Extraocular muscle action from brain stimulation in the macaque.

A study was made of the eye muscles activated by faradic stimulation of the frontal cortex, midbrain tegmentum, and oculomotor nerve of the macaque. The traditional view concerning the actions of the individual muscles was not confirmed. The vertical recti were found to be effective elevators and depressors of the eyes regardless of the position of the eye in the horizontal plane. At the brain levels stimulated, the vertical recti neither acted as adductors when the eye was partially adducted nor acted as torters. The superior oblique was an intorter regardless of the position of the eye in the orbit and did not behave as an effective depressor when the eye was adducted or as an abductor when the eye was partially abducted. In these experiments the superior oblique was activated only from midbrain stimulation. The inferior oblique acted as an elevator in the primary and adducted positions and as an extorter when the eye ivas abducted from both cortical and midbrain sites. The horizontal recti produced precise, balanced eye movements from cortical stimulation without the collaboration of the vertical muscles. Cocontraction of the vertical muscles was not demonstrated. In oblique conjugate movements from cortical stimulation the participation of two muscles was demonstrated. Torsion of the globe was not produced by cortical stimulation. An individual muscle is capable, when acting independently, of bringing the eye to different end positions (goals). Two individual or groups of muscles are capable, under certain circumstances, of bringing the eye to the same end position. It is suggested that these observations can be explained if it is assumed that an extraocular muscle is composed of motor units that subserve different functions (in terms of goal and speed of eye movement) and exert unequal forces along the muscle insertion.