Distinguishing between lateralized and nonlateralized brain activity associated with visual short-term memory: fMRI, MEG, and EEG evidence from the same observers.

Previous functional neuroimaging studies have shown that maintenance of centrally presented objects in visual short-term memory (VSTM) leads to bilateral increases of BOLD activations in IPS/IOS cortex, while prior electrophysiological work suggests that maintaining stimuli encoded from a single hemifield leads to a sustained posterior contralateral negativity (SPCN) in electrophysiology and magnetoencephalography. These two findings have never been investigated using the same physiological measures. We recorded the BOLD response using fMRI, magnetoencephalography (MEG), and electrophysiology (EEG), while subjects encoded visual stimuli from a single hemifield of a balanced display. The EEG showed an SPCN. However, no SPCN-like activation was observed in the BOLD signals. The BOLD response in parietal cortex remained bilateral, even after unilateral encoding of the stimuli, but MEG showed both bilateral and contralateral activations, each likely reflecting a sub portion of the neuronal populations participating in the maintenance of information in VSTM. Contrary to the assumption that BOLD, EEG, and MEG responses - that were each linked to the maintenance of information in VSTM - are markers of the same neuronal processes, our findings suggest that each technique reveals a somewhat distinct but overlapping neural signature of the mechanisms supporting visual short-term memory.