A NEUROMAGNETIC STUDY OF SELECTIVE AUDITORY ATTENTION

Auditory event-related magnetic fields and electrical potentials were recorded from subjects who were instructed to attend to a sequence of constant pitch tones presented to one ear and ignore a concurrent sequence of tones with a different pitch presented to the other ear. Subjects' task was to detect and count longer duration 'target' tones (P = 0.1) interspersed with 'standard' tones (P = 0.4) in the attended ear and to ignore tones (both standards and targets) in the other ear. All stimuli both attended and ignored elicited a prominent response approximately 100 msec after tone onset (N1m). Beginning approximately 150 msec following stimulus onset an attetion-dependent modulation of the magnetic response (Ndm) was observed for each subject. In 2 subjects whose magnetic field patterns were mapped in detail, equivalent current dipole (ECD) modeling was used to estimate the sources of N1m and Ndm activity. By transforming the coordinate systems for magnetic resonance images (MRIs) and ECD solutions, the locations and orientations of ECDs were determined relative to each subject's brain structures. ECDs for both N1m and Ndm were located in auditory cortex along the posterior regions of the sylvian fissure. Monte Carlo error analyses indicated that the ECD for Ndm is near, but significantly anterior to that for N1m.