The spatiotemporal dynamics of the face inversion effect: a magneto- and electro-encephalographic study

The neurophysiological basis of the face inversion effect was studied with magneto- and electro-encephalography in 10 normal subjects. Spatiotemporal analyses using dipole modeling was performed on combined evoked magneto and electro-encephalography data to hemifield presentation of upright and inverted faces and objects. Inferior temporal cortex, i.e. fusiform gyrus, and lateral temporal cortex near the superior temporal sulcus were activated simultaneously, but independently, at 140-200 ms post-stimulus to upright and inverted unfamiliar faces. Right hemisphere inferior temporal cortex and lateral temporal cortex were active in all subjects, and in the left hemisphere in half the subjects. Latencies to inverted relative to upright faces were longer in the right hemisphere, and shorter in the left hemisphere. For right hemifield stimulation ipsilateral activation delay was around 18-19 ms for both upright and inverted faces and was calculated from all 10 subjects. For left hemifield stimulation, and the data from 7 of 10 subjects, it was 22 and 29 ms to upright and inverted faces, respectively. In sum, the methods used in this study did not identify clear differences in anatomical location of activated regions to upright and inverted faces. We believe, however, that the differences in processing upright versus inverted faces are attributable to temporal processing differences rather than to processing of information by different brain regions.