Neuronal coding of interaural transient envelope disparities

Sensitivity to interaural time differences in the inferior colliculus of cochlear implanted rats with or without hearing experience

For deaf patients cochlear implants (CIs) can restore substantial amounts of functional hearing. However, binaural hearing, and in particular, the perception of interaural time differences (ITDs) with current CIs has been found to be notoriously poor, especially in the event of early hearing loss. One popular hypothesis for these deficits posits that a lack of early binaural experience may be a principal cause of poor ITD perception in pre-lingually deaf CI patients. This is supported by previous electrophysiological studies done in neonatally deafened, bilateral CI-stimulated animals showing reduced ITD sensitivity. However, we have recently demonstrated that neonatally deafened CI rats can quickly learn to discriminate microsecond ITDs under optimized stimulation conditions which suggests that the inability of human CI users to make use of ITDs is not due to lack of binaural hearing experience during development. In the study presented here, we characterized ITD sensitivity and tuning of inferior colliculus neurons under bilateral CI stimulation of neonatally deafened and hearing experienced rats. The hearing experienced rats were not deafened prior to implantation. Both cohorts were implanted bilaterally between postnatal days 64-77 and recorded immediately following surgery. Both groups showed comparably large proportions of ITD sensitive multi-units in the inferior colliculus (Deaf: 84.8%, Hearing: 82.5%), and the strength of ITD tuning, quantified as mutual information between response and stimulus ITD, was independent of hearing experience. However, the shapes of tuning curves differed substantially between both groups. We observed four main clusters of tuning curves - trough, contralateral, central, and ipsilateral tuning. Interestingly, over 90% of multi-units for hearing experienced rats showed predominantly contralateral tuning, whereas as many as 50% of multi-units in neonatally deafened rats were centrally tuned. However, when we computed neural d' scores to predict likely limits on performance in sound lateralization tasks, we did not find that these differences in tuning shapes predicted worse psychoacoustic performance for the neonatally deafened animals. We conclude that, at least in rats, substantial amounts of highly precise, "innate" ITD sensitivity can be found even after profound hearing loss throughout infancy. However, ITD tuning curve shapes appear to be strongly influenced by auditory experience although substantial lateralization encoding is present even in its absence.

Temporal effects in interaural and sequential level difference perception

Temporal effects in interaural level difference (ILD) perception are not well understood. While it is often assumed that ILD sensitivity is independent of the temporal stimulus properties, a reduction of ILD sensitivity for stimuli with a high modulation rate has been reported (known under the term binaural adaptation). Experiment 1 compared ILD thresholds and sequential-level-difference (SLD) thresholds using 300-ms bandpass-filtered pulse trains (centered at 4 kHz) with rates of 100, 400, and 800 pulses per second (pps). In contrast to the SLD thresholds, ILD thresholds were elevated at 800 pps, consistent with literature data that had previously been attributed to binaural adaptation. Experiment 2 showed better ILD sensitivity for pulse trains than for pure tones, suggesting that amplitude modulation enhances ILD sensitivity. The present ILD data and binaural adaptation data from the literature were predicted by a model combining well-established auditory periphery front-ends with an interaural comparison stage. The model also accounted for other published ILD data, including target ILD thresholds in diotic forward and backward fringes and ILD thresholds with different amounts of interaural correlation. Overall, a variety of temporal effects in ILD perception, including binaural adaptation, appear to be largely attributable to monaural peripheral auditory processing.

Latency modulation of collicular neurons induced by electric stimulation of the auditory cortex in Hipposideros pratti: In vivo intracellular recording

In the auditory pathway, the inferior colliculus (IC) receives and integrates excitatory and inhibitory inputs from the lower auditory nuclei, contralateral IC, and auditory cortex (AC), and then uploads these inputs to the thalamus and cortex. Meanwhile, the AC modulates the sound signal processing of IC neurons, including their latency (i.e., first-spike latency). Excitatory and inhibitory corticofugal projections to the IC may shorten and prolong the latency of IC neurons, respectively. However, the synaptic mechanisms underlying the corticofugal latency modulation of IC neurons remain unclear. Thus, this study probed these mechanisms via in vivo intracellular recording and acoustic and focal electric stimulation. The AC latency modulation of IC neurons is possibly mediated by pre-spike depolarization duration, pre-spike hyperpolarization duration, and spike onset time. This study suggests an effective strategy for the timing sequence determination of auditory information uploaded to the thalamus and cortex.

Influence of envelope waveform on ITD sensitivity of neurons in the auditory midbrain

Using single-neuron electrophysiology, we show that the precise shape of a sound’s “energy envelope” is a critical factor in determining how well midbrain neurons are able to convey information about auditory spatial cues. Consistent with human behavioral performance, sounds with rapidly rising energy and relatively long intervals between energy bursts are best at conveying spatial information. The data suggest specific sound energy patterns that might best be applied to hearing devices to aid spatial listening.

Interaural time differences (ITDs) conveyed by the modulated envelopes of high-frequency sounds can serve as a cue for localizing a sound source. Klein-Hennig et al. (J Acoust Soc Am 129: 3856, 2011) demonstrated the envelope attack (the rate at which stimulus energy in the envelope increases) and the duration of the pause (the interval between successive envelope pulses) as important factors affecting sensitivity to envelope ITDs in human listeners. Modulated sounds with rapid attacks and long pauses produce the lowest ITD discrimination thresholds. The duration of the envelope’s sustained component (sustain) and the rate at which stimulus energy falls at the offset of the envelope (decay) are only minor factors. We assessed the responses of 71 single neurons, recorded from the midbrains of 15 urethane-anesthetized tri-colored guinea pigs, to envelope shapes in which the four envelope components, i.e., attack, sustain, decay, and pause, were systematically varied. We confirmed the importance of the attack and pause components in generating ITD-sensitive responses. Analysis of neural firing rates demonstrated more neurons (49/71) show ITD sensitivity in response to “damped” stimuli (fast attack and slow decay) compared with “ramped” stimuli (slow attack and fast decay) (14/71). Furthermore, the lowest threshold for the damped stimulus (91 μs) was lower by a factor of 4 than that for the temporally reversed ramped envelope shape (407 μs). The data confirm the importance of fast attacks and optimal pause durations in generating sensitivity to ITDs conveyed in the modulated envelopes of high-frequency sounds and are incompatible with models of ITD processing based on the integration of sound energy over time.

NEW & NOTEWORTHY Using single-neuron electrophysiology, we show that the precise shape of a sound’s “energy envelope” is a critical factor in determining how well midbrain neurons are able to convey information about auditory spatial cues. Consistent with human behavioral performance, sounds with rapidly rising energy and relatively long intervals between energy bursts are best at conveying spatial information. The data suggest specific sound energy patterns that might best be applied to hearing devices to aid spatial listening.

Sensitivity to Interaural Time Differences Conveyed in the Stimulus Envelope: Estimating Inputs of Binaural Neurons Through the Temporal Analysis of Spike Trains

Sound-source localization in the horizontal plane relies on detecting small differences in the timing and level of the sound at the two ears, including differences in the timing of the modulated envelopes of high-frequency sounds (envelope interaural time differences (ITDs)). We investigated responses of single neurons in the inferior colliculus (IC) to a wide range of envelope ITDs and stimulus envelope shapes. By a novel means of visualizing neural activity relative to different portions of the periodic stimulus envelope at each ear, we demonstrate the role of neuron-specific excitatory and inhibitory inputs in creating ITD sensitivity (or the lack of it) depending on the specific shape of the stimulus envelope. The underlying binaural brain circuitry and synaptic parameters were modeled individually for each neuron to account for neuron-specific activity patterns. The model explains the effects of envelope shapes on sensitivity to envelope ITDs observed in both normal-hearing listeners and in neural data, and has consequences for understanding how ITD information in stimulus envelopes might be maximized in users of bilateral cochlear implants-for whom ITDs conveyed in the stimulus envelope are the only ITD cues available.

Ongoing temporal coding of a stochastic stimulus as a function of intensity: time-intensity trading

Stimulus-locked temporal codes are increasingly seen as relevant to perception. The timing of action potentials typically varies with stimulus intensity, and the invariance of temporal representations with intensity is therefore an issue. We examine the timing of action potentials in cat auditory nerve to broadband noise presented at different intensities, using an analysis inspired by coincidence detection and by the binaural "latency hypothesis." It is known that the two cues for azimuthal sound localization, interaural intensity or level differences and interaural time differences (ITDs), interact perceptually. According to the latency hypothesis, the increase in intensity for the ear nearest to a sound source off the midline causes a decrease in response latency in that ear relative to the other ear. We found that changes in intensity cause small but systematic shifts in the ongoing timing of responses in the auditory nerve, generally but not always resulting in shorter delays between stimulus onset and neural response for increasing intensity. The size of the temporal shifts depends on characteristic frequency with a pattern indicating a fine-structure and an envelope response regime. Overall, the results show that ongoing timing is remarkably stable with intensity at the most peripheral neural level. The results are not consistent in a simple way with the latency hypothesis, but because of the acute sensitivity to ITDs, the subtle effects of intensity on timing may nevertheless have perceptual consequences.

Level-dependent latency shifts quantified through binaural processing

The mammalian binaural system compares the timing of monaural inputs with microsecond precision. This temporal precision is required for localizing sounds in azimuth. However, temporal features of the monaural inputs, in particular their latencies, highly depend on the overall sound level. In a combined psychophysical, electrophysiological, and modeling approach, we investigate how level-dependent latency shifts of the monaural responses are reflected in the perception and neural representation of interaural time differences. We exploit the sensitivity of the binaural system to the timing of high-frequency stimuli with binaurally incongruent envelopes. Using these novel stimuli, both the perceptually adjusted interaural time differences and the time differences extracted from electrophysiological recordings systematically depend on overall sound pressure level. The perceptual and electrophysiological time differences of the envelopes can be explained in an existing model of temporal integration only if a level-dependent firing threshold is added. Such an adjustment of firing threshold provides a temporally accurate neural code of the temporal structure of a stimulus and its binaural disparities independent of overall sound level.

The roles of local inhibition mediated by gamma-aminobutyric acid (GABA)-A receptor in duration tuning in the inferior colliculus of guinea pigs

CONCLUSION

GABA-mediated inhibition is responsible for the duration tuning in the inferior colliculus (IC) of guinea pigs, a non-echo-locating mammal. Duration tuning in this species is better demonstrated in an appropriate short time window.

OBJECTIVES

To investigate the role of GABA-mediated inhibition in duration tuning of neurons in the IC of guinea pigs.

MATERIALS AND METHODS

Duration tuning pattern was recorded by measuring the spikes of single neurons in response to broadband noise of different durations. The effect of GABA-mediated inhibition was verified by comparing the responses with and without the use of the GABA-A receptor blocker bicuculline (BIC), which was applied using micro-iontophoresis.

RESULTS

In addition to overall increase in responsiveness, the application of BIC was found to significantly reduce or eliminate the duration selectivity in 44 of the 67 neurons that showed clear duration tuning from a sample of total 340 neurons.

Is duration tuning a transient process in the inferior colliculus of guinea pigs?

Duration selectivity appears to be a fundamental neural encoding mechanism found throughout the animal kingdom. Previous studies reported that band-pass duration-tuned neurons typically show offset responses and occupy a small portion of auditory neurons in non-echolocation mammals relative to echolocation bats. Therefore, duration tuning is generally weaker in non-echolocation mammals. In the present study, duration tuning was analyzed for 207 neurons recorded in the inferior colliculus (IC) of guinea pigs. Duration tuning was found to be stronger in the onset component of the responses from sustained, on-off and pause neurons than had been reported previously, when a short analysis window was applied. The need for an appropriate time window for duration tuning analysis was also supported by the fact that the on and off responses from an on-off neuron may show different duration tuning features. Therefore, duration tuning appears to be a transient neural coding process in the IC of guinea pigs. Duration tuning for these types of neurons may have been blurred by the use of a relatively unselective, long window.

A neural correlate of stochastic echo imaging

Bats quickly navigate through a highly structured environment relying on echolocation. Large natural objects in the environment, like bushes or trees, produce complex stochastic echoes, which can be characterized by the echo roughness. Previous work has shown that bats can use echo roughness to classify the stochastic properties of natural objects. This study provides both psychophysical and electrophysiological data to identify a neural correlate of statistical echo analysis in the bat Phyllostomus discolor. Psychophysical results show that the bats require a fixed minimum roughness of 2.5 (in units of base 10 logarithm of the stimulus fourth moment) for roughness discrimination. Electrophysiological results reveal a subpopulation of 15 of 94 recorded cortical units, located in an anterior region of auditory cortex, whose rate responses changed significantly with echo roughness. It is shown that the behavioral ability to discriminate differences in the statistics of complex echoes can be quantitatively predicted by the neural responses of this subpopulation of auditory-cortical neurons.

Encoding of virtual acoustic space stimuli by neurons in ferret primary auditory cortex

Recent studies from our laboratory have indicated that the spatial response fields (SRFs) of neurons in the ferret primary auditory cortex (A1) with best frequencies > or =4 kHz may arise from a largely linear processing of binaural level and spectral localization cues. Here we extend this analysis to investigate how well the linear model can predict the SRFs of neurons with different binaural response properties and the manner in which SRFs change with increases in sound level. We also consider whether temporal features of the response (e.g., response latency) vary with sound direction and whether such variations can be explained by linear processing. In keeping with previous studies, we show that A1 SRFs, which we measured with individualized virtual acoustic space stimuli, expand and shift in direction with increasing sound level. We found that these changes are, in most cases, in good agreement with predictions from a linear threshold model. However, changes in spatial tuning with increasing sound level were generally less well predicted for neurons whose binaural frequency-time receptive field (FTRF) exhibited strong excitatory inputs from both ears than for those in which the binaural FTRF revealed either a predominantly inhibitory effect or no clear contribution from the ipsilateral ear. Finally, we found (in agreement with other authors) that many A1 neurons exhibit systematic response latency shifts as a function of sound-source direction, although these temporal details could usually not be predicted from the neuron's binaural FTRF.

Mechanisms underlying the sensitivity of neurons in the lateral superior olive to interaural intensity differences

The initial processing of interaural intensity differences (IIDs), the major cue to the azimuthal location of high-frequency sounds in mammals, is carried out by neurons in the lateral superior olivary nucleus (LSO) that receive excitatory input from the ipsilateral ear and inhibitory input from the contralateral ear (IE neurons). The "latency" hypothesis asserts that it is the effects of intensity differences on the latency, and hence the relative timing, of the synaptic inputs to these neurons that is the basis of their sensitivity to IIDs, while other models assign the major role to changes in the relative amplitude of the inputs. To test the latency hypothesis and to determine the contributions of changes in the relative timing and amplitude of synaptic inputs to the IID sensitivity of LSO neurons, a method was developed of generating sets of stimuli that produced either the same changes in the relative timing of inputs without any change in their amplitude (equivalent interaural time difference stimuli) or the same differences in amplitude without any difference in timing (delay-cancelled IID stimuli) as a given range of IIDs. Data were obtained from a sample of IE neurons in the LSO of anesthetized rats using these stimulus paradigms and click and tone-burst stimuli. For click stimuli, the IID sensitivity of a small proportion of neurons was explained entirely by sensitivity to differences in input timing, but the sensitivity of most neurons reflected either sensitivity to the relative amplitude of inputs or to the joint operation of both factors. In neurons whose sensitivity was tested at a number of different absolute sound pressure levels (SPLs), the relative contributions of the two factors tended to differ at different SPLs. The IID sensitivity of onset responses to tone stimuli could be classified into the same three categories but was explained for a larger proportion of neurons by sensitivity to differences in input timing. The IID sensitivity of the late response component of neurons with sustained responses to tones in all cases reflected sensitivity to the relative amplitude of the inputs. The results confirm the contribution of changes in latency produced by intensity changes to the IID sensitivity of the onset responses of many IE neurons in LSO but require rejection of the strong form of the latency hypothesis, which asserts that this factor alone accounts for such sensitivity.

Temporal processing in sensory systems

The idea that sensory information is represented by the temporal firing patterns of neurons or entire networks, rather than by firing rates measured over long integration times, has recently gained increasing experimental support. A number of mechanisms that help to preserve temporal information in ascending sensory systems have been identified, and the role of inhibition in these processes has been characterized. Furthermore, it has become obvious that temporal processing and the representation of sensory events by temporal spike patterns are highly dependent upon the behavioral state of the animal or experimental subject.