A topological hypothesis for the functional connections of the cortex : A principle of the 'cortical graphs' based on neuroimaging

• Published in: Medical hypotheses 1999-09, Vol.53 (3), p.263-266
• Main Authors: GOMEZ M, J. F, LOPERA R, F. J
• Format: Article
• Language: English
• Subjects: Biological and medical sciences; Cerebral Cortex - diagnostic imaging; Cerebral Cortex - physiology; Degenerative and inherited degenerative diseases of the nervous system. Leukodystrophies. Prion diseases; Electroencephalography; Humans; Medical sciences; Models, Neurological; Nerve Net - physiology; Neurology; Tomography, Emission-Computed
• ISSN: 0306-9877; 1532-2777

Combined EEG and PET techniques show three activation levels of the cortex: deep sleep, relaxed state and alert. We propose, correspondingly, that a cortical module can be in one of three equivalent states: inactivated, pre-activated, and activated. Neuroimaging techniques can show activated cortical regions in detail. However, the functional connections (FCs) among them are not shown in the image. They can be found by EEG-coherence functions. This can be seen as a 'three-level- cortical graph'. A cortical graph is a mathematical representation where the cortical units (modules or regions) are represented by points (nodes) and the FCs are represented by lines between these points. At the upper level, activated modules can establish FCs implying high electrical coherence (they are the winners of a competitive process between preactivated modules at the middle level). We propose that, during alert state, the activated nodes and the dynamic switching among them always form connected graphs. It means that, for any possible configuration, there always exists a path (direct or indirect) between any couple of nodes. We base our view on (1) analysis of simple tasks by PET; (2) the existence of coordinated behavior in normal subjects; (3) cortical topologies previously proposed; and (4) computer simulations of cortico-cortical connections. We also suggest that disengaged (nonconnected) cortical graphs, produce 'functional disconnection syndromes' which cause some symptoms in schizophrenia, and Alzheimer disease.