The gradient of spatial auditory attention in free field: an event-related potential study.

Young adult subjects attended selectively to brief noise bursts delivered in free field via a horizontal array of seven loudspeakers spaced apart by 9 degrees of angle. Frequent "standard" stimuli (90%) and infrequent "target/deviant" stimuli (10%) of increased bandwidth were delivered at a fast rate in a random sequence equiprobably from each speaker. In separate runs, the subjects' task was to selectively attend to the leftmost, center, or rightmost speaker and to press a button to the infrequent "target" stimuli occurring at the designated spatial location. Behavioral detection rates and concurrently recorded event-related potentials (ERPs) indicated that auditory attention was deployed as a finely tuned gradient around the attended sound source, thus providing support for gradient models of auditory spatial attention. Furthermore, the ERP data suggested that the spatial focusing of attention was achieved in two distinct stages, with an early more broadly tuned filtering of inputs occurring over the first 80-200 msec after stimulus onset, followed by a more narrowly focused selection of attended-location deviants that began at around 250 msec and closely resembled the behavioral gradient of target detections.