Self Comes to Mind

individual experiences in unique environments get to work on that first connection pattern, pruning it away, making certain connections strong and others weak, thickening or thinning the cables in the network, under the influence of our own activities. Learning and creating memory are simply the process of chiseling, modeling, shaping, doing, and redoing our individual brain wiring diagrams. The process that began at birth continues until death makes us part with life, or some time before, if Alzheimer’s disease disrupts the process.
    How does one uncover the design of the wiring diagrams? Until quite recently, research on this problem required brain specimens, largely postmortem material from either humans or experimental animals. Samples of brain tissue would be fixed and stained with identifiable dyes, and very thin slices of tissue could be analyzed under the microscope. There is a venerable tradition of such studies in experimental neuroanatomy, and they have yielded most of the knowledge we have today about the brain’s networking. But our knowledge of neuroanatomy remains embarrassingly incomplete, so there is an urgent need for such studies to continue, making use of considerable progress in the available stains and in the power of modern microscopes.
    Recently, new possibilities have opened with the use of magnetic resonance methods in living humans. Noninvasive methods such as diffusion imaging are allowing us a first glimpse of in vivo human connection networks. Although the techniques are still far from satisfactory, they promise to yield fascinating revelations.
    How do the billions of neurons inside a human brain and the trillions of synapses they form manage to produce not just the actions that constitute behaviors but also minds—minds of which each owner can be conscious and minds that can give rise to cultures? To say that so many neurons and synapses do the job by massive interactivity and by the ensuing complexity is not a good answer. Interactivity and complexity must surely be present, but interactivity and complexity are not amorphous. They derive from the varied designs of local circuit arrangements and the even more varied ways in which such circuits create regions and regions become affiliated in systems. How each region is made, internally, determines its function. A region’s location in the overall architecture is important too, because its place in the global plan determines its partners in the system—the regions that talk to a particular region and to which it talks back. To make matters even more complicated, the opposite is also true: to a certain extent the partners that it interacts with determine where its place is going to be. But before we go any further, we should give a brief account of the materials used to construct brain architecture.
BRICKS AND MORTAR
    The mind-making brain is made of neural tissue, and neural tissue, like any other living tissue, is made of cells. The principal type of brain cell is the neuron , and for reasons that I alluded to in Chapters 1, 2, and 3, the neuron is a distinctive cell in the universe of biology. Neurons and their axons are embedded —suspended might be a better term—in a scaffolding made up of another type of brain cell, the glial cell . Besides providing neurons with physical support, glial cells also provide part of their nourishment. Neurons cannot survive without glial cells, but everything indicates that neurons are the critical brain unit as far as behavior and mind are concerned.
    When neurons use their axons and send messages to muscular fibers, they can produce movements; and when neurons are active within very complex networks of map-making regions, the result is images, the main currency of mental activity. Glial cells, as far as we know, do nothing of the sort, although their full contribution to the operation of neurons has not been fully elucidated. On a somber note, glial cells are the origin of the most deadly brain tumors, the gliomas, for which there is no cure to date. Even worse, for reasons that are entirely unclear, the incidence of malignant gliomas is rising worldwide, unlike practically all other malignancies. The other common origin of brain tumors is the cells of the meninges—the skinlike membranes that cover brain tissue. Meningiomas tend to be benign, although, by dint of their location and unchecked growth, they can compromise brain function seriously and are anything but innocent.
    Each neuron