Position of acoustic stimulus modulates visual α activity.

It has been repeatedly shown that a unimodal stimulus can modulate oscillatory activity of multiple cortical areas already at early stages of sensory processing. In this way, an influence can be exerted on the response to a subsequent sensory input. Even though this fact is now well established, it is still not clear whether cortical sensory areas are informed about spatial positions of objects of modality other than their preferred one. Here, we test the hypothesis of whether oscillatory activity of the human visual cortex depends on the position of a unimodal auditory object. We recorded electroencephalogram while presenting sounds in an acoustic free-field either at the center of the visual field or at lateral positions. Using independent component analysis, we identified three cortical sources located in the visual cortex, showing stimulus position-specific oscillatory responses. The most pronounced effect was an immediate α (8-12 Hz) power decrease over the entire occipital lobe when the stimulus originated from the center of the binocular visual field. Following a lateral stimulation, the amplitude of α activity decreased slightly over contralateral visual areas, while at the same time a weak α synchronization was observed in corresponding ipsilateral areas. Thus, even in the absence of visual stimuli, the visual cortex is differentially activated depending on the position of an acoustic sound source. Our results show that the visual cortex receives information about the position of auditory stimuli within the visual field.