Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Brain and Cognitive Sciences, 2003.Includes bibliographical references.Face recognition is one of the most important problems our visual system must solve. Here I used magnetoencephalography (MEG) in an effort to characterize the sequence of cognitive and neural processes underlying this remarkable ability. This work is designed to answer several questions. First, how long does it take for the human visual system to recognize a stimulus as a face? Second, what are the stages of processing in face perception? Finally, what is the nature of representations extracted at each of these stages? MEG provides an ideal tool for addressing these questions, as its high temporal resolution enables us to separately measure perceptual operations that may occur only a few tens of milliseconds apart from each other. Yet, unlike single-unit recording, it can be used in normal human subjects. Three new findings about human face recognition will be reported in this thesis. First, a face stimulus begins to be categorized as a face within 100 ms after stimulus onset in humans, substantially faster than previously thought. Second, face recognition occurs in two distinct stages: an initial stage at which the stimulus is categorized as a face, and a stage that occurs 70 ms later at which the individual identity of the face is extracted. Finally, the representations extracted at these two stages differ not only in specificity, but also in the aspects of a face represented at each stage.by Jia Liu.Ph.D.