Mental concentration and the pattern reversal visual evoked response

The pattern reversal visually evoked response (VER) was recorded using a small check size stimulus. Subjects viewed the VER stimulus with the left eye and a light emitting diode (LED) with the right eye. They were asked to concentrate on either the VER stimulus or the LED. The two targets were arranged so that the left eye remained accurately focused on, and directed to, the center of the VER stimulus throughout the recordings. The VER wave became attenuated when the subject concentrated on the LED. The wave was reduced to around two-thirds of the amplitude present when concentrating on the VER stimulus.