N400 potential generators in response to visually presented images are spread across both cerebral hemispheres.

N400 is a negative EEG potential evoked by a meaningful stimulus. It is usually modulated by semantic relatedness of consecutive linguistic and non-linguistic stimuli such as images. Recent research showed that neuronal generators of N400 in response to these two types of stimuli differ. By using electrocorticography (ECoG) recordings, we show direct evidence about the localization of the N400 generator in response to semantically-related image pairs. Methods: A semantic image association task was conducted on a patient with refractory epilepsy with depth and subdural grid electrodes implanted in/above the following areas: left (8 electrodes) and right (8 electrodes) hippocampus, right posterior (4 electrodes) and anterior (4 electrodes) temporo-basal neocortex, left temporal neocortex (4×8 grid) and right temporo-lateral neocortex (8 electrodes). Simultaneously with ECoG, EEG was recorded from 27 scalp electrodes (10 – 20 system). Both EEG and ECoG signals were referenced to a common average reference but for the ECoG case, this was done per grid. All signals were filtered between 1 and 15Hz. Trials were corrected using a 100ms pre-stimulus baseline. Results: Student’s t-test showed for ECoG a difference between N400 amplitudes in response to semantically related and unrelated images in the following areas: left hippocampal area (t(1,417)=2.04, p<0.05), right hippocampal area (t(1,417)=4.1, p<0.0001), right posterior (t(1,417)=3.75, p<0.0005) and anterior (t(1,417)=2.8, p=0.005) temporo-basal neocortex and left temporal neocortex (t(1,417)=3.26, p=0.001). The EEG signal showed a clear centro-parietal N400 effect of semantic relatedness. Conclusion: N400 generators for image processing are spread across both hemispheres, unlike the ones for linguistic stimuli, which according to some localization studies, are more confined in the left temporal neocortexstatus: publishe