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Nicolas Robitaille, Stephan Grimault, Jay Todd, René Marois, Douglas Cheyne, Pierre Jolicoeur; Multiple physiological markers of visual short-term memory: convergence and divergence. Journal of Vision 2009;9(8):595. doi: 10.1167/9.8.595.
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Maintenance of centrally presented objects in visual short-term memory (VSTM) leads to bilateral increases of the BOLD response in IPS/IOS cortex (Todd & Marois, 2004), while maintaining stimuli encoded from a single hemifield leads to a sustained posterior contralateral negativity (SPCN) in electrophysiology (Vogel & Machizawa, 2004). We recorded the BOLD signal using fMRI, magnetoencephalography (MEG), and electroencephalography (EEG) while participants encoded visual stimuli from a single hemifield. The EEG recorded over parietal cortex showed the SPCN, as did MEG activation in superior IPS. However, no SPCN-like activation (meaning a contralateral-ipsilateral difference that increases with the increase of mnemonic load) was observed in the BOLD signal for superior IPS, disconfirming the hypothesis that the SPCN and BOLD activation in IPS are two markers of the same neuronal process. Furthermore, left inferior occipital (IO) cortex was prominent by being the only BOLD activation showing an SPCN-like pattern: an increase of activation with higher load, only for right hemifield stimuli. Right IO, however, showed an increase of the BOLD signal for stimuli encoded from both hemifields. Given that the SPCN and the magnetic equivalent (the SPCM) have been shown to occur bilaterally, IO cannot be the locus of SPCN. The different results will be discussed in regard to the various physiological markers of VSTM.
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