Modulation of human auditory spatial scene analysis by transcranial direct current stimulation

Localizing and selectively attending to the source of a sound of interest in a complex auditory environment is an important capacity of the human auditory system. The underlying neural mechanisms have, however, still not been clarified in detail. This issue was addressed by using bilateral bipolar-balanced transcranial direct current stimulation (tDCS) in combination with a task demanding free-field sound localization in the presence of multiple sound sources, thus providing a realistic simulation of the so-called "cocktail-party" situation. With left-anode/right-cathode, but not with right-anode/left-cathode, montage of bilateral electrodes, tDCS over superior temporal gyrus, including planum temporale and auditory cortices, was found to improve the accuracy of target localization in left hemispace. No effects were found for tDCS over inferior parietal lobule or with off-target active stimulation over somatosensory-motor cortex that was used to control for non-specific effects. Also, the absolute error in localization remained unaffected by tDCS, thus suggesting that general response precision was not modulated by brain polarization. This finding can be explained in the framework of a model assuming that brain polarization modulated the suppression of irrelevant sound sources, thus resulting in more effective spatial separation of the target from the interfering sound in the complex auditory scene.