Will-driven saccad

Volunteer saccad

Saccad, will-driven

Will-controlled saccades are triggered from the frontal gaze direction center. The frontal gaze direction center sends direct threads to the opposite side's pontina gaze direction center. At the same time, the frontal gaze direction center signals to the nucleus lobe , which then inhibits the tonic inhibition exerted by the substantia nigras pars reticulata on parts of the colliculus superior and on parts of the brainstem's gaze direction centers. This disinhibition of gaze direction centers activates the motor neurons that bring about the desired gaze direction change.

The frontal gaze direction center, in turn, is controlled from the bark of the parietal lobe by nerve cells that are responsible for the brain's attention to phenomena in the visual field (visual attention). Visual attention appears to be particularly well developed within the right parietal lobe. This may explain both the brain's special interest in events in the left hemisphere of vision and the severe left-sided neglect phenomenon that is often seen after damage to the right parietal lobe.

NOTE. The nerve fibers coming from the inner (nasal) half of the retina cross cross over the mean line of the optic nerve cross (chiasma opticum) and join the (uncrossed) threads coming from the outer (temporal) retinal half of the eye. This, together with the lens of the eye refracting light so that the outer half of the field of view falls on the inner (nasal) part of the retina and vice versa, means that the left half of the field of view, as perceived by each eye, will be projected solely to the visual cortex of the right hemisphere of the brain and vice versa.