Single neuron spiking and gamma

Single neuron spiking and gamma

Coupling between neuronal firing rate, gamma and fMRI is related to interneuronal correlations

Rafael Malach et al, Dept. of Neurobiology, Weizmann Institute

Current Biology, 17, pp. 1275-85, August 7 2007

http://www.cell.com/current-biology/

INTRODUCTION: Closing in on Consciousness? Recently rather similar experiments have demonstrated a strong correlation between, firstly, consciousness and the global gamma synchrony and secondly consciousness and a jump from baseline to 50 Hz spiking in single neurons. The problem here is to discover the link, if any, between these two correlations. Is it the gamma synchrony which drives the activity of single neurons, or is it the other way round, or just a coincidence, and what if any is the significance for consciousness studies?

The authors ask to what extent the spiking activity of individual neurons is related to the gamma local field potential. Earlier studies had shown a confusing variation in the degree of correlation between neuronal spiking and gamma activity, with some studies showing a strong correlation and others showing only a weak correlation. The authors here think that they have a resolution to the arguments that have arisen around this confusing data.

Their study demonstrates that most of the variability in the data can be explained in terms of whether or not the activity of individual neurons is correlated to the activity of neighbouring neurons. A relationship with gamma synchrony is apparent where there is correlated activity in neighbouring neurons. The link between individual neurons that are associated with other active neurons and the gamma synchrony is apparent, both when the brain is receiving sensory stimulation, and when activity is more introspective. P. The gamma synchrony is considered to arise from the dendritic activity of a large number of neurons over an extensive area of the cortex. This study shows that the relation between the activity of individual neurons and gamma correlates with the extent to which the activity of the neuron is linked to the firing rate of its neighbouring neurons. This establishes a relationship between gamma activity and a large number of individual neurons distributed over a region of the cortex.

In this study, subjects watched a film. During this, scanning showed a high correlation between the spiking of individual neurons and gamma activity that arose at the same time. But this did not happen in all cases. It was found that the main factor relating to whether or not neuronal spiking related to gamma activity was the degree of correlation in spiking between neighbouring neurons. The authors stress that these relationships are not just a function of sensory stimuli because a similar although less pronounced relationship between individual neurons, the spiking of their neighbours and gamma synchrony were apparent when the brain was in a more introspective state and not receiving pronounced sensory input. This study was based on recording the activity of several individual neurons. It was shown that the correlation between the spiking of the individual neuron and gamma synchrony could be predicted from the level of correlations between the activity of neighbouring neurons. When neurons were not correlated with their neighbours gamma activity was at a low level.

CONCLUSION: This tends to look like pieces of a jigsaw puzzle, and unfortunately one that we may not get much help in assembling. We know that the global gamma synchrony correlates to consciousness. We know that a jump to 50 Hz spiking in individual neurons correlates to consciousness. We also now know that the spiking in the individual neurons correlates to gamma if the spiking of the individuals correlates to their neighbours.

From the point of view of recent findings relative to quantum coherence in organic matter it has become most plausible to think in terms of consciousness arising within individual neurons, but the road there may involve feed forward and feedback as is often the case in brain processing. Processing in one neuron as a result of external signals may set off other neighbouring neurons, which ultimately broaden into a neuronal assembly oscillating as a local gamma synchrony. Longer range signals to other neuronal assemblies would set up global gamma synchrony. It might only be at that point that signals went back to individual neurons triggering quantum coherent activity within the neuron. This might account for the 500ms time lag for signals to come into consciousness (the Libet half second), while at the same time being compatible with the femto and pico second timescales of functional quantum activity in biological systems. Very speculative, but perhaps this at least provides a starting point or framework for thinking about the consciousness problem.