The present study examined infants' brain activity in response to upright and inverted faces using near infrared spectroscopy (NIRS), which can non-invasively record hemodynamic changes of the brain. NIRS is particularly useful for recording in infants, since recordings can be made, even while the infants are awake, without fixing their body and brain. For this objective, we used newly developed sensor probes of NIRS for recording in infants. We measured changes in cerebral oxygenation in 10 5-8-month-olds' left and right lateral areas while they were looking at upright and inverted faces. The results are summarized as follows: (1) the concentration of oxyhemoglobin (oxy-Hb) and total hemoglobin (total-Hb) increased significantly in the right lateral area during the upright face condition, (2) the concentration of total-Hb in the right lateral area differed significantly between the upright and inverted conditions, (3) hemodynamic changes were maximal in the temporal region, probably in the superior temporal sulcus (STS) in both hemispheres, and (4) the right hemisphere seems to be more important for recognizing upright faces. This is the first evidence showing that there is an inter-hemispheric difference on the effect of face inversion in the infant brain using a hemodynamic method.

The objective of the present study was to determine whether a developmental difference occurs in brain activity when infants look at frontal and profile views using near-infrared spectroscopy (NIRS), which is an optical imaging technique used to measure changes in the concentrations of oxyhemoglobin (oxy-Hb), deoxyhemoglobin (deoxy-Hb), and total hemoglobin (total-Hb). For this objective, we compared NIRS results in two age groups, 5- and 8-month-old infants, while they were looking at frontal views, profile views, and objects. We found that the concentration of oxy-Hb and total-Hb in the 5-month-old group increased for only frontal views in the right temporal regions. In contrast, the concentration of oxy-Hb and total-Hb in the 8-month-old group increased for both frontal and profile views in the right temporal regions. Therefore, the present study indicated that the right hemisphere was dominant for the perception of profile views as well as frontal views. In addition, the most important and interesting finding was that the infants' brain activity of the face area would become view-invariant at the age of 8 months but not at 5 months. The developmental period for view-invariant face recognition has been discussed in previous psychological studies, but this is the first objective study to confirm that the period is between 5- and 8-months by measuring the blood flow in the brain using NIRS.

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In this paper, we summarize a series of studies on the developmental changes in social cognition in mother-raised infant chimpanzees from birth to around 2 years old. The infants preferentially tracked a photograph of their mother's face at 1 month but showed indifferent preferences to faces at 2 months old. This change in facial recognition was correlated with a decrease in neonatal spontaneous smiling, increase in social smiling and a decline in neonatal imitation of facial expressions. Also at around 2 months, the infants began to show preferences for directed-gaze faces over averted gazes, and the amount of