Shufflebrain

Today there are entire libraries, whole university departments, and specialized learned societies devoted exclusively to storing, disseminating, and promoting the wisdom of the "neurosciences." So vast is knowledge about the nervous system that the study of human neuroanatomy alone requires a completely separate course. Facts abound on the brain's chemical composition, anatomical organization, and electrophysiological activities. The main routes for incoming sensory messages, for example, have been plotted and replotted--the signals enabling us to see a sunrise, hear a sparrow, smell a rose, taste a drop of honey, feel the sting of a wasp, appreciate the texture of another human hand. The images of these words, for instance, land on the retinas of the reader's eyes and trigger well-worked-out photochemical reactions, which, in turn, detonate electrical signals within the receptor cells--the rods and cones. The retina itself begins sorting, integrating, and encoding the signals into messages, which it transmits through highly specific routes via the optic nerves and optic tracts to relays in the core of the brain. From the relays, the message moves to specific cells in what are called the occipital lobes of the cerebrum, and there establishes point-for-point communication between loci out in the visual fields, and particular input stations in the brain.

Much is known, too, of outflow pathways used in carrying direct orders to the effectors of our overt behavior--the muscles and glands that let us walk, talk, laugh, blush, cry, sweat, or give milk. In spite of admittedly vast gaps among the facts, enough is known today to fill in many of the blanks with plausible hypotheses about circuits used in language, emotions, arousal, and sleep--hypotheses for many of our actions and even a few of our feelings and thoughts. Damage to a known pathway yields reasonably predictable changes or deficits in behavior, perception, or cognition. Neurological diagnoses would be impossible, otherwise. For example, a person with partial blindness involving the upper, outer sector of the visual field, with accompanying hallucinations about odors and with a history of sudden outbursts of violence, quite likely has a diseased temporal lobe of the cerebrum --- the forward part of the temporal lobe on the side opposite the blindness, in fact. Or a person who suffers a stroke, cannot speak but understands language, and is paralyzed on the right side of the body almost assuredly has suffered damage at the rear of the cerebrum's frontal lobe--the of the left frontal lobe, to be precise.

For a quick pictorial neuroanatomy lesson, go here.

Up to a point, in other words, the brain fits neatly and simply into the anatomical scheme of things. But throughout history, the battle-ax, shrapnel, tumors, infections, even the deliberate stroke of the surgeon's knife, have paralyzed, blinded, deafened, muted, and numbed human beings, via the brain, without necessarily destroying cognition, erasing memory, or fractionating the mind. It wasn't that anatomists couldn't link specific functions to particular parts of the brain. Far from it. But when we reached for the dénouement, for an explanation of the most pivotal features of the brain, the structural argument teetered under the weight of contradictory evidence.

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Consider a paradox about vision known as macular sparing. No part of the human brain has been worked on more exhaustively and extensively than the visual system. Nor, seemingly, could any structural realist ask for a more explicit relationship between form and function than one finds there. Every locus in our fields of view corresponds virtually point-for-point with microscopic routes through our visual pathways. As you can demonstrate to yourself with gentle pressure on an eyelid, you form an image of right and left fields in both retinas. (The nose blocks the periphery of the eye's inner half and the right eye's outer half; vice versa for the right field). For optical reasons however, the images of the field do a 180-degree rotation in projecting onto the two retinas. Thus the left field registers on the left eye's inner half and the right eye's outer half; vice versa for the right field. The fibers from the retina, which form the optic nerve strictly obey the following rule: those from the inner half of the retina cross to the opposite side of the brain; those from the outer half do not. Thus all information about the visual fields splits precisely down the middle and flashes to the opposite side of the brain. Corresponding fibers from the two eyes join each other in the centers of our heads (at a structure known as the optic chiasm) and form what are called optic tracts--the right tract carrying messages about left field exclusively, and the left tract carrying information about right field. If an optic tract is totally destroyed, we become blind to the entire opposite visual field.