Abstract
We live in complex auditory environments, in which we are confronted with multiple competing sounds, including the cacophony of talkers in busy markets, classrooms, offices, etc. The purpose of this article is to synthesize observations from a series of experiments that focused on how spatial hearing might aid in disentangling interleaved sequences of sounds. The experiments were unified by a non-verbal task, “rhythmic masking release”, which was applied to psychophysical studies in humans and cats and to cortical physiology in anesthetized cats. Human and feline listeners could segregate competing sequences of sounds from sources that were separated by as little as ∼10°. Similarly, single neurons in the cat primary auditory cortex tended to synchronize selectively to sound sequences from one of two competing sources, again with spatial resolution of ∼10°. The spatial resolution of spatial stream segregation varied widely depending on the binaural and monaural acoustical cues that were available in various experimental conditions. This is in contrast to a measure of basic sound-source localization, the minimum audible angle, which showed largely constant acuity across those conditions. The differential utilization of acoustical cues suggests that the central spatial mechanisms for stream segregation differ from those for sound localization. The highest-acuity spatial stream segregation was derived from interaural time and level differences. Brainstem processing of those cues is thought to rely heavily on normal function of a voltage-gated potassium channel, Kv3.3. A family was studied having a dominant negative mutation in the gene for that channel. Affected family members exhibited severe loss of sensitivity for interaural time and level differences, which almost certainly would degrade their ability to segregate competing sounds in real-world auditory scenes.
Highlights
Everyday listening situations require us to isolate sounds of interest amid competing sounds
How does a listener piece string together the syllables from one talker amid the tangle of competing voices at a cocktail party or any other complex auditory scene? What are the brain mechanisms that enable such a task? In this review, we have focused on a series of experiments that were unified by use of a common psychophysical task, “rhythmic masking release”, and its corresponding stimulus set
We summarize some of the key conclusions from those previous studies
Summary
Everyday listening situations require us to isolate sounds of interest amid competing sounds. That is roughly the spatial acuity with which cortical neurons in the anesthetized cat auditory cortex could segregate streams of noise bursts from alternating source locations, according to the results from Middlebrooks and Bremen (2013) The latter authors extended that observation by testing the target-masker separation at which target rhythm could be identified on the basis of firing patterns of single cortical neurons. In those empirical tests, stimuli consisted of sequences of broad-band noise bursts presented as Rhythm 1 or Rhythm 2, which were essentially equivalent to the broad-band condition in the human psychophysical experiments (Middlebrooks and Onsan, 2012). That acuity of single cortical neurons is remarkably close to the psychophysical thresholds of feline (and human) listeners
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