Induced and evoked responses in MEG

Dear all The first slide in the attached Powerpoint presentation was generated by Piers for a paper based on data gathered at Aston. The stimuli were written words. The figure shows the time-frequency plots for virtual electrodes inserted at 4 different sites in the brain, with power represented in the colour. There are 3 plots for each site showing evoked power (left), total power (middle) and induced power (right). These figures were generated in Fieldtrip where the time-frequency plots are computed with respect to a baseline. Piers has added a couple of slides which shows the steps in Fieldtrip that he used. The left middle occipital gyrus (MOGl) site is closest to primary visual cortex where the words first arrive. You will notice that the power there is mostly evoked and that the induced component is small. The same is true at the left mid fusiform gyrus (= the 'visual word form area'). Contrast that with the left and right inferior frontal gyri (IFGl), which are further from primary visual cortex. There is some evoked power at those sites, but most of the total power is induced. It seems to me that the lessons could be: 1. If you plot total power, do not automatically assume that it will be mostly induced. There are conditions and sites (close to primary sensory cortex?) where the total power may be mostly evoked. 2. Do not plot total power and call it "induced". It may not be. 3. It could be that the more synapses a response passes through, the more the power shifts from evoked to induced. That could be because different stimuli complete the passage from early to late sites at different rates in different participants with the result that the response gets jittered in time, disappears in the evoked averaging, and appears instead in the induced response. Induced responses may not always be qualitatively different from from evoked responses - they may simply be smeared out in time. (Note: This is not to deny the existence of induced responses which are NOT the result of stereotypical waveforms + jitter, but rather represents increased power in several oscillators within a given time period.) 4. It would be a good thing if the VE tools we develop in YNiC for generating time-frequency plots were capable of distinguishing evoked from induced responses and plotting them separately. Please discuss in rare pre-Christmas moments of sobriety. Andy