Of all of nature, with its myriad of animals, it is the mammalian brain that has proven most adaptive. It adapts during the postnatal period, and continues to adapt, learning from new experiences. In the 60’s, a series of studies demonstrated that rats, when placed in different, more complex environments, grew thicker brains and new synapses. This study showed that the once popular belief that learning and memory are additive processes involved in the formation of new synaptic connections and the strengthening of existing connections.
It has also been suggested that there is more than just additive synapses in learning. J. Z. Young, in 1964, posited that learning could be the result of eliminated neuronal connections. Years later, J. S. Albus suggested that pattern storage was accomplished by weakening synaptic weights rather than strengthening them. Richard Dawkins, the famous evolutionary biologist, speculated that the death of neurons could underlie the storage of memory.
This brings us to an intriguing, but surprising, question. How can the elimination of neurons be involved in learning and memory? One would surely think the contrary: new knowledge being stored in ever increasing cabinets of information, categorized by the sensory impression the idea or memory primarily influenced. Memories are often associative: as we saw with Little Albert, fear was created by association with a startling noise. It is difficult to imagine that the learning of a new skill, roller skating, or speaking a foreign language, the formation of new memory, such as learning the words to a song, could be made possible by the loss of synapses.
But how could a subtractive process of neuron elimination be involved in learning and memory? It is particularly difficult to understand how the learning of a new skill, such as riding a bicycle or speaking a foreign language, or acquisition of new memories, such as learning the words to a poem or song, could be made possible by loss of synapses.
We have seen that, in the development and maturation of the brain, connections are rarely used and are weakened, or eliminated, whereas those in active neural pathways are retained and strengthened. Subtractive processes make sense when one deals with an overwired, immature brain, a brain that may have twice the amount of synapses than an adult. How can this subtractive process work for a mature adult–the adult brain–that has already undergone synaptic pruning?
It is, more than likely, a process involving the addition of new synapses, in the case of learning language, or at least the reorganization of current ones, which would be necessary for the learning to take place. In dealing with this subject, there is another problem we must reconcile; (humans are quite fond of ‘how’ and ‘why’ questions) Can we understand how the brain knows which new synapse to add or modify?
There has to be some sort of synaptic change when someone attempts to learn a foreign language and many adults learn English and other languages, and this kind of learning must be the result of changes in the synaptic connections of the brain. Which synapses, or combination of synapses, will do the trick? It would appear that the brain would sort out, or try a number of new combinations and select the best. To select the best, that is, the most fit, implies a source of variation. Perhaps it works in the same manner as the initial variation of synaptic connections present in an immature brain.
A possible solution, one to which I’ve already alluded to, was proposed by Jean-Pierre Changeux in 1983 in his book, ‘L’homme Neuronal’ (Neuronal Man.) In it, Changeux suggested a type of natural selection, a ‘Darwinism’ of the synapses to account for the developing mind and what it undergoes within its cultural environment. In this manner, culture makes a progressive impression. There are 10,000, give or take, synapses per cortical neuron which are not established immediately. They propagate in different waves during periods of development: from birth to puberty. In each stage there is ‘transient’ redundancy.
He was suggesting that changes in the adaptive brain, or those occurring between birth and puberty in humans, involve the elimination of preexisting pathways, although these preexisting synapses were not necessarily all present at the same time. He hypothesized that, from birth to puberty, waves of synaptic growth would occur, in essence eliminating a useless and redundant synapse. Waves of synaptic overproduction provided variation on which ‘synaptic selection’ could operate. This learning has resulted in an absolute increase in synaptic growth and numbers over time. The growth isn’t constant, but was envisioned as analogous, repeatedly taking two steps forward, two steps back, randomly adding new synapses.
Changeux had little evidence for this hypothesis, the hypothesis that synaptic variation in the form of overproduction would ensure the elimination of synapses as part of the brain’s restructuring to permit the learning of new skills and acquisition of knowledge. Such evidence was found a few years after the publication of his book.
William Greenough and his associates, whose work on the development of the maturing rat brain, noted earlier, also conducted research on changes within the brain induced by placing adult specimens in special, enriched environments. In one study this resulted in a 20% increase in the number of synapses per neuron in the upper layers of the visual cortex. Further research demonstrated such dramatic increases were not limited to the rat’s visual cortex.
These discoveries, amid others, led Greenough’s group to suggest that waves of synapse proliferation, first demonstrated by Changeux, could be elicited by the complex demands on the adult brain in new, challenging environments. This process is referred to as ‘experience dependent’ development. It depends on the environment, and more especially, triggering the formation of new synaptic growth on which the selection pressure can act.
Greenough’s conception of the adult brain’s ability to learn and form new memories offer an appealing solution to problems concerning the additive and subtractive process. The additive component results in the blooming of new synapses in response to the animal’s attempt to control new and complex environment. The brain appears to know what part of itself is involved in synapse-construction. It need not, indeed it could not, know which connections to make.
Forming a large variety of new connections, the brain can select the combinations that work best, in the same way that the immature, developing brain retains useful connections from the initial oversaturation of unfit synapses. The long-term result is overall addition to the number of synapses. The selection process fine-tunes the connections are ‘selected’ and retained, while less useful ones are eliminated.
Clear evidence exists for synaptic increase in learning, however, there is no such evidence in mature learning for an overproduction of synapses, those to be pruned away. However, research has found evidence for an overproduction of dendrites in mature rats, when healing from a brain injury, suggests synaptic overproduction may be involved. These findings correlate nicely with subtractive synapse finding on brain maturation and provide a solution for how the brain could know exactly which new synaptic connections to establish to enable it to acquire new knowledge, skills, and memories.
There are few neuroscientists who have opted for the selectionist approach to their research and theorizing. Changeux and Greenough, and their associates are not the only ones whose research suggests that the adult brain develops, learns, and does this through a cumulative process of neural variation and selection. This theory has been embraced and given support by several neuroscientists. William Calvin refers to the brain as a ‘Darwin machine’ that follows the general constituents of natural selection on a neuronal level: ‘The brain makes lots of random variants by brute force, bashing about and then select the good ones.’
Gerald Edelman, awarded the Nobel prize in 1972 for his research on the chemical structure of antibodies in the immune system. His remarkable output, having written several books describing the aspects of his, ‘Neuronal group selection theory’ of brain development and learning, a selectionist process he refers to as ‘Neural Darwinism.’ Noted psychologist and neuroscientist Michael Gazzaniga, who is best known for his ground-breaking research on humans with split brains, recently embraced a selectionist account of brain functioning and development.
Current research is under way to determine whether unambiguous physical evidence can be found for the overproduction and elimination of newly formed synapses in the adult brain in response to environmental changes. Such a finding would place the brain alongside the immune system as another striking example of how cumulative variation and selection processes during the lifetime of an organism make it possible to adapt to complex, changing environments.
We have seen that understanding the adapted complexity of the human brain involves three questions: how did the brain become a biological organ? How does it developed from a fertilized egg into a mature brain? And how is it able, when mature, to rewrite itself to form and adapt to environmental changes?
A lot of work must first be done before we have the detail answers to these questions. Substantial progress, however, has already been made; we are living in an age where scientists are spending more and more work on the brain and much more time on defining its methodology of function. To a large extent, progress has consisted of rejecting providential and instructionist explanations for these puzzles to fit, and finding evidence in favor for selectionist explanations. The process of cumulative blind variation and selection working over millions of years is not only the only reasonable theory for the biological evolution of the brain, yet we find it has surfaced again and again in different but recognizable forms as a working theory for the brain’s embryonic growth and continued development during its–relativity brief–lifetime.
It is here that the ‘Darwinism of the synapses’ replaces the ‘Darwinism of genes.’ To close the circle, it should be noted, that a consequence of joint effects among-organism genetics and within-organism synaptic selection is the brain’s understanding of itself and the selection pressures responsible for its extraordinary abilities.