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Appendix 2. Observations from the direct stimulation of the cerebral cortex

Neurosurgeon Wilder Penfield invented a procedure in which he treated patients with severe epilepsy by destroying nerve cells in the brain where the seizures originated. Before operating, Penfield stimulated the brain with electrical probes while the patients were conscious on the operating table, and observed their responses. In this way he could more accurately target the areas of the brain responsible, reducing the side-effects of the surgery. This technique also allowed him to create maps of the somatosensory and motor cortices of the brain showing their connections to the various limbs and organs of the body (see the illustration of the “cortical homunculus“). In the somatomotor cortex, the hands occupy a larger cortical real estate compared to the rest of the body. A larger cortical representation enables fine motor control of the hands. In the somatosensory cortex, the face is represented by a large part of the cortex because of the high number of sensory nerves found in the face. Note that the “homunculus” representation in the cortex is somewhat topographical: the nose is represented between the mouth and eyes, the fingers are represented near the palm, and so on.

Temporal lobe stimulation

In order to avoid damaging language areas, Penfield had to explore the specialization of the temporal lobe (although the general pattern of cortical organization is common in all people, each of us has a unique map of functional specialization representation in the cortex). Accordingly, Penfield stimulated the temporal lobe neurons in 520 patients. Stimulation of the temporal lobe triggered vivid recall of memories in 40 patients. Here is a quote from Penfield, W. “The Mystery of the Mind”, Princeton University Press (1975) pages 24-26, describing responses of patient M.M. (a young woman 26 years of age) upon stimulation of several points in the right temporal lobe. The point number is in bold:

11 - “I heard something, I do not know what it was.”

11 - (stimulation repeated without warning) “Yes, Sir, I think I heard a mother calling her little boy somewhere. It seemed to be something that happened years ago.” When asked to explain, she said, “It was somebody in the neighborhood where I live.” Then she added that she herself “was somewhere close enough to hear.”

12 - “Yes. I heard voices down along the river somewhere-a man’s voice and a woman’s voice calling…I think I saw the river.”

15 - “Just a tiny flash of a feeling of familiarity and a feeling that I knew everything that was going to happen in the future.”

17c - (a needle is insulated except at the tip was inserted to the superior surface of the temporal lobe, deep in the fissure of Sylvius, and the current was switched on) “Oh! I had the same very, very familiar memory, in an office somewhere. I could see the desks. I was there and someone was calling me, a man leaning on a desk with a pencil in his hand.”

I warned her I was going to stimulate, but I did not do so. “Nothing.”

18a - (stimulation without warning) “I had a little memory - a scene in a play - they were talking and I could see it - I was just seeing it in my memory.”

Here is another quote from Penfield, W. “The Mystery of the Mind”, page 21:

“It was evident at once that these [patient’s recollections] were not dreams. They were electrical activations of the sequential record of consciousness, a record that had been laid down during the patient’s earlier experience. The patient “re-lived” all that he had been aware of in that earlier period of time as in a moving-picture “flashback”.

On the first occasion when one of these flashbacks was reported to me by a conscious patient (1933), I was incredulous. On each subsequent occasion, I marveled. For example, when a mother told me she was suddenly aware, as my electrode touched the cortex, of being in her kitchen listening to the voice of her little boy who was playing outside in the yard. She was aware of the neighborhood noises, such as passing motorcars, that might mean danger to him.

A young man stated he was sitting at a baseball game in a small town and watching a little boy crawl under the fence to join the audience. Another was in a concert hall listening to music. “An orchestration,” he explained. He could hear the different instruments...

D.F. could hear instruments playing a melody. I re-stimulated the same point thirty times (!) trying to mislead her, and dictated each response to a stenographer. Each time I re-stimulated, she heard the melody again. It began at the same place and went on from chorus to verse. When she hummed an accompaniment to the music, the tempo was what would have been expected.”

Let us attempt to understand Penfield’s observations within the context of the theory presented in this book. Electrical stimulation of the temporal lobe in conscious patients triggered a recall of a series of memory frames that included both visual (“I think I saw the river”, “I could see the desks”, “a man leaning on a desk with a pencil in his hand”, “I had a little memory - a scene in a play-they were talking and I could see it - I was just seeing it in my memory”) and auditory (“mother calling her little boy“, “instruments playing a melody”) sensations. It is likely that each conscious frame described by the patient was represented by an increased activity of a large group of neurons (neuronal ensemble) firing in synchrony with each other and with the attention rhythm. The neuronal ensemble likely included neurons located in visual cortex (V1, V2, V4, ITL, MT), as well as in auditory cortex. It is likely that these were the same parts of cortex that were active and synchronized with the attention rhythm during the original conscious experience of the physical world.

We can depict the neuronal ensembles activated by the stimulation of the temporal lobe as pyramids. Each side of the pyramid represents one sensual modality. The neurons at the base of the pyramid are the least specific - these are the neurons located in the primary visual area, primary auditory area and so on. The neurons in the temporal lobe stimulated by Penfield are located at the tip of the pyramid. They are the most specific. These neurons have high probability of triggering synchronous activity of a complete neuronal ensemble. These neurons are binding the ensemble together. The memory about an event is not located in any single neuron. The memory is encoded in the connections between the neurons of the complete ensemble (hundreds of thousands of neurons). Thus the memory is distributed throughout the cerebral cortex. The activity of the complete ensemble can be triggered by activity in any part of the ensemble. However, activity in the binding neurons located in the temporal lobe have the greatest probability of activating the complete neuronal ensemble. Damage to a part of the cortex may interfere with the memory about one aspect of the event. However, damage to the binding neurons of the temporal lobe makes it especially hard for the patient to activate the complete neuronal ensemble. Thus, damage to the temporal lobe can be especially detrimental to patient memory. Again, not because memory is located in the temporal lobe, but because binding neurons are located there.

Finally, I should mention that humans can voluntarily modify any part of the neuronal ensemble representing the memory of an event. Humans can use mental synthesis to change the color of the objects, replace objects, and change any auditory or other characteristics of the event.

Maps of the somatosensory and motor cortices of the brain showing their connections to the various limbs and organs of the body (cortical homunculus):

Stimulation of language cortex

Here is a quote from Penfield, W. “The Mystery of the Mind”, Princeton University Press (1975) pages 50-51, describing responses of patient C.H. upon stimulation of his language cortex in the left temporal lobe:

“… We have found that a gentle electrical current interferes with the function of the speech mechanism. One touches the cortex with a stimulating electrode and, since the brain is not sensitive, the patient does not realize that this has made him aphasic until he tries to speak, or to understand speech, and is unable to do so.

One of my associates began to show the patient a series of pictures on the other side of the sterile screen. C.H. named each picture accurately at first. Then, before the picture of a butterfly was shown to him, I applied the electrode where the speech cortex to be. He remained silent for a time. Then he snapped his fingers as though in exasperation. I withdrew the electrode and he spoke at once:

“Now I can talk,” he said. Butterfly. I couldn’t get that word ‘butterfly,’ so I tried to get the word ‘moth!’ ”

It is clear that while the speech mechanism was temporarily blocked he could perceive the meaning of the picture of a butterfly. He made a conscious effort to “get” the corresponding word. Then, not understanding why he could not do so, he turned back for a second time to the interpretive mechanism, which was well away from the interfering effect of the electrical current, and found a second concept that he considered the closest thing to a butterfly. He must then have presented that to the speech mechanism, only to draw another blank.”

From this experiment it is clear to us that the visual recognition cortex (visual department) can function independently from the auditory cortex (auditory/ linguistic department). The patient C.H. was obviously able to recognize the butterfly without any help from the linguistic cortex. When the linguistic department was not able to name the butterfly, C.H. was able recall the image of a moth (that is associated with the butterfly), again with no help from the linguistic cortex. We can depict the neuronal activation pattern in C.H. as follows:

(1) A picture of a butterfly is shown on display. Neurons in V1 (light green line) are activated.
(2) The butterfly is recognized. The part of the neuronal ensemble representing the butterfly in the visual cortex is activated.
(3) However the auditory neurons that are part of the butterfly ensemble (located in the auditory plane of the light green pyramid) are inactivated by electrical current. Consequently, this part of the ensemble is not activated and the patient fails to speak the word ‘butterfly’.
(4) The patient searches for a different neuronal ensemble that is closely associated with the neuronal ensemble of the butterfly. The neuronal ensemble representing a moth is activated and the patient experiences the mental image of a moth (blue).
(5) Again the patient attempts to activate the auditory neurons. However the auditory cortex is still inactivated by electrical current and the patient fails to speak the word ‘moth’.
(6) As soon as the electrical stimulation of the auditory cortex is stopped, the complete neural ensemble that includes the auditory neurons can form and the patient is able to speak.