Experimental procedures.

<p>During the measurements, participants were asked to gaze at the fixation point in the center of the screen (+). First, a picture of a face either with or without expression (emotion task), or a face or picture of a hand (categorization task) was randomly presented for 700 ms. Next, a blank screen appeared for 300 ms and was followed by a “Go” signal. Then the participants had to indicate whether the face was with or without expression (or whether the picture was a face or hand) by pressing pre-assigned button 1 or 2 using the right hand. Finally, a blank screen was presented for a random time interval (1000 ms±300 ms). Then the next trial started. The assignment of the response buttons (1 or 2) was counterbalanced across participants. The photographs in the figure are not the original images used in the study, but similar images used for illustrative purposes only. The subject of the photographs has given informed consent to publication, as outlined in the PLOS consent form.