John K. Cullen

Dichotic speech perception: An interpretation of right‐ear advantage and temporal offset effects

In two experiments on normals we presented CV nonsense syllables both dichotically and monotically, with onsets of the syllables separated by 0, 15, 30, 60, and 90 msec (first experiment) and 0, 90, 180, 250, and 500 msec (second experiment). We found that when one of the CVs trailed the other by 30–60 msec, the trailing CV became more intelligible than when it was given simultaneously; the leading syllables intelligibility dropped from its “simultaneous” level when leading by 15 and 30 msec. The leading message was more intelligible between 15 and 250 msec when the two channels were mixed monotically. In the dichotic simultaneous conditon, voiceless consonants were more intelligible than voiced, especially in voiced‐voiceless pairs. When the voiced CV trailed the voiceless CV, the former became almost as intelligible as its voiceless counterpart. A left hemisphere “speech processor” was postulated, with suppression of information from ipsilateral sources during contralateral stimulation. The postulated...

Is Speech “Special”? Perhaps the Temporal Lobectomy Patient Can Tell Us

When dichotic nonsense syllables are presented to temporal lobectomy patients at equal intensities, the ear contralateral to the site of the lesion performs more poorly than the ipsilateral ear; if the ipsilateral ear is stimulated below threshold, the contralateral ear performs near 100%. However, if the intensity of speech in the ipsilateral ear is increased above SRT, the contralateral scores drop markedly as intelligibility increases in the ipsilateral ear. This “trade off” does not occur when noise is the competing stimulus. This phenomenon is interpreted as a sign of “speech identification” and is suggested as a potential technique for differentiating speech from nonspeech elements.

Acoustic Problems in Dichotic Listening Tasks

Publisher Summary This chapter discusses acoustic problems in dichotic listening tasks. It reviews principles that are applicable to dichotic speech perception. Dichotic listening tasks, in which competing signals are presented to both ears, are used to study central auditory perception in children and adults. The acoustic signals used in dichotics are of many forms, covering pure tones, nonsense syllables, words, or sentences, depending upon the central process being studied. Furthermore, control of level, frequency response, subject hearing screening, and so on are generally pertinent to dichotic experiments in which signals other than speech are employed. The signal level and signal-to-noise (S/N) ratio affect performance in dichotic experiments much as these factors affect performance in monaural or binaural perception tasks. However, in dichotic tasks, investigators are often interested in between-ear performance differences as well. In dichotic speech tasks, for example, the difference in right-ear and left-ear performance (so-called right-ear advantage, or REA) is often the primary dependent variable. It is difficult to set criteria as to what constitutes normal REA. However, if experimenters consider acoustic factors irrelevant and present stimuli at levels that have varying effects on the accuracy of recall, they magnify the difficulties of interpretation.

Rate discrimination of high-pass-filtered pulse trains.

Difference limens for trains of 30-microseconds pulses were determined for repetition rates of 50, 100, 200, 400, and 800 pulses per second under conditions of no filtering and high-pass filtering (115 dB/oct) with corner frequencies of 2.5, 5.0, 7.5, and 10 kHz. Low-pass-filtered noise was mixed with the trains of impulses to preclude discrimination on the basis of potential low-frequency signal components. Measures were obtained from four trained listeners at a signal level of 30 dB SL relative to individually determined thresholds for each filter condition and repetition rate. The data support the hypothesis that resolution of pulse-train repetition rate involves both temporal- and frequency-based processes--the latter becoming ineffective when frequency resolution of the ear is insufficient to resolve separate harmonics of the signal. Inter- and intra-individual differences are interpreted as reflecting frequency resolution capacity.

Intensity difference limens at high frequencies

Thresholds for amplitude modulation detection were obtained from four subjects at frequencies of 2, 4, 6, 8, and 10 kHz for sensation levels of 15, 30, 45, and 60 dB and modulation rates of 2, 4, and 8 Hz. High-frequency difference limens calculated from amplitude modulation thresholds were found to change nonmonotonically as a function of sensation level, independent of modulation rate. This nonmonotonic relation stemmed mainly from a gradual reduction of the difference limen at the lowest sensation level with increasing frequency. Difference limens for pulsed tone discrimination were also measured in two of the subjects at 2, 6, and 10 kHz and sensation levels of 15, 30, 45, and 60 dB. The relation between intensity discrimination and sensation level was similar to that found for amplitude modulation detection. These findings are interpreted as indicating that the nonmonotonic relation between sensation level and intensity resolution is a general characteristic of stimulus processing at higher frequencies.

The Effect of Varied Bandwidth, Signal‐To‐Noise Ratio, and Intensity on the Perception of Consonant‐Vowels in a Dichotic Context: Additivity of Central Processing

Normal subjects were presented simultaneously aligned consonant‐vowel stimuli in three forced‐choice dichotic experiments in which the information content of one signal was changed by (1) varying the intensity, (2) low‐pass filtering, and (3) alterations in signal‐to‐noise ratios. The results of all three experiments were complementary. As signal characteristics were varied to produce a reduction of information to one ear, and thus a decrement in performance, an increment in performance of the unaltered ear was observed. The trade‐off ratios of decrement and increment were such that total performance (summed ear scores) remained virtually constant. Dichotic ear effects (e.g., right ear outperforming left ear) appear to be orthogonal to this phenomenon. These observations indicate that perception of dichotically presented consonant‐vowel depends upon the central additivity of information from two quasi‐independent channels, overall performance being limited by the capacity of the central processor. [Work s...

Effects of background noise level on detection of tone glides

Detection thresholds were obtained for short duration rising tone glides and falling tone glides for normal hearing listeners in quiet and with varied levels of background, broadband noise. For signal durations 50 ms, or greater, pure tones were detectable at lower levels than were rising and falling tone glides; no level effects were observed. For signal durations less than 50 ms an interaction between background noise level, signal duration, and signal class was observed. The previously reported tendency for short duration rising glides to be detected at lower sound pressures than pure tones or falling tone glides appears to be unique to a limited range of noise levels (40-75 dB).

Temporal integration of two‐component tone glides

Temporal integration of complex signals composed of two linearly combined tone glides was studied in a 60-dB SPL noise background. When both glide tones increased or decreased in frequency, integration functions differed for durations shorter than 50 ms in a manner similar to results obtained for single-component tone glides. That is, rising two-component glides were detected at lower thresholds than were falling two-component glides. However, when one component increased in frequency and the other decreased, thresholds were the same, regardless of which component increased and which decreased.

Auditory signal processing in hearing‐impaired persons with residual high‐frequency hearing

A number of persons with severe sensorineural hearing losses in the frequency range below 6000 Hz have been shown to have near‐normal thresholds for frequencies above 6000 Hz. This paper reports on measures of high‐frequency signal processing in seven persons with this type of heating impairment. Measures taken include estimates of difference limens for intensity and frequency, gap detection, rate discrimination, and narrow‐band masking functions. Although little normative data exist for comparative purposes, measures of frequency, intensity, and time resolution are within the range of what one would anticipate by extrapolation. Intensity and frequency differences limens, and wideband and narrow‐band masking functions also were obtained in the region of impaired hearing {i.e., 3–6 kHz}; narrow‐band masking measures and difference limens indicated impaired frequency processing and varied between subjects. Intensity discrimination fell within previously published ranges. [Work supported by the National Inst...

The Effects of Delayed Auditory Feedback on Ictal‐Speech Automatisms

The experiments reported in this paper were designed to provide information about whether speech can be elaborated without closed‐loop auditory‐feedback control. The experiments involve precipiation of focal, psychomotor seizure activity in patients known to talk during their seizures (ictal‐speech automatisms). Metrazol (pentylenetetrazol) is injected intravenously, under EEG control, in order to induce seizure activity. The effects of delayed auditory feedback on speech elaborated during seizure activity are compared to the effects of delayed auditory feedback on comparable speech samples obtained under normal speaking conditions. Observations made on one patient demonstrate that speech elaborated during seizure activity can sound quite normal; observations fail to show any evidence of the increase in phonation time and increase in mean sound‐pressure level characteristic of speech under delayed auditory feedback. The same speech, elaborated before and after the seizure, shows marked prolongation of phonation time and increase in mean sound‐pressure level under delayed auditory feedback conditions. From these observations we infer that the speech elaborated during psychomotor seizures can be organized in a normal fashion without closed‐loop auditory‐feedback control.