การทดสอบฟองThe Bubbles experiment was adapted from Gosselin and Schyns.22 The experiment was programmed with the Psychophysics Toolbox30 and the Pyramid Toolbox31 for Matlab. The original facial stimuli32 came from 5 Caucasian females and 5 Caucasian male posers. Each poser displayed both a fearful and a happy facial expression, yielding 20 different facial expressions. Faces were normalized for location of the eyes and mouth and lighting and were presented through an elliptical aperture to exclude features outside a face. Figure 1 illustrates the experimental stimulus-generation procedure for a given trial (for more details, see 22,33). First (top row), the original facial stimuli (upper left image) were decomposed into 5 bands of spatial frequency bandwidth of one octave each (85.3–42.7, 42.7–21.3, 21.3–10.6, 10.6–5.3, and 5.3–2.6 cycles per face width; scale 1–5 from the highest to lowest) using a Laplacian transform. The remaining bandwidth of <2.6 cycles per face served as constant background. Second (middle row), we created a number of randomly positioned Gaussian apertures (i.e., the bubbles). Each aperture had a SD of 3 cycles per image such that the size of the bubbles at each spatial scale was adjusted accordingly. This ensured that bubbles revealed similar amount of visual information across spatial frequencies. Third (bottom row), randomly sampled Gaussian apertures were independently applied to decomposed face images at each of the 5 spatial frequency bandwidths to reveal partial facial information at each spatial frequency. Finally, the information revealed at each spatial frequency was summed to generate an experimental stimulus for each trial (lower right image at the bottom row). This procedure ensured that for each trial, facial information revealed by the bubbles (i.e., facial features at each spatial frequency) was randomly sampled.
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