Tanya L. Arbogast

Note on informational masking (L)

Informational masking (IM) has a long history and is currently receiving considerable attention. Nevertheless, there is no clear and generally accepted picture of how IM should be defined, and once defined, explained. In this letter, consideration is given to the problems of defining IM and specifying research that is needed to better understand and model IM.

The effect of spatial separation on informational and energetic masking of speech.

The effect of spatial separation of sources on the masking of a speech signal was investigated for three types of maskers, ranging from energetic to informational. Normal-hearing listeners performed a closed-set speech identification task in the presence of a masker at various signal-to-noise ratios. Stimuli were presented in a quiet sound field. The signal was played from 0 degrees azimuth and a masker was played either from the same location or from 90 degrees to the right. Signals and maskers were derived from sentences that were preprocessed by a modified cochlear-implant simulation program that filtered each sentence into 15 frequency bands, extracted the envelopes from each band, and used these envelopes to modulate pure tones at the center frequencies of the bands. In each trial, the signal was generated by summing together eight randomly selected frequency bands from the preprocessed signal sentence. Three maskers were derived from the preprocessed masker sentences: (1) different-band sentence, which was generated by summing together six randomly selected frequency bands out of the seven bands not present in the signal (resulting in primarily informational masking); (2) different-band noise, which was generated by convolving the different-band sentence with Gaussian noise; and (3) same-band noise, which was generated by summing the same eight bands from the preprocessed masker sentence that were used in the signal sentence and convolving the result with Gaussian noise (resulting in primarily energetic masking). Results revealed that in the different-band sentence masker, the effect of spatial separation averaged 18 dB (at 51% correct), while in the different-band and same-band noise maskers the effect was less than 10 dB. These results suggest that, in these conditions, the advantage due to spatial separation of sources is greater for informational masking than for energetic masking.

The effect of spatial separation on informational masking of speech in normal-hearing and hearing-impaired listeners

The ability to understand speech in a multi-source environment containing informational masking may depend on the perceptual arrangement of signal and masker objects in space. In normal-hearing listeners, Arbogast et al. [J. Acoust. Soc. Am. 112, 2086-2098 (2002)] found an 18-dB spatial release from a primarily informational masker, compared to 7 dB for a primarily energetic masker. This article extends the earlier work to include the study of listeners with sensorineural hearing loss. Listeners performed closed-set speech recognition in two spatial conditions: 0 degrees and 90 degrees separation between signal and masker. Three maskers were tested: (1) the different-band sentence masker was designed to be primarily informational; (2) the different-band noise masker was a control for the different-band sentence; and (3) the same-band noise masker was designed to be primarily energetic. The spatial release from the different-band sentence was larger than for the other maskers, but was smaller (10 dB) for the hearing-impaired group than for the normal-hearing group (15 dB). The smaller benefit for the hearing-impaired listeners can be partially explained by masker sensation level. However, the results suggest that hearing-impaired listeners can use the perceptual effect of spatial separation to improve speech recognition in the presence of a primarily informational masker.

Evidence for spatial tuning in informational masking using the probe-signal method

Auditory spatial attention is one mechanism that may contribute to the ability to identify one sound source in a multi-source environment. The role of auditory spatial attention in a multi-source environment was investigated using the probe-signal method. The experiment took place in a quiet room with seven speakers arranged in a semi-circle in front of the listener. The speakers were placed at 30-degree intervals at a distance of 5 ft from the listener. The signal was comprised of eight contiguous, 60-ms pure-tone bursts arranged in either a rising or falling frequency pattern. Masker components were also comprised of eight contiguous pure-tone bursts but with durations that varied randomly from 20 to 100 ms. The six maskers were played with the signal and were constructed in order to result in informational rather than energetic masking. The frequency of each masker component was chosen randomly on each burst from a narrow frequency band, independent from the signal frequency band. The task was 1I-2AFC fixed-level identification with response time measurement. The listener was instructed to focus attention on a specified speaker (expected location) for a block of trials. Accuracy and response time were compared across two conditions: (1) signal presented at the expected location and (2) signal presented at an unexpected location. Results indicate a significant increase in accuracy and faster response time when the signal was presented at the expected location as compared to an unexpected location. These results suggest that auditory spatial attention plays an important role in multi-source listening, especially when the listening environment is complex and uncertain.

The role of reverberation in release from masking due to spatial separation of sources in speech recognition

Arbogast et al. [ARO Mtg. (2002)] found a large release from masking obtained by spatial separation of a target talker and competing speech masker. Both stimuli were sentences from the Coordinate Response Measure corpus [Bolia et al., J. Acoust. Soc. Am. (2000)] processed by extracting the envelopes of 15 narrow frequency bands and using the envelopes to modulate carrier tones at the center of each band. By playing nonoverlapping subsets (6–8) of bands from signal and masker they minimized the energetic component while maximizing the informational component of masking. This study extends that work to determine the interaction between reverberation, masker type, and spatial release from masking. Stimuli were processed and presented as above. The target sentence was played at 0‐deg azimuth while the masker sentence was played at 0 or 90‐deg azimuth. Noise–masker controls were also tested. The listening environment was an IAC booth having dimensions of 12 ft×13 ft. Acoustic extremes were achieved using Plexi...

Informational masking: Toward improved understanding

This paper is concerned with simultaneous informational masking of tones by tone complexes. Past work distinguishes between energetic (peripheral) masking, which occurs when target and masker overlap in the peripheral frequency channels, and informational (central) masking, which can occur even when there is no such overlap but there is uncertainty in the frequency spectrum of the stimulus. We suggest that this distinction is (1) applicable to domains other than the frequency domain (e.g., the spatial domain) and (2) relative to the level of processing in the auditory system considered. We further suggest that this distinction can be rigorously defined for any domain and any level by comparing performance of the ideal detector operating on the inputs at that level to human psychophysical performance. We also argue that informational masking is strongly influenced by target‐masker similarity and segregation difficulty, not only by stimulus uncertainty. Experiments designed to probe this issue are discussed...

Signal–masker similarity and perceptual segregation in informational masking: Some examples

In a companion paper at this meeting (Durlach et al.) it was suggested that changes in similarity and/or perceptual segregation of a signal and masker could affect the amount of informational masking in ways that would not be predicted based on the amount of uncertainty. In that paper, several simple stimulus manipulations are used as illustrative examples of the predicted changes. Although these were suggested as gedanken (thought) experiments, data are presented here confirming these suggestions. Perceptual segregation of the signal was accomplished by varying the degree of similarity or coherence between signal and masker along a relevant stimulus dimension. For example, when the masker consisted of a set of randomly drawn upward glides, listeners perceptually segregated a downward gliding signal. This reduced the amount of informational masking obtained relative to the condition in which both signal and masker were similar upward glides. The same was true for asynchrony of onsets, difference in percei...

Increasing informational masking by contralateral stimulation

Although informational masking is thought to reflect central mechanisms, substantial reductions in informational masking generally occur when the target and masker are presented to different ears. Recently, Brungart and Simpson [J. Acoust. Soc. Am.] found that the intelligibility of a target phrase in a monaural two‐talker stimulus was severely degraded by the addition of an unrelated speech signal to the contralateral ear. Their results suggest that the presence of an informational masker in the unattended ear can increase informational masking in the target ear. In this study parallel experiments using nonspeech informational maskers determined whether similar results are obtained in nonspeech detection tasks. The stimuli were sequences of multitone bursts arranged in spectrotemporal patterns that produce very large or very small amounts of informational masking. The signal was a sequence of 1000‐Hz tone bursts played synchronously with the maskers. Combinations of effective and ineffective informationa...