Motion-picture film is a strip of discrete, still pictures but produces the visual impression of continuous movement. Stationary light bulbs coming on one after the other over the theatre entrance also produce an impression of steady movement. In part, such effects of apparent movement (called the visual phi phenomenon) depend on persistence of vision: visual response outlasts a stimulus by a fraction of a second. When the interval between successive flashes of a stationary light is less than this visual-persistence time, the flicker will appear to fuse into a continuous light. The flicker frequency at which this occurs is called the perceiver’s flicker-fusion frequency (or critical flicker frequency) and represents the temporal resolving power of his visual system at the time. Another process on which apparent movement depends is a tendency (called visual closure or phi) to fill in the spaces between adjacent visual objects. This means that the movement detectors of the visual system are triggered as effectively by a closely spaced pair of lights alternately going on and off as by a single light moving back and forth. It would seem that two aspects of visual function (flicker fusion and phi) make the motion-picture industry possible.

Stroboscopic effect

When a rotating electric fan is illuminated by a flashing light source (called a stroboscope) so that a flash arrives whenever a fan blade passes a fixed position, the blades will seem to stand still. This is a useful way of observing fast-moving objects such as machinery or insect wings. If the flashes occur less frequently, the object will seem to move slowly in its actual direction; when the flashes arrive more frequently, the object will seem to move backward, as stagecoach wheels may do in the cinema.

Visual movement in depth

An object moving directly away from an observer provides fewer visual cues of movement than it would be moving across the field of view. However, changes in retinal-image size are produced that give a clue to its movement. Thus a stationary, but shrinking, luminous object in the dark is seen as if it were receding. Other clues to movement in depth are changes in the convergence angle of the two eyes, in the focussing mechanism, and in the haziness and brightness of the object.

Auditory

Sound waves travel well in water, and fish are accordingly able to rely heavily on acoustic cues to detect moving objects. Land animals, although typically more visually oriented, also make some use of such cues, including changes in intensity (loudness) and small differences in the time at which the wave reaches each ear. Some animals (e.g., rabbit, horse) have mobile external ears that track moving sound sources. Bats vocalize high-frequency waves and are able to detect objects by sonic reflection (a technique similar to sonar).

Kinesthetic

Kinesthesis here refers to experiences that arise during movement from sense organs in the membranes lining the joints and from the sense of effort in voluntary movement; receptors in muscles seem to have little role in the perception of bodily movements. Depending on speed of motion and the joint involved, blindfolded people can detect a passive joint movement as small as a quarter of a degree. People vary widely in the accuracy with which they can actively produce movement of a given extent; this ability also varies with direction of movement and the opposing friction, mass, and springiness.

Kinesthetic perception may persist for a limb that has been amputated, giving rise to a hallucinatory experience known as the phantom limb. The patient may experience vividly the “movement” of the absent part; a recent amputee may attempt to stand on his missing leg or to grasp with his missing hand.

Vestibular system

Vestibular structures, enclosed in a fluid-filled cavity in the region of each inner ear, include the utricle, a small sac containing minute sensitive hairs associated with tiny sandlike granules called otoliths. The utricle functions as a linear accelerometer. When the head tilts relative to gravity or is accelerated, the relatively dense otoliths deflect the hair cells and nerve impulses are transmitted to the brain. At constant velocity the otoliths become stable, stimulation ceases, and a person must rely on other cues (e.g., by observing the passing scene) to detect his motion.

Vestibular structures for each ear also include three fluid-filled semicircular canals, each in a different plane. Each canal has a swelling (ampulla) that contains the cupula, a cluster of sensitive hairs embedded in a jellylike mound. As the head moves in the plane of a given canal, motions of the fluid deflect the cupula to produce nerve impulses. These travel through the brainstem to other brain and spinal centres that mediate equilibrium or balance and that generate nystagmic eye movements.

Taken together, the semicircular canals serve as a rotary accelerometer. If a person is rotated at constant velocity and then is suddenly stopped, the cupula is redeflected to give a feeling of rotation in the opposite direction; this also gives rise to dizziness and postrotational nystagmus. Dancers and skaters learn to overcome such effects by concentrating on some fixed, visible object; with their eyes closed they are as likely to fall as anyone.

Overstimulation of the vestibular system (e.g., on a ship or airplane) may induce motion sickness. A person with vestibular function totally destroyed is not subject to motion sickness; but if the vestibular mechanism is impaired only on one side, each movement of the head can be nauseating. Such a patient takes a long time to compensate for this imbalance.

In outer space there is no gravitation to mediate feelings of up or down, although these still may arise from visual cues. The utricles and vestibular canals still respond to movements of the head, however, and serve orientation within the spacecraft.