[Frontiers in Bioscience 14, 2779-2800, January 1, 2009]

Consciousness: physiological dependence on rapid memory access

Arthur J. Hudson

Department of Clinical Neurological Sciences, University Hospital, London Health Sciences Center, University of Western Ontario, London, Ontario, Canada, N6A 5A5


1. Abstract
2. Introduction
3. Dendritic information processing
3.1.Theta- and gamma-frequency oscillations
3.2.Local circuit neuron (LCN)
3.3.Gap junctions
3.4.Dendrodentritic synapses
4. Axon conduction velocity
5. Axons and field effects
5.1.Mauthner and basket cells
5.2.Hippocampal population spikes
6. Modules, minicolumns and dendritic bundles
6.3.Dendritic bundles and honeycomb-like structure
7. Reticular activating (arousal) system
7.1.Dorsal and ventral hypothalamic pathways
7.2.Orexin nuclei
7.3 .Suprachiasmatic nucleus (SCN) and the sleep/wake cycle
8. Thalamocortical relays and the thalamic reticular nucleus
9. Neural basis of memory
9.1.The hippocampus
9.2.The amygdala
9.3.The prefrontal cortex
10. Neonatal development of consciousness
10.1.Pre- and postnatal myelination of the brain
10.2.Postnatal sensory development
10.3.Sensory deprivation in the newborn
10.3.1. Blindness of ocular origin
10.3.2. Cochlear hearing loss
10.3.3. Somatosensory deafferentation
11. Memory and consciousness consolidation
12. Summary and perspective
13. Acknowledgments
14. References


Consciousness develops from birth during the early months as the senses and other nervous system functions mature sufficiently to receive, process and store information. Among these is the ascending reticular activating (arousal) system in the brain stem that is responsible for wakefulness and was proposed by Penfield and Jasper more than 50 years ago as the "controlling mechanism for states of consciousness". This concept has remained the most advanced physiological interpretation of consciousness although recent developments offer greater insights into its nature. The ascending arousal system is the source of activation of the thalamocortical and cortical mechanisms for sensory input and facilitates the rapid matching of sensory input and the binding of memory during cognitive processing. Nonetheless, it is proposed that memory is the critical element through which our connection with the world exists without which, despite a fully functional arousal system, consciousness as we know it could not exist. Evidence is presented in support of this concept in addition to the physiological difficulties that must be resolved if consciousness is to be understood.