Examination of a linear model in an informational masking study

Informational masking in the modulation domain

Uncertainty regarding the frequency spectrum of a masker can have an adverse effect on the ability to focus selective attention on a target frequency channel, yielding informational masking (IM). This study sought to determine if uncertainty regarding the modulation spectrum of a masker can have an analogous adverse effect on the ability to focus selective attention on a target modulation channel, yielding IM in the modulation domain, or "modulation IM." A single-interval, two-alternative forced-choice (yes-no) procedure was used. The task was to detect 32-Hz target sinusoidal amplitude modulation (SAM) imposed on a broadband-noise carrier in the presence of masker SAM imposed on the same carrier. Six maskers, spanning the range from 8 to 128 Hz in half-octave steps, were tested, excluding those that fell within a two-octave protected zone surrounding the target. Psychometric functions (d'-vs-target modulation depth) were measured for each masker under two conditions: a fixed (low-uncertainty/low-IM) condition, in which the masker was the same on all trials within a block, and a random (high-uncertainty/high-IM) condition, in which it varied randomly from presentation-to-presentation. Thresholds and slopes extracted from the psychometric functions differed markedly between the conditions. These results are consistent with the idea that IM occurs in the modulation domain.

Sub-optimal construction of an auditory profile from temporally distributed spectral information

When spectral components of a complex sound are presented not simultaneously but distributed over time, human listeners can still, to a degree, perceptually recover the spectral profile of the sound. This capability of integrating spectral information over time was investigated using a cued informational masking paradigm. Listeners detected a 1-kHz pure tone in a simultaneous masker composed of six random-frequency tones drawn on every trial. The spectral profile of the masker was cued using a precursor sound that consisted of a sequence of 50-ms bursts, separated by inter-burst intervals of 100 ms. Each burst in the precursor consisted of pure tones at the masker frequencies with tones appearing at each of the masker frequencies at different presentation probabilities. As the presentation probability increased in different conditions, the detectability of the target improved, indicating reliable precursor cuing regarding the spectral content of the masker. For many listeners, performance did not significantly improve as the number of precursor bursts increased from 2 to 16, indicating inefficient integration of information beyond 2 bursts. Additional analyses suggest that when intensity of the bursts is relatively constant, the contribution of the precursor is dominated by information in the initial burst.

Informational masking of negative masking

Negative masking (NM) is a ubiquitous finding in near-"threshold" psychophysics in which the detectability of a near-threshold signal improves when added to a copy of itself, i.e., a pedestal or masker. One interpretation of NM suggests that the pedestal acts as an informative cue, thereby reducing uncertainty and improving performance relative to detection in its absence. The purpose of this study was to test this hypothesis. Intensity discrimination thresholds were measured for 100-ms, 1000-Hz near-threshold tones. In the reference condition, thresholds were measured in quiet (no masker other than the pedestal). In comparison conditions, thresholds were measured in the presence of one of two additional maskers: a notched-noise masker or a random-frequency multitone masker. The additional maskers were intended to cause different amounts of uncertainty and, in turn, to differentially influence NM. The results were generally consistent with an uncertainty-based interpretation of NM: NM was found both in quiet and in notched-noise, yet it was eliminated by the multitone masker. A competing interpretation of NM based on nonlinear transduction does not account for all of the results. Profile analysis may have been a factor in performance and this suggests that NM may be attributable to, or influenced by, multiple mechanisms.

Development of auditory selective attention: why children struggle to hear in noisy environments

Children's hearing deteriorates markedly in the presence of unpredictable noise. To explore why, 187 school-age children (4-11 years) and 15 adults performed a tone-in-noise detection task, in which the masking noise varied randomly between every presentation. Selective attention was evaluated by measuring the degree to which listeners were influenced by (i.e., gave weight to) each spectral region of the stimulus. Psychometric fits were also used to estimate levels of internal noise and bias. Levels of masking were found to decrease with age, becoming adult-like by 9-11 years. This change was explained by improvements in selective attention alone, with older listeners better able to ignore noise similar in frequency to the target. Consistent with this, age-related differences in masking were abolished when the noise was made more distant in frequency to the target. This work offers novel evidence that improvements in selective attention are critical for the normal development of auditory judgments.

Estimates of internal templates for the detection of sequential tonal patterns

In this experiment, listeners detected sequential tonal patterns embedded in multitone multiburst random maskers. The maskers consisted of eight 30 ms bursts of random-frequency tones. The signal, when present, occupied the central six bursts and was centered at 1000 Hz. The six sequential signal tones formed several spectro-temporal patterns: an equal-frequency pattern, three ascending patterns with frequency ranges spanning 0.5-, 1-, and 2-equivalent rectangular bandwidths (ERBs), and a random pattern with frequencies drawn at random from the range of 925-1075 Hz. The total number of tones in each burst, m, was varied to determine detection threshold. The detectability of the signal pattern declined as the frequency range of the signal pattern increased, and when the signal was random. Relative weights as a function of time and frequency, interpreted as listeners' internal templates, depended systematically on the properties of the signal pattern tested. The templates indicated that when sensitivity was poor, listeners integrated increasingly broad spectro-temporal regions around the signal frequencies, and sometimes integrated energy from the final burst even though the signal tones never occupied the final burst.

Estimates of effective frequency selectivity based on the detection of a tone added to complex maskers

In Experiment 1, the validity of parameters associated with the roex(p, r) auditory filter shape was examined for three different types of maskers: (a) A noise masker, (b) a random 12-tone masker whose frequencies varied on a burst-by-burst basis [multiple-burst different (MBD)], and (c) a random 12-tone masker whose frequencies were the same across bursts [multiple-burst same (MBS)]. First, the power spectrum model of masking was used to estimate auditory filter shapes for four observers. Second, the resulting auditory filter shapes were used in a computer simulation that provided an estimate of internal noise for each observer. Third, relative weights across frequency were estimated for each observer and each masker type. For the noise masker, these analyses provided predictions and relative weights that were consistent across the three analyses. For the MBD and MBS maskers, there was little consistency; neither the estimated internal noise nor the estimated relative weights reliably supported a single-filter model of detection. In Experiment 2, the time course for the detection of a tone added to an MBD masker was evaluated by estimating relative weights jointly in time and frequency. The relative weights at the signal frequency formed a rough inverse "U" across time.

Informational masking of speech in children: effects of ipsilateral and contralateral distracters

Using a closed-set speech recognition paradigm thought to be heavily influenced by informational masking, auditory selective attention was measured in 38 children (ages 4-16 years) and 8 adults (ages 20-30 years). The task required attention to a monaural target speech message that was presented with a time-synchronized distracter message in the same ear. In some conditions a second distracter message or a speech-shaped noise was presented to the other ear. Compared to adults, children required higher target/distracter ratios to reach comparable performance levels, reflecting more informational masking in these listeners. Informational masking in most conditions was confirmed by the fact that a large proportion of the errors made by the listeners were contained in the distracter message(s). There was a monotonic age effect, such that even the children in the oldest age group (13.6-16 years) demonstrated poorer performance than adults. For both children and adults, presentation of an additional distracter in the contralateral ear significantly reduced performance, even when the distracter messages were produced by a talker of different sex than the target talker. The results are consistent with earlier reports from pure-tone masking studies that informational masking effects are much larger in children than in adults.

Informational masking for simultaneous nonspeech stimuli: psychometric functions for fixed and randomly mixed maskers

Sensitivity d' and response bias beta were measured as a function of target level for the detection of a 1000-Hz tone in multitone maskers using a one interval, two-alternative forced-choice (1I-2AFC) paradigm. Ten such maskers, each with eight randomly selected components in the region 200-5000 Hz, with 800-1250 Hz excluded to form a protected zone, were presented under two conditions: the fixed condition, in which the same eight-component masker is used throughout an experimental run, and the random condition, in which an eight-component masker is chosen randomly trial-to-trial from the given set of ten such maskers. Differences between the results obtained with these two conditions help characterize the listener's susceptibility to informational masking (IM). The d' results show great intersubject variability, but can be reasonably well fit by simple energy-detector models in which internal noise and filter bandwidth are used as fitting parameters. In contrast, the beta results are not well fit by these models. In addition to presentation of new data and its relation to energy-detector models, this paper provides comments on a variety of issues, problems, and research needs in the IM area.

Observer weighting strategies in interaural time-difference discrimination and monaural level discrimination for a multi-tone complex

Two experiments measured listeners' abilities to weight information from different components in a complex of 553, 753, and 953 Hz. The goal was to determine whether or not the ability to adjust perceptual weights generalized across tasks. Weights were measured by binary logistic regression between stimulus values that were sampled from Gaussian distributions and listeners' responses. The first task was interaural time discrimination in which listeners judged the laterality of the target component. The second task was monaural level discrimination in which listeners indicated whether the level of the target component decreased or increased across two intervals. For both experiments, each of the three components served as the target. Ten listeners participated in both experiments. The results showed that those individuals who adjusted perceptual weights in the interaural time experiment could also do so in the monaural level discrimination task. The fact that the same individuals appeared to be analytic in both tasks is an indication that the weights measure the ability to attend to a particular region of the spectrum while ignoring other spectral regions.

Masker-first advantage for cues in informational masking

The detectability of a pure-tone signal may be reduced by adding a small number of randomly drawn masker tones remote from the signal frequency, an effect attributed to informational masking. A pretrial cue consisting of either the upcoming signal or masker releases informational masking, but a pretrial cue of the signal-plus-masker stimulus does not. In these experiments the relative potency of pre- and posttrial cues in releasing informational masking was examined. In separate conditions the masker-alone and signal-plus-masker stimuli were cues. The results indicated a masker-first advantage, i.e., sensitivity was superior when a masker cue preceded a yes/no trial interval compared to (a) when a signal-plus-masker preceded the trial, and (b) when either cue type followed the yes/no trial interval. A masker-first advantage was also obtained when the results from a two-interval forced-choice same/different task were examined. In contrast, a masker-first advantage was not obtained when the frequency of the signal to be detected was random. For detection tasks using random multi-tone maskers there may be differences in processing efficiency depending on the order in which stimuli are presented. The "masker-first advantage" may depend, in part, on observers maintaining their attention at the signal frequency.

Cuing effects for informational masking

The detection of a tone added to a random-frequency, multitone masker can be very poor even when the maskers have little energy in the frequency region of the signal. This paper examines the effects of adding a pretrial cue to reduce uncertainty for the masker or the signal. The first two experiments examined the effect of cuing a fixed-frequency signal as the number of masker components and presentation methods were manipulated. Cue effectiveness varied across observers, but could reduce thresholds by as much as 20 dB. Procedural comparisons indicated observers benefited more from having two masker samples to compare, with or without a signal cue, than having a single interval with one masker sample and a signal cue. The third experiment used random-frequency signals and compared no-cue, signal-cue, and masker-cue conditions, and also systematically varied the time interval between cue offset and trial onset. Thresholds with a cued random-frequency signal remained higher than for a cued fixed-frequency signal. For time intervals between the cue and trial of 50 ms or longer, thresholds were approximately the same with a signal or a masker cue and lower than when there was no cue. Without a cue or with a masker cue, analyses of possible decision strategies suggested observers attended to the potential signal frequencies, particularly the highest signal frequency. With a signal cue, observers appeared to attend to the frequency of the subsequent signal.