Trees of the Brain Roots of the Mind
by Giorgio A.Ascoli
The MIT Press, Cambridge, MA, 2015
248 pp., illus. 48 col. Trade, $30
Reviewed by Florence Martellini
Have you ever wanted to read about a scientific explanation for the brain - mind relationship? Trees of the Brain Roots of Mind is a book that aims to be both an introduction and a reference work on neuroscience. It is pleasantly and clearly written and illustrated; even if you are a novice, you will be able to enjoy it, but you will need a genuine interest in the field, stamina, and perseverance. The more expert readers will also find plenty to chew on, as this is a technical book. Giorgio Ascoli, the author, thoroughly and rigorously attempts to demonstrate scientifically how the activity of the neurons gives rise to our inner lives, our mental states. That body and mind are interconnected, that each of us perceives reality differently is no news for the artist, the philosopher, or the psychologist. The challenge, though, is in offering a scientific reason, in this case, concentrating on the micro level through the discipline of neuroscience. We can visualize the brain and its components but how exactly does this overwhelmingly complex machine engender our mental states?
The brain-mind relationship is far from being resolved; theories exist, but they are grossly incomplete. This book, bounded by the technological advancements at this point in time, claims that this relationship must ultimately be explained in terms of the structure, activity, and properties of the brain cells. A theoretical perspective on how anatomy and physiology engender our mental states is organized around three principles of how the brain and mind relate. These scenarios show how mental phenomena, emotional feelings and intuition are rooted in the brain by the neurons, which process, transmit and store information.
The author lays the foundation with a definition of a conscious mind: "it is what [the types of brain activity, that feel like something] feel like", explaining that feelings are observable and are linked to some types of brain activities. So, perhaps, the challenge would be to identify these specific activities (linked to feelings) to further studies on human consciousness. The first extremely useful and highly technical three chapters set out the basic functioning of the brain and how the information is transferred within and between neurons. Briefly, the latter are composed of the dendrites (receivers of information and computer powerhouse), the axons (simply transmit information) and the soma (body cell of the neuron from which axons and dendrites stem). Where the dendrites and axons belonging to different neurons overlap, they create synapses to transfer information between neurons. Then, the following chapters are organized around three principles of the brain's functioning mental states correspond to patterns of electric activity in the nervous system:
1. Learning is the acquisition of the ability to experience a previously inaccessible mental state and, knowing something is the ability to experience the corresponding mental state
2. The relative position of axons and dendrites in space determines our learning and how experience may or may not shape our memory.
The first principle is accepted by the majority of active neuroscientists. The author adds that, actually, electric activity can be compared to neuronal spikes (polarization and depolarization of the neuron), which are the carrier of information within the neurons. These electrical spikes cause biochemical changes that allow the passage of information between neurons in the areas of proximity, called synapses. So, mental states correspond to patterns of spikes firing or not. In effect, most mental states are made up of a sequences of shorter states e.g. a car approaching gives rise to a different pattern when far away or closer to you.
The connectivity within the brain, or connectome, can be analysed at both macro and micro levels. At macro level, we are looking at the connections between different regions of the brain, also known as projectome. At micro level, or cellular, we are looking at the connections between individual neurons, based on the activity of the synapses, also known as synaptome. The projectome is useful to quantify the brain activity but it provides only a snapshot of the corresponding mind's repertory. Physical connectivity of neurons does not by itself select mental states; the biophysical properties of the network (the activity of synapses) must also be taken into account to determine which firing patterns are possible and which are not. Why is that? Because each neuron is functionally connected to others, the firing of the first one systematically increases or decreases the probability that those connected to it will fire immediately afterwards, thus defining, through this pattern, what a person "knows". Associating the complete neuron-level physical connectome of a brain with the synaptome, which would show the neural "firing patterns", is more rigorous than doing the same with the physical connectivity map alone.
The second principle is about learning. We continuously learn as a consequence of our experience. Learning is different from the experience of reading a book; most content we read tends to be forgotten. Learning is the capacity to retrieve that content at a later time, not the experience of retrieving the knowledge; even without having to take any exam, we still have learned that material. How does this translate at neuron level? For the second principle, which meets some resistance because the proof of structural plasticity in the connection of synapses is recent, learning is the acquisition of the potential to represent a previously unknown mental state, corresponding to the change of connections in circuits of neurons. Up until recently, learning was thought to be a change in the strength of synapses rather than their presence or absence. However, the author claims that it is not only the effectiveness of synapses in allowing communication between two neurons that can vary, but also the connectivity map among neurons, i.e. the presence or absence of synapses. Therefore, the relationship between structure and activity in the brain is one of reciprocal cause and effect. This is what the author defines as plasticity.
We continuously experience mental states and with each experience the network of our brain changes slightly; as the Greek philosopher Heraclitus said, "everything is changing". Therefore, the continuous flow of activity steadily sculpts and re-sculpts the connectome and its synaptic weights, from before birth to death. Plasticity of neurons and synapses accompanies all activities and learning is a structural re-arrangement of the connectivity in the neuronal network. Much of what we learn and know is available to our mind, but we never "use" it or, simply, forget it. That implies that suitable synapses were in place in the brain to store this knowledge at an earlier time, but they since succumbed to rewiring. We are surrounded by events we fail to notice. Hence, we only learn a small proportion of the potential knowledge we are continuously exposed to.
If brains are reconfigured through experience, when or why do we learn something and when or why we do not? We tend to recall together thoughts or events that occurred together; the "fire together, wire together" of neurons. This means that axons and dendrites must also share the same space to be able to overlap. So, the third principle states that the relative position of axons and dendrites in space determines our learning and how experience may or may not shape our memory. However, since each mental state is represented by "cell assemblies" of neurons, there is no one-to-one correspondence between thoughts, concepts, feelings and individual neurons. Hence, the axons-dendrites overlaps, the existing connectivity, the potential to learn and the knowledge, explain why experts can grasp new concepts in their discipline much faster than novices. Thinking depends on what is known and what is known is a result of what is learned. Thinking, learning and knowing are intertwined.
As new learning is gated by prior knowledge, i.e. the connectome, so how can we acquire new knowledge? The author compares individual neurons to trees and the brain to a forest, where the branches of the individual trees come into close proximity. Although the vast majority of the neuronal arborization in an adult brain is fairly static, at any one time a few of the trees wiggle their limb by tiny bits; these small movements appear to be stimulated by repetition and rehearsal. Over time, they can amount to a comprehensive reorganization of the network, remoulding the blueprint of the machinery that can lead to certain cognitive or behavioral changes. Both the major rearrangements and the adaptation of the brain are a constant flux, nearly undetectable but nevertheless happening. These changes correspond to slow alterations of expertise and require persistent repetition.
By modifying the axons-dendrites overlaps, these arbor rearrangements fundamentally affect the capacity to form new synapses and thus the ability to learn further information. However, the regenerative ability of the brain is limited: the only region of the human brain that keeps producing neurons throughout adulthood is the hippocampus, responsible for learning and memory. The author also explains that each individual neuron is different from every other and because different individuals live different lives, even if they started with exactly the same connectome, they would diverge into distinct shapes and networks.
After this intense 'neuronal juggle', the author acknowledges that the application of this theoretical framework presents enormous challenges. We know enough about the functioning of the nervous system but the current technology puts limitations to measuring seamlessly all neural connections and communications in a brain. The author reviews the various measuring equipment weighing out their pros and cons and suggesting ways to overcome their limitations. But the technological challenge is huge; fortunately, scientific progress continues.
Supposing we could map all the individual connections in a brain, then what? An understanding of how the brain engenders the mind, the roots of the human soul, would have profound implications for religion, philosophy, science and nature. Will scientists' observation skills ever render visible humanists' intuition? It sounds though the further scientists venture into the micro world of the human machine the more they realize and appreciate its complexity and, beauty.