BackgroundPrevious work on the human auditory cortex has revealed areas specialized in spatial processing but how the neurons in these areas represent the location of a sound source remains unknown.Methodology/Principal FindingsHere, we performed a magnetoencephalography (MEG) experiment with the aim of revealing the neural code of auditory space implemented by the human cortex. In a stimulus-specific adaptation paradigm, realistic spatial sound stimuli were presented in pairs of adaptor and probe locations. We found that the attenuation of the N1m response depended strongly on the spatial arrangement of the two sound sources. These location-specific effects showed that sounds originating from locations within the same hemifield activated the same neuronal population regardless of the spatial separation between the sound sources. In contrast, sounds originating from opposite hemifields activated separate groups of neurons.Conclusions/SignificanceThese results are highly consistent with a rate code of spatial location formed by two opponent populations, one tuned to locations in the left and the other to those in the right. This indicates that the neuronal code of sound source location implemented by the human auditory cortex is similar to that previously found in other primates.

The ability to locate the direction of a target sound in a background of competing sources is critical to the survival of many species and important for human communication. Nevertheless, brain mechanisms that provide for such accurate localization abilities remain poorly understood. In particular, it remains unclear how the auditory brain is able to extract reliable spatial information directly from the source when competing sounds and reflections dominate all but the earliest moments of the sound wave reaching each ear. We developed a stimulus mimicking the mutual relationship of sound amplitude and binaural cues, characteristic to reverberant speech. This stimulus, named amplitude modulated binaural beat, allows for a parametric and isolated change of modulation frequency and phase relations. Employing magnetoencephalography and psychoacoustics it is demonstrated that the auditory brain uses binaural information in the stimulus fine structure only during the rising portion of each modulation cycle, rendering spatial information recoverable in an otherwise unlocalizable sound. The data suggest that amplitude modulation provides a means of "glimpsing" low-frequency spatial cues in a manner that benefits listening in noisy or reverberant environments.spatial hearing | binaural processing | auditory system | psychoacoustics | auditory MEG H uman listeners are able to determine the location of a talker against a background of competing voices, even in rooms where walls generate reflections that, taken together, can be more intense than sounds arriving directly from the source. The dominant cues for localization in such complex sound fields are the interaural time differences (ITDs) conveyed in the temporal fine structure (TFS) of low-frequency (<1,500 Hz) sounds (1); normal-hearing listeners can discriminate ITDs as low as 10-20 μs in 500-and 1,000-Hz pure tones to judge the source location (2). In addition to source localization-the focus of the current study-sensitivity to ITDs is also reported to contribute to "spatial unmasking": different spatial configurations of the signal and background noise enable sources to be heard out, increasing their intelligibility (3, 4).The majority of real-world sounds are strongly modulated in amplitude. Without these modulations humans are completely insensitive to the sound source location after its onset in reverberant environments [the Franssen effect (5)]. Human speech, for example, contains amplitude modulation (AM) rates ranging from those of syllables and phonemes to those conveying information about voice pitch (i.e., from 2 Hz up to about 300 Hz). These modulations act as potent grouping cues, enabling listeners to fuse sounds originating from a single talker, segregating them from competing talkers (6). Despite the importance of AM in real-world listening, however, behavioral measures of ITD sensitivity are commonly assessed for stimuli in which the amplitude is unmodulated. This is especially so when the focus of interest concerns ITDs conveyed in the TFS. Fig. 1 il...

A magnetoencephalography study was conducted to reveal the neural code of interaural time difference (ITD) in the human cortex. Widely used crosscorrelator models predict that the code consists of narrow receptive fields distributed to all ITDs. The present findings are, however, more in line with a neural code formed by two opponent neural populations: one tuned to the left and the other to the right hemifield. The results are consistent with models of ITD extraction in the auditory brainstem of small mammals and, therefore, suggest that similar computational principles underlie human sound source localization.

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The auditory system codes spatial locations in a way that deviates from the spatial representations found in other modalities. This difference is especially striking in the cortex, where neurons form topographical maps of visual and tactile space but where auditory space is represented through a population rate code. In this hemifield code, sound source location is represented in the activity of two widely tuned opponent populations, one tuned to the right and the other to the left side of auditory space. Scientists are only beginning to uncover how this coding strategy adapts to various spatial processing demands. This review presents the current understanding of auditory spatial processing in the cortex. To this end, the authors consider how various implementations of the hemifield code may exist within the auditory cortex and how these may be modulated by the stimulation and task context. As a result, a coherent set of neural strategies for auditory spatial processing emerges.

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