Neural correlates of sound localization in complex acoustic environments.

Listening to and understanding people in a "cocktail-party situation" is a remarkable feature of the human auditory system. Here we investigated the neural correlates of the ability to localize a particular sound among others in an acoustically cluttered environment with healthy subjects. In a sound localization task, five different natural sounds were presented from five virtual spatial locations during functional magnetic resonance imaging (fMRI). Activity related to auditory stream segregation was revealed in posterior superior temporal gyrus bilaterally, anterior insula, supplementary motor area, and frontoparietal network. Moreover, the results indicated critical roles of left planum temporale in extracting the sound of interest among acoustical distracters and the precuneus in orienting spatial attention to the target sound. We hypothesized that the left-sided lateralization of the planum temporale activation is related to the higher specialization of the left hemisphere for analysis of spectrotemporal sound features. Furthermore, the precuneus - a brain area known to be involved in the computation of spatial coordinates across diverse frames of reference for reaching to objects - seems to be also a crucial area for accurately determining locations of auditory targets in an acoustically complex scene of multiple sound sources. The precuneus thus may not only be involved in visuo-motor processes, but may also subserve related functions in the auditory modality

Activations of brain regions as revealed by the contrast of <i>“cocktail”</i> condition versus rest (<i>p</i>_FWE <0.05).

<p>STG, superior temporal gyrus; FEF, frontal eye fields; IPL, inferior parietal lobule; SPL, superior parietal lobule; IFG, inferior frontal gyrus; PrCu, precuneus), aIns, anterior insula; SMA, supplementary motor area. The color code refers to <i>t</i>-values.</p

Regions with significant activation for each contrast, main analysis.

<p>Local maxima employing a voxel-level threshold of <i>p</i>≤0.05, FWE-corrected for multiple comparisons and cluster size of more than 10 voxels (except for the “cocktail”>“sequence” comparison). FEF, frontal eye fields; SMA, supplementary motor area; STG, superior temporal gyrus; IPL, inferior parietal lobule; SPL, superior parietal lobule; IFG, inferior frontal gyrus.</p

Auditory and visual stimuli.

<p>(<b>A</b>) Example of one virtual auditory scene used for the <i>“cocktail”</i> and <i>“passive”</i> conditions. Each sound was presented as coming from a different location. (<b>B</b>, <b>C</b>) Visual stimuli. Each box represented a sound source (<i>“single” and “cocktail”</i> conditions) or from left to right the total number of sounds presented in a sequence of sounds (<i>“sequence”</i> condition). Subjects were instructed to perform a saccade as the response in each trial. In the “active” tasks (i.e., <i>“single”, “cocktail”,</i> and <i>“sequence”</i> conditions), the slot in the circle, which served as saccade starting point, was presented horizontally (neutral) hence not cueing any particular direction. (<b>B</b>). In the <i>“passive”</i> condition, the saccade direction was cued by the slot in the circle (<b>C</b>).</p

Activations of brain regions as revealed by the contrast of <i>“cocktail”</i>><i>“passive”</i> (<i>p</i>_FWE <0.05).

<p>STG, superior temporal gyrus; IFG, inferior frontal cortex; FEF, frontal eye field; SMA, supplementary motor area.</p

Results of analyses <i>(a)</i> employing performance as covariate and <i>(b)</i> modelling separately the first trial of each block to account for cueing effects.

<p>Local maxima employing a voxel-level threshold of <i>p</i>≤0.05, FWE-corrected for multiple comparisons and cluster size of more than 10 voxels. STG, superior temporal gyrus; IFG, inferior frontal gyrus.</p

Areas related to the localization of sounds in a “cocktail-party situation”.

<p>(<b>A</b>) Activations of brain regions as revealed by the contrast of <i>“cocktail”</i>><i>“single”</i> (<i>p</i>_FWE <0.05). The contrast resulted in a major activation of auditory cortex, specifically in the planum temporale (PT). Two further small clusters in right PT and left inferior frontal gyrus (IFG) were observed. (<b>B</b>) Activation for the contrast of <i>“cocktail”</i>><i>“sequence”</i> (<i>p</i>_FWE <0.05). The only areas active with this contrast were the precuneus (PrCu) bilaterally and a small cluster in left PT.</p

Experimental procedure.

<p>(<b>A</b>) Trial structure. Each trial began with the presentation of the auditory stimulus while the subject fixated at the fixation cross for 2 s. The auditory stimulus was followed by a 400 ms interstimulus interval and, subsequently, by the presentation of the visual saccadic-response stimulus for 1 s. The intertrial-interval lasted 600 ms. (<b>B</b>) Experimental run. Each run comprised 5 blocks of each condition, and each block consisted of a sequence of 5 trials. Between blocks, a 20-s rest period was inserted. Conditions were presented in a fixed order: “<i>single” – “cocktail” – “sequence” – “passive”</i>.</p