In search of neural signatures of visual binding : a MEG/SSVEF study

Abstract

Visual processes are distributed in numerous specialized cortical areas. How neural responses are bound into assemblies to elicit a unified perception remains a central issue in cognitive neurosciences. To address this issue, we conducted an MEG experiment to characterize neural mechanisms of visual coherent motion integration. Stimuli consisting of 2 verticals and 2 horizontals disconnected bars oscillating at 2.3Hz and 3Hz, respectively, were arranged in a “square” like shape. Such periodic stimulations elicit stereotypical oscillatory MEG activities phase-locked to visual stimulation at the first and second harmonics in cortical areas processing the stimulus elements (SSVEP). Subjects' perception, either rigid square or non-rigid square, was modulated by subtle changes in luminance distribution along the bars, resulting in 4 experimental conditions. This design allows separating SSVEP relative to element computation (f, 2f, 4f…etc) and SSVEP linked to motion integration at intermodulation frequencies (nf1 + nf2). The behavioural results confirmed that bars with low-luminance line-ends enhance rigid square perception. MEG analyses reveal focal and percept independent activities at first harmonics on occipital sensors, sources reconstruction showing retinotopic segregation between 2.3Hz and 3Hz activities in V1. We also found more widespread percept independent activities at second harmonics over occipital and parietal sensors. Finally, contrast between rigid and non-rigid percept showed fourth-order intermodulation frequency enhancement localized in the right frontal cortex, likely frontal eye fields (FEF). Overall tagged stimulation and SSVEP analysis provide a precise localisation of cortical activities related to the computation of focal visual elements, consistent with retinotopy. This methodology further highlights an electrophysiological signature of shape processing from ambiguous motion components at fourth-order intermodulation frequency in FEF.