engleski

MEG study of an early face-object dissociation

Many event-related potential (ERP) and magnetoencephalography (MEG) studies have reported a face-distinctive response around 170 ms (N170/M170). Recent studies [1, 2] found a face-selective MEG and ERP response just 100 ms after the stimulus onset. Differences between faces and other object categories at these early latencies might be related to the low-level visual differences [3]. The aim of this study was to investigate early dissociation of MEG responses to faces and objects, avoiding differences in low-level physical features of the stimuli. Eleven subjects with structural MRIs available participated in the study. Gray-scale stimuli were presented in the centre of the visual field for a duration of 150 ms with an interstimulus interval of 450 ms. The stimulus set consisted of nine different stimuli: face, flower, and 7 meaningless stimuli. The stimuli were created so that if they were superimposed in certain groups they would produce the same summed stimuli. Sums of responses to these superimposition groups were compared. Neurodynamic measurements were conducted at the BioMag Laboratory with a 306-channel Vectorview system (Elekta Neuromag Ltd., Helsinki). Neuromag software was used in the data analysis that included also spatio-temporal modeling. Eight out of the 11 subjects showed reliable face– object dissociation of MEG responses around 120 ms. The difference in responses can not be attributed to the low-level characteristics of the stimuli because the compared sums were composed of the responses to the stimuli that added together would give an identical image. When the summed responses included only responses to the meaningless stimuli no reliable difference was found. Spatio-temporal localization of the difference of superimposition groups containing the face and the flower revealed that the face– object dissociation takes place in occipital extrastriate regions. These results indicate an early separation of face and object processing that is not related to the physical features of the stimuli.

MEG; face processing; early dissociation

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