Coherence Potentials


Posted on March 24, 2013 by Tara Thiagarajan in Science. Comments Off on Coherence Potentials

Coherence Potentials

This section provides a detailed description of coherence potentials.

A coherence potential is a complex negative-positive waveform in the local field potential with a duration between 50 and 250 ms, reflecting aggregate neuronal activity in the local field that has exceeded a particular amplitude threshold and is therefore able to propagate to a large number of sites in the cortex without distortion of its temporal structure or substantial loss of amplitude. This is seen as a synaptic transmission dependent occurrence of the identical LFP waveform at different sites in the cortex with millisecond delays. In stark contrast, waveforms with peak amplitudes below the threshold traverse through the cortex with progressive distortion.

The path of propagation of coherence potentials is not wave like but rather jumps across the cortex to non-contiguous regions and stops abruptly. Generally they can start anywhere and will travel to a variable but usually large number of sites and only rarely the full measured extent. Importantly, while coherence potentials are identical in waveform within a propagated sequence, they are highly variable in their temporal structure across sequences. Generally, with measurements spanning a maximum of 60 sites or local fields, we see a successive stream of well distinguished coherence potential sequences as shown in the figure below. However, sometimes, multiple such sequences occur in a temporally intermingled manner and can easily be distinguished from one another on the basis of their waveform.
Image

This figure shows coherence potentials (CPs) arising within a 1 second long piece of recording. Here there were 60 coherence potentials (i.e. suprathreshold negative-positive LFP excursions or nLFPs) that occurred in a series of tight temporal clusters. The first matrix shows the time between the peaks of these coherence potentials, i.e. how rapidly they occurred one after another. The negative peaks of the first four CPs occurred within 5 ms (indicated in black) of one another, followed by a period >150 ms long (indicated in white) where no CP peaks were seen and then another cluster of 24 CPs within 5 ms on one another etc. The CPs in each cluster could have occurred at any site where we measured. This figure says nothing about the spatial pattern of their occurrence. The matrix below shows the similarity in the waveforms of these same 60 CPs. What is clear is that the waveform in each cluster is virtually identical (with correlations between 0.9 and 0.95 indicated by red-black), while the waveforms between clusters are generally < 0.6 which is the median correlation between randomly selected nLFPs. These boxes on the right show waveforms from each CP cluster overplotted within one box. The clusters corresponding to the 60 CPs shown in the matrices are marked by arrows.

We named them coherence potentials because the phenomenon was identified in the local field potential as a transient coherence between sites. We have used the term coherence not to refer specifically to spectral coherence but in its general sense which, according to Merriam-webster means : “the quality or state of cohering: as (a) systematic or logical connection or consistency (b) integration of diverse elements, relationships, or values”. However, there is likely to be broad spectral coherence as well since the waveforms at multiple sites are virtually identical in their shape at least up to 100 Hz which is the cut-off used to filter the local field potential signal.

Publications

Tipping Points in the Brain (Scientific American India)

Coherence potentials encode simple human sensorimotor behavior. Parameshwaran D, Crone NE, Thiagarajan TC.PLoS One. 2012;7(2):e30514.

Coherence potentials: loss-less, all-or-none network events in the cortex.Thiagarajan TC, Lebedev MA, Nicolelis MA, Plenz D.  PLoS Biol. 2010 Jan12;8(1)

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