Effects of ipsilateral and contralateral precursors on the temporal effect in simultaneous masking with pure tones.

In tone-on-tone masking, thresholds often decrease as the onset of the signal is delayed relative to the onset of the masker, especially when the frequency of the masker is higher than the frequency of the signal. This temporal effect was studied here by using a tonal "precursor," whose offset preceded the onset of the tonal masker (and signal). Under the right conditions, the precursor can reduce or eliminate the temporal effect by decreasing the threshold for a signal at masker onset, presumably for the same reason that the threshold decreases as a signal is delayed relative to the onset of a masker. In the present study, the frequency of the signal was 4000 Hz, and the frequency of the masker and precursor was typically 5000 Hz. In experiment 1, the precursor was presented to the ear receiving the masker and signal (ipsilateral precursor); in experiment 2, it was presented to the opposite ear (contralateral precursor). The results from experiment 1 can be summarized as follows: the ipsilateral precursor (a) reaches its maximum effectiveness (in reducing the temporal effect) for precursor durations of 200-400 ms; (b) is ineffective once the delay between its offset and the onset of the masker reaches about 50-100 ms; (c) is generally ineffective when its level is 10 or more dB lower than the level of the masker, but is effective when its level is equal to or greater than the level of the masker; and (d) becomes progressively less effective as its frequency is either increased or decreased relative to the frequency of the masker. The results from experiment 2 can be summarized simply by stating that the contralateral precursor is ineffective in reducing the temporal effect. These results suggest that the effect of the precursor may be mediated peripherally.

Spectral restoration of speech: intelligibility is increased by inserting noise in spectral gaps.

In order to function effectively as a means of communication, speech must be intelligible under the noisy conditions encountered in everyday life. Two types of perceptual synthesis have been reported that can reduce or cancel the effects of masking by extraneous sounds: Phonemic restoration can enhance intelligibility when segments are replaced or masked by noise, and contralateral induction can prevent mislateralization by effectively restoring speech masked at one ear when it is heard in the other. The present study reports a third type of perceptual synthesis induced by noise: enhancement of intelligibility produced by adding noise to spectral gaps. In most of the experiments, the speech stimuli consisted of two widely separated narrow bands of speech (center frequencies of 370 and 6,000 Hz, each band having high-pass and low-pass slopes of 115 dB/octave meeting at the center frequency). These very narrow bands effectively reduced the available information to frequency-limited patterns of amplitude fluctuation lacking information concerning formant structure and frequency transitions. When stochastic noise was introduced into the gap separating the two speech bands, intelligibility increased for "everyday" sentences, for sentences that varied in the transitional probability of keywords, and for monosyllabic word lists. Effects produced by systematically varying noise amplitude and noise bandwidth are reported, and the implications of some of the novel effects observed are discussed.

The vowel-sequence illusion: intrasubject stability and intersubject agreement of syllabic forms.

Earlier studies have found that listeners presented with a loud and clear repeating sequence of brief steady-state vowels typically report hearing two voices with distinctly different timbres repeating different syllables that either are English words or occur in English words. One of the simultaneous voices is generally based upon frequencies below, and the other above, the "crossover frequency" at approximately 1500 Hz that divides normal speech into regions contributing equally to intelligibility. It has been hypothesized that the lack of linguistic content halts the processing of vowel sequences at the syllabic level, and that the spectral splitting corresponding to the concurrent voices reflects a mechanism for independent processing of different frequency regions that can lead to increased intelligibility under difficult listening conditions. The present study employed twelve randomly selected arrangements of the same six 70-ms vowels, and it was determined that: (1) individuals reported the same perceptual organizations the following week; (2) insertion of a brief silent gap between restatements of a sequence resulted in reports of similar (and occasionally identical) syllables by different listeners hearing the same sequence; and (3) when two listeners' responses differed, they could nevertheless identify the particular vowel sequences corresponding to each other's verbal forms. Spectrograms of vowel sequences were compared with time-aligned spectrograms of a speaker's synchronous production of the forms as they were being heard, and some common features of the acoustic patterns were noted. It is suggested that vowel sequences provide a reliable and useful tool for probing aspects of the perceptual organization of speech sounds that are normally obscured by additional linguistic processing.

Auditory induction: reciprocal changes in alternating sounds.

When portions of a sound are replaced by a potential masker, the missing fragments may be perceptually restored, resulting in apparent continuity of the interrupted signal. This phenomenon has been examined extensively by using pulsation threshold, auditory induction, and phonemic restoration paradigms in which two sounds, the inducer and the inducee, are alternated (ABABA...), and the conditions required for apparent continuity of the lower amplitude inducee are determined. Previous studies have generally neglected to examine concomitant changes produced in the inducing sound. Results from the present experiments have demonstrated decreases in the loudness of inducers using inducer/inducee pairs consisting of tone/tone and noise/noise, as well as the noise/speech pairs associated with phonemic restorations. Interestingly, reductions in inducer loudness occurred even when the inducee was heard as discontinuous, and these decreases in loudness were accompanied by graded increases in apparent duration of the inducee, contrary to the conventional view of auditory induction as an all-or-none phenomenon. Under some conditions, the reduced loudness of the inducer was coupled with a marked alteration in its timbre. Especially profound changes in the inducer quality occurred when the alternating stimuli were tones having the same frequency and differing only in intensity--it seems that following subtraction of components corresponding to the inducee, an anomalous auditory residue remained that did not correspond to the representation of a tone.