Abstract
Gamma rhythms have been proposed to promote the feed forward or "bottom-up" flow of information from lower to higher regions in the brain during perception. On the other hand, beta rhythms have been proposed to represent feed back or "top-down" influence from higher regions to lower. The pedunculopontine nucleus (PPN) has been implicated in sleep-wake control and arousal, and is part of the reticular activating system (RAS). This review describes the properties of the cells in this nucleus. These properties are unique, and perhaps it is the particular characteristics of these cells that allow the PPN to be involved in a host of functions and disorders. The fact that all PPN neurons fire maximally at gamma band frequency regardless of electrophysiological or transmitter type, make this an unusual cell group. In other regions, for example in the cortex, cells with such a property represent only a sub-population. More importantly, the fact that this cell group's functions are related to the capacity to generate coherent activity at a preferred natural frequency, gamma band, speaks volumes about how the PPN functions. We propose that "bottom-up" gamma band influence arises in the RAS and contributes to the build-up of the background of activity necessary for preconscious awareness and gamma activity at cortical levels.
Highlights
Bottom-up or feed forward brain processes depend on sensory events as stimuli activate lower brain centers and the information rises to succeeding higher centers to promote perception
This review is concerned with questions about bottomup processes, where does the gamma activity arise? Is the gamma band activity generated only at the level of the cortex, or does it arise from lower centers to interact with ongoing cortical activity? What mechanisms generate activity at such frequencies? Can synaptic circuits maintain such frequencies for any length of time, or are there other mechanisms involved? Which lower centers generate gamma band activity, and is it coherent with cortical gamma band activity?
While cortical interneurons can generate membrane potential gamma oscillations through the activation of voltage-dependent, persistent sodium channels [8], in thalamocortical neurons, the main mechanism responsible for gamma band activity involves high threshold P/Q-type voltage-gated calcium channels located in the dendrites [19]
Summary
Bottom-up or feed forward brain processes depend on sensory events as stimuli activate lower brain centers and the information rises to succeeding higher centers to promote perception. Top-down or feedback processing refers to the influence imposed by higher centers on the perception of and attention to incoming stimuli. Recent studies suggest that feed forward and feedback signaling use different frequency channels, gamma and beta frequencies, respectively [1]. This review is concerned with questions about bottomup processes, where does the gamma activity arise? Is the gamma band activity generated only at the level of the cortex, or does it arise from lower centers to interact with ongoing cortical activity? What mechanisms generate activity at such frequencies? Which lower centers generate gamma band activity, and is it coherent with cortical gamma band activity? This review is concerned with questions about bottomup processes, where does the gamma activity arise? Is the gamma band activity generated only at the level of the cortex, or does it arise from lower centers to interact with ongoing cortical activity? What mechanisms generate activity at such frequencies? Can synaptic circuits maintain such frequencies for any length of time, or are there other mechanisms involved? Which lower centers generate gamma band activity, and is it coherent with cortical gamma band activity?
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