OBRA GRÁFICA ORIGINAL
4. Conservación de obra gráfica original
Admirable as the mechanical functioning of these chambers and tubes may be, when it comes to the pickup of information the system has its
limitations. In some circumstances it fails to specify different events as different and permits ambiguity .
• ILLUSIONS OF PASSIVE TRANSPOSITION. With respect to the chambers, the utricle and saccule, it is clear that an organ with a heavy statolith cannot by itself distinguish for its possessor between a
state of rest
and auniform motion in a straight line,
to use terms invented by Newton. It can specify starts and stops, pushes and pulls, or positive and negative accelerations in a horizontal direction. It can probably register short motions from place to place, but not motion as such. It is sensitive only totransitions
between Newtonian steady states. This is why, so long as the jet airplane moves uniformly, the traveler cannot feel movement. The vestibular organ is suited to detect active locomotion or passive starts and stops but not constant-velocity transportation. For millions of years animals moved by rhythmic pushes, not as Newtonian bodies and not in railroad cars or airplanes. It is therefore reasonable that an individual should be susceptible to vestibular illusions when passively transported in a vehicle.The statocyst was said to be an unfailing indicator of the direction "down." And so it is, except when subjected to a constant and sustained horizontal acceleration such as a speed-up, a slow-down, and a centrif- ugal force. The horizontal force then combines with the vertical force of gravity to yield a new vector resultant, and a shift in the apparent direc- tion of "down." The statocyst cannot be expected to separate a gravi- tational from a sustained inertial force. The situation is diagrammed in Figure 4.7. The individual will feel himself tilted in the direction oppo- site the passive acceleration. In an airplane, for example, when speeding up one feels that he is in a climb, and when slowing down that he is in a dive, even though the line of flight may be perfectly horizontal. Dur- ing an unbanked turn, one feels that he is tilted outward because the centrifugal force shifts the apparent direction of gravity. In such situa- tions the pilot does well to control the airplane by his view of the horizon, or by his instruments. "Flying by the seat of his pants" is likely to get him in trouble. The research on illusions of the vertical during flight is well described in
Celdard's
book on the senses(1953,
pp. 265ff.) .• ILLUSIONS OF PASSIVE HEAD ROTATION. With respect to the semicircu- lar canals of the vestibular apparatus, it is clear that this organ by itself cannot specify the difference between a state of rest and a uniform rotary motion, or spin, once it is established. This fact is of no conse- quence to an individual who turns around, or changes direction while walking, since the rotary acceleration of the head passes over into de- celeration without pause, and the flexible cupula of the horizontal canal first swings backward and then swings forward to its former neutral
70 /
THE SENSES CONSIDERED AS PERCEPTUAL SYSTEMS, /
Acceleration of Supporting Surface _
Figure 4.7 The simplified statocyst of an individual whose support is undergoing acceleration. The pull of gravity and the push of the surfaces of support both have been inclined from the normal direction by the fact of constant acceleration. Compare this diagram with Figures 4.1, 4.2, and 4.3.
position. The act is correctly specified. But it is of consequence to some- one who observes his feeling of rotation while sitting in the rotating chair of the classical laboratory experiment.
The commonest way of demonstrating illusions of rotation is to seat the subject, start him, turn him around a number of times, and then stop him. During the period of constant spin, the flexible cupula in the stimulated canal probably tends to recover its null position spontaneously and thus ceases to specify a turn. During the negative acceleration, the gate swings ahead of the null position, thus falsely specifying a rotation
in the opposite direction. Although the head is motionless, the organ continues to specify rotation until it returns again to neutral, yielding an illusory perception and also (as we shall observe in Chapter 9) forcing the eyes to compensate for a nonexistent turn of the head. The eyes show "after-nystagmus" and the observer experiences vertigo. This nystagmus should not be confused, however, with the compensatory eye- movements at the start of rotation, which are adaptive.
This experiment is instructive, although often misunderstood. The swinging gate has been deceived, as it were, into signalling a start with what is in reality a stop. But the deception results from the experimental intrusion of a coasting period between the real start and stop. The organ was evolved for the needs of active locomotion, not for swivelling chairs.
The illusion is hardly the fault of the organ, in view of its structure; the psychologist has simply