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Frédéric J.A.M. Poirier, Jocelyn Faubert; Face space is not linear: Empirical evidence of curvature and compression. Journal of Vision 2012;12(9):973. doi: https://doi.org/10.1167/12.9.973.
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© ARVO (1962-2015); The Authors (2016-present)
Face perception studies and models often assume that face space is linear, that is, that changes in gender or emotion produces linear changes in facial features. In the current study, we test the linear face space assumption by measuring additivity (i.e. whether the combination of gender and emotion is predicted by the sum of its components), proportionality (i.e. whether the ratio of internal-to-physical changes is constant, or whether there are compression or expansion effects at higher gender or emotion intensities), and directionality (i.e. whether increasing gender or emotion intensity introduces qualitative changes). Participants were told to produce faces that corresponded to various intensities along male/female and happy/sad continuums, including combinations of the two dimensions. They produced these faces using Poirier and Faubert (VSS 2010; in revision)’s technique, using sliders to adjust 53 components of facial expression including the shape and position of eyes, eyebrows, mouth, nose, and head. Data from 7 participants show that (1) variability along gender and emotion was captured by 3 dimensions: gender, emotion, and curvature (see below), accounting for 70.7% of variability in features (F(61, 500)=1.8, p=.0002), (2) gender and emotion were linearly additive within the range tested (F(12,560)=1.2, p=.30), (3) there was evidence of compression at high intensities for both gender (F(5, 30)=3.6, p=.012) and emotion (F(5,30)=5.0, p=0.002), (4) maximum expansion occurs at 0.5x female and 0.5x happy, and (5) the face space is significantly curved along both gender and emotion (R2s=49.1% & 57.4%, Fs(1,10)=9.7 & 13.5, ps=.011 & .0043 respectively) meaning that intensity changes introduced qualitative changes that cannot be captured as linear feature changes. The presence of deviations from linearity (e.g. compression, expansion, and curvature) implies that linear morphs as commonly used in experiments and virtual reality introduce both quantitative and qualitative systematic distortions, at least for variations involving gender and/or happy/sad.
Meeting abstract presented at VSS 2012
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