Body size estimation: multichannel or opponent process?

While body image distortion is a widespread issue both Australia and worldwide, little research has focussed solely on how humans visually encode body size. For example, it is not yet known whether body size is encoded through a multichannel (three or more neural channels) or an opponent process (where only two neural channels encode the stimulus). The present study aims to determine which of these processes encodes body size, through the useof visual after effects, which are defined as a change in appearance of a test stimulus after prolonged exposure to an adapting stimulus. Twenty participants viewed either expanded/‘fat’ bodies, or contracted/‘thin’ bodies at four separate levels of adaptor extremity. Following a baseline and a two-minute adaptation phase, participants were asked to select which of two distorted images (one expanded and one contracted by 3%) appeared more ‘normal’. Pre- to Post-Adaptation Scores (PPAS), or the change in after effect magnitude, was the main dependent variable across all levels (1-4) and group (expanded/contracted adaptors). Although the pattern of results was consistent with that of an opponent process in the expanded group only (after effect magnitude linearly increased as a function of adaptor extremity), no significant results in the contracted group were obtained. Limitations and reasons for non-significance are discussed, and plans to rectify these issues are outlined.