Kevin Whittingstall

Pharmacological dissociation of EEG and LFP: Effects of neural synchrony

Introduction and Scientific Aims

Electroencephalography (EEG) is one of the most commonly used methods to measure brain activity in humans. However, despite its widespread use, we still lack a clear understanding of how EEG signals are related to the spatio-temporal organization of underlying neuronal activity.  The dominant theory is that the surface EEG is not only dependent on the magnitude of LFP actvivity, but also on the degree of its temporal synchronization across space.  It is therefore theoretically possible that increases in cortical neural synchrony alone can enhance surface EEG signals without concomitant changes in the amplitude of LFP activity [1].  However, experimental confirmation of this hypothesis remains elusive, partly due to the difficulty in reliably separating intracortical measures of signal amplitude and spatial synchrony. 

The aim of this study is establish the contribution of local neural synchrony to measured surface EEG signals.  More specfically, can increases in EEG activity be detected when concomitant LFP activity is supressed?

Methods

In this study, we made simultaneous intracortical recordings of local field potentials (LFPs) with simultaneous surface EEG measurements in the primary visual cortex of two behaving non-human primates.  We made local injections of lidocaine, which has previously been shown to reduce LFP, though paradoxically increase EEG. Monkeys were trained to fixate during the presentation of a 5-second movie clip.  In each trial, we computed the EEG and LFP signal power, along with the temporal synchrony between micro-electrode pairs (LFP Spatial Coherence or SC).  These three measures where then normalized to their respective trial-average.

Results and Initial Conclusions

Figure 1 shows the relationship between high-gamma (60-100Hz) EEG, LFP and SC in the drug-free condition (no injection).  The upper left quadrant of this graph shows that in trials where LFP is negatively modulated, both EEG and SC are significantly positively modulated (p<0.01).  This result is more obvious during pharmacological manipulation (Figure 2).  Here, strong increases in EEG better follow SC compared to LFP, especially in the high-gamma frequency range.  In both drug and drug-free conditions, we found that SC explained a portion of the EEG which could not be explained by LFP alone. 



Figure 1 – Trial-to-Trial variations in EEG, LFP and SC.  EEG and LFP are represented as % change from the trial-average and SC is represented as a difference.
Figure 2 – With lidocaine, LFP power (green) is suppressed while SC (red) and high-frequency EEG (blue) is enhanced.

Our work provides the first experimental evidence demonstrating that the surface EEG signal is strongly influenced by the degree of underlying neural synchrony.  Of particular interest, is that robust EEG signals can be observed even though LFP is unchanged or even suppressed.  This finding is extremely important for the interpretation of EEG signals in both basic and clinical research, especially when they are compared to other neuroimaging modalities (e.g. fMRI [2,3]).

Supervises Students and Collaborators

For this project, I supervised Simon Musall (M.Sc. student) and collaborated with Dr. Alexander Rauch (MPI for Biological Cybernetics)

References

1. Cosandier-Rimele, D., Merlet, I., Badier, J.M., Chauvel, P., and Wendling, F. (2008). The neuronal sources of EEG: modeling of simultaneous scalp and intracerebral recordings in epilepsy. Neuroimage 1, 135-46.

2. Nunez, P.L., and Silberstein, R.B. (2000). On the relationship of synaptic activity to macroscopic measurements: does co-registration of EEG with fMRI make sense? Brain Topogr 2, 79-96.

3. Musall, S.F., VPfostl, V.,  I., Rauch, A., Logothetis N.K., Whittingtsall K., (submitted).  Effects of neural synchrony on surface EEG