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Neural Darwinism: A New Approach to Memory and Perception

The importance of the borders formed by the CAM-linked collectives was shown in a spectacular series of experiments in Edelman’s laboratory on the emergence of a single feather in the chicken. At each stage in the feather’s morphogenesis, borders were found between groups of cells linked together with different kinds of CAM. Following the formation of the borders each group of cells changed, those on one side into one kind of cell, and those on the other into another kind of cell. For example, the final feather, with its regular pattern of branching (a pattern that varies from feather to feather), is carved out of a cylinder of tissue. The distinctive feather pattern arises after alternating groups of cells (i.e., L-CAM-linked cell groups alternating with N-CAM-linked cell groups) make their appearance. Groups of cells with N-CAM linkages then die, while the alternating groups with L-CAM linkages become hardened, or keratinized. The result, a feather. The edges of the barbs on the feather were the borders between the L- and N-CAM cell groups. Even more dramatically, when changes were induced experimentally in the linkages made by one kind of CAM, cell groups that were linked by a completely different kind of CAM were altered.

From the workings of such epigenetic mechanisms we can see why knowing the entire genetic repertoire of an animal would not alone permit us to predict its final detailed morphology. Identical twins are never absolutely identical. And just as every feather has a different pattern of branching, every brain would be expected to have a different pattern of connections.

The demonstration that there are molecular reasons why no two brains could be identical is central to Edelman’s view that the brain functions as a system based on selection—the CAM mechanism creates diversity in the anatomical connections of an individual’s brain. The context and history of cellular development thus largely determine the structure of the brain; and therefore context and history are also important in brain function. Because development, structure, and function are related, it would not be surprising if the functional activities of a connected group of cells in the brain depended both on the activities of neighboring cell groups and on the past history of the particular group itself.

To show that brain function, like structure, also depends on context and history and not on localized functions and fixed memories is the burden of Edelman’s theory of neuronal group selection. What emerges is a new approach to the biological basis of psychology.

A major claim of this approach is that the unit of selection in the brain is a neuronal group, a set of interconnected neurons that function together. The patterns of connections that are established among neurons vary from group to group because of the changes in dynamics of the CAMs during development. The brain thus contains large numbers of different neuronal groups.

Neuronal groups are connected to one another as well as to the sensory receptors for light, touch, and sound in the eyes, skin, and ears. In general, neighboring groups of neurons in the brain receive input from neighboring sensory receptors (for example, on the second and third fingers of the hand); two neighboring groups in the brain can in fact receive input from the same sensory receptor. Although the inputs can overlap, the responses of each group to the stimuli will be different. Because each group of neurons has its own pattern of internal connections, which differs from those of other groups, each group will respond differently, even to identical stimuli.

The activities of a group of interconnected neurons would acquire significance not just because of the anatomical connections and physiological mechanisms on which the functioning of that group depends, but because of its context and the history of its received signals as well. If this is true, then a given “memory” cannot be stored in a specific place in the brain, since neighboring activities would of necessity change, and therefore the “context” of any neuronal cell group is never constant. If one were to assume that a memory was in fact stored as it is in a computer, altering this process would irreparably destroy it.

The embryonic processes I have described involving CAMs are central in creating a large repertoire of different neuronal groups. But after birth the principle of selection changes. Instead of alternations of CAMs, changes in the strength but not the pattern of connections occur; such changes determine the paths over which neural signals will flow.2 Environmental stimuli may cause one group to respond with greater activity than other groups receiving the same input. And when this happens, laboratory experiments show that the connections between the neurons in that group (the synaptic junctions) can be strengthened.

Edelman and his colleagues have worked out a set of possible rules that might govern these synaptic alternations. Molecular changes take place within neurons and at the synaptic junctions so that the neurons tend to be activated by similar stimuli on subsequent occasions. Particular variants within the brain’s population of neuronal groups are selected by the stimulus. Indeed, a group that responds to a stimulus might do more than this. As its connections are strengthened it might alter the strength of its links to other groups and, by competing with other groups, integrate neurons from them into its own response activity. The strengthening of the synaptic connections creates what Edelman calls a secondary repertoire: this is made up of neuronal groups that respond better to specific stimuli because they have been selected and their connections strengthened.

In their responses to stimuli, neuronal groups could be likened to a set of radio receivers, each tuned to a small band of frequencies. One radio might receive frequencies in the 1600 to 1700 kilocycle range, and another might receive frequencies in the 1550 to 1650 kilocycle range. Depending on the broadcast frequencies in the area, some of the receivers will respond to one broadcast, others to several broadcasts, and others to none at all. Analogously to an animal moving its head or body, moving from New York to Peking, for example, would change the response patterns of the individual receivers. The receiver’s purpose depends on where it is: in Peking it receives Radio Peking, in New York WCBS, and in Moscow Radio Free Europe, Radio Moscow, and a lot of jamming simultaneously.

Like the radio receivers, a given neuronal group can respond to more than one stimulus—what is called a degenerate response. In our analogy, a given radio receiver might pick up Radio Peking better than Radio Moscow, but it can pick up either station. (Of course, stimuli are not organized into coherent pieces of information like radio broadcasts. At higher levels in the brain, the stimuli must be organized in ways that will be meaningful and useful for the organism.)

The brain performs this organizing operation by using maps made up of neuronal groups. A map is a collection of neuronal groups in the brain which are arranged in a way that preserves the pattern of relationships either between a sheet of sensory receptors (such as those in the skin of the hand) and a sheet of neural tissue in the brain to which the sensory stimuli have been transmitted, or between two sheets of neural tissue. Groups are arranged in maps that “speak” back and forth to one another so as to create categories of things and events. Different kinds of maps are found in different parts of the brain, and an analysis of how such maps interact is an essential and final part of Edelman’s theory.

Because the brain has to be prepared for unpredictable events it must map stimuli in a variety of ways. Brain maps sort incoming stimuli by similarity (same frequency of sound, same intensity of sound, etc.) as well as by a mixture of properties. The main evolutionary principle at work here is that stimuli are organized into patterns that will help the organism cope with its environment.

In 1870 two young Germans, Gustav Theodor Fritsch and Eduard Hitzig, first discovered that the brain maps motor stimuli. They reported that touching discrete areas on a dog’s brain activated specific parts of the body. By the end of the century motor and sensory maps were a well-established feature of neuroanatomical teaching. These maps were assumed to be permanent and more or less identical in all members of a species.

It was therefore surprising when in 1983 Michael Merzenich and his collaborators at the University of California at San Francisco discovered that the sensory maps showed considerable variation in the brains of normal monkeys. The brain maps in a particular monkey’s brain varied over time. And there was considerable variation in corresponding maps in different monkeys as well. Subsequent experiments demonstrated that the maps became rearranged, even within short periods of time, following injury to a nerve supplying sensory input from one of the monkey’s fingers.

Merzenich’s work gives powerful support to Edelman’s claim that particular combinations of neuronal groups are selected competitively from the general population of neuronal groups by sensory input. Since the nerves from the different areas of the skin of the hand are connected eventually by overlapping branches to the same receiving areas in the brain, the part of the skin surface represented in a particular brain by a group of neurons depends on selective competition. Neuronal groups in one area of the brain may, for example, receive overlapping input from both the back and the palm of the hand, and the stimuli from the palm may more effectively select particular neuronal groups, establishing a dominant representation of the front of the hand in the map in that part of the brain.

Should the incoming nerves be damaged, reducing or eliminating the input from the palm (as in Merzenich’s experiments), the groups that could respond to the stimuli from the back of the hand will then be able to be expressed in the absence of competition from the neuronal groups in the palm. Thus, based on the inherent variation among neuronal groups, a new representation emerges in that area of the brain. The continuity of the new map’s general activity nonetheless can still represent, in some abstracted form, the activity at the sensory receptors.

Information in the brain is distributed among many maps, and according to Edelman’s theory, there must be incessant reference back and forth among them for categorization to occur. Sounds, for example, can be categorized as speech, noise, and music; or they can be used to locate things in space. Recent research shows that such localization requires a number of interacting maps. Owls, like human beings, use sounds to locate moving animals, for instance a mouse they might attack. The important sensory clues are the different arrival times of a sound at each ear, and its intensity. Since the owl’s brain cannot directly map the different times of arrival of a sound at each ear, two initial sensory maps represent the frequencies heard by the owl: one maps those heard in the right ear and the other maps those heard in the left ear. These representations are then combined in another map where the arrival times (called sound disparities) of a given frequency in one ear are compared to those in the other ear.

The sounds made by a mouse in a field can by this means be categorized according to the disparities in sound. These disparities can be used to help determine the sources of the sounds. Specific neuronal groups within a map may be activated by a difference in arrival time between the two ears of, for example, one thousandth of a second. In itself the activity of the neuronal groups will not tell the owl’s brain the source of the sound. But the entire pattern of activity of the map, the ways in which other neuronal groups are activated as well, will represent in the owl’s brain the location of the source of the sound. This pattern will have to be extracted in a further mapping, which could for example characterize certain patterns of activity indicating that sources of sound were, say, at 30 degrees to the left, others at 60 degrees to the right, etc. Finally the mapping that has derived the place of a source of sound is connected to a visual map of space, created from the owl’s visual receptors. The visual map is thus related to a map that recategorized auditory sensory input to place sounds in space. By relating the two sensory modalities, the owl’s brain has created a general map (auditory and visual) of space and the owl can respond to a variety of sensory inputs.

A particular pattern of activity will lead to a motor response in which the owl dives for its prey. If the owl is successful, it will associate that mapping and that pattern of activity with the particular motor act of attacking. If it fails, however, it will try other responses until it finally succeeds in capturing its prey. This was shown in the series of experiments by E.I. and P.F. Knudsen in which young owls were raised with one ear plugged, thus shifting the perceived location of sound relative to its actual location. Within four to six weeks these owls learned to localize sound accurately. The owls adjusted to the altered mapping of sound apparently by rearranging their internal mappings. Recognition therefore depends on mapped and remapped patterns of activity.3

No single map contains all the information necessary for the owl’s movements, and as I have said there must be a constant reference back and forth from neuronal groups in one map to neuronal groups in the other by means of so-called reentrant connections, i.e., nerves traveling in both directions to link the maps. According to Edelman’s theory, this is how the brain creates its categories and generalizations. Of course, the owl brain may also use the initial mapping of sound frequencies for higher maps that do not represent spatial disparities, but rather the actual sequence of sounds (to identify the kind of animal making the noise, perhaps). This will eventually create other kinds of categories for sound information.

The brain has many different kinds of maps and ways of mapping other maps that categorize “inputs” in many ways. The purpose of the maps is to create perceptual categorizations that permit the animal to act in appropriate ways.4 The environments in which an animal might find itself will of course change and so the perceptual categories must also change. But this is exactly what the multiple mappings are best suited for: the maps interact with one another and constantly recategorize information. And by referring the more abstract mappings back to the primary sensory maps that have a continuous relationship with external stimuli, the brain can effectively keep track of its various regroupings of the sensory inputs.

That mappings can be related to one another without any preestablished instructions has been demonstrated by Edelman and his colleague George Reeke, Jr., who built a new kind of automaton, based on the principles of selection, to simulate the mapping activity of the brain. The automaton abstracted from the mappings of visual inputs a variety of categorizations, such as for letters of the alphabet, without having been given specific instructions to do so. This provided further evidence that the interacting maps are essential for categorizing perceptions in a selective system.

A spy sitting in a music hall might want to locate the person he just heard say, “Nine o’clock tomorrow,” and he may also want to enjoy the singer’s “Casta Diva.” One set of brain maps will locate the person who said “nine o’clock,” while another set of maps will permit him to hear the “Casta Diva” for his own pleasure. Sounds have been categorized in different ways by his brain in accordance with his adaptive needs: business and pleasure.

Later that evening the spy may have forgotten the time he overheard mentioned by the person he was shadowing during the concert. Annoyed, he hums the melody of “Casta Diva” and suddenly recalls the nine o’clock assignation. Or he might recall it when he sees the announcement for a nine o’clock movie. This suggests that memory is not an exact repetition of an image of events in one’s brain, but a recategorization. Recategorizations occur when the connections between the neuronal groups in different maps are temporarily strengthened. Recategorization of objects or events depends upon motion as well as upon sensation, and it is a skill acquired in the course of experience. We recollect information in different contexts; this requires the activation of different maps interacting in different ways from those of our initial encounter with the information and it leads to its recategorization. We do not simply store images or bits but become more richly endowed with the capacity to categorize in connected ways.

Memory as recategorization is one of the deep consequences of Edelman’s theory of neuronal group selection. In a remarkable book published in 1932, the English psychologist Frederic C. Bartlett sketched out the view to which Edelman’s work has given a precision and a physiological justification. In Remembering Bartlett wrote:

Remembering is not the re-excitation of innumerable fixed, lifeless and fragmentary traces. It is an imaginative reconstruction, or construction, built out of the relation of our attitude towards a whole active mass of organised past reactions or experience, and to a little outstanding detail which commonly appears in image or in language form. It is thus hardly ever really exact, even in the most rudimentary cases of rote recapitulation, and it is not at all important that it should be so.

It is this quality of memory that Freud, too, sought to capture. Believing that memories must leave permanent traces, and unable to see how a perceptual structure could remain open to new perceptions if it were altered by previous stimuli, he constructed a theory quite different from the view of brain function presented here.

Unable to accept that fragmentary memories may well be fragmentary, Freud assumed memories were fixed in the same way that Newton had assumed time was absolute. Einstein eliminated absolute time and thereby presented a larger view of space and time. In dispensing with fixed memories and replacing them with memory as categorization, Edelman’s theory represents a radical departure from previous thought and may well open the possibility of a broader and deeper view of human psychology.

Each person according to his theory is unique: his or her perceptions are to some degree creations and his or her memories are part of an ongoing process of imagination. A mental life cannot be reduced to molecules. Human intelligence is not just knowing more, but reworking, recategorizing, and thus generalizing information in new and surprising ways. It could be that inappropriate categorizations from damaged maps may cause psychoses, just as the inability to correlate the succession of objects or events in time may be largely responsible for the loss of specific memories in the case of amnesia already mentioned.

Of course, language is acquired in society, but our ability to use it, to constantly reconceive the world around us, is at least in part a reflection of the multiple mappings and remappings that appear to be central to brain function. Such a view reinforces the idea that no two brains can be, or ever will be, alike. Edelman’s theory of neuronal group selection challenges those who claim that science views individual human beings and other animals as reproducible machines and that science is little concerned with the unique attributes of individuals and the sources of that uniqueness. Humanism never had a better defense.5

  1. 2

    This is analogous to the process by which antibodies that have been selected are then produced in large numbers, or cloned. The two processes resemble each other in effect, but the mechanisms are different: in the brain the strengths of synaptic connections are increased; in the immune system the number of cells is increased through cloning.

  2. 3

    E.I. Knudsen and P.F. Knudsen, “Vision Guides the Adjustment of Auditory Localization in Young Barn Owls,” Science 230 (November 1, 1985), pp. 545–548.

  3. 4

    The origin of perceptual categories by neuronal group selection is in some ways analogous to the origin of species by natural selection in evolutionary time. Much as unpredictable events over a long period of time may result in the selection of certain characteristics in organisms, unpredictable environmental events in an animal’s lifetime may result in the selection of certain neuronal groups leading to the formation of perceptual categories.

  4. 5

    I would like to thank Leif Finkel for his invaluable assistance in the preparation of this article.

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