26

The utricle and the saccule sense head position, and the semicircular canals sense head movement, but balance is affected by more than just vestibular information. Both proprioceptive information (pressure sensors and kinesthetic information) and visual information become integrated and contribute to our sense of balance. Yet there is no place in the cerebral cortex that has been discovered thus far as being dedicated to interpreting balance. There are, however, vestibular nuclei in the brainstem (near the 4th ventricle and brainstem) that receive vestibular information as well as proprioceptive and optic flow information.

Optic flow is the perceived visual motion of objects as the observer moves relative to them. To an observer driving a car, a sign on the side of the road would move from the center of their vision to the side, growing as they approached. If they had 360 degree vision, this sign would proceed to move quickly past their side to their back, where it would shrink. This motion of the sign is its optic flow.

This allows a person to judge how close they are to certain objects, and how quickly they are approaching them. It is also useful for avoiding obstacles: if an object in front of an observer appears to be expanding but not moving, they are probably headed straight for it, but if it is expanding but moving slowly to the side, they will probably pass by it. Since optic flow relies only on relative motion, it remains the same when they are moving and the world remains still, and when they are standing still but everything they can see is moving past them. These properties have made the concept useful for robot designers writing visual navigation routines. It also appears to be used by certain insects, especially flying ones, where a large optic flow (indicating a quickly approaching obstacle) triggers muscles to move away.

When moving in a particular direction, an observer’s optical flow field expands from a singular point, called the focus of expansion (Fig.3.6.1). This point of expansion tells us our heading; this is the one place in our visual field where there is no motion when we’re moving (because we’re moving straight toward it).

Optic flow thus provides us with information concerning coordinated motion in the visual field. Whereas semicircular canals respond to rotations and otolithic organs (utricle, saccule) sense linear accelerations—thus contributing to our sense of balance—optic flow tells us about our motion when we’re not accelerating (i.e., when our vestibular and kinesthetic information is useless).