Recent Work On The Neural Substrates Of Consciousness Brief Overview Of Anatomical Connectivity Neuromodulatory And Neurodynamic Aspects

There has been much progress since early conceptions of a reticular activating system by Moruzzi and Magoun. Although there is still no universal agreement on many basic theoretical issues, a broad-based theoretical confluence is slowly emerging. Progress has been made in defining methods for rigorous empirical work; many studies now use a contrastive analysis methodology (first advocated by Baars 1994) in which consciousness becomes a dependent variable (i.e., studying conscious vs. unconscious forms of visual or emotional processing to map the difference). Many streams of research and theory about consciousness emphasize fundamental and intrinsic integrative processes that operate across widely distributed networks. Foundations for these integrative processes may be found in the structures comprising the extended reticular activating system (discussed shortly in detail), and in its thalamic extensions in "nonspecific" thalamic systems, such as the intralaminar nuclei and the counterposed inhibitory gating system organized by the nucleus reticularis thalami (nRt). This "expanded RAS" has been dubbed the extended reticular thalamic activating system (or ERTAS by Newman and Baars, 1993). These distributed structures are thought to facilitate thalamocortical resonances and widespread thalamocortical communications that are likely informed and guided by brainstem and homeostatic mappings. Although there are important differences in the details of various theories about consciousness by leading theorists and investigators, such as Frith (1989), Rees et al. (2002), Schiff and Plum (2000), Freeman (1999), Hobson & Pace-Schott (2002), Damasio (1999), Baars (1996, 2002), Llinas et al. (1999), Newman & Baars (1993), Ribary (1991, 2000), Edelman (1987, 1999), Tononi (1998, 2000), Taylor (1999), Singer (1998), and others, many theories support the assumption that consciousness reflects globally integrative processes derivative of neurodynamical interactions between multiple contributing brain systems, particularly communication between the critical triad of thalamus, brainstem, and cortex.

As an extension and modification of these ERTAS and thalamocortical theories, Damasio (1999) recently broadly proposed that consciousness reflects second-order mappings between structures that correlate ongoing changes in primary or first-order mappings emerging out of brainstem and other homeostasis-monitoring systems in conjunction with object mappings supported in various sensory systems. Structures contributing to body mapping in all of its many dimensions constitute proto-self-systems. This includes the extended set of reticular structures in the brainstem plus hypothalamus, parabrachial nucleus, periaqueductal gray, and several somatosensory systems, including the insula. Proto-self body-mapping systems at various levels of the neuroaxis include visceral, proprioceptive, musculoskeletal, and primary somatosensory systems and also structures in the brainstem responsible for maintaining homeostasis. In Damasio's theory, this broad group of body mapping proto-self-systems interact with object-mapping regions in secondary mapping systems located in thalamus, cingulate, and superior colliculi. In this formulation, consciousness emerges from correlational ongoing neurodynamic mappings of current objects in the world and the current state of body. These correlations facilitate enhanced mapping of the particular object, "popping it out" from the unconscious background, and are proposed to be the foundations for the "movie in the brain," the integrated multimodal sensory fields of conscious states.

Another basic principle broadly supported in the consciousness literature is that consciousness is a resource-intensive and "limited-bandwidth" system, and that the brain works endlessly to offload as much of its processing as possible to unconscious processes. Novel tasks that initially demand the resources of consciousness are quickly learned and become increasingly supported in basal ganglia and cerebellar systems that underwrite the consolidation of habits, procedural memories, and complex motor skills, thus allowing consciousness the luxury, so to speak, of focusing on the most essential and novel adaptive demands facing the organism.

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