A wagon wheel like pattern of eight grey spokes pointing towards but not
reaching the center is presented as a background image on a computer monitor.
This `wagon-wheel` configuration of the fixation stimulus is designed to
help patients with unstable or eccentric fixation to fixate in the optical
axis. The image is calibrated to span exactly 18° diameter of the visual
field. At 33 locations, centrally and on concentric rings of 2°, 4°, 6°,
and 8° eccentricity, single letter stimuli are presented. The presentation
duration is 250 ms to minimize saccadic eye movements towards the stimulus.
Topographical map: According to the answer entry a topographical map is
calculated indicating at which location a letter was not detected (black
symbol) detected but not recognized (grey symbol) or had been correctly
identified (white symbol).