When I imagine a sort-of-core to my being, knowing this to be an will-o-the-wisp, what I find is rhythms: breathing, heartbeat, walking, speech cadence and so on, all those cyclic or tick-tocking things my body does. I have never taken that feeling too seriously but looking at consciousness seems to re-enforce the motif of rhythm.

 

Mehta’s group has just published a paper (see citation) on the effect of movement on hippocampal gamma waves. The results in this study are very clear cut and convincing.

Here is the abstract:

Cortical and hippocampal gamma oscillations have been implicated in many behavioral tasks. The hippocampus is required for spatial navigation where animals run at varying speeds. Hence we tested the hypothesis that the gamma rhythm could encode the running speed of mice. We found that the amplitude of slow (20–45 Hz) and fast (45–120 Hz) gamma rhythms in the hippocampal local field potential (LFP) increased with running speed. The speed-dependence of gamma amplitude was restricted to a narrow range of theta phases where gamma amplitude was maximal, called the preferred theta phase of gamma. The preferred phase of slow gamma precessed to lower values with increasing running speed. While maximal fast and slow gamma occurred at coincident phases of theta at low speeds, they became progressively more theta-phase separated with increasing speed. These results demonstrate a novel influence of speed on the amplitude and timing of the hippocampal gamma rhythm which could contribute to learning of temporal sequences and navigation.

 

Conscious awareness appears to predict the near future but I have not encountered a clear mechanism or description of what parts of the content of consciousness is projected forward. This paper shows that the navigation system in the hippocampus changes with the speed of movement. In effect it seems possible that the ‘here’ spot in a mental place map is biased by the speed of movement. So the faster we are traveling the more we feel projected up the road. This would be a start to understanding how the prediction involved in consciousness is produced.

 

Another thing that I found interesting although not entirely new was the idea of control via the phases of a slower wave. The theta wave is in the region of 6 Hz (or has about 6 peaks per second) and the gamma waves are much faster say 60 Hz for example (or 60 peaks per second). In the case of gamma in this region of the brain, they occur with the highest amplitude as the theta wave nears its trough. This type of mechanism can give very complex cyclical activity with different activities happening in sequence as the longer wave runs from a peak to a trough and then back to a peak. It even seems, on the basis of this study, that the sequence can be affected by some input such as the speed of movement. Consciousness is characterized by cycles of increasing and then decreasing gamma wave synchrony between cortical areas, possibly due to phrase locking to a theta-type rhythm. We are so far from understanding but a step closer.

 

 

Chen, Z., Resnik, E., McFarland, J., Sakmann, B., & Mehta, M. (2011). Speed Controls the Amplitude and Timing of the Hippocampal Gamma Rhythm PLoS ONE, 6 (6) DOI: 10.1371/journal.pone.0021408