Читать книгу The Form Within - Karl H Pribram - Страница 68

We experience the world in which we navigate as being “out there,” not within our sense organs or in our brain. This phenomenon is called “projection.” I experienced an interesting instance of projection that anyone can repeat in his own bathroom. I noted that the enlarging mirror (concave 7X) in my bathroom reflected the ceiling light, not at the mirror’s surface as one would expect, but instead, the image of the light was hovering in mid-air above the mirror. The image seemed so real that I tried to grasp it, to get my hand behind it, only to catch thin air.

Our experience of waveforms, as they create images reflected by mirrors, refracted by prisms or gratings, often seem equally “hard to grasp.” But the power that generates such waveforms can be experienced in a very real way, as in a tsunami sweeping ashore. My research has been motivated by the excitement of “getting behind” the images we experience—to try to understand how the experience is created. Cosmologists spend their days and nights trying to “get behind” the images they observe with their telescopes and other sensing devices. Brain scientists are, for the most part, not yet aware of the problem.

In order to understand the process by which we perceive images, we need to regard vision as akin to other senses, such as hearing. Harvard physiologist and Nobel laureate Georg von Békésy began his classical experiments, summarized in his 1967 book Sensory Inhibition, at the Hungarian Institute for Research in Telegraphy in 1927. His initial experiments were made on the basilar membrane of the cochlea, the receptor surface of the ear. Békésy began by examining how the arrival of pressure waves becomes translated into a neural code that reaches our brain. “The results suggested similarities to Mach’s law of [visual] brightness contrast. At the time my attempt to introduce psychological processes into an area that was held to be purely mechanical met with great opposition on the part of my associates in the Department of Physics.

. . . And yet I continued to be concerned with the differences between perceptual observations and physical measurements. My respect for psychological observation was further enhanced when I learned that in some situations, as in the detection of weak stimuli, the sense organs often exhibit greater sensitivity than can be demonstrated by any purely physical procedure. (Ernst Mach was a venerated Viennese scientist/philosopher active at the turn of the 20th century.)

When I met Békésy at Harvard, he was frustrated by the small size of the cochlea and was searching for as large a one as he could get hold of. He knew that I was working (in Florida) with porpoises at the time, and he asked me to bring him a porpoise cochlea if I came across one of the animals that had died. I did, but Békésy wanted something still larger.

Békésy had noted at the time—just as we would later come to view the eye as similar to the ear—that the skin was also like the ear in many respects. (The mammalian ear is derived from the lateral line system of fish, a system that is sensitive to vibrations transmitted through water.) Therefore, Békésy set out to make an artificial ear by aligning five vibrators that he could tune to different frequencies. He placed this artificial cochlea on the inside surface of the forearm, and he adjusted the vibrations so that the phase relations among the vibrations were similar to those in a real cochlea. What one felt, instead of the multiple frequencies of the five vibrators, was a single point of stimulation. Békésy showed that this effect was due to “sensory inhibition;” that is, the influence of the vibrators on the skin produced “nearest neighbor” effects, mediated by the network of nerves in the skin, that resulted in a single sensation. Changing the phase relations among the vibrators changed the location on the forearm where one felt the point.

Subsequently, in my laboratory at Stanford, a graduate student and I set to examining whether the brain cortex responded to such individual vibratory stimuli or to a single point as in the sensation produced in Békésy’s experiment. We did this by recording from the brain cortex of a cat while the animal’s skin was being stimulated by several vibrators. Békésy, who had moved to Hawaii from Harvard (which belatedly offered him a full professorship after he received his Nobel Prize) served on my student’s thesis committee; he was delighted with the result of the experiment: The cortical recording had reflected the cat’s sensory and cortical processing—presumably responsible for the cat’s perceptions—not the physical stimulus that had been applied to the skin on the cat’s lips.

On another occasion, still at Harvard, Békésy made two artificial cochleas and placed them on his forearms. I was visiting one afternoon, and Békésy, full of excitement, captured me, strapping the devices on my arms with great enthusiasm. He turned on his vibrators and adjusted the phase relations among the vibrations within each of the “cochleas” so that I experienced a point sensation on each arm. Soon, the point sensations began to alternate from arm to arm: when I felt a point in one arm, I did not feel it in the other. Interesting.

Békésy had concluded from this experiment that sensory inhibition must occur not only in our skin but somewhere in the pathways from our skin to our brain, a conclusion confirmed by the results of our cat experiments.

But this was not all. Békésy asked me to sit down and wait a while as the point sensations kept alternating from one arm to the other. I began reading some of his reprints. Then, suddenly, after about ten minutes, I had the weirdest feeling: The sensation of a point had suddenly migrated from my arms and was now at a location in between them, somewhat in front of me. I was fascinated.

I asked, how could this be? How can I feel, have a tactile sensation, outside of my body? Békésy replied with a question: “When you do surgery, where do you feel the tissue you are touching with your forceps and scalpel?” “Of course,” I answered, “out there where the tissue is—but the forceps and scalpel provide a solid material connection.” We all have this experience every time we write with pen or pencil, feeling the surface of the paper through the mediation of the implement.

But we also hear sounds without any obvious solid material connection with the source that produces them: for instance, the loudspeakers in a stereo system. I once heard the sound so realistically coming out of a fireplace flanked by two stereo-speakers that I actually looked up the chimney to see if there was another speaker hidden up there.

There are important advantages to our ability to project our perceptions away from our bodies in this fashion. In another experiment in which he set out to test the value of our stereo capability, Békésy plugged his ears and used only one loudspeaker, strapped to his chest, to pick up sounds in his environment. He tried to cross a street and found it almost impossible because he could not experience an approaching car until it was almost literally upon him. The impact of the vibrations on his chest from the approaching sound of a car hit him so suddenly that he clutched his chest and was nearly bowled over.

We may experience the world we navigate as being “out there,” but the apparatus that makes our experience possible is within our skin. My eyes and brain enable me to see—but the location of what I see is projected out, away from and beyond those bodily structures that make my seeing possible. Békésy’s experiments demonstrated the laws of projection create an anticipatory frame, a protective shell that surrounds us and allows us to choose an action before an oncoming event overwhelms us.

36. Localization of Perception Between Fingers

While reading Békésy’s notes on his experiments, I saw that he had scribbled some equations next to the text.

I asked him about them, and he answered that they were some new-fangled math that he did not fully understand as yet. I felt that, even if he understood the equations, I certainly would not. To my surprise, that understanding was to come later: they were the equations describing Gabor’s application to sensory processing of the mathematics of quantum physics.

My encounters with Békésy were among the most rewarding in my career: they greatly enhanced my appreciation of the issues to be faced in deciphering how we navigate our world.

The experimental results presented here showed that the perceptual process is far from a simple input of “what’s out there” to the brain. Rather, by means of the transformations produced by receptors, structures such as objects are formed from a holographic-like background within which every-“thing” is distributed “everywhere and everywhen.” The receptor then transforms these “objects”—in the case of vision by virtue of the quantum nature of retinal processing—into a constrained holographic-like process, a set of wavelets that Dennis Gabor called “quanta of information.” Békésy’s experiments, as well as my own, demonstrated that, in turn, these brain processes influence receptors as much as does the input from the-world-out-there that we navigate. More on this in the next chapter.