Hemispheric differences in evoked potentials to faces and words.

Sixteen right-handed subjects (8 male and 8 female) were asked to compare two faces or two words successively presented at the centre of the visual field. The brain's electrical activity was recorded from the scalp at symmetrical points of the left and right occipital lobes (0(1) and 0(2)) and posterior temporal lobes (T5 and T6). The reference electrode was placed on the scalp vertex (Cz). A multi-factor analysis of variance revealed significant hemispheric differences of the N243 and P406 amplitudes. For the N243 the opposite asymmetry was found for faces and words. For the face matching the N243 amplitude was higher in the right hemisphere, whereas for word matching it was higher in the left hemisphere. For the P406 the asymmetry was in the same direction both for faces and words, with higher amplitude in the left hemisphere. In the case of face matching the hemispheric difference in the P406 was more pronounced, due to a negative shift of the potential in the left hemisphere in the latency range of 200-1,500 ms. Functional asymmetry of the brain in face perception thus appears to be reflected in the brain's electrical activity. We conclude that differentiation in hemispheric functions takes place while encoding information about stimulus in short term memory.