Stability of ocular counterrolling and Listing's plane during static roll-tilts.

PURPOSE To investigate whether habituation occurs in ocular counterrolling (OCR), how stable shifted Listing's plane is, and what effects visual stimulation and alertness exert on the OCR and on Listing's plane. METHODS Two monkeys (Macaca fuscata) were engaged in this experiment. A dual scleral search coil method was used for recording three-dimensional eye movements. Spontaneous eye movements were recorded for 2 hours in dark and light, while each monkey was held in different static roll positions (up to +/-34 degrees) with its head fixed. Eye movements were also recorded during the monkeys' drowsy periods in the dark. RESULTS In alert conditions, OCR gains showed some fluctuations but did not change consistently for 2 hours in seven of seven sessions in the dark and in seven of nine sessions in the light. The OCR gains in the light did not differ from those in the dark. The thickness of shifted Listing's plane during the static roll-tilt was also stable for 2 hours and was within the range of the thickness of Listing's plane in the upright position in the light. During drowsy periods, the thickness of Listing's plane increased, and the gain in OCR decreased. CONCLUSION Ocular counterrolling shows no habituation for 2 hours of static roll-tilts in the alert monkey. In the same condition, Listing's plane consistently maintains its precision. Visual input does not affect OCR, but alertness is necessary to keep the stability of OCR and Listing's plane.