Neuromagnetic fields were recorded from human subject as they listened selectively to sequences of rapidly presented tones in one ear while ignoring tones of a diferent pitch In the oppoite ear. in the interval 20-50 msec (the P20-50) was also found to be enlarged with attention (6, 7).These ERP results support the view that the flow of auditory sensory information can be altered by attention at a relatively early stage of processing. They do not specify, however, the brain structures in which this stimulus selection takes place. One approach to address this question is to study neuromagnetic recordings (ERFs), which offer an advantage over ERPs for localization of the anatomical sources of evoked brain activity in cortical sulci (e.g., auditory cortex on the supratemporal plane). This advantage is due to ERF recordings being selectively sensitive to activity from such sources and due to magnetic fields being less distorted by the skull (8, 9).Several studies have applied source localization techniques to ERF recordings and concluded that at least part of the enhanced activity elicited by attended sounds in the N100 latency range arises from the vicinity of auditory cortex (10, 11). However, the precise anatomical source(s) of this attention effect have yet to be verified by superposition of calculated source coordinates onto magnetic resonance (MR) images of the subjects' brains.11 Even less information is available regarding the neural generator(s) ofthe P20-50 ERP attention effect, as no magnetic counterpart ofthis very early ERP modulation has yet been reported.In the current study, neuromagnetic and MR imaging techniques were combined to localize the neuroanatomical origins of the early effects of attention on tone-evoked brain activity. The results provide evidence that focused auditory attention exerts selective control over early sensory processing in the auditory cortical areas on the supratemporal plane beginning at 20 msec poststimulus. METHODSSelective auditory attention was studied using the same fast-rate dichotic listening paradigm that we have used previously in ERP studies (6, IThere is considerable evidence that at least some of the "exogenous" tone-evoked activity (i.e., non-attention-related) in the 30-to 150-msec latency range arises from neural generators in the vicinity of the auditory cortex on the supratemporal plane (12)(13)(14)(15). Recent neuromagnetic studies have localized portions ofthis early sensoryevoked activity to the supratemporal-plane auditory cortex as visualized on subjects' MR scans (16-18). 8722