Behind the scenes of auditory perception

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‘Auditory scenes’ often contain contributions from multiple acoustic sources. These are usually heard as separate auditory ‘streams’, which can be selectively followed over time. How and where these auditory streams are formed in the auditory system is one of the most fascinating questions facing auditory scientists today. Findings published within the past two years indicate that both cortical and subcortical processes contribute to the formation of auditory streams, and they raise important questions concerning the roles of primary and secondary areas of auditory cortex in this phenomenon. In addition, these findings underline the importance of taking into account the relative timing of neural responses, and the influence of selective attention, in the search for neural correlates of the perception of auditory streams.

Introduction

We are usually surrounded by multiple sound sources. The sound waves produced by these sources mingle before reaching our ears, creating complex vibration patterns on our eardrums. An essential function of the auditory system is to analyze these patterns to recover the sound sources that generated them (Figure 1a). This is known as the ‘auditory scene analysis’ problem [1] or, more colloquially, as the ‘cocktail party’ problem [2]. Understanding how the auditory system solves this problem is one of the most fascinating tasks facing auditory scientists today. During the past decade, studies devoted to exploring how, and where, auditory scenes are analyzed in the brain have multiplied, using techniques ranging from single-unit recordings to electroencephalography (EEG), magnetoencephalography (MEG), and functional magnetic resonance imaging (fMRI). Although the results of some of these studies were reviewed in earlier publications [3, 4, 5, 6, 7, 8, 9, 10, 11, 12], during the past two years, several new findings have emerged. Some of these findings qualify the conclusions of earlier studies, and raise important questions for future research.

The aim of this article is twofold: first, to provide a brief overview of the current state of knowledge concerning the neural basis of auditory scene analysis, for readers who are not familiar with this research; second, to summarize the three (in our view) most significant questions that have emerged from findings published on this topic within the past two years. Being brief, this review is also, necessarily, selective. It focuses on an important aspect of auditory scene analysis, which the auditory-perception and auditory-neuroscience literatures commonly refer to as ‘auditory streaming’ (Box 1).

Section snippets

Do auditory streams emerge below, in, or beyond the auditory cortex?

The auditory system is a multi-storey building (Figure 1b). As sensory information ascends from the cochlea to the primary auditory cortex, it passes through several nuclei. Neurons at each level exhibit complex response properties, which reflect sophisticated signal-processing operations. An important task for auditory researchers is to clarify the role of these different processing stages in auditory scene analysis. Most of the studies that have been performed during the past decade

How are auditory streams formed in the brain? The role of temporal coherence

Auditory streaming has traditionally been studied using sequences of pure tones at two frequencies (A and B), which are played in alternation (forming a repeating AB or ABA pattern)—an audio demo can be found at: http://www.tc.umn.edu/∼cmicheyl/demos.html. The probability that the A and B tones are heard as separate streams usually increases with their frequency separation, the pace of tone presentation, and – provided that the sequence is continuously attended to by the listener – the time

How does attention influence auditory stream formation at the neural level?

The influence of attention on the formation of auditory streams has inspired several studies during the past decade. EEG studies have identified neural indices of stream segregation, such as the mismatch negativity (MMN), which are modulated by attention, but can be detected in averaged responses even when the listeners are engaged in a task that draws their attention away from the evoking sounds [21, 38, 39, 40]. The ‘object related negativity’ (ORN), a neural index of the perceived

Conclusions

Over the past decade, a rapidly increasing number of studies have started to explore where and how auditory streams are formed in the brain. Neural correlates of auditory streaming have been identified in, below, and even beyond the auditory cortex—in cortical regions not traditionally associated with auditory processing. This suggests that the formation of auditory streams involves a broadly distributed neural network. An important goal for future research will be to clarify the roles of

References and recommended reading

Papers of particular interest, published within the annual period of review, have been highlighted as:

  • • of special interest

  • •• of outstanding interest

Acknowledgements

This work was supported by NIH R01 DC 07657 and Advanced Acoustic Concepts.

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