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Early distinction between MEG responses to faces and objects

Recent MEG and EEG studies [1, 2] have reported a face-distinctive response already at 100 ms after stimulus onset. Differences between responses for faces and other object categories at these early latencies might be related to low-level visual differences [3]. The primary visual cortex and extrastriate visual areas are sensitive to the location of the stimuli in the visual field, as well as spatial frequency and luminance contrast of the visual input [e.g., 4– 6]. The aim of this study was to investigate early distinction between MEG responses to faces and objects, avoiding differences in low-level physical features of the stimuli. Eleven subjects (5 females and 6 males, age range 23– 34 years, one left-handed) participated in the study. Gray-scale stimuli were presented in the centre of the visual field for a duration of 200 ms with an onset-to-onset interstimulus interval of 1500 ms. The stimulus set consisted of nine different line drawings: a face, a flower, and 7 meaningless figures (Fig.1) that would produce the same image when superposed in groups of 2 or 3 stimuli (e.g., 1+4+7=2+5+8, Fig.2). Sums of responses corresponding to these superposition groups were compared. Measurements were conducted at the BioMag Laboratory with a 306-channel whole-head magnetometer. Data analysis was carried out with the Elekta Neuromag software. Eight out of the eleven subjects showed distinction between MEG response to faces and objects already around 100 ms. Spatio-temporal localization of the difference of superposition groups containing the face and the flower revealed that the distinction between MEG responses to faces and objects takes place in occipital extrastriate regions (Fig.3 ; note the simple dipolar structure of the difference pattern). The nonlinearity in early MEG responses was larger for meaningful stimuli. Our study provides evidence that early distinction between MEG responses to faces and objects [1, 2] cannot be attributed to the differences in the low-level physical features of the stimuli. Our data also suggest that the brain response to the complex visual stimulus is not merely the sum of the responses to its constituent parts. References: [1] Liu J, Harris A, Kanwisher N. Nature Neurosci 2002, 5:910– 916. [2] Herrmann MJ, Ehlis AC, Ellgring H, Fallgatter AJ. J Neural Transm. 2005, 112:1073– 1081. [3] Halgren E, Raij T, Marinkovic K, Jousmäki V, Hari R. Cereb Cortex. 2000, 10:69– 81. [4] Aine CJ, Supek S, George JS, Ranken D, Lewine J, Sanders J, Best E, Tiee W, Flynn ER, Wood CC. Cereb Cortex 1996 ; 6:354– 361. [5] Singh KD, Smith AT, Greenlee MW. Neuroimage 2000 ; 12:550– 564. [6] Haynes JD, Lotto RB, Rees G. Proc Natl Acad Sci USA 2004 ; 101:4286– 4291.

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

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