Distinct differences in the pattern of hemodynamic response to happy and angry facial expressions in infants--a near-infrared spectroscopic study.

Recognition of other people's facial expressions of emotion plays an important role in social communication in infants as well as adults. Evidence from behavioral studies has demonstrated that the ability to recognize facial expressions develops by 6 to 7 months of age. Although the regions of the infant brain involved in processing facial expressions have not been investigated, neuroimaging studies in adults have revealed that several areas including the superior temporal sulcus (STS) participate in the processing of facial expressions. To examine whether the temporal area overlying the STS is responsible for the processing of facial expressions in infants, near-infrared spectroscopy (NIRS) was used to measure the neural activity in the temporal area overlying the STS as infants looked at happy and angry faces. NIRS provides a non-invasive means of estimating cerebral blood flow in the human brain and does not require severe constraints of head-movement. According to the International 10-20 system for EEG electrode placement, the measurement area was located in the bilateral temporal area centered at positions T5 and T6, which correspond to the STS. The time-course of the average change in total-Hb concentration revealed a clear difference in the pattern of hemodynamic responses to happy and angry faces. The hemodynamic response increased gradually when infants looked at happy faces and was activated continuously even after the disappearance of the face. In contrast, the hemodynamic responses for angry faces increased during the presentation of angry faces, then decreased rapidly after the face disappeared. Moreover, the left temporal area was significantly activated relative to the baseline when infants looked at happy faces, while the right temporal area was significantly activated for angry faces. These findings suggest hemispheric differences in temporal areas during the processing of positive and negative facial expressions in infants.