"Thomas A. Easton - Let There Be Light" - читать интересную книгу автора (Easton Thomas A)

and tried to leap backward, away from the falling "wall," even though the "visual" information upon which
he was acting was being impressed upon the skin of his back. Not only did he learn the first time what a
"looming" stimulus meant, but he localized it correctly in space, precisely as if he had functional eyes.
This is all the more startling when we consider the nature of the stimulus: an array of point stimuli, 20
x 20 in the latest models, only some of which are activated by the light reflected from an object and
entering the camera. Figure 2 shows, schematically, what a beer stein would look like with this system.
The figure is most accurate as a representation of the earlier mechanical stimulation—the dots of the
image are black and white and the corresponding vibrators are either on or off. Gray-scale information is
only now becoming available in those models which use electrical stimuli.



Figure 2.
A pictorial representation of a TVSS image. The large black dots are those activated by the
light entering the TV camera and are felt by the subject as vibrating points.

Obviously, a 400-point array cannot convey much information, but because the subject can move the
camera, either by its controls as in Figure 1, or by moving his head as when the camera is mounted in the
glasses, he can carry out what astronomers call occlusion studies and get a very accurate idea of what the
actual contours of an object are.
As presently designed, the system is cyclopean, or monocular, and it has a narrow (two-to-ten
degrees) field of view in order to let as much detail as possible reach the dot picture of the array. But as
long as the subject can control the movements of the camera, he can not only "see" the object, but, as
indicated above, he can "see" it as located in front of him, even though the signals that reach his brain are
originating on the skin of his back. He senses it not as a pattern of vibration, but as an externally localized
object, just as sighted persons do those objects they sense through their eyes.
It helps that the subject can correlate changes in the stimulus pattern with his own motor output—if
he makes a motion in connection with an object he knows is there, then stimulus changes become
identified with the object. It is in just this way that infants may learn to identify external objects as external
by correlating changes in appearance with eye and head movements.
But this similarity only disguises the nature of the problem to which the TVSS seems to be a solution.
Sensory substitution depends on the ability of one sensory system to assume the functions of another,
where "system" means the peripheral receptors, the nerves linking them to the spinal cord and/or the
brain, and the central nervous system itself. The various systems differ from each other most obviously in
the nature of their receptors, though there are also important differences in the neural equipment that
processes the data received and passed on by them.
While no two sensory systems may do the same thing, one may be made to serve the same purposes
as another. It is quite possible to transform stimuli appropriate to one receptor into stimuli appropriate to
another, and that is just what the TVSS does: the TV camera absorbs light and acts as a transducer to
change that stimulus to the electrical or vibratory stimulus appropriate to skin receptors.
Now, most people are surprised when they are told that the skin can be made to serve as an eye.
"How?" they ask. "After all, the eye is very small and covered with millions of very sensitive light
receptors and the skin is big and its receptors don't seem very sensitive at all." If they know a little more,
they point out that most areas of the skin are not sensitive enough to tell the difference between one and
two pinpricks unless they are relatively far apart. But the sensitivity is greater than it seems, and the skin's
reaction to pinpricks is not entirely pertinent: patterns can be sensed, and a good example is the way your
"insensitive" back responds to the texture of a shirt. We don't use it, but the potential is there, and the
skin of a person with a sensory handicap such as blindness often becomes almost excruciatingly
sensitive—the blind are sometimes said to be able to "feel" walls and other obstacles either as light or as
sound.