, the 23- to 30-kHz representation, which comprises 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 >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 best frequency (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.

This report examines temporal features of facilitation and suppression that underlie spectrally integrative responses to complex vocal signals. Auditory responses were recorded from 160 neurons in the inferior colliculus (IC) of awake mustached bats. Sixty-two neurons showed combination-sensitive facilitation: responses to best frequency (BF) signals were facilitated by well-timed signals at least an octave lower in frequency, in the range 16-31 kHz. Temporal features and strength of facilitation were generally unaffected by changes in duration of facilitating signals from 4 to 31 ms. Changes in stimulus rise time from 0.5 to 5.0 ms had little effect on facilitatory strength. These results suggest that low frequency facilitating inputs to high BF neurons have phasic-on temporal patterns and are responsive to stimulus rise times over the tested range. We also recorded from 98 neurons showing low-frequency (11-32 kHz) suppression of higher BF responses. Effects of changing duration were related to the frequency of suppressive signals. Signals<23 kHz usually evoked suppression sustained throughout signal duration. This and other features of such suppression are consistent with a cochlear origin that results in masking of responses to higher, near-BF signal frequencies. Signals in the 23- to 30-kHz range-frequencies in the first sonar harmonic-generally evoked phasic suppression of BF responses. This may result from neural inhibitory interactions within and below IC. In many neurons, we observed two or more forms of the spectral interactions described here. Thus IC neurons display temporally and spectrally complex responses to sound that result from multiple spectral interactions at different levels of the ascending auditory pathway.

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The purpose of the present study was to develop a protocol for use in testing the hearing of profoundly involved multi-handicapped children. Behavioral observation audiometry, visual reinforcement audiometry, auditory brain stem response (ABR], and noisetonedifference tests were administered to 156 children with multiple handicaps. Eighty three percent of the children with normal middle ear function were estimated to haw normal heap ing or, a t worst, a mild hearing loss, by one or more of the tests. ABR was the single best test, followed by behavioral observation audiometry and noisetonedifference. However, only 65% of the children would have passed had they only been t e s l ed Hlith ABR. If agreement among two of the tests had been required, hearing loss would have been ruled out in only 34% of the children. A factor analysis revealed that the various hearing tests were associated with diffeFnt mudiial and physical conditions which might influence threshold estimation for multi-handicapped children. These results support use of a seriea=positii test battery proto= col in which individual tests are used sequentially with the purpose of ruling out moderate-profound sensorineural hearing loss.

ABR waveforms from 50 multi-handicapped children were analysed by nine judges in an investigation of scoring bias. Prior to estimating threshold for each subject, the judges were provided with either true or false ABR thresholds. This preliminary information was true in 25 of the cases and false in the other 25. The same ABR waveforms were evaluated one week later, but true/false biasing thresholds were reversed. Results revealed that while the more experienced judges were more accurate in their assessments, they were not necessarily less biased. The effects of bias were more predominant in those cases where the estimation of threshold was the most difficult.