engleski

Magnetoencephalographic studies on face processing

Magnetoencephalography (MEG) provides a unique way to explore spatio-temporal dynamics of brain activity evoked by visual stimulation. We used MEG recordings and source localization to examine cortical processing of face stimuli. Many ERP and MEG studies have reported face sensitive N170 ERP component and its magnetic counterpart M170. However, the main focus in our analysis was on the earliest face-related activity up to 140 ms post-stimulus. Eight to eleven subjects with available MRIs participated in our studies. In the first study, gray-scale face photographs were presented in an oddball paradigm. In the second study, we used simple drawings of face, object (flower) and meaningless patterns to determine the earliest face-related response that does not depend on low-level physical features of stimuli. MEG measurements were conducted at the BioMag Laboratory, Helsinki University Hospital with a 306-channel Vectorview system (Elekta Neuromag Ltd., Helsinki). Data analysis was carried out with the Elekta Neuromag software, BESA, and two packages developed at Los Alamos National Laboratory, MRIVIEW and MEGAN. Our data confirmed that early brain response about 100 ms was sensitive to face inversion [1]. However change in facial expression or face identity did not influence this early response. Spatio-temporal source localization was conducted for neuromagnetic responses evoked by standard faces in the oddball paradigm. Up to four sources were identified during the first 140 ms after stimulus onset in the occipital, occipito-temporal, and parietal regions [2]. We found dissociation between MEG response to faces and objects already around 100 ms in the occipital extrastriate regions that could not be related to the physical stimulus features. The nonlinearity in the brain response was larger for faces and objects than for the meaningless stimuli [3, 4]. Our studies suggest that some aspects of face-specific processing take place already at about 100 ms poststimulus. At these quite early latencies many brain areas are active. We found that distinction between faces and objects takes place in the occipital cortex. [1] Susac A, Ilmoniemi RJ, Pihko E, Supek S. Neurodynamic Studies on Emotional and Inverted Faces in an Oddball Paradigm. Brain Topogr 2004 ; 16:265-268. [2] Susac A, Ilmoniemi RJ, Supek S. Early visual responses to upright and inverted faces. Proceedings of the 14th International Conference on Biomagnetism Biomag 2004, Boston, 2004 ; 445-446. [3] Susac A, Ilmoniemi RJ, Pihko E, Nurminen J, Supek S. Early distinction between MEG responses to faces and objects. OHBM2007 (accepted) [4] Susac A, Ilmoniemi RJ, Pihko E, Nurminen J, Supek S. Early dissociation of face and object processing: a magnetoencephalographic study. (in preparation)

Magnetoencephalography (MEG); face processing; object recognition; human; visual; occipital cortex; M100; nonlinearity

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