Precise mapping of early visual responses in space and time.

We used magnetoencephalography (MEG) to record brain activity while subjects passively viewed stimuli presented at eight different locations in the parafoveal and peripheral visual fields (VF). For each subject, the experiment was repeated on 3 different days. The generators of the early MEG signal components were localized using tomographic source analysis together with statistical parametric mapping. We identified activations throughout visual cortex in the first 100 ms of stimulus onset. The earliest stimulus-evoked responses were registered in V1. Then, activity with largely overlapping latencies spread rapidly to V2, V3 and throughout the whole visual system. Unambiguous and focal activations with precise onset, peak latencies, and peak amplitudes for each subject and day were identified in V1, in one ventral and three dorsal stream areas. Activations in all areas were consistent in location and timing across subjects and for each subject they were highly reproducible across 3 experimental days. Localization precision was typically within 2 mm in all areas. Retinotopic organizations of the identified areas were in good agreement with other neuroimaging and animal studies. The localization accuracy, as evidenced by computer simulations, was in line with our earlier fMRI/MEG study. On average, it was around 2 mm. Here we report, with very high reproducibility, the dynamics of early visual area activations and their dependence on the stimulated location of the VF. These results show for the first time in humans, significantly shorter onset latencies in V1 for peripheral than parafoveal VF stimulations.