They did a study on 24 subjects, half of which were Western Caucasian, and the other being of East Asian heritage. Each participant was shown a succession of two faces displayed on a screen. Every pair had either faces of the one person or two unlike people, or one individuals face repeated in the series. Also, the expression would vary between the faces to make it a little harder to identify the same faces. The testers had to determine if the two faces are of the same individual or not.
In order to understand how the experiments worked, you should know that when a certain stimulus, as in this case the face, is shown two times to a person, his or hers neural activity related to the response decreases in the second instance, this mechanism is called the repetition suppression. In the brain, an electrophysiological signal, N170, is triggered when a human face comes into sight.
The repetition suppression works like this; when a second face is identified as being the same as the first by the brain, an N170 signal is decreased, but if the face is recognized as being different, the N170 should stay the same. What the researchers did was subtract the speed at which the signal was triggered by face number two from that which was triggered from the first so that they can ascertain the difference between the two stimuli. This measurement is the single-trial repetition suppression (stRS) value.
The common stRS value for these trials were the same for both and highest when subjected were shown a face of their race again. However, there was no distinct change in the values when an Asian saw a Caucasian face and vice versa; in fact they were almost the same as when a face of the same race was looked at. Hence, these results are first to suggest a neurophysiological basis for the other race effect.