Thomas E. Hanna

Detection and intensity discrimination of a sinusoid

Intensity discrimination thresholds were measured for gated 100-ms, 1000-Hz tones. Discrimination thresholds were measured at several intensities near absolute threshold as well as at 30, 60, and 90 dB SPL. Psychometric functions were obtained for several of these discrimination conditions, and for detection of the signal in quiet. The results showed that Webers law is approximately valid for standards as low as 0 dB SL. Small amounts of negative masking were observed even when the data were expressed in terms of increment energy. The psychometric functions for the discrimination conditions had a similar form and were shallower than the psychometric function for the detection of a signal in quiet. A similar set of conditions was run in the presence of a continuous, broadband noise. The results were generally in agreement with those obtained in quiet, but slight differences suggested that the variability which limits performance in the two conditions is different. The results are discussed in terms of the effects of nonlinear transduction, the effects of uncertainty, and contrast mechanisms as proposed by Laming [Sensory Analysis (Academic, London, 1986)].

Estimation of psychometric functions from adaptive tracking procedures

Because adaptive tracking procedures are designed to avoid stimulus levels far from a target threshold value, the psychometric function constructed from the trial-by-trial data in the track may be accurate near the target level but a poor reflection of performance at levels far removed from the target. A series of computer simulations was undertaken to assess the reliability and accuracy of psychometric functions generated from data collected in up-down adaptive tracking procedures. Estimates of psychometric function slopes were obtained from-trial-by-trial data in simulated adaptive tracks and compared with the true characteristics of the functions used to generate the tracks. Simulations were carried out for three psychophysical procedures and two target performance levels, with tracks generated by psychometric functions with three different slopes. The functions reconstructed from the tracking data were, for the most part, accurate reflections of the true generating functions when at least 200 trials were included in the tracks. However, for 50- and 100-trial tracks, slope estimates were biased high for all simulated experimental conditions. Correction factors for slope estimates from these tracks are presented. There was no difference in the accuracy and reliability of slope estimation due to -target-level-for the adaptive track, and only minor differences due to psychophysical procedure. It is recommended that, if both threshold and slope of psychometric functions are to be estimated-from the trial-by-trial tracking data, at least 100 trials should be included in the tracks, and a three- or four-alternative forced-choice procedure should be used. However, good estimates can also be obtained using the two-alternative forced-choice procedure or less than 100 trials if appropriate corrections for bias are applied.

Effects of masker waveform and signal‐to‐masker phase relation on diotic and dichotic masking by reproducible noise

The proportions of hits and false alarms were estimated for the detection of a 500-Hz sinusoidal signal in each of 25, reproducible samples of wideband, white, Gaussian noise. The effects of signal phase were investigated under diotic (MoSo) and dichotic (MoS pi) conditions and compared to the predictions of two major models of binaural hearing. Averaging the data over samples obscured important across-sample and across-subject differences in performance. The proportions of hits and false alarms for individual noise samples presented under the MoSo condition were highly correlated with those for the same noise samples under the dichotic MoS pi condition, suggesting that the cues determining performance under these conditions are related. Signal-to-masker phase had a large effect on the proportion of hits under the MoSo condition, but only a small effect under the MoS pi condition. The Vector model predicts a large effect of signal phase under the MoS pi condition, and is, therefore, imcompatible with this aspect of the data. The expected value of the decision variable of the EC model is independent of signal phase. However, when the variance of the decision variable is also considered, the EC model does predict changes in the proportion of hits with the phase of the signal, comparable to those observed here. Further, it was shown that, if minor changes in the form of the EC models decision variable or in the distribution of the internal noise parameters are assumed, the expected value of the decision variable also changes with the phase of the signal.

Discrimination and identification of modulation rate using a noise carrier

Modulation-rate perception was measured for three tasks: a fixed-standard, forced-choice discrimination task with a 500-ms interstimulus interval; a random-standard, forced-choice discrimination task with an 8-s interstimulus interval; and an identification task. Thresholds were obtained for modulation rates from 14-224 Hz using noise carriers bandpass filtered from 500-4000 Hz, 500-1600 Hz, 1700-2800 Hz, and 2900-4000 Hz. The four bands yielded similar results except for modulation rates greater than 100 Hz, where the 500- to 1600-Hz thresholds were higher. Fixed-standard discrimination thresholds were about 3 Hz for modulation rates up to 66 Hz. The increase of thresholds for modulation rates above 66 Hz could be due to temporal resolution limits with a time constant of about 2-3 ms. For modulation rates above 100 Hz, critical-band filtering may further decrease sensitivity for the 500- to 1600-Hz noise band. Resolution in the random-standard discrimination task was similar to that for the identification task. Thresholds were elevated relative to fixed-standard thresholds except at the extremes of the stimulus range. In the random-standard discrimination task, a pronounced criterion bias was present for stimuli near the extremes of the range. Durlach and Braidas model [N. I. Durlach and L. D. Braida, J. Acoust. Soc. Am. 46, 372-383 (1969)] describes the data well and provides quantitative measures for our data in good agreement with those for intensity perception.

Masking in three-dimensional auditory displays

The extent to which simultaneous inputs in a three-dimensional (3D) auditory display mask one another was studied in a simulated sonar task. The minimum signal-to-noise ratio (SNR) required to detect an amplitude-modulated SOO-Hz tone in a background of broadband noise was measured using a loudspeaker array in a free field. Three aspects of the 3D array were varied: angular separation of the sources, degree of correlation of the background noises, and listener head movement. Masking was substantially reduced when the sources were uncorrelated. The SNR needed for detection decreased with source separation, and the rate of decrease was significantly greater with uncorrelated sources than with partially or fully correlated sources. Head movement had no effect on the SNR required for detection. Implications for the design and application of 3D auditory displays are discussed.

Spatial and Spectral Release from Masking in Three-Dimensional Auditory Displays

The extent to which simultaneous sounds in three-dimensional (3D) auditory displays mask one another was examined as a function of their spectral proximity and spatial separation. A tonal signal of either 0.5 or 4 kHz was presented at 0 deg azimuth in the horizontal plane of the listeners head. Spectral proximity was varied by centering a notched-noise masker on the signal frequency and varying the notch width. The masker was presented at an azimuth of 0, 20, or 40 deg, and the listeners head was immobilized. Detection levels decreased with both masker notch width and spatial separation. Analysis of the results shows that spatial separation produces both a broadening of the band over which masking is effective and a decrease in the minimum signal-to-noise ratio needed to detect the signal at all notch widths. Both effects were greater at 0.5 kHz than at 4 kHz. Additional experiments showed that these mechanisms are disrupted when two maskers are positioned symmetrically around the signal and head. The results are interpreted in terms of the effects of spatial separation on binaural processing and the listeners ability to resolve the signal and masker in frequency.

Phase effects for a sine wave masked by reproducible noise

A tone‐in‐noise detection task was used to assess the filtering properties of the auditory system. In the first experiment, ten 150‐ms samples of reproducible noise were used as maskers. The signal was a 500‐Hz tone either 20 or 100 ms in duration. The 100‐ms signal was centered temporally in the noise; the 20‐ms signal occurred either at the beginning, center, or end of the noise. The starting phase of the signal was varied from 0°–315° in steps of 45°. Signal thresholds, collected by the method of adjustment, were a cyclical function of starting phase and could be described by an energy model [Green and Swets, Signal Detection Theory and Psychophysics (Krieger, Huntington, NY, 1966/1974)]. A vector description of these data revealed an invariant property of each sample of noise, which we call the ‘‘noise vector.’’ The relationship among the parameters of the noise vectors over the various signal conditions suggest the presence of temporal interactions due to narrow‐band filtering. These relationships ar...

Forward masking of diotic and dichotic clicks by noise

The first part of the present study measured click thresholds during forward masking as a function of masker level and the temporal relation of the 6‐kHz low‐pass filtered noise masker to the click (300‐ms duration with a 20‐ms temporal gap or 10‐ms duration with a 5‐ms gap). Also varied were the spectral content of the click (low‐pass filtered at 1 kHz or 5 kHz) and the interaural phase of the click (0° or 180°). The difference in frequency content had no effect on the amount of masking for the 300‐ms masker while greater masking was found for the 1‐kHz click with the 10‐ms masker. This combination (1 kHz, 10 ms) was also the only one to produce masking level differences (MLDs) when the click was presented dichoticly. The second part of the experiment investigated the effects of combining the maskers used in the first part. Additional masking (above that predicted by an energy sum) was found, as has been reported elsewhere [M. J. Penner and R. M. Shiffrin, J. Acoust. Soc. Am. 67, 617–627 (1980)]; however...

Absolute intensity discrimination using two responses

The observer was asked to judge whether a comparison sound was more or less intense than a standard sound of 60 dB SPL. The sounds were broadband noises of 100-ms duration. The independent variables were the range of intensities used in the set of comparison sounds and the presence or absence of feedback. The main dependent variable was the variability of such judgments, which was measured from the psychometric function. The variability, measured in terms of the Weber function, increased from about 1.5 to 4 dB when the stimulus range changed from 10 to 60 dB. Increases in stimulus range increased the variability of the binary judgments in this task, as they had in previous experiments where multiple responses were required. Feedback improved performance primarily at the largest ranges. Only a small fraction of these changes in variability stems from sequential effects.

Estimates of the ratio of external to internal noise obtained using repeatable samples of noise

Several experiments conducted in our laboratory have used repeatable noise as a masking stimulus. The proportion of trials on which a subject made identical decisions when presented with identical signal‐plus‐noise or noise‐alone stimuli was used to estimate the ratio of external to internal noise standard deviations (R). R was estimated using 2AFC; single interval, Yes‐No; and single interval, four‐category rating paradigms. The effect of both signal level and noise level on the obtained value of R were systematically investigated. Diotic and dichotic conditions were compared. In discussing these results, the assumptions necessary to estimate R are evaluated. Particular emphasis is placed on considerations of whether the internal noise is independent of the external noise. The forms of the internal and the external noise distributions are also examined. [Work supported by NSF.]