Mapping the bilateral visual integration by EEG and fMRI.

In the human visual system, the internal representation of the left and right visual hemifields is split at the midline of the two cerebral hemispheres. The present study aims to address the questions of when and where the lateralized cortical visual representations are merged to form an intact percept by using a multimodal neuroimaging approach. Visual evoked potential (VEP) and functional magnetic resonance imaging (fMRI) data were acquired from a group of healthy subjects presented with unilateral versus bilateral visual stimuli. Cortical activities involved in processing bilateral visual information are expected to be equally responsive to ipsilateral and contralateral stimuli, and demonstrate spatial nonlinearity in the response to bilateral stimuli. Utilizing these features, we performed integrative as well as separate analyses for both VEP and fMRI data. The present results suggest that i) the majority of cortical activity that integrates visual information across hemifields takes place at extrastriate areas during late visual processing, and that ii) the lateral occipito-temporal (LOT) regions (likely the MT+ complex) and the medial occipital cortex (i.e. V1) may contribute to bilateral visual integration during early visual processing. Our findings are generally in agreement with the bottom-up visual hierarchy, with the exception of the evidence suggesting an early activation of the higher-tier LOT areas and the influence from ipsilateral visual inputs upon the V1 response.