Latency as a function of intensity in auditory neurons: influences of central processing

Duration selectivity of neurons in the inferior colliculus of the big brown bat: tolerance to changes in sound level

At and above the level of the inferior colliculus (IC), some neurons respond maximally to a limited range of sound durations, with little or no excitatory response to durations outside of this range. Such neurons have been termed "duration tuned" or "duration selective." In this study we examined the effects of varying signal amplitude on best duration, width of tuning, and first spike latency of duration tuned neurons in the IC of the big brown bat, Eptesicus fuscus. Response areas as a function of stimulus duration and intensity took a variety of forms, including open (V-shaped), narrow and level tolerant (U-shaped), or closed (O-shaped). The majority (82%) of duration tuned neurons had narrow U-shaped or O-shaped duration response areas. Those with narrow U-shaped response areas retained their duration tuning across a broad dynamic range, < or = 50 dB above threshold, whereas those with O-shaped response areas were narrowly tuned to both stimulus duration and amplitude. For about one-half (55%) of the neurons with either a U- or O-shaped response areas, best duration (BD) changed by <1 ms across the range of suprathreshold amplitudes tested. Changes in BD most often took the form of a shift to slightly shorter durations as stimulus level increased. For the majority (65%) of U- and O-shaped neurons, 50% width of duration tuning changed by <2 ms with increasing amplitude. Latency of response at BD remained stable across changes in sound level, suggesting that the relative strengths of excitatory and inhibitory inputs to duration tuned neurons remain in balance over a wide dynamic range of sound pressure levels.

Leading inhibition to neural oscillation is important for time-domain processing in the auditory midbrain

A number of central auditory neurons exhibit paradoxical latency shift (PLS), a response characterized by longer response latencies at higher sound levels. PLS neurons are known to play a role in target ranging for echolocating bats that emit frequency-modulated sounds. We recently reported that early inhibition of unit's oscillatory discharges is critical for PLS in the inferior colliculus (IC) of little brown bats. The goal of this study was to determine in echolocating bats and in non-echolocating animals (frogs): 1) the detailed characteristics of PLS and whether PLS was dependent on sound level, frequency, and duration; 2) the time course of inhibition underlying PLS using a paired-pulse paradigm. We found that 22% of IC neurons in bats and 15% in frogs exhibited periodic discharge patterns in response to tone pulses at high sound levels. The firing periodicity was unit specific and independent of sound level and duration. Other IC neurons (28% in bats; 14% in frogs) exhibited PLS. These PLS neurons shared several response characteristics: 1) PLS was largely independent of sound frequency and 2) the magnitude of shift in first-spike latency was either duration dependent or duration tolerant. For PLS neurons, application of bicuculline abolished PLS and unmasked the unit's periodical firing pattern that served as the building block for PLS. In response to paired sound pulses, PLS neurons exhibited delay-dependent response suppression, confirming that high-threshold leading inhibition was responsible for PLS. Results also revealed the timing of excitatory and inhibitory inputs underlying PLS and its role in time-domain processing.

Interaural Level Difference Processing in the Lateral Superior Olive and the Inferior Colliculus

Interaural level differences (ILDs) provide salient cues for localizing high-frequency sounds in space, and populations of neurons that are sensitive to ILDs are found at almost every synaptic level from brain stem to cortex. These cells are predominantly excited by stimulation of one ear and predominantly inhibited by stimulation of the other ear, such that the magnitude of their response is determined in large part by the intensities at the 2 ears. However, in many cases ILD sensitivity is also influenced by overall intensity, which challenges the idea of unambiguous ILD coding. We investigated whether ambiguity is reduced from one synaptic level to another for 2 centers in the so-called ILD processing pathway. We recorded from single cells in the free-tailed bat lateral superior olive (LSO), the first station where ILDs are coded, and the central nucleus of the inferior colliculus (ICC), which receives a strong projection from the LSO, as well as convergent projections from many other auditory centers. We assessed effects of overall intensity by comparing ILD functions generated with different fixed intensities to the excitatory ear. LSO cells were characterized by functions that shifted in a systematic manner with increasing intensity to the excitatory ear. In contrast, significantly more ICC cells had functions that were stable across overall sound intensity, indicating that hierarchical transformations increase stability. Furthermore, a population analysis based on proportion of active cells indicated that stability in the ICC was greatly enhanced when overall population activity was considered.

Temporal masking reveals properties of sound-evoked inhibition in duration-tuned neurons of the inferior colliculus

The inferior colliculus (IC) is the first place in the central auditory pathway where duration-selective neurons are found. Previous neuropharmacological and electrophysiological studies have shown that they are created there and have led to a conceptual model in which excitatory and inhibitory inputs are offset in time so that the cell fires only when sound duration is such that onset- and offset-evoked excitation coincide; the response is suppressed by inhibition at other durations. We tested predictions from the model using paired tone stimulation and extracellular recording in the IC of the big brown bat, Eptesicus fuscus. Responses to a best duration (BD) tone were used as a probe to examine the strength and time course of inhibition activated by a nonexcitatory (NE) tone of the same frequency but differing in duration. As the relative time between the BD and NE tones was varied, the activity evoked by the BD tone was affected in ways comparable with backward, simultaneous, and forward masking. Responses to the BD tone were completely suppressed at short interstimulus intervals when the BD tone preceded the NE tone. Suppression was also seen when the stimuli temporally overlapped and summed and at intervals when the BD tone followed the NE tone. The results show that duration-selective neurons receive an onset-evoked, inhibitory input that precedes their excitatory input. The period of leading inhibition was correlated with BD and first spike latency. The results suggest how inhibition in the CNS could explain temporal masking phenomena, including backward masking.

A unifying basis of auditory thresholds based on temporal summation

Thresholds of auditory-nerve (AN) fibers and auditory neurons are commonly specified in terms of sound pressure only, implying that they are independent of time. At the perceptual level, however, the sound pressure required for detection decreases with increasing stimulus duration, suggesting that the auditory system integrates sound over time. The quantity commonly believed to be integrated is sound intensity, implying that the auditory system would have an energy threshold. However, leaky integrators of intensity with time constants of hundreds of milliseconds are required to fit the data. Such time constants are unknown in physiology and are also incompatible with the high temporal resolution of the auditory system, creating the resolution-integration paradox. Here we demonstrate that cortical and perceptual responses are based on integration of the pressure envelope of the sound, as we have previously shown for AN fibers, rather than on intensity. The functions relating the pressure envelope integration thresholds and time for AN fibers, cortical neurons, and perception in the same species (cat), as well as for perception in many different vertebrate species, are remarkably similar. They are well described by a power law that resolves the resolution-integration paradox. The data argue for the integrator to be located in the first synapse in the auditory pathway and we discuss its mode of operation.

A unified mechanism for spontaneous-rate and first-spike timing in the auditory nerve

Recent physiological experiments have provided detailed descriptions of the properties of first-spike latency and variability in auditory cortex and nerve in response to pure tones with different envelopes. The envelope-dependence of first-spike timing and precision in auditory cortical neurons appears to reflect properties established in the nerve. First-spike latency properties in individual auditory nerve fibers are strongly correlated with their spontaneous rate (SR). It is shown here that a minimal, plausible model of auditory transduction with two free parameters accurately reproduces the physiological data from the auditory nerve population. The model consists of a simple gain stage, a bandpass filter, a rectifying saturating non-linearity, and a lowpass filter in series. The output of the lowpass filter drives an inhomogeneous Poisson process. The shape of the non-linearity is determined by SR; in physiological terms, this shape depends upon the resting sensitivity of the synapse between the inner hair cell and the auditory nerve. An alternative model for SR generation, where SR is added to the stimulus-driven output of a fixed nonlinearity, fails to account for the data. The results provide a novel, comprehensive and physiologically-based explanation for the range of experimental results on the envelope-dependence of first-spike latency and precision, and its relationship with SR, in the auditory system.

Precision of neural timing: effects of convergence and time-windowing

We study the improvement in timing accuracy in a neural system having n identical input neurons projecting to one target neuron. The n input neurons receive the same stimulus but fire at stochastic times selected from one of four specified probability densities, f, each with standard deviation 1.0 msec. The target cell fires if and when it receives m inputs within a time window of epsilon msec. Let sigma(n,m,epsilon) denote the standard deviation of the time of firing of the target neuron (i.e. the standard deviation of the target neuron's latency relative to the arrival time of the stimulus). Mathematical analysis shows that sigma(n,m,epsilon) is a very complicated function of n, m, and epsilon. Typically, sigma(n,m,epsilon) is a non-monotone function of m and epsilon and the improvement of timing accuracy is highly dependent of the shape of the probability density for the time of firing of the input neurons. For appropriate choices of m, epsilon, and f, the standard deviation sigma(n,m,epsilon) may be as low as 1/n. Thus, depending on these variables, remarkable improvements in timing accuracy of such a stochastic system may occur.

Motion processing in the auditory cortex of the rufous horseshoe bat: role of GABAergic inhibition

This study examined the influence of inhibition on motion-direction-sensitive responses of neurons in the dorsal fields of auditory cortex of the rufous horseshoe bat. Responses to auditory apparent motion stimuli were recorded extracellularly from neurons while microiontophoretically applying gamma-aminobutyric acid (GABA) and the GABAA receptor antagonist bicuculline methiodide (BMI). Neurons could respond with a directional preference exhibiting stronger responses to one direction of motion or a shift of receptive field (RF) borders depending on direction of motion. BMI influenced the motion direction sensitivity of 53% of neurons. In 21% of neurons the motion-direction sensitivity was decreased by BMI by decreasing either directional preference or RF shift. In neurons with a directional preference, BMI increased the spike number for the preferred direction by a similar amount as for the nonpreferred direction. Thus, inhibition was not direction specific. BMI increased motion-direction sensitivity by either increasing directional preference or magnitude of RF shifts in 22% of neurons. Ten percent of neurons changed their response from a RF shift to a directional preference under BMI. In these neurons, the observed effects could often be better explained by adaptation of excitation rather than inhibition. The results suggest, that adaptation of excitation, as well as cortex specific GABAergic inhibition, contribute to motion-direction sensitivity in the auditory cortex of the rufous horseshoe bat.

Structure and function of the bat superior olivary complex

The superior olivary complex (SOC) is a mammalian auditory brainstem structure that contains several nuclei. Some of them are part of the ascending system projecting to higher auditory centers, others belong to the descending system projecting to the cochlear nuclei or the cochlea itself. The main nuclei of the ascending system, the lateral and medial superior olive (LSO, MSO), as well as the lateral and medial nuclei of the trapezoid body (LNTB, MNTB), have been traditionally associated with sound localization. Here we review the results of recent studies on the main SOC nuclei in echolocating bats. These studies suggest that some SOC structures and functions are highly conserved across mammals (e.g., the LSO, which is associated with interaural intensity difference processing), while others are phylogenetically highly variable in both form and function (e.g., the MSO, traditionally associated with interaural time difference processing). For the MSO, these variations indicate that we should broaden our view regarding what functions the MSO might participate in, since its function in echolocation seems to lie in the context of pattern recognition rather than sound localization. Furthermore, across bat species, variations in the form and physiology of the MSO can be linked to specific behavioral adaptations associated with different echolocation strategies. Finally, the comparative approach, including auditory specialists such as bats, helps us to reach a more comprehensive view of the functional anatomy of auditory structures that are still poorly understood, like the nucleus of the central acoustic tract (NCAT).