Cortical Association Areas and Neuronal Assemblies

Twenty-five per cent of the neurons of a young and healthy brain are located at definite regions, in the cortical lobes and in the olphatory bulbs, in charge of specific functions : the processing of stimuli and motor responses. However, the great majority of neurons in the cortex are not related to such functions. They constitute association areas, responsible for the integration of current informations to previous emotional and cognitive ones (Fig.2).

Fig.2 - As zonas incolores representam as áreas de associação da
cortex cerebral humana.

These neurons get together to form minicolumns which are perpendicularly disposed in relation to to the cortical surface. Each minicolumn connects to their neighbors , thus forming the so-called columns, which are the basic units of information processing. From these columns, when motivated by external or internal stimuli, specific neurons are recruited to form the neuronal assemblies.

FIG. 3 - Diagrama representativo da disposição "em colunas "
da cortex cerebral humana

Information Processing

The association areas of the parietal lobe process somatosensory data originated from the skin, muscles, tendons and joints, as well as those related to body posture and movements. The integration of these information with others received from visual and hearing centers, allows us to formulate a conscious thought about the precise position of our body, be it still or in motion.

The fusion of recent sensory information with messages coming from the memory, allows a conscious and correct feeling of images, sounds, smells, touch and tastes. The associations areas in the frontal cortex mediate decisions, establish priorities, plan the future and, by differentiating right from wrong, provide us, humans, with the concepts of ethics and morality. The frontal cortex also participates, with the temporal one, in the performance of other high functions.

Language, for instance, comprises large frontal and temporal areas of association, which extend backwards, until the occipital lobe. Some associative areas, located in the temporal cortex, decide which data must be transferred to the long - term memory and stipulate whether the recorded events should be considered pleasant or unpleasant. To Susan Greenfield, any conscious thought that occupies our mind, at any particular moment, is generated when neurons, from several columns get together, building an assembly and, by acting in complete unison, initiate the formation of a conscious state.

Assemblies can be formed anytime and anywhere within the association areas. Thus, consciousness is spatially multiple. However, at a single time, only one assembly is large enough to create the conditions necessary to initiate a conscious state. So, only one consciousness is realized at any one time. Therefore it is temporally unified.

The reasoning behind this last affirmation brings about a logic question : if there is always the possibility that, at least two assemblies can be formed simultaneously, why we do not display two consciousness at the same time ?

Susan Greenfield explains the fact by using the argument that the more the number of neurons recruited, the more will be the size of the assembly and, therefore, the more will be the consciousness in terms of intensity and duration. It follows that, if a small number of neurons are recruited, than the resulting consciousness will be also small in intensity and duration or, perhaps, it will never be formed.

She continues her reasoning by saying that since the pool of appropriate neurons from which consciousness is formed is vast but finite, the formation of a neural group, large enough for consciousness, may preclude the formation of any equally large second assembly at that particular time. And Dr, Greenfield finishes her explanation with an interesting analogy : "if in a group of fifteen people, eleven are recruited for a football team, there are now enough left to make up a second team simultaneously." The reasoning of the English neurologist seems logical, although highly speculative.

Nevertheless, in such an indefinite context, like that of consciousness, we feel that her hypothesis is acceptable, although "sub judice" . Thus, conscious phenomena succeed one another, continuously, each one having its own duration and intensity. Not always, however, the sequence of conscious thoughts are under our total control.

On certain occasions they are disturbed by a stimulus that does not come from the external environment or from any psychic activity produced by our will. It appears as result of a quantum casual phenomenon. Let us explain ; You are engaged in a conversation about, let us say, informatics, when, suddenly, the image of the actress Jodie Foster comes to your mind, who, of course, has nothing to do with the subject in question. Why such amazing "intrusion"? Perhaps because a protein, that retained the image of the actress, as a memory formed days, months or years ago, suddenly "decides" to liberate this visual information, which will be captured and processed by one neuronal assembly, giving rise to a remembrance that, although not voluntarily recalled, emerged to a conscious level.

The theory of neuronal assemblies, as proposed by Dr. Greenfield, allows us to reformulate the concept of consciousness in relation to dreams : While we dream, we are forming mini-assemblies, produced only by small stimuli originated inside the mind ( since the strong sensory stimuli that comes from outside the body are absent ). Therefore, as a rule, only fragmented and unstable scenarios are formed. By their very fragility, these dreams are just brief episodes, which are readily substituted by other mini-assemblies and, consequently, by other dreams.

This instability prevents the activation of the amount of neurons necessary to generate an intensity capable to produce a full and long standing conscious experience. However, when the core of the dream causes anxiety, this may induce the formation of a much larger number of neurons, thus giving rise to those conditions (as we shall learn later) that favors the development of a definite state of consciousness. In such situations, we become aware that we are " living" a bad dream; we say, inside our own mind, " I must wake up" and... we do wake up. An indisputable evidence of the participation of consciousness in the dreaming process.

In 1996, two propositions, similar to that of Dr. Greenfield, were presented at the Tucson Conference (this conference congregated the world greatest experts in neurosciences) : Rudolpho Llinas described the existence of bursts of oscillatory waves, on the 40 Hz band, which are re-set whenever a sensory event takes place and then travel deep into the brain in a dialogue with the cortex. Alan Hobson, another investigator in this field, has developed an experimental model based on the conception that "consciousness is the result of gradual integration of multiple cognitive functions, which allows an unified representation of the world, our bodies and ourselves."

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