Trends in Cognitive Sciences
ReviewThe shape of ears to come: dynamic coding of auditory space
Section snippets
Extraction of auditory localization cues in the brainstem
The neural processing of different auditory localization cues begins in largely separate pathways in the brainstem 4. Auditory nerve fibres leaving the cochlea terminate on neurones in different subdivisions of the cochlear nucleus. Cells in the ventral subdivisions project, either directly or indirectly, to the superior olivary complex (SOC) on both sides of the brainstem. The SOC is therefore the principal site of binaural convergence, and it is at this level that neuronal sensitivity to
Mapping auditory space in the midbrain
Free-field studies have shown that a map of auditory space exists within the deeper layers of the SC (Refs 10, 11, 12). This is defined by an orderly relationship between the preferred sound directions of auditory neurones and their location within the nucleus (Fig. 1). In contrast to most other relay stations between the cochlea and the cortex, the SC is not ‘tonotopically’ organized. Instead, spatial information is combined across different frequency-specific channels in the brainstem to
Alternative coding strategies in the cortex
Recent studies of the neural processing of auditory space have focused increasingly on the cortex. It has been known for a long time that temporal lobe damage can lead to impaired sound localization 17, 18. But lesion studies are not always easy to interpret, because it can be unclear whether an observed deficit is attributable to a sensory or perceptual loss, or an inability to generate or perform the correct response. Nevertheless, it has been concluded that, after large bilateral lesions of
Auditory localization is individually calibrated
The size and shape of the head and external ears, and therefore the values of the auditory localization cues corresponding to particular sound directions, can vary substantially between individuals 35, 36, 37. These cue values also undergo changes within a subject as the head and ears grow (Fig. 3). Accurate localization therefore requires that the neural code for auditory space be calibrated to these individual characteristics. Evidence for this has been provided in human psychophysical
Experience shapes the developing representation of auditory space
Most of the neurophysiological evidence for experience-driven adjustments in sound localization has come from developmental studies of the auditory space map in the SC. Experiments in which animals have been reared with modified auditory or visual inputs have revealed substantial plasticity in this representation 43, 44.
The potential for the sound localization pathway to be shaped by experience of the acoustic localization cues provided by the listener's own ears is illustrated by the
Plasticity of sound localization is not restricted to development
There is considerable evidence that neural circuits, and the behaviours to which they contribute, are particularly dependent on sensory experience during a ‘critical period’ of early life. The plasticity of the auditory space map in the SC is no exception to this, as the spatial tuning of these neurones is more susceptible to altered sensory inputs during infancy than later in life 43, 44. This makes good sense, as developmental plasticity is clearly necessary in order to adjust neuronal
Concluding remarks
The neural computations involved in determining the location of a sound source are established in the brainstem, but the manner in which this information is encoded changes at higher levels of the auditory system. The SC contains a topographic representation of auditory space, which contributes to the multisensory guidance of orienting behaviour. The steps leading to the formation of this neural map and its alignment with other sensory representations have provided a valuable and accessible
Questions for future research
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How are the various schemes that have been proposed for coding space in the auditory cortex affected by the presence of multiple sound sources, or under conditions in which other features of the sound source vary? Are these coding schemes emergent properties of the cortex or do they originate in the subcortical nuclei that feed into A1?
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Studies that emphasize the importance of temporal discharge patterns in auditory spatial coding have mostly been carried out on anaesthetized animals. What is
Acknowledgements
Our research is supported by the Wellcome Trust (Senior Research Fellowship to A.J.K.) and by Defeating Deafness (Dunhill Medical Trust Fellowship to J.W.H.S.). We are grateful to Tom Mrsic-Flogel and to Doris Kistler for their assistance with recording some of the data illustrated in this paper, and to David Moore, Tom Mrsic-Flogel and Carl Parsons for their comments on an earlier draft of the manuscript.
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