Forward Masking: Temporal Integration or Adaptation?

In this paper we describe a method for comparing frequency selectivity in simultaneous and forward masking. The method is designed to eliminate off-frequency listening, which may have had a confounding influence in earlier studies. Thresholds for 1-kHz sinusoidal signals were measured as a function of the width of a spectral notch, centered at 1 kHz, a noise masker. In experiment I thresholds were measured in forward masking for signal durations of 5, 15, and 45 ms, with a noise spectrum level of 40 dB SPL/Hz. Thresholds decreased with increasing notch width for all signal durations. However, the change was more gradual at longer signal durations. This is consistent with a model in which the process of decay of masking follows the auditory filter. For each signal duration, threshold was also measured as a function of the level of a noise without a spectral notch. Each signal threshold for the notch-noise condition was then expressed as the level of a flat-spectrum noise which would give the same masking. When transformed in this way, the data for the three signal durations coincide, and can be interpreted in terms of the same auditory filter. In experiment II the 5-ms signals, whose level was fixed, followed the masker with delays of 5, 15, or 25 ms. The noise spectrum level was varied to find threshold. The change in threshold with notch width was independent of signal delay, confirming that the process of decay of masking follows the auditory filter. Experiment III showed that short-term spectral changes produced by differences in the shapes of the envelopes of signal and masker did not influence the results. In experiment IV thresholds for a 5-ms signal were measured in simultaneous and forward masking, both for a fixed noise level and for a fixed signal level. The derived auditory filters are similar for the two types of measure for each type of masking. However, the auditory filters derived in forward masking have bandwidths 17% smaller and slopes 48% greater than those in simultaneous masking. The differences between simultaneous and forward masking are interpreted in terms of suppression.

The mechanisms of forward masking are not clearly understood. The temporal window model (TWM) proposes that masking occurs via a neural mechanism that integrates within a temporal window. The medial olivocochlear reflex (MOCR), a sound-evoked reflex that reduces cochlear amplifier gain, may also contribute to forward masking if the preceding sound reduces gain for the signal. Psychophysical evidence of gain reduction can be observed using a growth of masking (GOM) paradigm with an off-frequency forward masker and a precursor. The basilar membrane input/output (I/O) function is estimated from the GOM function, and the I/O function gain is reduced by the precursor. In this study, the effect of precursor duration on this gain reduction effect was examined for on- and off-frequency precursors. With on-frequency precursors, thresholds increased with increasing precursor duration, then decreased (rolled over) for longer durations. Thresholds with off-frequency precursors continued to increase with increasing precursor duration. These results are not consistent with solely neural masking, but may reflect gain reduction that selectively affects on-frequency stimuli. The TWM was modified to include history-dependent gain reduction to simulate the MOCR, called the temporal window model-gain reduction (TWM-GR). The TWM-GR predicted rollover and the differences with on- and off-frequency precursors whereas the TWM did not.

Temporal resolution and temporal integration in the auditory system, with their conflicting time constants, are often regarded as paradox phenomena. In this study a model is presented for the simulation of a number of temporal resolution experiments (e.g., gap detection, forward masking) and classical test tone integration data (cf. contributions by Dau et al.). The model consists of two major parts. A preprocessing stage that transforms the acoustic stimulus into an internal representation, simulating the peripheral processing in the auditory system. Processing errors are accounted for by adding internal noise which introduces detection limits. The second stage is an optimum detector to simulate a ‘central’ decision device. The preprocessing stage consists mainly of three subunits: a Gamma tone filterbank to simulate peripheral filtering; a set of adaptive feedback loops that derives an adapted envelope of every peripheral filter output; a modulation filterbank (per peripheral channel) that analyzes stimulus periodicities in these adapted representations. The model is able to simulate gap detection and test tone integration experiments quantitatively without changing its parameters. It can be shown that within this model the modulation filterbank is vital to achieve this performance. [Supported by the DFG (Az. Mu/1201).]

The frequency selectivity of an auditory filter system is often conceptualized as a bank of bandpass auditory filters. Over the past 30 years, many simultaneous masking experiments using notched-noise maskers have been done to define the shape of the auditory filters (e.g., Glasberg and Moore 1990; Patterson and Nimmo-Smith 1980; Rosen and Baker, 1994). The studies of Glasberg and Moore (2000) and Baker and Rosen (2006) are notable inasmuch as they measured the human auditory filter shape over most of the range of frequencies and levels encountered in everyday hearing. The advantage of using notched-noise masking is that one can avoid off-frequency listening and investigate filter asymmetry. However, the derived filter shapes are also affected by the effects of suppression. The tunings of auditory filters derived from data collected in forward masking experiments were apparently sharper than those derived from simultaneous masking experiments, especially when the signal levels are low. The tuning of a filter is commonly believed to be affected by cochlear nonlinearity such as the effect of suppression. In past studies, the tunings of auditory filters derived from simultaneous masking data were wider than those of filters derived from nonsimultaneous (forward) masking data (Moore and Glasberg 1978; Glasberg and Moore 1982; Oxenham and Shera 2003). Heinz et al. (2002) showed that a tuning is generally sharpest when stimuli are at low levels and that suppression may affect tuning estimates more at high characteristic frequencies (CFs) than at low CFs. If the suggestion of Heinz et al. (2002) holds, i.e., if suppression affects frequency changes, comparing the filter bandwidths derived from simultaneous and forward masking experiments would indicate this. In this study we attempt to estimate filter tunings using both simultaneous and forward masking experiments with a notched-noise masker to investigate how the effects of suppression affect estimates of frequency selectivity across signal frequencies, signal levels, notch conditions (symmetric and asymmetric), and signal delays. This study extends the study of Unoki and Tan (2005).

Decision Making in Recurrent Neuronal Circuits

A revised computational model of the inner-hair cell (IHC) and auditory-nerve (AN) complex was recently presented [Sumner et al., J. Acoust. Soc. Am. 111, 2178-2188 (2002)]. One key improvement is that the model reproduces the rate-intensity functions of low- (LSR), medium- (MSR), and high-spontaneous rate (HSR) fibers in the guinea-pig. Here we describe the adaptation characteristics of the model, and how they vary with model fiber type. Adaptation of the revised model for a HSR fiber is in line with an earlier version of the model [Meddis and Hewitt, J. Acoust. Soc. Am. 90, 904-917 (1991)]. In guinea-pig, poststimulus time histograms (PSTH) have been found to show less adaptation in LSR fibers. Evidence from chinchilla suggests that this is due to chronic adaptation resulting from short interstimulus intervals, and that fully recovered LSR fibers actually show more adaptation. However, the model is able to account for both variations of PSTH shape when fully recovered from adaptation. Interstimulus interval can also affect recovery in the model. The model is further tested against data previously used to evaluate models of AN adaptation. The tests are (i) recovery from adaptation of spontaneous rate and (ii) the recovery of response to acoustic stimuli ("forward masking"), (iii) the response to stimulus increments and (iv) decrements, and (v) the conservation of transient components. A HSR model fiber performs similarly to the earlier version of the model. However, there is considerable variation in response to increments and decrements between different model fibers.

Auditory neurophonic responses to amplitude-modulated tones: Transfer functions and forward masking

A computerized two-interval forced-choice method was used to measure the threshold for detection of the N1 peak of the compound action potential (CAP) recorded at the round window of the chinchilla in response to a 23-ms probe tone. A 100-ms, pure-tone masker of equal frequency preceded the probe tone. Threshold shifts produced by the masker were measured as the intensity of the masker and the time delay between the masker and the probe were varied. The techniques differ from prior studies in that forward masking of the compound action potential has been quantified as shifts in detection thresholds rather than decrements in the magnitude of the N1 peak. Results were compared to analogous measures for single neurons in the auditory nerve [E. M. Relkin and D. G. Pelli, J. Acoust. Soc. Am. 82, 1679–1691 (1987)] and analogous psychophysical measures of forward masking. Preliminary findings indicate that forward masking of the CAP more closely corresponds to that observed psychophysically than does the forward masking observed in the response of a single neuron. [Work supported by the Deafness Research Foundation and NIH.]

The temporal evolution of masking and frequency selectivity was studied in the goldfish using classical respiratory conditioning and a tracking psychophysical procedure. The temporal position of a brief tonal signal within a longer duration, tonal masker has little or no effect on signal detectability when the frequency of the masker is less than or equal to that of the signal. For masker frequencies above that of the signal, signal detectability improves as the signal onset is delayed relative to that of the masker. These patterns of tone-on-tone masking are quite similar to those observed for humans. These temporal masking patterns are qualitatively similar in shape to the peristimulus-time histogram profiles of the low-frequency saccular fibers thought to be used in this task. Frequency- and time-dependent changes in signal detectability result in specific changes in the sharpness of psychophysical tuning curves (PTC). In general, PTCs determined for signals occurring at masker onset are the most broadly tuned, and PTCs determined in forward masking are the most sharply tuned. The PTCs for signals temporally centered in the masker are intermediate. These results suggest that temporal tone-on-tone masking patterns and the temporal evolution of psychophysical tuning curves result from the response properties of peripheral auditory-nerve fibers.

Reliability and Representational Bandwidth in the Auditory Cortex

Previously, a discrepancy between the amount of forward masking in the auditory nerve and published behavioral data was reported by these laboratories [E. M. Relkin and C. W. Turner, J. Acoust. Soc. Am. 84, 584–591 (1988)]; in that physiological experiment, however, a 2-ms signal rise time was employed, while the previous behavioral data used longer rise times. In the present behavioral study, both 2-ms and 10-ms signal rise-fall times were employed. The results show that for 10-ms ramps, the maximum amount of masking is large (at least 30 dB), in agreement with previous data. In contrast, for 2-ms ramps, the maximum amount of masking is smaller (15 dB). This is similar to the amount of masking in the auditory nerve reported in Relkin and Turner (1988), and indicates little or no discrepancy between behavioral and auditory nerve forward masking. This difference is observed for wide- and narrow-band maskers, suggesting that the results are not simply explained by the short-term spectra of the ramps. [Work supported by NINCDS.]

Amniotes evolved a unique postsynaptic terminal in the inner ear vestibular organs called the calyx that receives both quantal and nonquantal (NQ) synaptic inputs from Type I sensory hair cells. The nonquantal synaptic current includes an ultrafast component that has been hypothesized to underlie the exceptionally high synchronization index (vector strength) of vestibular afferent neurons in response to sound and vibration. Here, we present three lines of evidence supporting the hypothesis that nonquantal transmission is responsible for synchronized vestibular action potentials of short latency in the guinea pig utricle of either sex. First, synchronized vestibular nerve responses are unchanged after administration of the AMPA receptor antagonist CNQX, while auditory nerve responses are completely abolished. Second, stimulus evoked vestibular nerve compound action potentials (vCAP) are shown to occur without measurable synaptic delay and three times shorter than the latency of auditory nerve compound action potentials (cCAP), relative to the generation of extracellular receptor potentials. Third, paired-pulse stimuli designed to deplete the readily releasable pool (RRP) of synaptic vesicles in hair cells reveal forward masking in guinea pig auditory cCAPs, but a complete lack of forward masking in vestibular vCAPs. Results support the conclusion that the fast component of nonquantal transmission at calyceal synapses is indefatigable and responsible for ultrafast responses of vestibular organs evoked by transient stimuli.SIGNIFICANCE STATEMENT The mammalian vestibular system drives some of the fastest reflex pathways in the nervous system, ensuring stable gaze and postural control for locomotion on land. To achieve this, terrestrial amniotes evolved a large, unique calyx afferent terminal which completely envelopes one or more presynaptic vestibular hair cells, which transmits mechanosensory signals mediated by quantal and nonquantal (NQ) synaptic transmission. We present several lines of evidence in the guinea pig which reveals the most sensitive vestibular afferents are remarkably fast, much faster than their auditory nerve counterparts. Here, we present neurophysiological and pharmacological evidence that demonstrates this vestibular speed advantage arises from ultrafast NQ electrical synaptic transmission from Type I hair cells to their calyx partners.

The Bayesian adaptive procedure proposed by Shen and Richards [J. Acoust. Soc. Am. 134(2), 1134–1145 (2013)] was extended to allow the rapid estimation of auditory filters that were asymmetric about their peak frequencies. The estimation of the auditory-filter shape (five free parameters) was achieved using single Bayesian adaptive tracks of 150–200 trials (approximately 15 min to complete). During the experimental track, listeners detected a tonal signal presented with either simultaneous or forward maskers. Both types of maskers consisted of two bands of noises, one on each side of the signal frequency. The Bayesian adaptive procedure iteratively updated the parameter estimates following each experimental trial and determined the stimulus that would maximize the gain of information on the following trial. The stimuli were adaptively manipulated along three dimensions: the masker level and the spectral location of the upper and lower masker bands. The proposed procedure allowed the reliable estimation of the auditory-filter shape for naïve normal-hearing listeners. The model predictions replicated the known effect of the masker-signal simultaneity on the auditory filter.

An organism’s ability to detect and discriminate sensory inputs depends on the recent stimulus history. For example, perceptual detection thresholds for a brief tone can be elevated by as much as 50 dB when following a masking stimulus. Previous work suggests that such forward masking is not a direct result of peripheral neural adaptation; the central pathway apparently modifies the representation in a way that further attenuates the input’s response to short probe signals. Here, we show that much of this transformation is complete by the level of the inferior colliculus (IC). Single-neuron extracellular responses were recorded in the central nucleus of the awake marmoset IC. The threshold for a 20-ms probe tone presented at best frequency was determined for various masker-probe delays, over a range of masker SPLs and frequencies. The most striking aspect of the data was the increased potency of forward maskers as their SPL was increased, despite the fact that the excitatory response to the masker was often saturating or non-monotonic over the same range of levels. This led to probe thresholds at high masker levels that were almost always higher than those observed in the auditory nerve. Probe threshold shifts were not usually caused by a persistent excitatory response to the masker; instead we propose a wide dynamic-range inhibitory mechanism locked to sound offset as an explanation for several key aspects of the data. These findings further delineate the role of subcortical auditory processing in the generation of a context-dependent representation of ongoing acoustic scenes.

In the diverse mechanosensory systems that animals evolved, the waveform of stimuli can be encoded by phase locking in spike trains of primary afferents. Coding of the fine structure of sounds via phase locking is thought to be critical for hearing. The upper frequency limit of phase locking varies across species and is unknown in humans. We applied a method developed previously, which is based on neural adaptation evoked by forward masking, to analyze mass potentials recorded on the cochlea and auditory nerve in the cat. The method allows us to separate neural phase locking from receptor potentials. We find that the frequency limit of neural phase locking obtained from mass potentials was very similar to that reported for individual auditory nerve fibers. The results suggest that this is a promising approach to examine neural phase locking in humans with normal or impaired hearing or in other species for which direct recordings from primary afferents are not feasible.