Gradient shifts with naturally occurring human face stimuli

The present research examined stimulus generalization and gradient shifts on a dimension involving human faces. Twenty undergraduates were instructed to examine the proportion of the total face length that lay between the tip of the nose and the end of the chin. The face stimuli were images of actual people shown on a computer screen; no face was shown more than once. All of the participants received discrimination training. The positive stimuli were faces with average proportions (ratios of 0.356 to 0.365). The negative stimuli were faces with very small proportions (ratios less than or equal to 0.315) for half of the participants, and faces with very large proportions (ratios greater than or equal to 0.405), for the other half. The generalization test consisted of images from 5 categories of proportions, ranging from very small to very large. The results of both conditions included a shift in the gradient away from the negative stimuli.