Eye movements are controlled by a circuit that is more complex than usual in the central nervous system. Cortical control of conjugate eye movements involves the inferior parietal lobe, the frontal eye field and the dorsolateral pré-frontal córtex. All control the cerebellum and pontine nucleii, via the superior colliculi.

When you move your eyes from one finger on your right to another on your left it is a saccade that does it with a velocity of a few hundred degrees of arc per second. When you follow a finger moving slowly in front of you, it is the smooth pursuit system that operates, with velocities up to 40 degrees of an arc per second. The third system that moves the eyes is the vestíbulo-ocular reflex, or doll’s head eye reflex, which is self-explanatory. It keeps the eys fixed on a target when the head moves from one side to the other, as fast as it may go.

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Bittencourt PRM, Wade P, Smith AT and Richens A. The relationship between peak velocity of saccadic eye movements and serum benzodiazepine concentration. British Journal of Clinical Pharmacology 12:  523-533, 1981

While we walk around the world all 3 systems works in conjunction. Many movements are reflex, very fast, somewhat like when a neurologist uses his hammer to provoque a knee jerk. The tendon reflex goes very fast to the spinal cord, connects with the motor system and the knee jumps. In the case of eye movements it is movements of the head and visual targets that provoque reflex movements. These are operated by the oculomotor and vestibular nuclei in the brain stem, with the suppport of the cerebellum, which balances the movements so they are made in an equilibrium. Just like our gait, as we walk along the world. The size of the saccades, the velocity of smooth pursuit, much is operated at the level of brainstem and cerebelar nucleii and cortex.

It is when the person makes a conscious decision to follow na object, or to change gaze from one object to the other, or to explore a larger surface, that the cortical regions come into operation. Vision, of course, goes to the occipital lobe, which keeps feeding information all the time, via the superior colliculus in both sides. This by itself is already very complex, as we use binocular overlapping visual fields. From the colliculi the visual information goes from the occipital lobes to the brainstem and cerebellum to control the direction and equilibrium of the movements. In the case of movements generated by visual reflex, the information goes from the visual fields to the brainstem and cerebellum directly.

It is when we see and consciously direct our eyes to move that the occipital lobe receives the information. The parietal lobe most likely operates as a sensory cortex, it senses vision and most likely harbours information like a library keeps books. The dorsolateral frontal lobe and the frontal eye fields are most likely where the whole thing starts when we make a conscious decision to move our eyes, thus operating the whole system.

Franziska S Thaler et al. Neurology 2019, 92:193-195

The frontal eye field, where it all starts, is between the central and precentral sulci, in front of the primary hand motor representation. It is where you would expect the eyes to be in the famous homunculus that locates the motor system in the precentral sulcus. Very close to the motor neurons for the head, and not far from the fingers. That would be Broadman área 6, and 8, as thought until recently.

Dr Paulo Bittencourt

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