“…Characteristically, the dominant eye movements are bilateral and horizontal, although on occasion a vertical nystagmus, which usually signifies an acquired disorder, has been reported as being inherited (Forsythe, 1955;Marmor, 1973). The intensity (amplitude x frequency) of the nystagmus may vary enormously from individual to individual, and it may alter for each direction of gaze (Dell'Osso, 1973;Abadi and Sandikcioglu, 1974).…”