The precedence effect.

In a reverberant environment, sounds reach the ears through several paths. Although the direct sound is followed by multiple reflections, which would be audible in isolation, the first-arriving wavefront dominates many aspects of perception. The "precedence effect" refers to a group of phenomena that are thought to be involved in resolving competition for perception and localization between a direct sound and a reflection. This article is divided into five major sections. First, it begins with a review of recent work on psychoacoustics, which divides the phenomena into measurements of fusion, localization dominance, and discrimination suppression. Second, buildup of precedence and breakdown of precedence are discussed. Third measurements in several animal species, developmental changes in humans, and animal studies are described. Fourth, recent physiological measurements that might be helpful in providing a fuller understanding of precedence effects are reviewed. Fifth, a number of psychophysical models are described which illustrate fundamentally different approaches and have distinct advantages and disadvantages. The purpose of this review is to provide a framework within which to describe the effects of precedence and to help in the integration of data from both psychophysical and physiological experiments. It is probably only through the combined efforts of these fields that a full theory of precedence will evolve and useful models will be developed.

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.

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.

[Endoscopic sclerotherapy of esophageal varices with 80% alcohol. Experience with 46 cases]. / Escleroterapia endoscópica de las várices esofágicas con alcohol al 80%. Experiencia en 46 casos.

We are presenting the results obtained by endoscopic sclerotherapy with 80% alcohol in fifty-five patients with upper GI bleeding caused by esophageal varices, with or without active bleeding at the moment of the endoscopic study. We obtained control of bleeding in 83.64% with a recurrence of bleeding of 16.36% during the first 15 days. At the same period of time we obtained a 16.36% mortality rate. In the forty six patients that concluded sclerotherapy, decrease in size of the varices occurred in 89.79% and eradication was obtained in 6.11% of the patients. We observed three types of complications: esophageal ulcers in 100% of the cases which healed in a period of three weeks. Retrosternal pain of short duration, in 45.65%, this pain appears when the sclerosant substance is injected. Esophageal stenosis in 5.45% which were treated with dilatations. Our conclusion is that 80% alcohol is an effective and safe sclerosant substance in the management of acute bleeding of esophageal varices and in the prevention of the immediate recurrence, but has the disadvantage of a great number of local complications.