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A Brain Structure Looking for a Function

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A Brain Structure Looking for a Function CONSCIOUSNESS REDUX HUB OF SENTIENCE A Brain Structure Looking for a Function Could a thin, enigmatic layer of nerve cells be a key component of the networks generating conscious experience? Point to any one organ in the body, and doctors can tell you something about what it does and what happens if that organ is injured by accident or disease or is removed by surgery—whether it be the pituitary gland, the kidney or the inner ear. Yet like the blank spots on maps of Central Africa from the mid-19th centu- ry, there are structures whose functions remain unknown despite whole-brain imaging, electroencephalographic re­­ cordings that monitor the brain’s ca­­ cophony of electrical signals and other advanced tools of the 21st century. Consider the claustrum. It is a thin, irregular sheet of cells, tucked below the neocortex, the gray matter that by white matter—the tracts, or wire reveal that it is a neural Grand Central allows us to see, hear, reason, think and bundles, that interconnect cortical re­­ Station. Almost every region of the cor- remember. It is surrounded on all sides gions with one another and with other tex sends fibers to the claustrum. These brain regions. The claustra—for there connections are reciprocated by other ) are two of them, one on the left side of fibers that extend back from the claus- Koch BY CHRISTOF KOCH the brain and one on the right—lie below trum to the originating cortical region. ( the general region of the insular cortex, Neuroanatomical studies in mice and CABE C M Christof Koch is chief underneath the temples, just above the rats reveal a unique asymmetry—each scientific officer at the Allen ears. They assume a long, thin wisp of a claustrum receives input from both cor- Institute for Brain Science SEAN shape that is easily overlooked when tical hemispheres but only projects back ); in Seattle. He serves on Scientific American Mind’ s inspecting the topography of a brain to the overlying cortex on the same side. board of advisers. image. Whether or not this is true in people is illustration Advanced brain-imaging techniques not known. Curiouser and curiouser, as ( Send suggestions for column topics to that look at the white matter fibers Alice would have said. [email protected] coursing to and from the claustrum Unlike most other parts of the brain, JON HAN 24 SCIENTIFIC AMERICAN MIND NOVEMBER/DECEMBER 2014 Exploring the riddle of our existence there are no reliable case studies of pa­­ all agree that one of the defining proper- but a fleeting moment until the next neu- tients with selective destruction of one ties of any subjective experience is that ronal assembly comes into being and a or both claustra from stroke, viral infec- it is unified. No experience can be re­­ new experience supersedes the old one. tion or other calamity. Lesioning the duced to independent components. Looking at the far-flung two-way structure in laboratory animals is chal- Every experience is irreducible. When I connections between the claustrum and lenging given its thin and elongated look at my wife’s face, I do not see two the cortex, Crick and I—for at that time nature. For the same reason, brain imag- eyes in a black-and-white picture with a in 2004, I was working closely with him ing has not been very useful: the small- disembodied layer of blue superimposed and had been for 16 years—hypothe- est spatial features distinguishable on top. No, I perceive her blue eyes as sized that this superhub of neuronal through positron-emission tomography one integral and seamless whole. Nor do activity could be pivotal for conscious- or functional MRI, two of the most I experience my Bernese mountain dog ness. Because every region of cortex pro- widely used imaging techniques, are two doing funny things with her snout while jected to its associated claustral target area, and this neural communications hub reciprocated the connection, the claustrum could serve as an integrator FRANCIS CRICK AND I HYPOTHESIZED THAT for crisscrossing electrical signals, pro- vided that all of this information could THIS SUPERHUB OF NEURONAL ACTIVITY be freely admixed within the structure. COULD BE PIVOTAL FOR CONSCIOUSNEss. We endlessly discussed various neuro- anatomical and biophysical means for the claustrum to achieve this integration and wrote a manuscript. to three millimeters across, bigger than a loud noise fills the room; no, I hear her Francis knew that he only had a lim- the claustrum’s width. And because it is bark. The experience of seeing the word ited amount of time left; he had end- embedded within white matter and “honeymoon” is not reducible to the stage colon cancer. He called me on the sandwiched between two very active experience of seeing “honey” on the left way to the hospital, calmly telling me neuronal tissues—below the neocortex and “moon” on the right. not to worry about the manuscript fol- and above the putamen, part of a larger We know that different groups of lowing our last brainstorming session region, the basal ganglia, lodged deep neurons become active in response to because he was going to make correc- within the brain—it is problematic to such commonly encountered features as tions to it (which he did, dictating them unambiguously pinpoint changes in colors and motion, faces and dogs, to his secretary from the clinic). Two blood flow to the claustrum and not to words, sounds, and so on. These cells are days later, on his deathbed, Francis hal- these nearby, large structures. dispersed among the 16 billion neurons lucinated a debate with me about the making up the cerebral cortex. Together role of the claustrum’s connection to Enter the Dragon the active and inactive cells give rise to consciousness, a scientist to the very In biology, a reliable guide to under- a conscious experience. Furthermore, end. The paper was published a year lat- standing function is to study structure. we know from introspection that what er in the world’s oldest scientific journal, Francis Crick and James Watson proved we are conscious of is in constant flux. the Philosophical Transactions of the this idea spectacularly in 1953. They Distracted by the sight of a passing Royal Society. inferred the key function of DNA, the motorboat on the lake outside my house, molecule of heredity—that is to say, stor- I am about to turn back to writing my Enter the Electrodes ing and copying genetic information— article when I suddenly recall that I In the intervening years, a handful of from its double-helical chemical structure. promised to pick up dog food, and then studies further delineated the molecular Half a century later Crick, by then biolo- my attention shifts without warning to neuroanatomy of the claustrum in ro­­ gy’s most respected sage, tried his hand at Richard Wagner’s “Liebestod” playing dents and a crude map of its connections the same game, linking a structure—the on the radio. Each of these sights, in people. One investigation focused on claustrum—to a function—the emergence sounds, memories or thoughts re­­quires the role of the claustrum in integrating of integrated, conscious experience. that the underlying electrical and chem- visual and auditory stimuli. Using mi­­cro- Whereas scholars of consciousness ical activity of a privileged set of neurons electrodes that recorded the electrical disagree about many aspects of this is rapidly bound to give rise to an inte- activity in awake monkeys, the investiga- most mysterious phenomenon, virtually grated conscious experience that lasts tors confirmed that part of the claustrum MIND.SCIENTIFICAMERICAN.COM SCIENTIFIC AMERICAN MIND 25 CONSCIOUSNESS REDUX A possible nexus of consciousness, the claustrum reveals itself through high-definition fiber tractography as a locus at which connections arrive from—and extend outward to—distinct regions of the cerebral cortex. tended to respond more to visual stimu- versity, the clinical team made a remark- stimulation started—behaviors such as li, whereas one of its nearby re­­gions was able observation: electrically stimulat- making repetitive tongue or hand move- sensitive to tones. But no in dividual neu- ing a single site with a fairly large cur- ments or repeating a word. Koubeissi rons responded to both visual and audi- rent abruptly impaired consciousness in was careful to monitor electrical activi- tory events, arguing against a multisen- 10 out of 10 trials—the patient stared ty throughout her brain to confirm that sory role for the claustrum, thereby leav- blankly ahead, became unresponsive to episodes of loss of consciousness did not ing it bereft of any obvious function. commands and stopped reading. As accompany a seizure. This seeming impasse may have soon as the stimulation stopped, con- Two aspects of this patient’s case had changed with a single dramatic case sciousness returned, without the patient never been seen before. First, no abrupt report. A 54-year-old woman who had recalling any events during the period and specific cessation and resumption of uncontrollable epileptic seizures had electrodes implanted deep within her brain to help pinpoint the exact origin University Pittsburgh of of her seizures. During this procedure, ELECTRICAL STIMULATION OF A SINGLE SITE electrodes can triangulate the focal area where the seizure originates so that it IN THE BRAIN IMPAIRED CONSCIOUSNESS IN IRANDA M can be surgically removed. They can EVERY ONE OF 10 EXPERIMENTAL TRIALS. also inject electric current to help map the brain, identifying areas responsible ERNANDEZ- F . for important functions such as speech C or movement and thus sparing them dur- when she was out. Note that she did not consciousness have previously been UAN ing the surgery.
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