Effect of hindbrain lesions on conjugate horizontal eye movements in cats

Abstract

The present study was instigated to determine whether horizontal conjugate deviations of different origins are separately mediated, or whether all types of horizontal eye movements utilize one common anatomical substrate or “centre”. In addition, an attempt was made to gain information on oculomotor pathways between midbrain and medulla. Functional deficits in horizontal conjugate eye movements were assessed following placement of acute and chronic lesions in the hindbrain of cats. Sites in brain stem and cortex were stimulated unilaterally to evoke eye movements in the encephale isole preparation. Pre-lesion responses from the same sites served as controls for the effect of acute lesions, and responses evoked from homotopic sites served as controls for the effect of chronic (3-week) lesions. Acute or chronic destruction of an area in the postero-medial medulla depressed or abolished eye movements evoked from the contralateral superior colliculus without altering responses evoked from the occipital cortex. In contrast, destruction of an area 2–3 mm antero-lateral, just posterior to the level of the sixth nucleus, depressed cortically evoked movements without affecting those from the superior colliculus. Lesions in the lateral pontine tegmentum depressed or altered responses to stimulation in the contralateral superior colliculus and, rarely, in the occipital cortex. More lateral pontine lesions altered conjugate deviations evoked from stimulation in the ipsilateral medulla. Although the majority of hindbrain lesions were followed by a deficit in one or more types of evoked conjugate deviations to the right, no one lesion abolished all horizontal eye movements. The results indicate that there is no single supranuclear area in the hindbrain of the cat the removal of which abolishes all types of lateral gaze. Rather, mediation of conjugate horizontal deviations of occipital cortical origin appears to require the integrity of neuronal elements distinct from those mediating collicular responses, possibly reflecting differential mediation of “phasic” and “tonic” components of eye movements. Oculomotor pathways between midbrain and medulla in the cat include a lateral component through the pontine level, whose interruption in some cases resulted in loss of abduction with preservation of adduction rather than the more typical conjugate defect following a supranuclear lesion.