Academic journal article Perception and Psychophysics

Gain Control in the Auditory System: Absolute Identification of Intensity within and across the Two Ears

Academic journal article Perception and Psychophysics

Gain Control in the Auditory System: Absolute Identification of Intensity within and across the Two Ears

Article excerpt

A number of studies have suggested that the auditory system employs nonlinear gain control to modulate its response to a broad range of stimulus intensities (e.g., Parker, Murphy, & Schneider, 2002), but the precise location and nature of this mechanism remains uncertain. To address these issues, we investigated the extent to which gain control in one ear was influenced by auditory events in the other ear. Listeners were asked to identify which of four sounds, varying in intensity (25, 30, 35, and 40 dB [SPL]), had been presented to the left ear. Subsequent test sets included an 80-dB (SPL) stimulus presented to either the left (ipsilateral) or the right (contralateral) ear. Ipsilateral presentations of the 80-dB (SPL) stimulus produced a greater reduction in identification accuracy among the original four sounds than did contralateral presentations, but both presentations caused a reduction in gain. In a second experiment, the perceived laterally of a sound was manipulated by presenting the same sound to both ears with an interaural time delay. When the 80-dB (SPL) stimulus was added to the set of quiet sounds (which were presented to both ears but were perceived to be in the left ear because of the interaural time delay), identification accuracy was reduced by the same amount, independently of whether the 80-dB (SPL) stimulus was perceived only in the ipsilateral ear or only in the contralateral ear.

Although the response range of individual primary auditory afferents in mammals is quite limited (e.g., 15-40 dB for the cat; Kiang, Watanabe, Thomas, & Clark, 1965), behaviorally, mammals can respond to intensity differences over a considerably larger range (e.g., >100 dB for the cat; Heffner & Heffner, 1985). To explain how a limited response range in primary auditory afferents can eventually yield a much larger behavioral dynamic range, many investigators have proposed the existence of a nonlinear auditory amplification system (a nonlinear amplifier) capable of expanding the dynamic range of hearing (Parker & Schneider, 1994; Robles & Ruggero, 2001; Schneider & Parker, 1990; Yates, 1995). Moreover, recent results have suggested that the gain of this nonlinear amplifier is under top-down control (e.g., Parker, Murphy, & Schneider, 2002). That is, the amount of amplification depends on such factors as the listener's expectations concerning the sound intensities and/or the range of stimuli expected. However, how and at what level this top-down control is exerted is not yet known. Here, we explore how gain is affected in the auditory system, specifically exploring whether the acoustic events in one ear can affect sensitivity to sounds in the contralateral ear.

Locus of the Nonlinear Amplifier

A number of electrophysiological and biomechanical studies have shown that there is a mechanism within the cochlea (called the cochlear amplifier) that amplifies very low-intensity stimuli, thereby increasing sensitivity and extending the range over which the auditory system can respond (for reviews, see Brownell, 1999; Rubles & Ruggero, 2001; Yates, 1995). However, given the complexity of the auditory system, it is also possible that gain adjustments are imposed at more central levels, ranging from the brainstem to the cortex. Hence, the cochlear amplifier may be only a component of a general system that uses nonlinear amplification to extend its effective dynamic range.

Top-Down Gain Control

Behavioral evidence has demonstrated that listeners are able to use top-down knowledge to accommodate an anticipated range of acoustic intensities (Parker et al., 2002; Parker & Schneider, 1994). Using an absolute identification paradigm, Parker et al. asked listeners to identify which one of four tones (e.g., 25, 30, 35, and 40 dB [SPL], 1000-Hz tones) was presented on a trial by pressing one of four buttons (e.g., Button 1 for the 25-dB [SPL] tone, Button 2 for the 30-dB [SPL] tone, etc. …

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