The 2005 IEEE International Conference on Neural Networks and Brain

ICNN&B'05

Computational Neuroscience

(ICNN&B'05 Invited Paper)

Working Mechanism of Brain Neural NetworkWeichang Chen Ziqiang Wang Zhihua I

China Japan Friendship Instituteof Medical Sciences,Beijing 100029, China

Email: chenweic@ht.rol.cn.net wangziqiang 1@sohu

Abstract The working mechanism of the brain and theorigin of consciousness is still the great challenge inneuroscience and computational science. Human cerebralcortex is composed of the typical six layers structure. Differentlayers possessed different functions. But from the functionalpoint of view, it is a unitary system composed of vertical chainsof neurons which formed the functional columns of thecerebral cortex. A neural network model was suggested andthe information flow chart within and among the functionalcolumns was deduced to describe the information processingmechanism of the cerebral functional column.

The reverberating circuits of the thalamus and cerebralcortex are of great importance to maintain the activation,consciousness and the normal function of the cerebral cortex.There are three kinds of thalamocortial projections: thespecific projection, the nonspecific projection, and the diffuseprojection. This ascending reticular projection system is ofgreat importance for the maintenance of wakefulness andconsciousness of the cerebral cortex.A functional consciousness neural network model was

suggested. The chief cortical areas involved in the workingmechanism of consciousness are the prefrontal cortex (PFC),the anterior cingulate cortex (ACC), and the intermediaterelated associate cortex. These three cortical areas constitutedthe intermediate working platform of consciousness in whichthe prefrontal cortex was served as the display screen anddecision making element of the consciousness. The anteriorcingulate cortex was served as the selective element to guidethe consciousness to the destination of interest (theintermediate working associate cortex). The information of theselected associate cortex was then allocated and entered theintermediate working platform of consciousness.Keywords-consciousness, functional column, thalamocor- ticalprojections, brain neural network, prefrontal cortex, anteriorcingulate cortex.

I. INTRODUCTnON

The working mechanism of the brain and the origin ofconsciousness is still the great challenge in neuroscience andcomputational science. Magoun first reported that theascending reticular system of the brain stem played animportant role for wakefulness and consciousness. One ofthe fundamental properties of consciousness is that it ishighly unified and integrated. Each conscious statecomprises a single scene that cannot be decomposed intoindependent components. We cannot be aware of twoincongruent scenes at the same timeEl1. The number of

Chen Zhiyi ChenNational Laboratory of pattern

Recognition, Institute ofAutomation,Academia Sinica, Beijing 100080,China

.com zhchen@ht.rol.cn.net

different conscious states that can be accessed within a shorttime is very large. The brain is a computer that interfaceswith a conscious entity. It is an apparatus for decoding,analyzing and exploring the 4-demensional universe that welive in. It is a circuit that is composed of a complex array ofneural networks that are being continuously modified by ourenvironment and the experiences which we learned fromthat environment. To understand consciousness, it isimportant to identify the underlying neural processes thatare both integrated and capable of such exceptionallyinformation differentiations[21.

II. FUNCTIONAL COLUMNS OF THE CEREBRAL CORTEX

The cerebral cortex is essential for the humanconsciousness. Human cerebral cortex is composed of thetypical six layers structure. The first layer is the molecularlayer which contains the tangential transverse neural fibersto connect the adjacent areas of the cerebral cortex. Thesecond and third layers are the external granular layer andpyramidal cells layer respectively which are composed ofsmall and medium pyramidal cells with axons running to thedeep layers and other areas of the cerebral cortex. Thefourth layer is the internal granular layer of small stellatecells which receive the afferent fibers from the thalamus andsent axons to other layers of the cerebral cortex. The fifthand sixth layers are ganglionic layer and polymorphic layerrespectively which receive the input information from otherlayers and areas of the cerebral cortex and sent axons toother areas of the cerebral cortex and the lower centers ofthe brain or the spinal cord. The reverberating circuitsamong the thalamus and cerebral cortex are of greatimportance to maintain the activation, consciousness and thenormal function of the cerebral cortex. There are three kindsof thalamocortical projections: 1. The specific projection.The specific thalamocortical projections originated from thethalamic relay nuclei and projected to the primary sensorycortical areas such as the visual, auditory and somaticsensory cortical areas. 2. The nonspecific thalamocorticalprojections originated from the association thalamic nucleiand projected to the associate cerebral cortical areas. 3. Thediffuse thalamocortical projection originated from thereticular system of the thalamus and the brainstem anddiffusively projected to a large extent of the cerebral cortex.This ascending reticular projection is of great importance for

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the maintenance of wakefulness and consciousness of thecerebral cortex.But from the functional point of view, it is a unitary

system composed of vertical chains of neurons whichformed the functional columns of the cerebral cortex 31.Functional columns were the functional units of the cerebralcortex and each column contained about 4000 neurons. Thediameter of each column was 200-400gm. It was estimatedthat the human cerebral cortex possessed three millions offunctional columns. A neural network model was suggestedto describe the information processing mechanism of thefunctional column of the cerebral cortex.

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Fig. 1. The neural network of the :functional column ofthe cerebral cortex

Fig.l1. showed the neurons and the neural connecfions ofthe funcfional colulmn of the cerebral cortex. In the layer Ithere was the horizontal cell of Cajal which connected thefunctional columns transversely. In the layer II and layerHI there were small and medium pyramidal cells whichconsfituted the inter- and intra connecfion of the :funcfionalcolumnms. Layer IV contained numerous stellate neuronswhich received the specific projecfion fibers from thethalamic relay nuclei and sent axons to the neurons in otherlayers of the funcfional columns. Layer V and Layer VIcontained large output neurons which received input signalsfrom the other layers of the functional column and sent longaxons to other areas of the cerebral cortex and the lowercenters of the brain. Layer V and Layer VI were not onlythe chief output layers of the funcfional column but also sent

collateral fibers back to other layers to form thereverberating circuits within the functional column. Thefunctional column of the cerebral cortex received threekinds of the thalamocortical projections. The specificprojection fibers were chiefly projected to the layer IV. Thenonspecific projection fibers were chiefly projected to thelayer II and layer IH while the diffusive projection fibersfrom the ascending reticular system were projecteddiffusively to all layers of the functional columns. The inputand output signals of the functional column could besummarized as Fig. 2.

Fig. 2. The information flow of the functional column of the cerebral cortex

In Fig.2, No.1 input represented the specificthalamocortical projections. No.2 input represented thenonspecific thalamocortical projections. No.3 inputrepresented the diffusive thalamocortical projections. NO.4input and output represented the cortico-corticalinterconnections. No.5 output represented the outputprojections from cerebral cortex to the lower centers of thebrain, while the activity of the reverberating circuits (No.6)within the functional column determined the degree ofexcitability of the functional column which was of greatimportance to maintain the conscious state and the functionof the cerebral cortex. So the activity function of afunctional column of the cerebral cortex Fc(A) was theconjugated function of the diffusive reticular input I(R) andthe activation intensity of the reverberating circuits I(A),Fc(A) = I(R) * I(A).

There are three kinds of neurons of discharge patterns inthe brain: the phasic discharge, the clustered (or thepolyphasic) discharge and the rhythmic(or the tonic)discharge. The phasic discharge neurons might be served toresponse the input stimulation from the sensory system andthe other cerebral functional columns. The clustereddischarge neurons might be served for the output of thecerebral functional columns to the other cerebral functionalcolumns or the lower neural centers of the brain. Therhythmic discharge neurons might be served as an internaltime unit to synchronize the activity of the neurons withinand among the functional columns. When the functional

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column was at the excitatory state, there would emerge ahigh frequency synchronous discharge within the functionalcolumn and the thalamocortical system and also thecorticocortical reentrant circuits. It was suggested that theactivity of each functional columns was the internal or thesubjective representation unit of conception or recognitionof the external environment. Functional columns might bethe storage units of long term memory.

III. NEURAL NETWORK MODEL OF CONSCIOUSI'ESS

It is suggested that the lower level neuronal processes inthe brain cause consciousness and consciousness is simply ahigher level feature of the human brain that is made up ofthe lower level neuronal elements. Many observationssupported that only certain distributed cortical areas wereactivated in response to a given task associated withconsciousness[5]. In order to emphasize both the integrationand the constantly changing activity patterns of theconsciousness, Giulio Tononi termed this unified neuralprocess underlying conscious experience as "dynamiccore"[61. The dynamic core is a functional cluster, itsparticipation neuronal groups are much more stronglyinteractive among themselves than with the rest of the brain.The dynamic core also has high complexity, its globalactivity patterns must be selected within less than onesecond out of a very large repertoire.The dynamic core might change in composition over time.

Some group of neurons might at time be part of the dynamiccore and underlie conscious experience, while at other timesit might not be part of the consciousness and thus be inunconscious processes. For example, the retina got theinformation from a fixation scene, the transmission of thevisual signal from LGN to V1, V4 and MT cortical areaswas independent of consciousness.A functional consciousness neural network model was

suggested as shown in Fig.3. Most activities of the cerebralcortex are subconscious activities which processed theexternal and internal information automatically without thecontrol of consciousness. Only certain brain areas areactivated in response to a given task under conscious control.The chief cortical areas involved in the working mechanismof consciousness are the prefrontal cortex (PEC), theanterior cingulated cortex (ACC), and some intermediaterelated associate cortex. These three cortical areasconstituted the dymic working platform of consciousness inwhich the prefrontal cortex is served as the display monitorand decision making element of the consciousness. Theanterior cingulate cortex is served as the selective element toguide the consciousness to the destination of interest (tosome functional columns of the associate cortex). Theanterior cingulated cortex is well known as the center ofattention. The information from the selected associate cortexis then allocated and entered the intermediate workingplatform of consciousness. The associate cortical areas ofinterest are the information elements in which the

information is processed and stored. Consciousness is justlike an acting play which the prefrontal cortex is the editorand the playing stage. The anterior cingulated cortex is justlike a director which selected the actors and directed theperformance of the scene. But the lifetime of one scene ofconsciousness is rather short which is about 500 millisecondin magnitude. Present scene of consciousness would beautomatically decayed or replaced by new information ofinterest through competition.

PFC

ACC

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Fig. 3. Functional consciousness neural network model

IV. CONCLUSION

A functional consciousness neural network model wassuggested based on the structure and function of the humanbrain. The brain neural network is hierarchically organizedand interconnected for information processing inside thebrain. The human consciousness was formed and improvedby learning and training. The brain control was achievedlocally and the response depended on the experience and theexternal and internal stimulus. So the neural network modelfor consciousness should be included two parts, the centralcore of working stage and the distributed modular cerebralnetworks relating to specific brain functions. Thishypothesis of consciousness mechanism model still neededto be improved and verified.

REFERENCES

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[3] P. 0. Kanold, P. Kara, R. C. Reid, and C. J. Shatz, "Role of subplateneurons in functional maturation of visual cortical columns", Science2003 Jul 25; 301(5632): 521-525.

[4] T. Klausberger, P. J. Magill, L. F. Marton ET al. "Brain state and celltype specific firing of hippocampal interneurons in vivo". Nature, 2003Feb 20; 421(6925): 797-799.

[5] F. Crick, and C. Koch, "Are we aware of neural activity in primaryvisual cortex?", Nature, 1995 May 11; 375(6527):121-123.

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