Neal F. Viemeister

Noncategorical perception of stop consonants differing in VOT.

The discriminability of bilabial stop consonants differing in VOT (the Abramson-Lisker bilabial series) was measured in a same-different task, an oddity task, and a dual response, discrimination--identification task. Subjects showed excellent within-category discrimination in all three tasks after a moderate amount of training in a same-different task with a fixed standard and with feedback. In addition, discrimination performance continuously improved with increasing stimulus difference for both intra- and intercategory comparisons. Also, subjects were able to alter their identification responses so that well-defined category boundaries fell at arbitrary values determined by the experiments. These results are not compatible with a strict interpretation of the categorical perception of stop consonants.

Played‐again SAM: Further observations on the pitch of amplitude‐modulated noise

The pitchlike sensation elicited by sinusoidally amplitude‐modulated (SAM) noise remains a controversial phenomenon. The controversy centers on two major points: (1) whether this sensation is ’’really’’ pitch rather than, e.g., roughness or intermittency, and (2) the possibility that any pitch sensation is mediated by short‐term spectral information rather than temporal information—thus nullifying an interesting aspect of the phenomenon. Three experiments employing SAM wideband noise, SAM wideband noise bandpass‐filtered after modulation, and a SAM 10 kHz pure tone were performed: (1) open‐set melody identification, (2) melodic dictation, and (3) musical‐interval adjustment. These experiments extend our earlier study [Burns and Viemeister, J. Acoust. Soc. Am. 60, 863 (1976)]. The results provide further evidence that SAM noise can, at suitable modulation frequencies, elicit a sensation of pitch (as defined by the ability to carry melodic information), and that this pitch represents a purely temporal pheno...

Intensity coding and the dynamic range problem.

The psychophysical data on intensity discrimination indicate that certain schemes are unlikely as general intensity codes at the level of the auditory nerve and indirectly suggest that the most likely code is one based upon the firing rates of frequency-localized groups of fibers. A detection-theory analysis of a rate-based intensity code indicates that information from very few fibers can, if the information is appropriately combined, account for psychophysical discrimination even at high intensities. This suggests that fibers with similar CFs can code intensity over a wide range and that complex spectra can be represented at the level of the auditory nerve by a rate-CF code over the dynamic range of hearing. The analysis also indicates, however, a substantial discrepancy between the psychophysical data on the dependence of discrimination thresholds on level and the predicted discrimination behavior of a representative population of auditory nerve fibers. Thus, if intensity coding is based on localized firing rate, this fundamental psychophysical behavior does not result solely from peripheral processes.

Cues for discrimination of envelopes

Thresholds for detection of sinusoidal amplitude modulation at a signal modulation frequency were measured in the presence of a masker modulation frequency, with broadband noise carriers. Broad tuning for modulation frequency was observed. For maskers half or twice the signal frequency, thresholds depended on the relative phases of the signal and masker. These results were used to determine what aspects of envelopes listeners might be using in making decisions. Simulations were performed using an envelope detector model, consisting of bandpass filtering, half-wave rectification, and low-pass filtering. Decisions were based on envelope statistics that have been used to predict other data. These statistics were (1) rms power, (2) ratio of maximum to minimum amplitude (max/min), (3) crest factor, (4) fourth moment, and (5) average slope. The max/min statistic was successful at predicting the major trends in the data, without requiring the presence of channels tuned to modulation frequency.

Simultaneous masking by gated and continuous sinusoidal maskers

Simultaneous masking of a 20-ms, 1-kHz signal was investigated using 50-ms gated and continuous sinusoidal maskers with frequencies below, at, and above 1 kHz. Gated maskers can produce considerably (5-20 dB) more masking than continuous maskers, and this difference does not appear to result from the spread of energy produced by gating either the masker or the signal. For masker frequencies below the signal frequency, this difference in masking is primarily due to the detection of the cubic difference tone in the continuous condition. For masker frequencies at and above the signal frequency, the difference appears to be an important property of masking. Implications of this frequency-dependent effect for measures of frequency selectivity are discussed.

Intensive and temporal effects in pure‐tone forward masking

Intensive and temporal characteristics of forward masking were assessed using a pure‐tone masker and a 1‐kHz probe. For a given interval between masker and probe, the slope of the masking function tends to decrease with increasing masker frequency, but is always less than unity. When the masker frequency equals that of the probe, increasing the masker‐probe interval decreases the slope of the masking function. Implications of these data for forward‐masked psychophysical tuning curves are discussed. A model, based on a linear combination of exponential decays and an initial logarithmic transform of stimulus intensity, predicts the general form of the masking function for varying interval between masker and probe, and suggests time constants which decrease with increasing masker level.

Intensity discrimination in normal‐hearing and hearing‐impaired listeners

Weber fractions (delta I/I) for gated 500-ms tones at 0.3, 0.5, 1, 2, and 3 kHz, and at levels of the standard ranging from absolute threshold to 97 dB SPL, were measured in quiet and in high-pass noise in five listeners with cochlear hearing loss and in three normal-hearing listeners. In regions of hearing loss, the Weber fractions at a given SPL were sometimes normal. When the Weber fractions were normal or near-normal, the addition of high-pass noise elevated the Weber fraction, strongly suggesting the use of spread of excitation to higher frequencies. Inversely, when the Weber fractions were elevated, the addition of high-pass noise produced no additional elevation, suggesting an inability to use spread of excitation. In general, the relative size of the Weber fractions, the effects of high-pass noise, and to a lesser extent, the dependence of the Weber fraction on level, were consistent with expectations based upon the audiometric configuration and the use of excitation spread. There were several notable inconsistencies, however, in which normal Weber fractions were seen at a frequency on the edge of a steep high-frequency loss, and in which elevated Weber fractions were observed in a flat audiometric configuration. Finally, when compared at the same SL, the Weber fraction was sometimes smaller in cochlear-impaired than in normal hearing listeners. This was true even in high-pass noise, where excitation spread was limited, and may reflect the unusually steep rate versus level functions seen in auditory nerve fibers that innervate regions of pathology.

Forward masking of amplitude modulation : Basic characteristics

In this study we demonstrate an effect for amplitude modulation (AM) that is analogous to forward making of audio frequencies, i.e., the modulation threshold for detection of AM (signal) is raised by preceding AM (masker). In the study we focused on the basic characteristics of the forward-masking effect. Functions representing recovery from AM forward masking measured with a 150- ms 40- Hz masker AM and a 50- ms signal AM of the same rate imposed on the same broadband-noise carrier, showed an exponential decay of forward masking with increasing delay from masker offset. Thresholds remained elevated by more than 2 dB over an interval of at least 150 ms following the masker. Masked-threshold patterns, measured with a fixed signal rate (20, 40, and 80 Hz) and a variable masker rate, showed tuning of the AM forward-masking effect. The tuning was approximately constant across signal modulation rates used and consistent with the idea of modulation-rate selective channels. Combining two equally effective forward maskers of different frequencies did not lead to an increase in forward masking relative to that produced by either component alone. Overall, the results are consistent with modulation-rate selective neural channels that adapt and recover from the adaptation relatively quickly.

The temporal course of simultaneous tone‐on‐tone masking

Threshold for a 20-ms, 1-kHz signal was measured as a function of its temporal position within a longer duration gated masker; masker frequencies were below, at, and above 1 kHz. For a masker frequency above the signal frequency, there is a sizable temporal effect: As the onset of the signal is delayed, threshold decreases rapidly but then increases again as the signal approaches masker offset. Similar results can be observed for a masker frequency below the signal frequency, but that temporal effect is due to the detection of the cubic difference tone. The implication of this frequency-dependent temporal effect for measuring psychophysical tuning curves is discussed.

Intensity discrimination of noise in the presence of band‐reject noise

Webers law holds over at least an 80‐dB range for intensity discrimination of 200‐msec bursts of noise. Webers law holds over a comparable range when information regarding intensive differences is effectively restricted to a limited frequency region by the addition of a relatively intense band‐reject noise. In particular, no failure of Webers law is observed at intensities for which the discharge rates of fibers innervating the frequency region of the stopband are presumably saturated. These results do not support the generally accepted notion that a spread of excitation along the cochlear partition with increasing intensity is necessary for the auditory system to maintain its large dynamic range.