In psychological experiments involving facial stimuli, it is of great importance that the basic perceptual or psychological characteristics that are investigated are not confounded by factors such as brightness and contrast, head size, hair cut and color, skin color, and the presence of glasses and earrings. Standardization of facial stimulus materials reduces the effect of these confounding factors. We therefore employed a set of basic image processing techniques to deal with this issue. The processed images depict the faces in grayscale, all at the same size, brightness, and contrast, and confined to an oval mask revealing only the basic features such as the eyes, nose, and mouth. The standardization was successfully applied to four different face databases, consisting of male and female faces and including neutral as well as happy facial expressions. An important advantage of the proposed standardization is that featural as well as configurational information is retained. We also consider the procedure to be a major contribution to the development of a de facto standard for the use of facial stimuli in psychological experiments. Such methodological standardization would allow a better comparison of the results of these studies.
Gronenschild, E. H. B. M., Smeets, F. G. B., Vuurman, E. F. P. M., van Boxtel, M. P. J., & Jolles, J. (2009). The use of faces as stimuli in neuroimaging and psychological experiments: a procedure to standardize stimulus features. Behavior Research Methods, 41(4), 1053-1060. https://doi.org/10.3758/BRM.41.4.1053