Sunday, November 6, 2011

Rita Carter, Mapping the Mind (Rev. 2010)

Five postings in 2011 on books and subjects related to the mind and brain, and I was consciously aware of that fact and a desire to move on to something new. But as I wrapped up V.S. Ramachandran's The Tell-Tale Brain (see previous post), his remark that as neuroscientists map the brain they are "grouping their way toward the periodic table of elements" reminded me that a book on The Bookshelf that I had purchased last year, Rita Carter's Mapping the Mind, was waiting to be read. I found this book at the bookshop at the conclusion of the American Museum of Natural History's exhibit on The Brain. I recalled that what impressed me toward a purchase was the exquisite drawings of the brain, many of which included arrows to illustrate the interconnectedness of specific brain regions to explain a specific neuronal process. For those who are not practicing neurologists, a picture can nicely supplement a thousand words.

By the end of Mapping the Brain, I wondered if this should have been the first book I ever read on the brain. Would I have better appreciated all the other material I have read on this subject if I had already read this book? I can't answer that, but studying Rita Carter's text after I had read these other books, many of which are discussed or mentioned in prior posts, was facilitated by my prior exposure to the subject. Either way, Mapping the Brain is a good overview and introduction to the brain and a good review as well.

Ramachandran's analogy that neuroscience's understanding of the brain is moving in the direction of establishing something akin to chemistry's periodic of table of elements is best left as a metaphor rather than a suggestion of equivalence (as I think the statement was intended). It is fair to say, like the periodic of table of elements, that the brain is organized, but it is not sequential in the same sense that the elements can be organized sequentially according to their atomic weight or related in their properties as part of one of 18 different groups. While thinking about this, it crossed my mind that evolutionary age would be one way of sequentially organizing the parts of the brain. Antonio Damasio did something like this in Self Comes to Mind (see April 8, 2011 post), describing the sequential evolution of the brain stem, the limbic system, and ultimately the cerebral cortex. But the brain is indeed very complex, as Carter notes in her closing paragraph, when she says that "today's mind voyagers are discovering a biological system of awe-inspiring complexity." One could also try to organize the parts of the brain sequentially, starting with a particular sensory input, and follow the connections to other parts of the brain to conscious awareness and action (or unconscious action, as the case may be) that ensues. In the end, however, that effort would not be particularly useful given the multi-layer network of sensory inputs that are processed simultaneously, including the presence of variable emotional reactions in connection with a particular sensory input that could stimulate a different behavioral outcome.

Both Carter and Ramachandran caused me to question a statement I made in a prior post (June 12, 2011 post) while discussing Michael Shermer's The Believing Brain. I wrote:

"I strongly suspect that if we dissected human brains and a network of connected neurons from a representative sample of humans, we would find a very high level of near identity among brains. There will be some differences due to DNA, and there will be some pathological differences as well, perhaps caused during embryonic development. But I believe that by and large we will find that human brains, neuron by neuron, are organized and folded and layered in substantially identical ways." There is substantial truth in my remark --- I did not say identical, but I did say "near identity" and "substantially identical." These words came at the risk of maybe overstating the case. Carter writes, "Human brains are constructed along fairly standard lines and so we all tend to see the world in a fairly standard way." And as I noted, there are differences due to DNA, pathological injury, and embryonic development. But I overlooked perhaps the largest exception --- experience (nurture) and its impact on memory -- and what is referred to as synaptic plasticity. And it is long-term memory -- enhanced by repeated experiences in some that others do not share --- that gives rise to our autobiographical self and what makes each one of us unique. So while we may "tend to see the world in a fairly standard way," Carter notes:

"Every brain constructs the world in a slightly different way from any other because every brain is different. The sight of an external object will vary from person to person because no two people have precisely the same number of motion cells, magenta-sensitive cells, or straight line cells. . . . An individual's view is formed both by their genes and by how their brain has been moulded by experience. Musicians, for instance, have brains which are physically different from others and which work differently when they play or hear music. . . . Extraordinary individual ways of seeing things may also arise from strange 'quirks' of brain development. Albert Einstein, for instance, had a very oddly constructed brain which might account for his astonishing insights into the nature of space and time."

Carter's treatment of language pretty much follows that of Ramachandran. And her treatment of memory restates much of what Antonio Damasio (see April 8, 2011 post) and Daniel Schacter (see September 20, 2011 post) discuss, but I still learned something new. For example, "Episodes that are destined for long-term memory are not lodged there right away. The process of laying them down permanently takes up to two years. Until then they are still fragile and may quite easily be wiped out. It is this replay from hippocampus to cortex and back again -- a process known as consolidation, that slowly turns fleeting impressions into long-term memories. . . Much of the hippocampal replay is thought to happen during sleep. Recordings from hippocampal cells show them engaging in a 'dialogue' with cortical cells, during which they signal one another, back and forth, in a call-and-reply formation. Some of this is known to take place during the 'quiet' phase of sleep, when dreaming, if it occurs at all, and is vague and instantly forgotten. Until memories are fully encoded in the cortex they are still fragile and may quite easily be wiped out. And even when they are established, they are not fixed. A memory is not, in fact, a recollection of an experience but the recollection of the last time you recalled the experience. Hence our memories are constantly changing and redeveloping. The process by which a memory changes is more or less the same as the consolidation process that lays it down for the first time. As we will see, each time we recall something, it is changed a little because it becomes mixed up with things that are happening in the present. Reconsolidation is a process by which this slightly altered memory effectively replaces the previous one, writing over it, so to speak, rather like re-recording over a rewritable DVD." I mentioned this phenomenon in the September 20, 2011 post discussing Daniel Schacter's discussion of the consistency bias, whereby the brain infers past beliefs from our current state and the reference to Joseph LeDoux's discussion of reconsolidation. Carter explains it better.

I also learned that not all memories are stored in cortical areas. While long-term memories are initially stored in the hippocampus, as described above, over the course of roughly two years they are transferred to the cortex and the hippocampus is no longer required for their retrieval. These memories are distributed in the same parts of the brain that encoded the original experience. So sounds are found in the auditory cortex, taste and skin sensory memories are found in the somatosensory cortex, and sight in the visual cortex. But procedural -- "how to" --- memories are stored outside of the cortex, in the cerebellum and putamen, and fear memories are stored in the amygdala.

Carter also addresses, albeit briefly, the subject of imagination, which I have touched on in several previous postings (see, for instance, July 30, 2011 post and May 22, 2011 post) and describes the connection to memory. "Our ability to conjure up scenarios which have not actually happened is prodigious. Imaginative capacity runs along a spectrum from the mundane skills required to envisage what your supper might taste like if you combined the onion, mushrooms, and left over chicken in the fridge with some curry sauce, through to the awe-inspiring visions of artists, writers, and excitable children. Even the humblest of these skills outranks the abilities --- as far as we can tell --- of every other species. . . . At first sight memory and imagination seem quite distinct: the first in concerned, after all, with what happened already whereas the second is all about what has not. But recent studies show that imagination is wholly dependent on memory, because memories are its building blocks. When we imagine something happening we root around in our memory and come up with experiences which seem likely to recur, then combine them, chop them, shake them and blend them until they come out as something entirely different." In my view, this is cannot be unrelated to the process of reconsolidation that I mentioned above and some of the biases that other writers have described (see September 20, 2011 post and June 12, 2011 post) whereby memories are altered.

A text box entitled "Remembering the Future" by Eleanor Maguire is inserted by Carter, which perceptively quotes Lewis Carroll's Alice in Wonderland: "It's a poor memory that only works backwards." Maguire cites an MRI study that found recalling past experiences and imagining future possible ones activated a network of brain regions in common including the hippocampus. She notes a new theory of 'scene reconstruction,' which allows for the internal rehearsal of events or scenes, which underpins the process of creating a simulated event. Maguire writes: "[I]n humans, the use of this scene reconstruction process goes far beyond simply predicting the future, to general evaluation of fitness for purpose and ultimately creativity. For example, a scriptwriter or novelist who is writing a passage in a film or book may play out the whole scene using their construction system, not with the idea of predicting the future, but instead for the purpose of evaluating its aesthetic suitability." This is the sort of discussion I was hoping to find in The Tell-Tale Brain (see previous post). What is missing from her discussion is the identification of the parts of the brain involved in imagination, although the text seems to identify the hippocampus as one candidate for assembling disparate memories. Also missing is the evolutionary basis for this ability. Obviously, the ability to plan for the future has survival value, and our planning capacity is lodged in the frontal cortex, where working memory occurs. The building blocks for understanding creative imagination are before us, and understanding a collateral capacity --- the capacity for deception, including self-deception --- by imaginatively rearranging memories and treating them as factual, when they are not, needs to be understood as well. Part of this is reflected in the previous discussions of mental biases. Carter notes that the subject of belief and non-belief is not unrelated to how the brain treats statements it believes to be true and those it believes to be false. Research, she says, shows that truth telling appears to be the default position for the human brain, and that telling lies involves extra cognitive effort. I am not sure this is entirely true, as bias mechanisms appear to be short-cuts for resolving conflicts in our memories.

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