Throughout our daily lives, there is continual movement of the head and eyes, and their coordination, which is essential for proper vision, requires complex interactions to deal with changes in the position of the head and shifts of gaze. We have discovered a region in the pontine and medullary reticular formation that, when electrically stimulated, leads to brisk head movements. The vast majority of these movements are in the ipsilateral direction. Horizontal rotatory movement predominates, but tilting and stabilizing movements also are elicited. Although eye movements frequently occur with head movements, stimulation of this region with the head restrained does not produce eye movements. Movements elicited from this area never change the direction of gaze, which is stablized by the vestibulo-ocular reflex. However, the elicited head movements are modulated by the initial position of the head, attentive fixation, and level of alertness. To understand the role of this region of the brainstem in the control of head movements further, we injected tracer compounds to reveal afferent and efferent connections. Areas that send axons to our head movement area include our previously identified head movement portions of the superior colliculus, those parts of the motor and premotor cortices that initiate head movements, the periaqueductal gray (extensively connected to limbic/emotional areas), and the interstitial nucleus of Cajal (long suggested to have head movement function). Our data demonstrate a central role for this part of the brainstem reticular formation in the integration of head movements. We also studied the efferent projections from the reticular formation head movement area by means of localizing terminal fibers, learning that the reticular formation sends axons to the motoneuronal regions of the upper cervical spinal cord. These connections provide direct access to the motoneurons that move the head. Furthermore, axonal endings within the caudal medulla, vestibular nuclei, and parvocellular reticular fields integrate head movements with the vestibulo-ocular reflex and other movements of the eyes. Together these studies demonstrate for the first time the critical role that the medullary and pontine reticular formation plays in the control of head movement. They show how it integrates influences from voluntary, stabilizing, emotional, and orienting systems.
