Terrence R. Dolan

Explanation of Masking‐Level Differences That Result from Interaural Intensive Disparities of Noise

The detectability of a monaurally presented 500‐cps signal was measured at each of several values of interaural correlation for a wide‐band noise masker. The results, when expressed in terms of masking‐level differences (MLDs), are in agreement with data reported by Whitmore and Wilbanks and show a function of similar form to that described by Robinson and Jeffress. Additionally, the detectability of a monaural signal was investigated as a function of the interaural intensive relations of a wide‐band noise masker. The data are in agreement with results originally shown by Hirsh, and since, replicated several times. That is, detectability is greatest when the level of the noise masker at the two ears is equal, and decreases as the level of the masker at the nonsignal ear is attenuated. The data are used to estimate parameters for a simple model that attempts to account for changes in detectability that occur when a monaural signal is presented with binaural, correlated noise having an interaural level difference. The model proposes that at low external noise levels, internal noise leads to a decrease in interaural‐noise correlation. Predictions from the model are in reasonable agreement with data reported previously.

Effect of Masker Spectrum Level on Masking‐Level Differences at Low Signal Frequencies

Most, if not all, models of binaural hearing predict a dependence of the magnitude of the masking‐level difference (MLD) on signal frequency. Specifically, the size of the MLD is expected to increase as the signal frequency is decreased. The results of many experiments in which signal frequency was varied, however, do not support the above prediction. The magnitude of the MLD appears to decrease at frequencies below 300 cps. The present experiment was designed to re‐examine the decrease in the size of the MLD at low frequencies and to test a qualitative explanation of this phenomenon based on the concept of internal noise. To do so, the size of the MLD in the N0Sπ masking condition and the N0S0 masking condition was estimated relative to the NMSM masking condition at signal frequencies of 150 and 300 cps at several masker spectrum levels. The results indicated that the magnitude of the MLD at both frequencies is dependent upon spectrum level of the masker. The results further indicate that the dependence ...

Effect of “backward” masker fringe on the detectability of pulsed diotic and dichotic tonal signals

The effects of backward masker “fringe” on performance in homophasic and antiphasic masking conditions were investigated. The results of the study indicate that (1) the presence of a backward masker fringe has only a small effect on performance in homophasic masking conditions; (2) under antiphasic masking conditions, the presence of a backward masker fringe improves performance; (3) similar to the results of studies investigating the effects of forward masker fringe, the magnitude of the improvement in performance increases as the duration of the fringe increases; and (4) the magnitude of the improvement caused by the presence of backward masker fringe is considerably smaller than the magnitude of the improvement caused by the forward masker fringe. It appears, then, that the presence of backward masker fringe may provide a baseline or reference phase similar to that which the forward fringe is presumed to provide. The presence of this reference phase after the offset of the signal apparently makes the phase shift associated with the signal-plus-noise waveform more discernible, thus leading to higher detectability. However, for reasons not yet clear, the presence of a baseline or reference phase after the offset of the signal (backward fringe conditions) does not make the signal-plus-noise phase shift as discernible as do either the presence of that same reference phase before the onset of the signal (forward fringe condition) or the presence of that same reference phase both before signal onset and after signal offset (continuous masker condition).

Binaural interaction in backward masking

The binaural auditory system exhibits certain advantages over the monaural system when detecting a tonal signal in a background of masking noise. These advantages have been described in detail and are referred to as masking-level differences, or MLDs. It has been demonstrated, for example, that performance in detecting a tonal signal that has been reversed in phase at one ear relative to the other ear is about 15–17 dB better than detection of the same signal in-phase at the two ears when masked by moderately intense masking noise that is in-phase at the two ears. The explanations for this phenomenon fall into two general categories, and both types of explanations are based upon the interaction of the tonal signal and masker when they are added together. In the present paper, data are presented which indicate that an MLD of at least 4–5 dB can be obtained in a binaural masking experiment in which the offset of the tonal signal precedes the onset of the noise masker.

The Behavioral and Histological Effects of Auditory Over‐exposure on the Cat

The behavioral and histological effects of overstimulation on the auditory system of the cat are being examined. Thus far, the exposure stimuli considered have been tones between 125 and 4000 Hz at several stimulus intensities and durations. Stimulation levels have been varied from 120 to 160 dB SPL, and durations between 1 and 4 h have been examined. One of the initial results is that the amount and locus of damage resulting from exposure and the associated effects on audibility are most difficult to predict at low exposure frequencies. At 125 Hz, for example, seven animals have been exposed for 4 h at intensities between 150 and 160 dB SPL. The exposure caused either no apparent damage and no associated hearing loss or almost total damage along the entire length of the cochlea and nearly complete hearing loss. These and other results from exposure frequencies of 1, 2, and 4 kHz will be described. [Research supported by a grant from the National Aeronautics and Space Administration.]

Effect of Signal Frequency on the MLD for Uncorrelated Noise

Rabiner, Laurence, and Durlach [J. Acoust. Soc. Amer. 40, 62–70 (1966)] reported that the masking‐level difference (MLD) for uncorrelated noise with signals in‐phase and reversed in‐phase at the ears was independent of signal frequency for frequencies between 150 and 1000 Hz. Since all other “MLD conditions” show marked signal‐frequency effects, and since the E‐C model predicts that such effects will also occur with uncorrelated noise, the present study reinvestigates the effect of signal frequency on the MLD for uncorrelated noise. The results indicate that the MLD for NUS0 decreases with increasing signal frequency.

Vibrotactile encoding of speech signals

Preliminary results of a research effort concerned with the sensory encoding of speech by humans through the vibrotactile system are described. The research involves the use of a hardware system which includes a frequency‐transformation device [see Finkenzeller, P., Habilitation Thesis, Universitat Erlangen, Erlangen, Germany (1973)] and a vibrotactile transducer which is used to present the transformed speech signal to the hand or arm. The transformation device operates on the speech signal essentially in real time and is capable of lowering the spectral content of the waveform while preserving part of the information available in the fine structure. In this paper, the transformation device and results of early speech‐discrimination experiments are described. These initial experiments have primarily involved the psychophysical measurement (using an A‐B‐X paradigm) of intelligibility of words selected from the W‐22 and Modified Rhyme Test lists. The results suggest that the utilization of appropriate spec...

Detection and recognition of alphabetic characters: Simultaneous and contiguous

The present experiment was designed to examine the temporal ordering of detection performance and recognition performance in the perception of alphabetic characters, employing a visual backward-masking by noise paradigm. The results suggest that the encoding processes that lead to a correct “detection” response are completed earlier than the encoding processes that lead to a correct “recognition” response.

Effect of stimulus bandwidth and type of masker presentation on the function relating detection performance to signal duration

It has been shown that, when a narrow‐bandpass filter is employed with a pulsed masker, signal detectability in the homophasic condition increases monotonically as signal duration decreases. In addition, pulsing the masker has been found to result in a decrement in antiphasic detection performance. The present study was designed to examine the effect of the interaction of stimulus bandwidth and type of masker presentation on the function relating detection performance to signal duration in homophasic and antiphasic conditions. Two interaural phase relations, N0S0 (homophasic) and N0Sπ (antiphasic), two types of masker presentation (continuous and pulsed), two bandpass filters (50 Hz and 200 Hz), and seven signal durations (5, 10, 20, 100, 500, and 1000 msec) were employed. The results of this study indicate that the changes in homophasic detection performance at short signal durations can be explained through consideration of the interaction of signal bandwidth with the assumed internal bandwidth of the a...

Psychophysical Investigation of Auditory Sensitization

The fact of “sensitization” is well known. In the auditory system, it is demonstrated by a larger amplitude for N1 in response to a click having a condensation followed by rarefaction as opposed to the reverse (Peake and Kiang). The Bekesy audiogram also shows fluctuations in threshold as a gated tonal stimulus is slowly moved in phase along a very low‐frequency masking tone (Shickman). We have attempted to demonstrate sensitization in a controlled psychophysical setting in which a very low frequency, usually 50 Hz, was adjusted in level to be 5–10 dB below classical absolute threshold. This served as the background; it cannot strictly be called a masker. A brief high‐frequency tone was gated coherently with the low frequency and placed in various phase relations to the low‐frequency tone. Percent‐correct judgments in two‐interval forced‐choice experiments was the dependent variable. Sensitization was demonstrated. [Work supported in part by the U. S. Office of Naval Research.]