Activation of the human occipital and parietal cortex by pattern and luminance stimuli: neuromagnetic measurements

We compared cortical reactivity to pattern and luminance stimuli byrecording evoked responses and spontaneous brain rhythms from 10 subjectswith a whole-scalp neuromagnetometer. Hemifield patterns (black-and-whitecheckerboards) elicited strong contralateral transient activation of theoccipital V1/V2 cortex, maximum at 65-75 ms, followed by sustainedactivation during the 2 s stimulus. Responses to hemifield luminancestimuli also had an occipital component, but they were dominated byactivation of the medial parieto-occipital sulcus (POS) 60- 70 ms later.The POS region was equally well activated by foveal and extrafovealstimuli. The occipital responses to hemifield luminance stimuli differedfrom those to pattern stimuli in two main aspects: the sustained activationwas significantly weaker, and the responses were almost symmetrical,indicating a surprisingly bilateral occipital activation. These effectswere similar with foveal and extrafoveal stimuli. The spontaneous 10 Hzalpha rhythm, originating predominantly in the POS region, was suppressedafter both stimulus onsets and offsets, more strongly for luminance thanpattern stimuli. Activation of the occipital cortex dominated after patternstimuli, whereas the effect of luminance stimulation was stronger in theparieto-occipital region. The distinct signal distributions in theoccipital and POS regions suggest that the two types of stimuli activatethe magno- and parvocellular pathways to a varying degree. These findingsare also in line with a stronger attention-catching value of the luminancethan pattern stimuli.