Excitation at widely dispersed loci in the cerebral cortex may represent a neural correlate of consciousness. Accordingly, each unique combination of excited neurons would determine the content of a conscious moment. This conceptualization would be strengthened if we could identify what orchestrates the various combinations of excited neurons. In the present paper, cholinergic afferents to the cerebral cortex are hypothesized to enhance activity at specific cortical circuits and determine the content of a conscious moment by activating certain combinations of postsynaptic (...) sites in select cortical modules. It is proposed that these selections are enabled by learning-related restructuring that simultaneously adjusts the cytoskeletal matrix at specific constellations of postsynaptic sites giving all a similar geometry. The underlying mechanism of conscious awareness hypothetically involves cholinergic mediation of linkages between microtubules and microtubule-associated protein-2 . The first reason for proposing this mechanism is that previous studies indicate cognitive-related changes in MAP-2 occur in cholinoceptive cells within discrete cortical modules. These cortical modules are found throughout the cerebral cortex, measure 1–2 mm2, and contain approximately 103–104cholinoceptive cells that are enriched with MAP-2. The subsectors of the hippocampus may function similarly to cortical modules. The second reason for proposing the current mechanism is that the MAP-2 rich cells throughout the cerebral cortex correspond almost exactly with the cortical cells containing muscarinic receptors. Many of these cholinoceptive, MAP-2 rich cells are large pyramidal cell types, but some are also small pyramidal cells and nonpyramidal types. The third reason for proposing the current mechanism is that cholinergic afferents are module-specific; cholinergic axons terminate wholly within individual cortical modules. The cholinergic afferents may be unique in this regard. Finally, the tapering apical dendrites of pyramidal cells are proposed as primary sites for cholinergic mediation of linkages between MAP-2 and microtubules because especially high amounts of MAP-2 are found here. Also, the possibility is raised that muscarinic actions on MAP-2 could modulate microtubular coherence and self-collapse, phenomena that have been suggested to underlie consciousness. (shrink)

Various neurophysiological experiments have revealed remarkable correlations between cortical neuronal activity and subjective experiences. However, the mere presence of neuronal electrical activity does not appear to be sufficient to produce these experiences. It has been suggested that the explanation for the neural basis of consciousness might lie in understanding the reason that some types of neuronal activity possess subjective correlates and others do not. Here I propose and develop the idea that this difference may be caused by the existence of (...) an elementary nonarbitrary linkage between temporal or spatiotemporal patterns of neuronal activity and their subjective attributes. I also show how cortical neural circuits capable of generating experience-coding patterns could emerge during evolution and brain development, due to the presence of spontaneous stochastic neuronal activity and activity-dependent synaptic plasticity. This hypothesis leads to several testable predictions, principal among which is the idea that the neural correlates of consciousness are essentially innate and universal. (shrink)