Figure 5 Pathways involved in the counterroll of the eyes (dotted arrows) during a head tilt

In this example, a left head tilt results in a counterclockwise (with respect to the examiner) torsional counterroll of the upper poles of the eyes. This response is mediated by a crossed otolith projection to the extorters in the patient's right eye (the inferior oblique and inferior rectus muscles) and a double crossed projection to the intorters of the left eye (the superior oblique and superior rectus muscles). These slow phases are punctuated by torsional fast phases that are mediated by the rostral interstitial nucleus of the medial longitudinal fasciculus. The interstitial nucleus of Cajal (INC) is also shown (without connections). This important midbrain structure contains circuitry important for neural integration of vertical and torsional gaze, eye-head coordination during roll movements, and contains inhibitory burst neurons for vertical eye movements.³ Lesions of these otolith projections result in the opposite reciprocal effects leading to intorsion and elevation of the right eye and extorsion and depression of the left eye, the so-called skew deviation. If the lesion occurs prior to the otolith pathway decussation (here on the left) then the lower left eye is on the side of the lesion. Alternately, if the lesion is within this pathway after the decussation (in the pons or midbrain), then the higher eye is on the side of the lesion.