Susan E. Hall

Detection of silent intervals between noises activating different perceptual channels: Some properties of “central” auditory gap detection

This article describes four experiments on gap detection by normal listeners, with the general goal being to examine the consequences of using noises in different perceptual channels to delimit a silent temporal gap to be detected. In experiment 1, subjects were presented with pairs of narrow-band noise sequences. The leading element in each pair had a center frequency of 2 kHz and the trailing elements center frequency was parametrically varied. Gap detection thresholds became increasingly poor, sometimes by up to an order of magnitude, as the spectral disparity was increased between the noise bursts that marked the gap. These data suggested that gap-detection performance is impoverished when the underlying perceptual timing operation requires a comparison of activity in different perceptual channels rather than a discontinuity detection within a given channel. In experiment 2, we assessed the effect of leading-element duration in within-channel and between-channel gap detection tasks. Gap detection thresholds rose when the duration of the leading element was less than about 30 ms, but only in the between-channel case. In experiment 3, the gap-detection stimulus was redesigned so that we could probe the perceptual mechanisms that might be involved in stop consonant discrimination. The leading element was a wideband noise burst, and the trailing element was a 300-ms bandpassed noise centered on 1.0 kHz. The independent variable was the duration of the leading element, and the dependent variable was the smallest detectable gap between the elements. When the leading element was short in duration (5-10 ms), gap thresholds were close to 30 ms, which is close to the voice onset time that parses some voiced from unvoiced stop consonants. In experiment 4, the generality of the leading-element duration effect in between-channel gap detection was examined. Spectrally identical noises defining the leading and trailing edges of the gap were presented to the same or to different ears. There was a leading-element duration effect only for the between channel case. The mean gap threshold was again close to 30 ms for short leading-element durations. Taken together, the data suggest that gap detection requiring a temporal correlation of activity in different perceptual channels is a fundamentally different task to the discontinuity detection used to execute gap detection performance in the traditional, within-channel paradigm.

Detection of static and dynamic changes in interaural correlation

This study examines the relation between a static and a dynamic measure of interaural correlation discrimination: (1) the just noticeable difference (JND) in interaural correlation and (2) the minimum detectable duration of a fixed interaural correlation change embedded within a single noise-burst of a given reference correlation. For the first task, JNDs were obtained from reference interaural correlations of + 1, -1, and from 0 interaural correlation in either the positive or negative direction. For the dynamic task, duration thresholds were obtained for a brief target noise of +1, -1, and 0 interaural correlation embedded in reference marker noise of +1, -1, and 0 interaural correlation. Performance with a reference interaural correlation of +1 was significantly better than with a reference correlation of -1. Similarly, when the reference noise was interaurally uncorrelated, discrimination was significantly better for a target correlation change towards +1 than towards -1. Thus, for both static and dynamic tasks, interaural correlation discrimination in the positive range was significantly better than in the negative range. Using the two measures, the length of a binaural temporal window was estimated. Its equivalent rectangular duration (ERD) was approximately 86 ms and independent of the interaural correlation configuration.

“Central” auditory gap detection: A spatial case

Normal listeners were tested for their temporal auditory gap detection thresholds using free-field presentation of white-noise stimuli delivered from the left (L) and right (R) poles of the interaural axis. The noise bursts serving as the leading and trailing markers for the silent period were presented in either the same (LL,RR) or different (LR,RL) auditory locations. The duration of the leading marker was a second independent variable. Gap thresholds for stimuli in which the markers had the same location were low, and usually were independent of the duration of the leading marker. Gap thresholds for the LR and RL conditions were longer. These gap thresholds were sensitive to the duration of the leading marker, and increased as the leading marker duration decreased. This finding is consistent with the hypothesis that a relative timing operation mediates gap detection when the markers activate different perceptual channels. The present data suggest that this timing process can operate on perceptual channels emerging from central nervous system processing.

Psychophysical evidence for adaptation of central auditory processors for interaural differences in time and level

Human listeners were studied for their ability to lateralize single target tones of each of two frequencies relative to midline clicks. They did so before and after exposure to adaptor tones of the same frequencies. The adaptor tones were strongly lateralized, and in opposite directions for each frequency, by either an interaural time difference (ITD, Experiment 1) or interaural level difference (ILD, Experiment 2). Following adaptation, psychometric functions for ITD (Exp. 1) and ILD (Exp. 2) were obtained for target tones for the two frequencies separately. These were found to be shifted in the direction of the fatigued side. In the case of ILD, this was in the absence of a shift in monaural sensitivity sufficient to account for the effect. For both ITD and ILD studies, shifts in perceived laterality were induced in opposite directions at two frequencies concurrently. This effect was induced with only seconds of intermittent exposure to the adaptor tones. The fact that it could be induced at two frequencies in opposite directions at the same time, suggests (a), that these data constitute new psychophysical evidence for the frequency specificity of ITD and ILD coding in the human brain, and (b), that the effect was not due to the introduction of some response bias at the decision level of perceptual judgement. The data are interpreted in terms of a two- or three-channel opponent process model.

Auditory saltation: A new measure for an old illusion

Auditory saltation is a mislocalization phenomenon in which click trains presented successively at two discrete spatial locations appear to originate from a smoothly varying series of source locations spanning the true source locations. The temporal parameters of this illusion are investigated with click stimuli delivered via headphones with the goal of establishing a portable test of the illusion. A simple rating scale was developed which required human participants to indicate the degree to which the perceived sound locations were heard as continuous (evenly distributed across space) versus discontinuous. Control conditions were devised to provide a baseline against which to compare the illusion. Auditory saltation is discussed in terms of perceptual grouping.

The effects of lateralized adaptors on lateral position judgements of tones within and across frequency channels

Two experiments examined the effect of highly lateralized adaptor tone pulses on the perceived intracranial location of subsequent test tones. In Experiment 1, adaptor tones of each of two frequencies, highly lateralized to opposite sides by a quarter-period interaural time difference (ITD), were found to shift the perceived intracranial location of test tones of each adaptor frequency away from the side of the adaptor. The shift in perceived location was seen for all test tone ITDs with the same sign as the adaptor tone, and sometimes extended to include test tones with small ITDs favoring the opposite ear. The generality of the effect across test tone ITDs of the same sign as the adaptor suggests that the human auditory lateralization system is built of two (left, right) hemifield-tuned azimuthal channels, and that perceived lateral location depends on the relative outputs of those two channels. In Experiment 2, the perceived location of test tones lateralized by ITD was studied in the same listeners at each of the same two frequencies, but after selective adaptation with tone pulses of only one frequency and laterality. The perceived lateral position of test tones with the same frequency as that of the adaptor underwent the same changes as seen in Experiment 1. The perceived lateral position of test tones of the nonadapted frequency usually shifted weakly in the opposite direction, i.e., in the direction expected if the second adaptor from Experiment 1 had actually been present. These data have implications both for the processes mediating selective adaptation using contingent stimuli, and for the azimuthal tuning of auditory spatial channels in man.

Interaction in the perceptual processing of interaural time and level differences

Phillips and Hall [Psychophysical evidence for adaptation of central auditory processors for interaural differences in time and level, Hear. Res., 202 (2005) 188-199.] recently described the frequency-specific, selective adaptation of perceptual channels for interaural differences in level (ILD) and time (ITD). Psychometric functions for laterality based on ITD or ILD were obtained before and after exposure to adaptor tones of two frequencies presented alternately and highly lateralized to opposite sides. Following adaptation, points of perceived centrality (PPCs) were displaced towards the sides of the adaptor tones, and in opposite directions for the two frequencies. That is, laterality judgements showed a shift away from the adapted side, particularly for test cue values near the middle of the range. These data were congruent with a two-channel, opponent-process model of sound laterality coding. The present study used the same general paradigm to explore the independence of perceptual ITD and ILD processing. Psychometric functions for laterality based on ITD or ILD were obtained for each of two frequencies concurrently, before and after exposure to adaptor tones lateralized using the complementary cue. Once again, PPCs derived from the psychometric functions were displaced towards the sides of the adaptor tones, consistent with an opponent-process account of sound laterality coding. The size of the adaptation effect was at least as great as that described in the earlier study. Thus, a quarter cycle ITD adapting stimulus effected a 3 dB shift in the mean ILD-based PPC, and a 12 dB ILD adapting stimulus effected a 100 micros shift in the mean ITD-based PPC. These data offer new evidence concerning interaction in the processing of ITDs and ILDs.

ADDITIVITY OF PERCEPTUAL CHANNEL-CROSSING EFFECTS IN AUDITORY GAP DETECTION

Five normal listeners were tested in detail for their auditory gap-detection thresholds, using stimuli in which the narrow-band noise markers of the gap differed in one or both of two auditory dimensions (frequency composition and ear stimulated). Gap thresholds for stimuli in which the markers differed along either single dimension averaged about 18 ms, while thresholds for markers differing across both dimensions were closer to 28 ms. These data suggest that the perceptual relative-timing operation that mediates between-channel gap detection is shared across auditory dimensions.

Spatial Stimulus Cue Information Supplying Auditory Saltation

Auditory saltation is a misperception of the spatial location of repetitive, transient stimuli. It arises when clicks at one location are followed in perfect temporal cadence by identical clicks at a second location. This report describes two psychophysical experiments designed to examine the sensitivity of auditory saltation to different stimulus cues for auditory spatial perception. Experiment 1 was a dichotic study in which six different six-click train stimuli were used to generate the saltation effect. Clicks lateralised by using interaural time differences and clicks lateralised by using interaural level differences produced equivalent saltation effects, confirming an earlier finding. Switching the stimulus cue from an interaural time difference to an interaural level difference (or the reverse) in mid train was inconsequential to the saltation illusion. Experiment 2 was a free-field study in which subjects rated the illusory motion generated by clicks emitted from two sound sources symmetrically disposed around the interaural axis, ie on the same cone of confusion in the auditory hemifield opposite one ear. Stimuli in such positions produce spatial location judgments that are based more heavily on monaural spectral information than on binaural computations. The free-field stimuli produced robust saltation. The data from both experiments are consistent with the view that auditory saltation can emerge from spatial processing, irrespective of the stimulus cue information used to determine click laterality or location.

Sensitivity of unanesthetized chinchilla auditory system to noise burst onset, and the effects of carboplatin

The gross near-field responses of the auditory nerve and inferior colliculus to noise burst stimuli were recorded through intracranially implanted electrodes in six unanesthetized chinchillas. Responses were studied as a function of stimulus plateau amplitude and rise time, both before and after a systemic dose of 75 mg/kg of carboplatin. Both recording sites showed sensitivity to stimulus level and rise time. Increases in stimulus level and decreases in stimulus rise time each produced increases in the response magnitude, and decreases in response latency. When the stimuli were re-specified as rate of pressure change at sound onset (Pa/s), the amplitude and latency of responses at each site were found to be a direct function of rate of sound pressure change. These data provide the first confirmation in unanesthetized animals of previous single unit observations in barbiturate-anesthetized cats. Carboplatin treatment resulted in a 20-80% loss of inner hair cells, a modest threshold elevation, and a 50-75% reduction in peak response amplitudes. The general patterns of sensitivity to stimulus level and rise time were not markedly affected by carboplatin, nor was the fashion in which response parameters (amplitude and latency) were ruled by rate of pressure change at sound onset.