PubMed Health⌕ Search

Biomedical subjects

V M Richards

Publications and source records attributed to V M Richards.

9 recordsLinked to original sources
PubMed1992

The detectability of a tone added to narrow bands of equal-energy noise.

The ability to detect a 2000-Hz tone added to bands of noise centered at 2000 Hz was measured using a two-interval, forced-choice, pulsed-masker paradigm. The stimuli ranged in duration from 50-200 ms, and the maskers ranged in bandwidth from 5-320 Hz. In one condition, the bands of noise had equal energy across the two intervals of each trial and in a second condition the levels of the stimuli were independently and randomly chosen from a 30-dB range on a presentation-by-presentation basis. The energy model failed to predict the data obtained either in the presence or in the absence of level variation. Control experiments showed that exposure to level variation yielded an overall reduction in sensitivity, suggesting that the presence of level variation leads to changes in the listeners' detection strategies. Computer simulations indicated that changes in either the fine structure or envelopes of the waveforms were sufficient to account for detection when changes in stimulus energy were not reliable.

Attention↗
PubMed1991

The detection of a tone added to a narrow band of noise: the energy model revisited.

In an effort to determine whether cues related to changes in energy contribute to the detection of a tone added to a narrow band of noise, we examined the effect of level variation on detection thresholds. In the first experiment, the level of each waveform was randomly varied on each presentation. Level variation had only marginal effects on performance. In addition, detection thresholds were obtained using bands of noise with equal energy across intervals. Neither increasing nor decreasing the variance of the noise-alone and tone-plus-noise energy difference distributions altered the detectability of a tone added to noise. Thus, the changes in energy that are concomitant with the addition of the tone are not the sole cue for the detection of the tone. In a second experiment, three psychometric functions were measured. One function was determined using no level variation, one was measured in the presence of level variation, and one was measured in the context of level variation, but for trials without level variation. The context of level variation slightly reduced detectability. In a third experiment, we compared detectability in three conditions: no level variation, across-trial level variation, and across-interval level variation. The thresholds obtained in the absence of level variation were superior to those measured in the presence of level variation, regardless of the manner in which the level variation was incorporated.

Attention↗
PubMed1990

Sensitivity to envelope coherence.

Measurements are reported on the ability of observers to discriminate whether the envelope of two amplitude-modulated sinusoids are in phase or out of phase. Spacing between the two carriers was either 2/3 or 4/3 octave, and the depth of modulation was varied to determine threshold. Discrimination performance improved as the level of the carriers increases up to about 60 dB SPL. The frequency locus of the two carriers (geometric mean of the two frequencies), which varied from 500 to 8000 Hz in different experiments, had little effect on discrimination accuracy. Discrimination performance was relatively constant for modulation rates below 100 Hz and deteriorates for higher modulation rates. These results are compared with data obtained from comodulation masking release experiments.

Acoustic Stimulation↗
PubMed1990

The role of single-channel cues in synchrony perception: the summed waveform.

Human observers are able to discriminate between simultaneously presented bands of noise having envelopes that are identical (synchronous) rather than statistically independent (asynchronous). The possibility that the detection of envelope synchrony is based on cues available in a single critical band, rather than on a simultaneous comparison of envelopes extracted via independent critical bands, is examined. Two potential single-channel cues were examined, both relying on the assumption that information present in the envelope of the summed bands is available to the listener. One such single-channel cue is the rms of the envelope of the summed waveform; the envelope is more deeply modulated for the summed synchronous bands than for the summed asynchronous bands. The second cue examined was envelope regularity; the envelope of the summed synchronous bands has periodic envelope minima, while the summed asynchronous bands exhibit aperiodic envelope minima. Psychophysical results suggest that such within-channel cues may be both available to, and utilized by, the listener when the component bands are separated by less than one-third of an octave.

Attention↗
PubMed1989

Auditory profile analysis: potential pitch cues.

The ability to detect changes in spectral shape, or profile analysis, was measured for both complex and simple changes in the power spectrum of a complex equal-amplitude standard. In an effort to determine whether detectability was mediated by changes in pitch that are concomitant with changes in spectral shape, the pitch of the stimuli were altered on a trial by trial basis. For moderate-range pitch randomizations, thresholds were on average 3 dB poorer than when no pitch randomization was employed. For large-range pitch randomizations, threshold changes were larger, but performance levels remained above chance levels. The psychophysical data, coupled with computer simulations of Feth's envelope-weighted average instantaneous frequency pitch model, indicate that changes in pitch contribute little to the discriminability of complex spectra.

Acoustic Stimulation↗
PubMed1989

Signal uncertainty and psychometric functions in profile analysis.

Experiment 1 was conducted to compare the effects of signal frequency uncertainty on the detection of a change in spectral shape and on the detection of a tone in wideband noise. Results indicate that for both tasks the uncertainty effect was small, being on average about 3 dB. In a second experiment, psychometric functions were measured for the detection of changes in the spectral shape of multicomponent complexes. Psychometric functions for profile tasks have a 25-dB range and are similar to those measured for the detection of an increment in the level of a single sinusoid. These psychometric functions are different from those found when detecting a signal in noise, which typically have a 10-dB range. Three equations for the shape of the psychometric functions were compared. The difference in the resulting fits was small, thus preventing an unambiguous choice of functional form.

Acoustic Stimulation↗
PubMed1988

Components of monaural envelope correlation perception.

The ability of listeners to discriminate between simultaneously presented bands of noise whose envelopes were either the same or statistically independent was determined. Bands of 100-Hz wide noise were employed which had low and high center frequencies of (2500, 2750), (2500, 3000), (2500, 3500) and (4000, 4400) Hz. Average discriminations were above 90% correct except for the (2500, 3500) Hz condition, which yielded an average of 77% correct. Next, a factorial stimulus design was employed in order to determine the relative importance of envelope and power spectrum cues. The results indicate that in the absence of power spectrum cues, bands with the same envelopes could be discriminated from bands with statistically independent envelopes. When the envelopes were always the same, listeners were able to discriminate between power spectra that were either the same or different. In contrast, when the envelopes were always different, listeners were unable to discriminate between the same and different power spectra.

Cues↗
PubMed1987

Monaural envelope correlation perception.

The ability to discriminate between simultaneously presented 100-Hz-wide bands of noise with envelopes that were either similar or dissimilar was measured. The center frequencies of the noise bands, fL and fL + delta f Hz, were systematically varied. When the bands of noise were separated by an octave, delta f = fL, discriminations were at chance levels. For frequency separations less than an octave, delta f less than fL, discrimination was best for fL = 2500 and 4000 Hz, somewhat poorer for fL = 1000 Hz, and impossible for fL = 350 Hz. Listeners were also asked to discriminate between bands of noise with envelopes that were either perfectly or partially correlated, and bands with envelopes that were either uncorrelated or partially correlated. The data suggest that, when transformed to an equal-variance scale (Fisher's z), equal changes in Fisher's z lead to equal changes in detectability, regardless of the correlation of the envelopes of the reference signal.

Acoustic Stimulation↗