The contribution of midbrain circuits in the control of gaze

The midbrain contains several structures important for the generation of torsional and vertical eye movements including the rostral interstitial nucleus of the MLF (riMLF) and the interstitial nucleus of Cajal (iC). While the riMLF is the immediate premotor structure for the generation of torsional and vertical saccades, the iC is considered a major part of the neural integrator for torsional and vertical eye movements. Experiments in monkeys show that a unilateral inactivation of the riMLF with muscimol leads to spontaneous contralesional torsional nystagmus, whereas an iC inactivation causes ipsilesional torsional nystagmus. In addition, inactivation of either structure leads to a tonic ocular torsion to the contralesional side. While the deficits after a riMLF lesion are thought to result from an imbalance of the saccade generator, a vestibular imbalance probably causes the deficits after an iC lesion. Contralesional and ipsilesional torsional nystagmus is also found in patients with unilateral mesencephalic lesions. A detailed analysis of the lesions from MRI scans shows a preferential involvement of the riMLF for patients with contralesional torsional nystagmus, and a major involvement of iC in cases with ipsilesional torsional nystagmus. Thus, the direction of torsional nystagmus appears to be a valuable topodiagnostic sign for patients with midbrain lesions.