Functionally independent components of early event-related potentials in avisual spatial attention task

Spatial visual attention modulates the first negative-going deflection in t he human averaged event-related potential (ERP) in response to visual targe t and non-target stimuli (the N1 complex). Here we demonstrate a decomposit ion of N1 into functionally independent subcomponents with functionally dis tinct relations to cask and stimulus conditions. ERPs were collected from 2 0 subjects in response to visual target and non-target stimuli presented at five attended and non-attended screen locations. Independent component ana lysis, a new method for blind source separation, was trained simultaneously on 500 ms grand average responses from all 25 stimulus-attention condition s and decomposed the nontarget N1 complexes into five spatially fixed, temp orally independent and physiologically plausible components. Activity of an early laterally symmetrical component pair (N1a(R) and N1a(L)) was evoked by the left and right visual field stimuli, respectively. Component N1a(R) peaked ca. 9 ms earlier than N1a(L). Central stimuli evoked both components with the same peak latency difference, producing a bilateral scalp distrib ution. The amplitudes of these components were not reliably augmented by sp atial attention. Stimuli in the right visual field evoked activity in a spa tio-temporally overlapping bilateral component (N1b) chat peaked at ca. 180 ms and was strongly enhanced by attention. Stimuli presented at unattended locations evoked a fourth component (P2a) peaking near 240 ms. A fifth com ponent (P3f) was evoked only by targets presented in either visual field. T he distinct response patterns of these components across the array of stimu lus and attention conditions suggest that they reflect activity in function ally independent brain systems involved in processing attended and unattend ed visuospatial events.