CCNi Debates

GAMMA OSCILLATIONS

This Debate on “Gamma Oscillations” was primarily held in the light of the recent paper published in Neuron by Yuval-Greenberg et al. (2008) that highlighted a potential eye-artefact problem with induced gamma frequency band effects. In their own investigations by means of electroencephalography (EEG) the authors had found that minimal eye movements (so-called microsaccades) were not necessarily captured by standard eye artefact rejection/identification methods for EEG data pre-processing affecting gamma oscillations even when they appeared at posterior sites (e.g. over occipital cortex). That is, such gamma effects could reflect the high-frequency activity of eye muscles rather than gamma oscillations of neurons in the brain. There could also be a combination of both. The extent of this ‘contamination’ of posterior gamma effects by microsaccades further depends on the employed reference, i.e. nose- in contrast to average reference (Yuval-Greenberg et al., 2008). In the Debate held at the CCNi, Catherine Tallon-Baudry and Jean-Philippe Lachaux, who are two of the most prominent researchers on gamma oscillations, obtained the opportunity to discuss this general issue with Shlomit Yuval-Greenberg. In essence, the Debate clarified the conditions under which gamma effects could indeed be contaminated by microsaccades and under which conditions genuine ‘brain’ gamma effects might be observed. Firstly, it was highlighted that MEG was less succeptible to such contamination due to the nature of the sensors, the resulting topographies, and the lack of a reference electrode. Furthermore, it was clarified that the time-frequency signature of a microsaccade-induced gamma effect was transient and rather broadband (comprising a quite large frequency range, e.g. 60-120 Hz), hence, frequency-specific and sustained gamma effects (e.g. Hoogenboom et al., 2006), would be unlikely to reflect eye movements. Nevertheless, it was also apparent that a few published results fitted the general profile of microsaccade-induced gamma effects, yet, due to a lack of high-resolution eye movement data (i.e. by means of an appropriate eye tracker) it could not be resolved whether these effects reflected brain oscillations or eye muscle activity, leaving the stain of a doubt.