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Shedding light on infant brain function: the use of near‐infrared spectroscopy (NIRS) in the study of face perception

The human face provides a wealth of socially relevant information. Adults readily detect faces and decode all kinds of information from the face such as age, gender, familiarity, race, gaze direction, emotion, etc. Given the multitude of information that can be gleaned from the face, it is not surprising that face perception is mediated by a complex and highly specialized distributed neural system in human adults. The functional organization of this system can be broadly characterized by a distinction between (a) the representation of invariant structural aspects of faces, which constitutes the basis of recognizing individuals, and (b) the interpretation of dynamic changes of faces such as eye gaze and emotional expression that are used in face-to-face communication with others (1,2). Although neuropsychological and neuroimaging work have helped identify a distributed network of specialized brain areas involved in adults’ face processing, the more basic question remains, how do these adult abilities develop and what are their precursors? It is thus crucial to look at the earliest stage of face processing: infancy. So far, infant face perception has been predominantly investigated using electroencephalography (EEG) and eventrelated potentials (ERPs) (see (3)), which offer good temporal but relatively poor spatial resolution. However, in recent years, researchers have begun to use near-infrared spectroscopy (NIRS) to study infant brain function, permitting more precise localization of brain activation by measuring haemodynamic responses (for a review see (4)). In their article, Carlsson et al. (5) present an innovative study that uses this optical imaging technique to examine the brain processes that underlie 6to 9-month-olds’ recognition of their mother’s face. NIRS measurements were obtained from two locations over the right hemisphere, one over posterior