Sandra J. Guzman

Responses of ventral cochlear nucleus units in the chinchilla to amplitude modulation by low‐frequency, two‐tone complexes

For a tone that is amplitude modulated by two tones (fmod1 and fmod2), neither the stimulus waveform nor the half-wave rectified waveform has spectral energy at the envelope beat frequency (fmod2-fmod1). The response of ventral cochlear nucleus units in the chinchilla were recorded for best frequency tones that were amplitude modulated by low-frequency, two-tone complexes. Fourier analysis of poststimulus time histograms shows spectral peaks at fmod2-fmod1 in addition to the peaks at fmod1 and fmod2. The peaks in the neural spectra arise from compressive nonlinearities in the auditory system. The magnitudes of these spectral peaks are measures of synchrony at each frequency component. For all units, synchrony at fmod1 and fmod2 is greater than the synchrony at fmod2-fmod1. For a given unit, synchrony at fmod1 and fmod2 remains relatively constant as a function of overall level, whereas synchrony at fmod2-fmod1 decreases as the level increases. Synchrony was quantified in terms of the Rayleigh statistic (z), which is a measure of the statistical significance of the phase locking. In terms of z, phase locking at fmod1 and fmod2 is largest in chopper units, whereas onset-chopper units and primarylike units having sloping saturation in their rate-level functions show the smallest amount of phase locking. Phase locking at fmod2-fmod1 is also largest in chopper units, and smallest in onset-chopper units and primarylike units with sloping saturation.

Observer weighting of interaural delays in source and echo clicks

A correlational analysis was used to assess the relative weight given to the interaural differences of time (IDTs) of source and echo clicks for echo delays ranging from 1-256 ms. In three different experimental conditions, listeners were instructed to discriminate the IDT of the source, the IDT of the echo, or the difference between the IDTs of the source and echo. The IDT of the target click was chosen randomly and independently from trial-to-trial from a Gaussian distribution (mu = 0 microsecond, sigma = 100 microseconds). The IDT of the nontarget click was either fixed at 0 microsecond or varied in the same manner as the IDT of the target. The data show that for echo delays of 8 ms or less, greater weight was given to the IDT of the source than to that of the echo in all experimental conditions. For echo delays from 16-64 ms, the IDT of the echo was weighted slightly more than that of the source and the weights accounted for a greater proportion of the responses when the echo was the target, indicating that the binaural information in the echo was dominant over the binaural information in the source. The data suggested the possibility that for echo delays from 8-32 ms, listeners were unable to resolve the temporal order of the source and echo IDTs. Listeners were able to weight the binaural information in the source and echo appropriately for a given task only when the echo delay was 128 ms or greater.

The relationship between localization and the Franssen effect

The relationship between localization and the Franssen effect was studied for noise and tones in a sound-deadened and in a live room. The noise was wideband and the tones were 250, 500, 1000, 1500, 2500, and 4000 Hz. Listeners were asked to determine the location of the stimuli in a localization task and to discriminate the difference between a pair of stimuli used to generate the Franssen illusion and a steady-state tone in a Franssen-effect discrimination task. Poor performance in the Franssen-effect discrimination task is consistent with the stimulus conditions leading to a strong Franssen illusion. Poor performance in both the Franssen effect and localization tasks was obtained for midfrequency tones (near 1500 Hz) and in the live room. Thus, the Franssen illusion is strongest for a live room and for midfrequency tones consistent with the difficulty listeners have in localizing sounds under these conditions. These results are consistent with those of Hartmann and Rakerd [J. Acoust. Soc. Am. 86, 1366-1373 (1989)] and support their suggestion of a correlation between the Franssen effect and localization in rooms.

Observer weighting of binaural information in source and echo clicks

Relative weights given to interaural differences of time (IDTs) of source and echo clicks were computed for a range of echo delays (1–256 ms) for stimuli presented over headphones. On each trial, source and echo IDTs were selected randomly and independently (μ=0 μs, σ=100 μs). Listeners were instructed to indicate the direction of the IDT, left or right, of the source or echo (in separate conditions). Weights were obtained by computing the correlation between the source or echo IDT and the listener’s binary responses. In all conditions, little weight was given to the echo at short echo delays ( 32 ms, listeners gave greater weight to the source or echo, as appropriate. For all echo delays, when listeners were instructed to indicate the direction of the source IDT, the percent correct was lower when the echo DT varied across trials than when it was fixed at 0 μs, indicati...

Precedence and plausibility

The ‘‘Clifton effect’’ [R. K. Clifton, 1834–1835 (1987)] was studied in a sound‐deadened room with seven loudspeakers. One loud speaker produced a source click while other loudspeakers produced delayed copies simulating echoes (delays: 2–30 ms). Each combination of source and echoes is one click event and was presented as a train of click events (1–20 click events). A train was presented to listeners who made two judgments for the LAST click event presented: (1) The number of loudspeakers which produced sounds for the last click event, and (2) the loudspeaker location for each perceived source. ‘‘Catch trials’’ were introduced to make sure listeners used all possible responses and were able to locate the loudspeaker sources. When more than 10 click events were presented, a switch in conditions was introduced between the 10th and 11th click event. If the switch was plausible for a natural source and its echoes, responses indicated that listeners processed delayed clicks as echoes. If the change was implaus...

Breakdown of echo suppression with multiple echoes

If a sound is followed a few milliseconds later by its echo, the echo is often suppressed. If a train of repeating sources and echoes is established and then the source and echo are reversed, there is a temporary breakdown of echo suppression. A procedure has been developed for studying echo suppression in which listeners attend to the last sounds in a series of repeating sounds. Using this procedure the effect of multiple echoes (up to seven) on echo suppression and the breakdown of echo suppression was investigated. In all cases there is a breakdown in echo suppression when the echoes are changed in a number of ways. The results indicate an interaction among number of echoes, spatial location of the echoes, and the timing among the echoes in both echo suppression and its breakdown. The results will be discussed in terms of the concept that echo suppression depends on the listener’s prior experience in the acoustic environment. [Work supported by NIDCD and AFOSR.]

Relationship of working memory to the effect of listener training on pitch and rhythm perception

Experience and training can affect discrimination of tonal sequences. Recent work has demonstrated differences in processing multidimensional tonal sequences defined both by pitch contour and sequence rhythm. With groups defined by skill level on a pattern reconstruction task, a significant interaction between group and sequence-task condition suggested a difference in the manner of integration of pitch and rhythm information, with the poorer performing group showing greater integration across dimensions. Current work investigated the effect of training over sessions on pitch and rhythm processing of four-tone sequences by trained musicians and audio-arts students experienced in critical listening. Sequence tones either had a fixed duration (212 ms) with frequency randomly selected from a logarithmically scaled distribution (400–1750 Hz), a fixed frequency (837 Hz) with a randomly selected logarithmically scaled duration (75–600 ms), or a random frequency and duration. In initial conditions, the task was ...

Effect of listener experience on pitch and rhythm perception with tonal sequences

Experience and training can influence discrimination of tonal sequences. Current work investigated pitch and rhythm processing of four-tone sequences by audiology and speech students, audio-arts students experienced in critical listening, and trained musicians. Sequence tones either had a fixed duration (212 ms) with frequency randomly selected from a logarithmically scaled distribution (400–1750 Hz), a fixed frequency (837 Hz) with a randomly selected log scaled duration (75–600 ms), or a random frequency and duration. In initial conditions, the task was to assemble sequence elements to recreate the target sequence for each of the three sequence types. To evaluate effect of extraneous randomization, both frequency and duration were randomized in the final two conditions with only one of the two attributes defining the target sequence. Audio-arts students performed significantly better than audiology and speech students in the reconstruction task. In conditions involving joint processing of sequence pitch...

Relationship between pitch and rhythm perception with tonal sequences

Past work has shown varying degrees of relationship between pitch and rhythm perception. Current work investigated the relationship between pitch and rhythm processing in a four-tone sequence-reconstruction task which places additional demands on short-term memory. Sequence tones either had a fixed duration (212 ms) with frequency randomly selected from a logarithmically scaled distribution (400–1750 Hz), a fixed frequency (837 Hz) with a randomly selected log scaled duration (75–600 ms), or a random frequency and duration. In initial conditions, the task was to assemble sequence elements to recreate the target sequence for each of the three sequence types. To evaluate effect of extraneous randomization, both frequency and duration were randomized in the final two conditions with only one of the two attributes defining the target sequence. When only one stimulus attribute was randomized, performance was significantly better with sequences defined by pitch rather than rhythmic variation. Combining pitch an...

Magnitude estimation of perceptual fusion of multiple sound sources

The precedence effect states that the first sound wave arriving at a listener has more influence on the perception of location than later arriving echoes. When a source plus echoes is perceived approximately at the location of the source, the image is said to be ‘‘fused.’’ The following set of experiments introduces a new procedure to estimate fusion of clicks. Listeners are presented with a source click plus a single echo click at a variety of delays. Stimuli are rated between a value of ‘‘0’’ and ‘‘100,’’ with ‘‘0’’ indicating a single source and ‘‘100’’ indicating more than one source. These ratings are used to determine delays which are perceived as a single source (receive a rating of ‘‘0’’) and delays which are perceived as multiple sources (receive a rating of ‘‘100’’). These anchor stimuli are then presented on each trial. The stimulus which received a rating of ‘‘0’’ is presented first, followed by the sound to be rated. The stimulus which received a rating of ‘‘100’’ is presented last. This procedure was used to determine fusion as a function of location and number of echoes presented. [Research supported by a program project grant from NIDCD.]