Inferior Colliculus Neurons Research Articles

Bat inferior collicular neurons have the greatest frequency selectivity when determined with best-duration pulses

During hunting, insectivorous bats such as Eptesicus fuscus progressively increase the pulse repetition rate, shorten the pulse duration, and lower the frequency and amplitude of emitted pulses as they search, approach and finally intercept insects or negotiate obstacles. As such, analysis of an echo parameter by the bat is inevitably affected by other co-varying echo parameters. The present study examined the effect of pulse duration on frequency selectivity of neurons in the central nucleus of the inferior colliculus (IC) of the big brown bat. A family of iso-level frequency tuning curves of each IC neuron was first measured with tone bursts of different durations. The bandwidth of iso-level frequency tuning curves within each family was then compared. Our data show that most IC neurons discharge maximally to a particular pulse duration which is defined as the best duration (BDu). The iso-level frequency tuning curves of these duration-selective neurons have the narrowest bandwidth when measured with the BDu pulse than with non-BDu pulses. They also have the narrowest bandwidth when measured with a short than with a long BDu pulse. These data suggest that frequency selectivity of duration-selective IC neurons becomes sharper when short echo duration at the final phase of hunting is encoded by IC neurons that have short BDu.

Tinnitus and inferior colliculus activity in chinchillas related to three distinct patterns of cochlear trauma

A longstanding hypothesis is that tinnitus, the perception of sound without an external acoustic source, is triggered by a distinctive pattern of cochlear hair cell (HC) damage and that this subsequently leads to altered neural activity in the central auditory pathway. This hypothesis was tested by assessing behavioral evidence of tinnitus and spontaneous neural activity in the inferior colliculus (IC) after unilateral cochlear trauma. Chinchillas were assigned to four cochlear treatment groups. Each treatment produced a distinctive pattern of HC damage, as follows: acoustic exposure (AEx): sparse low-frequency inner hair cell (IHC) and outer hair cell (OHC) loss; round window cisplatin (CisEx): pronounced OHC loss mixed with some IHC loss; round window carboplatin (CarbEx): pronounced IHC loss without OHC loss; control: no loss. Compared with controls, all experimental groups displayed significant and similar psychophysical evidence of tinnitus with features resembling a 1-kHz tone. Contralateral IC spontaneous activity was elevated in the AEx and CisEx groups, which showed increased spiking and increased cross-fiber synchrony. A multidimensional analysis identified a subpopulation of neurons more prevalent in animals with tinnitus. These units were characterized by high bursting, low ISI variance, and within-burst peak spiking of approximately 1,000/sec. It was concluded that cochlear trauma in general, rather than its specific features, leads to multiple changes in central activity that underpin tinnitus. Particularly affected was a subpopulation ensemble of IC neurons with the described unique triad of features.

Functional characterization of acid-sensing ion channels in cultured neurons of rat inferior colliculus

Acid-sensing ion channels (ASICs), which are widely distributed in the mammalian brain, the spinal cord and the peripheral sensory organs, are ligand-gated cation channels activated by extracellular protons. Abundant experimental evidence shows that ASICs play important roles in physiological/pathological conditions, such as sensory transduction, learning/memory, retinal function, seizure and ischemia. In the auditory system, however, there are only a few studies available describing ASICs in hair cells, the spiral ganglion and the vestibular ganglion. In particular, functional ASICs have not been assessed in the central auditory region, although there is evidence to show their transcription in the inferior colliculus (IC). In the present study, we characterized ASIC-like currents in cultured IC neurons of rats with whole-cell patch-clamp techniques. A rapidly decaying inward current was induced by exogenous application of acidic solution in cultured IC neurons with a response threshold around pH 6.9 and a half activation pH value at 5.92. The current was sensitive to amiloride half-maximal inhibition concentration (IC50)=20.4+/-0.4 microM), an ASIC blocker, and its reversal potential was close to the theoretical Na+ equilibrium potential, indicating that the recorded current was mediated by ASICs. Further experiments revealed the presence of homomeric ASIC1a channels in IC neurons: (1) the ASIC-like current was partially carried by Ca2+ as demonstrated with an ion-substitution protocol and Ca2+ imaging; (2) the current was inhibited by the tarantula venom Psalmotoxin (PcTX1), a specific blocker for homomeric ASIC1a channels; (3) the current could be inhibited by extracellular Ca2+ (IC50=2.31 mM) and Pb2+ (10 microM), confirming the presence of ASIC1a subunit. The presence of functional ASIC2a containing channels was revealed by the Zn2+ (300 microM)-induced enhancement of ASIC-like currents and the absence of functional ASIC3 channels was indicated by the insensitivity of ASIC-like currents to salicylate (1 mM), an ASIC3 subunit blocker. Finally, we show that activation of ASICs by a pH drop could induce membrane depolarization and evoke neuronal firing in IC neurons. Our study clearly demonstrates that functional homomeric ASIC1a channels and ASIC2a-containing channels, but not ASIC3 channels, are present in the IC. We suggest that ASICs should be taken into consideration for their possible functional roles in information processing and pathological processes in the central auditory system.

Whole cell recordings of intrinsic properties and sound-evoked responses from the inferior colliculus

Response features of inferior colliculus (IC) neurons to both current injections and tone bursts were studied with in vivo whole cell recordings in awake Mexican free-tailed bats. Of 160 cells recorded, 95% displayed one of three general types of discharge patterns in response to the injection of positive current: 1) sustained discharges; 2) adapting discharges; and 3) onset-bursting discharges. Sustained neurons were the most common type (N=78), followed by onset-bursting (N=57). The least common type was adapting (N=17). In 90 neurons the profiles of synaptic and discharge activity evoked by tones of different frequencies at 50 dB SPL were recorded. Three major tone-evoked response profiles were obtained; 1) neurons dominated by excitation (N=32) in which tones evoked excitatory post-synaptic potentials (EPSPs) or EPSPs with discharges over a range of frequencies with little or no evidence of inhibitory post-synaptic potentials (IPSPs) evoked by frequencies that flanked the excitation; 2) neurons that had an excitatory frequency region in which discharges were evoked that was flanked by frequencies that evoked predominantly IPSPs (N=26); 3) neurons in which all frequencies evoked IPSPs with little or no depolarizations (N=32). The question we asked is whether IC cells that express a particular profile of PSPs and discharges to acoustic stimulation also have the same current-evoked response profile. We show that, with one exception, the intrinsic features of an IC neuron are not correlated with the pattern of its synaptic innervation; the two features are unrelated in the majority of IC cells. The exception is a subtype of inhibitory dominated cell where most frequencies evoked IPSPs to both the onset and to the offset of the tone bursts. In those cells injected current steps always evoked an onset-bursting response.

Sensitivity of Inferior Colliculus Neurons to Interaural Time Differences in the Envelope Versus the Fine Structure With Bilateral Cochlear Implants

Bilateral cochlear implantation seeks to improve hearing by taking advantage of the binaural processing of the central auditory system. Cochlear implants typically encode sound in each spectral channel by amplitude modulating (AM) a fixed-rate pulse train, thus interaural time differences (ITD) are only delivered in the envelope. We investigated the ITD sensitivity of inferior colliculus (IC) neurons with sinusoidally AM pulse trains. ITD was introduced independently to the AM and/or carrier pulses to measure the relative efficacy of envelope and fine structure for delivering ITD information. We found that many IC cells are sensitive to ITD in both the envelope (ITD(env)) and fine structure (ITD(fs)) for appropriate modulation frequencies and carrier rates. ITD(env) sensitivity was generally similar to that seen in normal-hearing animals with AM tones. ITD(env) tuning generally improved with increasing modulation frequency up to the maximum modulation frequency that elicited a sustained response in a neuron (tested </=160 Hz). ITD(fs) sensitivity was present in about half the neurons for 1,000 pulse/s (pps) carriers and was nonexistent at 5,000 pps. The neurons that were sensitive to ITD(fs) at 1,000 pps were those that showed the best ITD sensitivity to low-rate pulse trains. Overall, the best ITD sensitivity was found for ITD contained in the fine structure of a moderate rate AM pulse train (1,000 pps). These results suggest that the interaural timing of current pulses should be accurately controlled in a bilateral cochlear implant processing strategy that provides salient ITD cues.

Effects of salicylate application on the spontaneous activity in brain slices of the mouse cochlear nucleus, medial geniculate body and primary auditory cortex

Salicylate is a well-known substance to produce reversible tinnitus in animals and humans as well. It has been shown that systemic application of salicylate changes the neuronal spontaneous activity in several parts of the auditory pathway. The effects observed in central auditory structures in vivo could be based upon the changed afferent cochlear input to the central auditory system or in addition by a direct action of salicylate onto neurons within the auditory pathway. A direct influence of local salicylate application on spontaneous activity of central auditory neurons has already been described for the inferior colliculus (IC) in brain slice preparations. As spontaneous activity within all key structures of the central auditory pathway could play an important role in tinnitus generation, the present study investigated direct effects of salicylate superfusion on the spontaneous activity of the deafferented cochlear nucleus (CN), medial geniculate body (MGB), and auditory cortex (AC) in brain slices. Out of 72 neurons, 73.4% responded statistically significantly to the superfusate by changing their firing rates. 48.4% of them increased and 51.6% decreased their firing rates, respectively. The mean change of firing rate upon salicylate superfusion was 24.4%. All responses were not significantly different between the brain areas. The amount of neurons which responded to salicylate and the mean change of firing rate was much higher in the IC than in the CN, MGB and AC. This contributes to the hypothesis that salicylate-induced tinnitus is a phantom auditory perception mainly related to hyperexcitability of IC neurons. However, the present results suggest that the individual, specific salicylate sensitivity of CN, MGB and AC neurons can modulate the salicylate-induced generation of tinnitus.

The role of broadband inhibition in the rate representation of spectral cues for sound localization in the inferior colliculus

Previous investigations have shown that a subset of inferior colliculus neurons, which have been designated type O units, respond selectively to isolated features of the cat’s head-related transfer functions (HRTFs: the directional transformation of a free-field sound as it propagates from the head to the eardrum). Based on those results, it was hypothesized that type O units would show enhanced spatial tuning in a virtual sound field that conveyed the full complement of HRTF-based localization cues. As anticipated, a number of neurons produced representations of virtual sound source locations that were spatially tuned, level tolerant, and effective under monaural conditions. Preferred locations were associated with spectral cues that complemented the highly individualized broadband inhibitory patterns of tuned neurons. That is, higher response magnitudes were achieved when spectral peaks coincided with excitatory influences at best frequency (BF: the most sensitive frequency) and spectral notches fell within flanking inhibitory regions. The directionally dependent modulation of narrowband ON-BF excitation by broadband OFF-BF inhibition was not a unique property of type O units.

Glycinergic “Inhibition” Mediates Selective Excitatory Responses to Combinations of Sounds

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 [(+/-)-3-(2-carboxypiperazin-4-yl)-propyl-1-phosphonic acid], alone (n = 14) or combined with AMPA receptor antagonist NBQX (n = 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.

Laterality, intensity, and frequency of electrical intracochlear stimulation are differentially mapped into specific patterns of gene expression in the rat auditory brainstem

Specific patterns of sensory activity may invoke neuroplastic remodelling in the young and adult mammalian brain. Among the indicators for the initiation of neuronal remodelling is the expression of immediate early genes (IEGs). The IEG c-Fos encodes a monomer of the transcription factor dimer AP-1 that triggers cascades of activity-dependent protein synthesis. In order to investigate activity-dependent neuroplasticity in the central auditory system, we applied unilateral electrical intracochlear stimulation (EIS), adjusted to evoke amplitudes of the auditory brainstem response corresponding to about 40 dB acoustic stimulation in adult urethane-anaesthetized rats for 2 h. The intracochlear electrode was baso-dorsally located and the stimulation was specified with respect to intensity (100 or 1500 impulses per min) and frequency (1.6, 50, 100 or 400 Hz). Following EIS, we detected and counted neurons in cochlear nucleus, superior olive, inferior colliculus, and medial geniculate body that initiate stimulat...

The study of the response properties of GABAergic neurons of the mammalian inferior colliculus to sound stimuli

The study of the response properties of GABAergic neurons of the mammalian inferior colliculus to sound stimuli

A classification of NOergic neurons in the inferior colliculus of rat according to co-existence with classical amino acid transmitters

Since the localization of nitric oxide synthase (NOS) can be identified by enzyme histochemistry for NADPH-diaphorse (NADPH-d), this method has been used widely for mapping NOS-containing (presumably NOergic) neurons in the central nervous system. So far several studies suggest that NADPH-d is present in distinct neuronal populations in the inferior colliculus (IC), a major processing center for both the ascending and descending auditory pathway, and NO may play an important role in audition. On one hand, there is evidence from several lines of research that the IC makes extensive use of the neuroactive amino acids, in particular the inhibitory transmitter g-aminobutyric acid (GABA) and the excitatory amino acid glutamate (GLU). However, lacking is a description of the distribution of NOergic neurons to which traditional neurotransmitters may be linked. The present research utilized NADPH-d enzyme histochemistry in combination with immunocytochemistry to determine if NO may colocalize with either or both GABA and glutamate in distinct subpopulations of IC neurons. The NADPH-d positive neurons were predominantly found in two main subdivisions of the IC: the external cortex (ECIC) and the dorsal cortex (DCIC). The large numbers of these NADPH-d positive neurons appeared immunostained for GLU while only a small number, seemed to belong to the small cells (somatic area < 100 microm2) similarity to stellate cells group was positive for GABA throughout the cortex of the IC. Owing to no coexistence between GABA and GLU in the same NADPH-d positive neuron in the pairs of adjacent sections of the IC by the mirror-image technique, the present results consequently support that NOergic neurons could be subdivided into at least three distinct populations with a large proportion of about 77% being GLUergic, much lower frequency of about 11% being GABAergic and the remaining 12% expressing non-GABA and non-GLU. In summary, the existence of two functionally distinct populations of NO/GABAergic and NO/GLUergic neurons in the NOergic neurons of IC suggest that at least two differential pattern of GLU-mediated excitatory NO transmission and GABA-mediated inhibitory NO transmission are involved in the networking of auditory communication in the cortex of IC.

Local inhibition shapes duration tuning in the inferior colliculus of guinea pigs

Neural tuning to sound durations is a useful filter for the identification of a variety of sounds. Previous studies have shown that the interaction between excitatory and inhibitory inputs plays a role in duration selectivity in echolocating bats. However, this has not been investigated in non-echolocating mammals. In the inferior colliculus (IC) of these mammals, it is recognized that the excitatory responses to sounds are mediated through AMPA and NMDA receptors while the inhibitory input is mediated through γ-aminobutyric acid (GABA) and glycine receptors. The present study explores the potential interplay between inhibitory and excitatory inputs and its role in the duration selectivity of IC neurons in guinea pigs. It was found that the application of bicuculline (BIC, a GABA A blocker) and/or strychnine (STRY, a glycine blocker) eliminated or reduced duration tuning in most units that were duration tuned (32 out of 39 for BIC, 50 out of 64 for STRY, respectively). The inhibitory input (either by GABA or by glycine) appeared to have a stronger regulating effect on the early excitation mediated by AMPA than on later excitation by NMDA. This is more distinguishable in neurons that show duration selectivity. In conclusion, the inhibitory effect on the early responses appears to be the main contributor for the duration selectivity of the IC in guinea pigs; potential mechanisms for this duration selectivity are also discussed.

Duration-Sensitive Neurons in the Inferior Colliculus of Horseshoe Bats: Adaptations for Using CF-FM Echolocation Pulses

The present study examines duration-sensitive neurons in the inferior colliculus (IC) of the least horseshoe bat, Rhinolophus pusillus, from China. In contrast to other bat species tested for duration selectivity so far, echolocation pulses emitted by horseshoe bats are generally longer and composed of a long constant-frequency (CF) component followed by a short downward frequency-modulated (FM) sweep (CF-FM pulse). We used combined CF-FM pulses to analyze the differential effects that these two pulse components had on the duration tuning in neurons of the horseshoe bat's IC. Consistent with results from other mammals, duration-sensitive neurons found in the least horseshoe bat fall into three main classes: short-pass, band-pass, and long-pass. Using a CF stimulus alone, 54% (51/95) of all IC neurons showed at least one form of duration selectivity at one or more stimulus intensities. In 65 of the 95 IC neurons tested with CF pulses, we were also able to test their duration selectivity for a combined CF-FM pulse, which increased the ratio of duration-sensitive neurons to 66% (43/65). Seven to 15 neurons that failed to show duration tuning for CF bursts became duration sensitive for CF-FM pulses, with most of them exhibiting short-pass (depending on stimulus intensity, between 4 and 8 neurons) or band-pass tuning (1-3 neurons). Increasing stimulus intensities did not affect the duration tuning in 53% (23/43) of duration-sensitive neurons for CF bursts and in about 26% (7/27) for CF-FM stimuli. In the remaining neurons, increasing sound levels generally reduced the ratio of duration-sensitive neurons to 33% for CF and 37% for CF-FM stimulation. In those that remained duration sensitive, louder CF bursts shortened best durations in band-pass neurons and cutoff durations in short- and long-pass neurons, whereas louder CF-FM stimuli reduced the cutoff durations only in short-pass neurons. Bandwidths of band-pass neurons were not significantly affected by any stimulus configuration, with only a slight trend for increasing bandwidths for louder CF bursts (but not CF-FM stimuli). Best durations and cutoff durations reached higher values than those in the other bat species examined so far and roughly match the longer durations of echolocation pulses emitted by horseshoe bats. Therefore presentation of a CF-FM stimulus improved the duration sensitivity in IC neurons by increasing the ratio of duration-tuned neurons and making them less susceptible to changes in signal intensity.

Responses of cochlear nucleus neurons to harmonic and mistuned complex tones

Preliminary measurements of the representation in the cochlear nucleus (CN) of harmonic tones, harmonic tones with mistuned components, and double harmonic tones are reported. These data indicate that, unlike auditory nerve fibers and IC neurons, neurons in the CN may exhibit one of several qualitatively different response patterns when stimulated with mistuned tones. Primary-like neurons synchronized their discharges to 2–3 individual stimulus components, much like auditory nerve fibers do. Chopper neurons tended to respond with the periodicity of envelopes produced by interactions between adjacent stimulus components but exhibited little or no response synchronized to individual stimulus components. A small proportion of CN neurons exhibited complex slowly-modulated discharge patterns similar to those that are commonly observed in the inferior colliculus (IC). The patterns obtained from CN neurons with different pure tone discharge patterns were generally consistent with expectations based on previous studies with other stimuli. The measurements provided additional insight into the hierarchical processing stages that result in the highly patterned responses of IC neurons to harmonic and mistuned tones.

Sensorineural Hearing Loss and Neural Correlates of Temporal Acuity in the Inferior Colliculus of the C57bl/6 Mouse

Perception of complex sounds depends on the encoding of the dynamic and static structures within the ongoing stimulus by the auditory system. Aging has been associated with deficits in both areas, thus, the difficulty that the elderly have in speech comprehension could due to hearing loss, or to a loss of temporal sensitivity, or some combination of both. We investigated the effects of sensorineural hearing loss (SNHL) on neural correlates of temporal resolution by recording the responses of inferior colliculus neurons to a gap detection paradigm. We used C57BL/6 (C57) strain of laboratory mouse, which carries the Ahl deafness gene that initiates a progressive high frequency SNHL beginning at about 2 months of age and rapidly progresses to total deafness by 18 months. We compared gap encoding from inferior collicular neurons from young, normal-hearing C57 mice and middle-aged, hearing-impaired, C57 mice, quantifying minimal gap threshold, and recovery functions. The proportion of unit types, spontaneous rates and degree of monotonicity were comparable between young and middle-aged C57 mice. As expected, single unit thresholds were elevated by 30-40 dB in middle-aged C57 mice. However, no significant differences in mean minimal gap thresholds or in the slopes of the gap recovery functions were found between the two age groups. Thus, the results suggest that moderate high frequency SNHL does not affect temporal processing as measured by the gap detection paradigm.

Visual- and saccade-related signals in the primate inferior colliculus.

The inferior colliculus (IC) is normally thought of as a predominantly auditory structure because of its early position in the ascending auditory pathway just before the auditory thalamus. Here, we show that a majority of IC neurons (64% of 180 neurons) in awake monkeys carry visual- and/or saccade-related signals in addition to their auditory responses (P < 0.05). The response patterns involve primarily excitatory visual responses, but also increased activity time-locked to the saccade, slow rises in activity time-locked to the onset of the visual stimulus, and inhibitory responses. The presence of these visual-related signals suggests that the IC plays a role in integrating visual and auditory information. More broadly, our results show that interactions between sensory pathways can occur at very early points in sensory processing streams, which implies that multisensory integration may be a low-level rather than an exclusively high-level process.

Responses to Social Vocalizations in the Inferior Colliculus of the Mustached Bat Are Influenced by Secondary Tuning Curves

Neurons in the inferior colliculus (IC) of the mustached bat integrate input from multiple frequency bands in a complex fashion. These neurons are important for encoding the bat's echolocation and social vocalizations. The purpose of this study was to quantify the contribution of complex frequency interactions on the responses of IC neurons to social vocalizations. Neural responses to single tones, two-tone pairs, and social vocalizations were recorded in the IC of the mustached bat. Three types of data driven stimulus-response models were designed for each neuron from single tone and tone pair stimuli to predict the responses of individual neurons to social vocalizations. The first model was generated only using the neuron's primary frequency tuning curve, whereas the second model incorporated the entire hearing range of the animal. The extended model often predicted responses to many social vocalizations more accurately for multiply tuned neurons. One class of multiply tuned neuron that likely encodes echolocation information also responded to many of the social vocalizations, suggesting that some neurons in the mustached bat IC have dual functions. The third model included two-tone frequency tunings of the neurons. The responses to vocalizations were better predicted by the two-tone models when the neuron had inhibitory frequency tuning curves that were not near the neuron's primary tuning curve. Our results suggest that complex frequency interactions in the IC determine neural responses to social vocalizations and some neurons in IC have dual functions that encode both echolocation and social vocalization signals.

Responses of inferior colliculus neurons to double harmonic tones.

The auditory system can segregate sounds that overlap in time and frequency, if the sounds differ in acoustic properties such as fundamental frequency (f0). However, the neural mechanisms that underlie this ability are poorly understood. Responses of neurons in the inferior colliculus (IC) of the anesthetized chinchilla were measured. The stimuli were harmonic tones, presented alone (single harmonic tones) and in the presence of a second harmonic tone with a different f0 (double harmonic tones). Responses to single harmonic tones exhibited no stimulus-related temporal pattern, or in some cases, a simple envelope modulated at f0. Responses to double harmonic tones exhibited complex slowly modulated discharge patterns. The discharge pattern varied with the difference in f0 and with characteristic frequency. The discharge pattern also varied with the relative levels of the two tones; complex temporal patterns were observed when levels were equal, but as the level difference increased, the discharge pattern reverted to that associated with single harmonic tones. The results indicated that IC neurons convey information about simultaneous sounds in their temporal discharge patterns and that the patterns are produced by interactions between adjacent components in the spectrum. The representation is "low-resolution," in that it does not convey information about single resolved components from either individual sound.

Activation of the serotonin 1A receptor alters the temporal characteristics of auditory responses in the inferior colliculus

Serotonin, like other neuromodulators, acts on a range of receptor types, but its effects also depend on the functional characteristics of the neurons responding to receptor activation. In the inferior colliculus (IC), an auditory midbrain nucleus, activation of a common serotonin (5-HT) receptor type, the 5-HT 1A receptor, depresses auditory-evoked responses in many neurons. Whether these effects occur differentially in different types of neurons is unknown. In the current study, the effects of iontophoretic application of the 5-HT 1A agonist 8-OH-DPAT on auditory responses were compared with the characteristic frequencies (CFs), recording depths, and control first-spike latencies of the same group of IC neurons. The 8-OH-DPAT-evoked change in response significantly correlated with first-spike latency across the population, so that response depressions were more prevalent in longer-latency neurons. The 8-OH-DPAT-evoked change in response did not correlate with CF or with recording depth. 8-OH-DPAT also altered the temporal characteristics of spike trains in a subset of neurons that fired multiple spikes in response to brief stimuli. For these neurons, activation of the 5-HT 1A receptor suppressed lagging spikes proportionally more than initial spikes. These results suggest that the 5-HT 1A receptor, by affecting the timing of the responses of both individual neurons and the neuron population, shifts the temporal profile of evoked activity within the IC.

Cerebral Responses to Change in Spatial Location of Unattended Sounds

The neural basis of spatial processing in the auditory cortex has been controversial. Human fMRI studies suggest that a part of the planum temporale (PT) is involved in auditory spatial processing, but it was recently argued that this region is active only when the task requires voluntary spatial localization. If this is the case, then this region cannot harbor an ongoing spatial representation of the acoustic environment. In contrast, we show in three fMRI experiments that a region in the human medial PT is sensitive to background auditory spatial changes, even when subjects are not engaged in a spatial localization task, and in fact attend the visual modality. During such times, this area responded to rare location shifts, and even more so when spatial variation increased, consistent with spatially selective adaptation. Thus, acoustic space is represented in the human PT even when sound processing is not required by the ongoing task.