Single vs. paired visual stimulation: superposition of early neuromagneticresponses and retinotopy in extrastriate cortex in humans

Neuromagnetic techniques were used in conjunction with magnetic resonance i maging (MRI) techniques to: (1) localize and characterize cortical sources evoked by visual stimuli presented at different locations in the lower righ t visual field; (2) examine the superposition of cortical responses by comp aring the summation of responses to the presentation of single stimuli with responses to paired stimuli; and (3) examine the spatial resolution of mag netoencephalographic (MEG) techniques by comparing the identified source lo cations evoked by the presentation of single vs. paired stimuli. Using mult i-dipole, non-linear minimization analyses, three sources were localized fo r each stimulus condition during the initial 80-170 ms poststimulus interva l for all subjects. In addition to an occipital source, two extrastriate so urces were identified: occipital-parietal and occipital-temporal. Each sour ce evidenced a systematic shift in location associated with changes in stim ulus placement parallel to the vertical meridian. To our knowledge, this is the first demonstration of retinotopic organization of extrastriate areas, using non-invasive neuromagnetic techniques. The paired presentation of st imuli reflected superposition of the responses evoked by single stimuli but only for early activity up to 150 ms poststimulus. Undersummation was evid ent after 150 ms. All sources identified for single stimuli were also ident ified in the paired-stimulus responses; but at the expense of larger errors for some of the estimated parameters. (C) 1999 Elsevier Science B.V. All r ights reserved.