Mechanism for preconscious awareness.
PI/Investigator: Francisco J. Urbano, Ph.D., University of Buenos Aires, Visiting Scientist; E. Garcia-Rill, Ph.D., COBRE PI.
Institution: Center for Translational Neuroscience (COBRE), Department of Neurobiology and Developmental Sciences, University of Arkansas for Medical Sciences, Little Rock, AR
Background: Freud identified three different parts of the mind based on our level of awareness, the conscious mind, the preconscious mind, and the subconscious mind. The preconscious mind includes those things of which we are aware, but we are not paying attention to them. If we choose to pay attention, we bring them to the conscious mind. Urbano et al. suggest that gamma band activity in the reticular activating system (RAS) mediates preconscious awareness, that it is the maintained high frequency activity across various nuclei that keeps the “stream of consciousness” as William James, called it, flowing across our preconscious mind. This mechanism allows us to survive in a complex world, being aware of the traffic and pedestrians around us as we commute to work. When a stimulus introduces potential danger, for example, an oncoming bus, we instantly bring it into conscious control in order to respond. Gamma waves are neural oscillations in the range of 30-100 Hz, on average ~40 Hz. These waves have been implicated in creating the unity of conscious perception, or binding, and are related to awareness, problem solving, and REM sleep.
Advance: These studies from the Center for Translational Neuroscience at UAMS during an ongoing collaboration with Dr. Urbano’s lab. at the University of Buenos Aires, Argentina, refer to a series of recent results that convincingly demonstrate the presence of gamma band activity in cells of the pedunculopontine nucleus (PPT), the intralaminar thalamic parafascicular nucleus (Pf), and the pontine subcoeruleus nucleus dorsalis (SubCD). The PPT is involved in modulating waking and REM sleep, while the Pf can affect cortical arousal, and the SubCD is thought to control REM sleep, all states marked by high frequency activity. These authors found high threshold calcium channels in every PPT and every Pf cell, such that activation of these cells drive them to fire faster and faster, but then plateau at gamma band frequencies. The cells simply do not fire any faster, which is not the case in many CNS regions. Every SubCD cell studied exhibited sodium-dependent subthreshold oscillations to maintain firing in the beta/gamma range. Classic studies by Steriade had established that cells in the PPT and intralaminar thalamus could fire in the gamma band range in vivo.
The authors call the RAS a “gamma making machine” and explain a concept that few of us realize, that, in order to maintain a circuit firing at gamma band frequencies (~40 Hz), synaptic interactions by themselves cannot hope to maintain the rhythm due to the likely failure of multiple synapses at such high frequencies. That is, it is probably the presence of both synaptic interactions and intrinsic membrane properties (e.g. subthreshold oscillations) that can maintain such fast activity.
Support: Project support and Core Facilities provided by COBRE award P20 GM104325.
Public Health Impact: The importance of the discovery of these mechanisms behind gamma band activity, therefore, is critical to understanding the function of the RAS. In addition, knowing the mechanism behind gamma band activity in the RAS will allow the development of novel stimulants and anesthetics, and agents that modulate awareness. The implications of this work are profound, and we should pay conscious attention to these findings.
Citation and links: Subject of an editorial Commentary.
Urbano, F.J., Kezunovic, N., Hyde, J., Simon, C., Beck, P., and Garcia-Rill, E. 2012 Gamma band activity in the reticular activating system (RAS). Frontiers in Neurology; Sleep and Chronobiology 3: 6, 1-16, 2012. http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3269033