The Behavioral and Neural Indicators of Face Specific Processing: Holistic Processing and the N170

Abstract

Faces are visually similar to one another, and we have extensive contact experience with them, which jointly render them a special visual stimulus for humans. One of the ways in which face processing is special is that faces are processed in a holistic manner (that is, multiple facial parts are obligatorily integrated into a whole or Gestalt). Faces also elicit some specific neural signatures, such as the N170 component of the event-related brain potential. In my thesis, I explore different aspects of these behavioral and neural face-specific indices. In the first study, I examine whether holistic face processing can occur when facial information that is presented unconsciously. Results revealed that the composite effect, as measured by the complete design of the composite task, was observed in the unconscious condition, but was smaller and less consistent than that in the conscious condition. This conclusion reflects my observation that when the bottom halves of faces are displayed unconsciously, they still influence the processing of top halves of aligned faces. However, much of the conscious composite face effect was disrupted when the bottom facial parts were rendered subliminally. The second study explores the implicit, but seldom tested, assumption that the distributions of amplitudes are normally distributed within conditions. This assumption underlies the logic of deriving the N170 by averaging all similar trials. Violations of these assumptions could have significant implications for the interpretation of the N170. Data used in this study came from three experiments in which intact and scrambled faces and houses were displayed for different durations. N170 data were analyzed with modified ex-Gaussian functions and linear mixed models. Results showed that N170 amplitudes in most conditions were normally distributed which, for the first time, provides empirical evidence supporting the assumptions for averaging single-trial amplitudes. The third study examines whether the N170 is generated in a graded or an all-or-none manner by looking into the influence of stimulus duration on the manifestation of the N170, as well as how the N170 is related to subjective perception. Participants categorized the stimuli as faces or houses, and reported their subjective confidence. The data demonstrated that N170 amplitudes evoked by stimuli with different durations did not differ when high subjective confidence was reported. These findings indicate that the strongest N170 is generated for faces that are perceived with high subjective confidence, supporting the notion that the N170 is generated in an all-or-none fashion. The above evidence brings new insights to holistic face processing and the N170, enriching the knowledge of distinct facets of face-specific processing.