ACTIVATION OF THE HUMAN OCCIPITAL AND PARIETAL CORTEX BY PATTERN AND LUMINANCE STIMULI - NEUROMAGNETIC MEASUREMENTS

We compared cortical reactivity to pattern and luminance stimuli by re cording evoked responses and spontaneous brain rhythms from 10 subject s with a whole-scalp neuromagnetometer. Hemifield patterns (black-and- white checkerboards) elicited strong contralateral transient activatio n of the occipital V1/V2 cortex, maximum at 65-75 ms, followed by sust ained activation during the 2 s stimulus. Responses to hemifield lumin ance stimuli also had an occipital component, but they were dominated by activation of the medial parieto-occipital sulcus (POS) 60-70 ms la ter. The POS region was equally well activated by foveal and extrafove al stimuli. The occipital responses to hemifield luminance stimuli dif fered from those to pattern stimuli in two main aspects: the sustained activation was significantly weaker, and the responses were almost sy mmetrical, indicating a surprisingly bilateral occipital activation. T hese effects were similar with foveal acid extrafoveal stimuli. The sp ontaneous 10 Hz alpha rhythm, originating predominantly in the POS reg ion, was suppressed after both stimulus onsets and offsets, more stron gly for luminance than pattern stimuli. Activation of the occipital co rtex dominated after pattern stimuli, whereas the effect of luminance stimulation was stronger in the parieto-occipital region. The distinct signal distributions in the occipital and POS regions suggest that th e two types of stimuli activate the magno- and parvocellular pathways to a varying degree. These findings are also in line with a stronger a ttention-catching value of the luminance than pattern stimuli.