Compensatory eye movements in the presence of conflicting canal and otolith signals

Abstract

Orbital motion of the head with the face directed towards the axis of rotation is a stimulus to the otolith organs which is in the opposite rightwards-left-wards sense to the rotational stimulus to the semicircular canals. This can be experienced, for example, by a child held at arms length "en face' and swung from side to side. As one swings, say to the right, the child's head rotates to its right yet moves linearly to its left. Eye movement responses to a transient orbital movement were observed whilst subjects fixated earth-fixed targets. i) a "near target" placed between the head and the axis whose relative displacement is in the same direction as head rotation, and ii) a "far target" placed beyond the axis whose relative motion is in the opposite direction to head rotation. The motion stimuli evoked slow phase eye movements at 45 ms latency, always in the opposite direction to head rotation, thus compensating for the motion of the far target but in the wrong direction for fixating the near target. Theoretically, fixating the near target demands a predominance of the otolith ocular-reflex, which would give an eye movement in the correct direction. However, despite visual cues, it seems that if the canal and otolith-ocular reflexes are evoked in opposing directions, the otolith reflex fails to operate at a sensitivity sufficiently high to reverse the direction of the canal-reflex.