A case of ocular tilt reaction and torsional nystagmus due to direct stimulation of the midbrain in man. 
A case is presented of a patient who underwent insertion of a periaqueductal grey electrode for the relief of chronic pain.  Shortly after insertion, the patient developed a left-sided ocular tilt reaction (OTR).  The electrode tip, initially to the right of the midline, was then withdrawn slightly so that it was now on the left side.  Stimulation at this point gave rise to a worsening of the tilt reaction, in addition to bilateral counter-clockwise torsional nystagmus.  Computerized tomography and stereotactic coordinates indicated that the tip of the electrode was finally situated in the region of the left interstitial nucleus of Cajal (INC).  The production of an OTR by stimulation in this region is similar to the phenomenon previously reported in monkeys and cats.  The side of the OTR is consistent with previous evidence, suggesting that the utricular pathways cross between medulla and midbrain.