The effect of curve adaptation on the perceived expression of the real-face set derived from two Ekman faces (experiment 2). a , Psychometric functions from a naive subject under the following conditions. 0-f, No adaptation baseline (blue); f-f, adaptation to the saddest face (green); c-f, adaptation to the concave curve, whose curvature and length matched those of the saddest face (red). For each condition, the fraction of happy responses was plotted as a function of the proportion of happiness in the morphed test faces. A proportion of 0 or 1 corresponded to the original sad or happy image taken from the Ekman database. b , Summary of all four subjects' data. For each condition, the average PSE relative to the baseline condition and the SEM were shown. The p value for each condition in the figure was calculated against the baseline condition using the two-tailed paired t test.