Abstract: A method and system for presenting two successive images on a video display unit as a dichoptic stimulus of substantially equal intensity, wherein a first image is displayed at a predetermined intensity so as to be seen by a first eye of an observer only, an optical shutter being used to prevent the displayed image from reaching the second eye of the observer. The first image is then extinguished and the state of the optical shutter reversed so that the first eye can no longer see the video display unit whilst a second image is presented to the second eye. In order to prevent the second eye from seeing an after image of the first image, a neutral density filter or polarizer is disposed within the light path and the second image is illuminated at a correspondingly higher intensity so as to compensate for the attenuation factor of the neutral density filter or polarizer.