Fig. 2.

An illustration of the matching-to-sample (MTS) facial expression discrimination task. The subject is first presented with a sample stimulus, a facial expression (in this case, a bared-teeth display) and a cross-shaped cursor on the computer screen over a black background (Fig 2a). The sample stimulus is the image to match, and the subject first must orient toward it by touching it with the joystick-controlled cursor. After this, the sample clears the screen and the subject is presented with two alternative stimuli (Fig. 2b)—one matches the sample by showing the same category of expression made by a different individual (left), while the other shows a different expression (relaxed-lip face, right side). The correct choice is to select the stimulus that looks most like the sample.