THE BRAIN FROM TOP TO BOTTOM

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The Sense of Self

Brain Imaging

"Grandmother Cells", or Synchronous Discharges of Neurons?

Brain Rhythms: The Oscillations That Bind

The intralaminar nuclei of the thalamus have neurons with long axons that send projections throughout the entire cortex. In return, the neurons in all the areas of the cortex that receive these projections send their own projections back to the intralaminar nuclei, thus creating a possible feedback loop.

The neurons of the intralaminar nuclei fire at a frequency of about 40 Hz, and Llinas believes that their rhythmicity is the source of the rhythm that he detects at the surface of the cortex. At every cycle, a wave would originate in the intralaminar nuclei and spread out through the entire cortex, somewhat like the way that the sweepline on an old-fashioned radar screen illumined all the objects that it passed through in a cycle.

Though cases of brain damage affecting only the intralaminar nuclei are rare, we do know that if these nuclei are damaged, the individual falls into a coma, which supports the idea that these structures play a role in conscious phenomena.

The human brain has several different oscillatory states that correspond to various functional states. For example, when the neurons of the thalamus are firing at a rate of two cycles per second (2 Hz), the brain is in a state of deep sleep. At 10 Hz, the brain is awake but not paying any attention to the outside world. And at 40 Hz, the brain is either awake and attentive, or in the process of dreaming.

From the standpoint of the thalamocortical system, the general functional states present during REM sleep and during wakefulness are fundamentally equivalent, even though the ways that the brain processes sensory and cortical information in the two states are different.

REM sleep may thus be regarded as a modified attentive state in which the attention is diverted from the sensory inputs and instead focused on memory.

As a corollary, wakefulness may be regarded as a state of dreaming modulated by the constraints imposed by specific sensory inputs. Thus, to paraphrase Llinas, people’s waking lives would be the equivalent of dreams guided by their senses.

NEURONAL ASSEMBLIES AND SYNCHRONIZATION OF BRAIN ACTIVITY

The famous“binding problem”—the problem of how we mentally represent distinct objects, each with its own specific characteristics—is at least partly resolved by the phenomenon of neuronal synchronization around 40 Hz. But that does not tell us how the representation of a given object enters our consciousness while the representations of others remain unconscious. In other words, it does not tell us what determines which of the numerous assemblies of neurons oscillating at various frequencies (and corresponding to various representations of different objects) is going to come to our conscious attention.

In the 1990s, Rodolfo Llinas and his colleagues performed a series of detailed studies on thalamocortical interactions. From these studies, they developed a theory that simultaneously integrates the data from the conscious state of waking and from dreaming, addresses the binding problem, and provides a criterion for determining which conscious representation is going to be selected at any given time.

Though Llinas’s model is close to several others, it is unique in that it proposes an original solution to this last question. Llinas and his collaborators developed their model using a highly sensitive brain-imaging device called a magnetoencephalograph (MEG), which measures the electrical currents in the brain indirectly (follow the Tool module link to the left ). Using an MEG, Llinas observed phased oscillations running from the anterior to the posterior part of the cortex. Each of these waves lasted about 12.5 milliseconds (ms) and was followed by a rest interval of 12.5 ms, for a total time of 25 ms per cycle, which meant that this cycle occurred approximately 40 times per second. In other words, we are talking about the same 40-Hz gamma oscillations associated with the problem of binding the attributes of an object.

These oscillations are apparently produced by the non-specific nuclei of the thalamus, such as the intralaminar nuclei (see sidebar and diagram to the left), whose projections pass through the cortex from front to back.

Llinas’s model is based on the interaction between two families of oscillators. One is this system of diffuse projections from the thalamus that are responsible for the waves that sweep through the cortex and provide a “context” for conscious perception 40 times per second. The other family is the other well known system that connects the specific thalamic nuclei to the corresponding specialized areas in the cortex and helps to bind the various attributes of a single object. (For example, one of these nuclei, the lateral geniculate nucleus, receives visual information from the retina and then passes this information on to the primary visual area in the occipital cortex.)

In short, Llinas’s hypothesis is that the neuronal assemblies that correspond to a particular piece of conscious content are those that oscillate in phase not only with one another (to bind the various characteristics of that content together), but also with the non-specific oscillations that sweep through the cortex.

Llinas has shown that the presence of these cycles correlates with coherent, conscious experiences: these cycles occur continuously during wakefulness and dreaming but not during deep sleep.

For example, a subject who is awake and engaged in any cognitive task will display robust neuronal activity around 40 Hz. Thus, during wakefulness, the two families of oscillators are coupled, and the specific circuits respond to external signals. During dreaming, the two families of nuclei are coupled, but the specific nuclei do not respond practically at all to external signals. The conscious contents come from the inside, from memories stored in the brain (see sidebar). Lastly, in deep sleep, the two families of oscillators are decoupled, and there are no conscious contents.

Certain experiments have also shown that when a human subject is awake, a piercing sound will interrupt the non-specific sweeping wave and start a new one, but when the subject is in REM sleep, this same sound will produce a cortical response without resetting the sweeping wave to zero. These findings might reflect the fact that a stimulus of this kind will attract our conscious attention when we are awake, but not when we are in REM sleep (and even less so when we are in deep sleep, during which the sweeping wave is either absent or considerably attenuated).

In another set of experiments, the sound that the subjects heard was a pair of clicks, separated by an interval ranging from 3 to 30 ms. When the interval was 13 ms or more, the subjects could distinguish the two clicks, but when it was shorter, they perceived only one. In addition, the MEG recordings made during these experiments showed that when the intervals were shorter than 12 ms, the sweeping oscillations returned to zero only once, but when the intervals were longer, they did so twice. These results seem to indicate that consciousness is discrete rather than continuous, with the 12-ms interval representing the “quantum of consciousness”—the basic time unit of conscious experience.

One equivalent example in a non-laboratory setting would be someone who is thinking as she walks down the street, so that her brain is generating oscillations at around 40 Hz. So long as her mental representation of her external environment is consistent with her actual surroundings, her brain will continue to update this scene at a steady rate. But if she suddenly hears a dog barking angrily close by, her 40-Hz cycle will be immediately reset to zero to incorporate this new stimulus into the overall scene, so that she can take this new information into account.

Every wave that sweeps across the cortex in 12.5 ms thus creates a new image, but these images succeed each other so quickly that they seem continuous to us, just as the still images on motion-picture film create the appearance of fluid motion if they are run through the projector quickly enough.

Many neurobiological models of consciousness hypothesize that neuronal assemblies engage in a form of competition that acts as a process for selecting conscious contents. Examples of such models included the one developed by Crick and Koch starting around the year 2000, as well as the model of Edelman, Dennett, and Baars.

But neurophysiologist William Calvin may be the scientist who most fully incorporates Darwinian selection processes into his model of brain function. His “mental Darwinism” is based on the concept of a cerebral code which is the analog of the body’s genetic code, but serves the purpose of selecting and reproducing conscious representations. According to Calvin, each of our thoughts becomes our dominant brain activity pattern for the moment that we are having it, as the result of a process in which it is selected from among numerous other variants that remain unconscious.

In Calvin’s view, the physical substrate of this cerebral code, the basic unit of our thoughts, consists of the smallest neuronal assemblies in the cortex, and these assemblies consist of about 10 000 neurons each, comprised in some 100 microcolumns. These small neuronal assemblies would be similar in size to the 0.5 mm macrocolumns found in the associative cortex or to the ocular dominance columns in the primary visual cortex, though their connections may be less genetically determined.

According to Calvin’s hypothesis, it is these columns’ electrical activity patterns, forming a mosaic of hexagonal cells covering the entire cortex, that enter into competition to become a conscious mental image. And in the process of replicating itself to spread through the cortical space, this cerebral code introduces variants that may be the source of better-adapted behaviours.

This concept of the brain as an evolutionary system not only is consistent with the principle of natural selection but also fits very nicely with the concepts of neuronal assemblies proposed by Hebb and even by Dawkins.

Lastly, in developing his model, Calvin also acknowledged that ultimately, our thoughts must always serve the purpose of action. And like many others before him (such as Varela and Llinas), he summarizes this imperative with a shocking statement: that far from being a combination of sensations and memories, in the end our thoughts are nothing more than movements that have not yet been actualized.