Elsevier

Physics Letters A

Volume 266, Issues 4–6, 28 February 2000, Pages 303-308
Physics Letters A

Phase transitions in the human brain revealed by large SQuID arrays: Response to Daffertshofer, Peper and Beek

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Introduction

In their Letter [1], Daffertshofer, Peper and Beek (referred to in the following as DPB) report on findings from an attempt to replicate an experiment by one of us (S.K.) in 1989 which demonstrated a phase transition at both brain (MEG) and behavioral levels of description [10], [12], [3]. Although DPB reproduce most of the original results, they fail to observe a frequency doubling in the brain signals which occurs across the transition. This apparent absence triggers a lengthy discussion of possible artifacts that may happen in spectral analysis of time series in the presence of noise. Here we show the results of our own replication of the original experiment using a state of the art full-head magnetometer in order to illustrate our point of view with respect to the arguments made in DPB. We find that the results from our earlier and recent experimental studies are in excellent agreement even though they use different subjects. Moreover, our results agree with well-established features of evoked auditory and motor fields in the neurophysiological literature whereas it is not clear to us how the findings of Daffertshofer et al. fit into this picture.

Section snippets

Comparing the experiments

In all three experiments (the original, DPB and our recent one) a transition in coordination behavior from syncopation to synchronization [11] was used to prepare and probe neural activity in human subjects. The subjects' task was to perform a flexion movement of the preferred index finger in between two consecutive tones of an auditory metronome, i.e. to syncopate with the stimulus. It is well-known that by increasing the presentation rate of the stimuli as a control parameter a point is

Results from our recent experiment

Our results from the 143-channel device are summarized in Figs. 2–5. Fig. 2 shows the average time series in all sensors (N≈120 after behavioral editing and artifact rejection) for one coordination cycle prior to the switch in behavior (frequency plateau II, 1.25 Hz) and following the switch (plateau VII, 2.5 Hz). Underlaid in color is the spatial pattern at maximum amplitude, i.e. 90 ms and 45 ms after stimulus onset for pre- and post-transition, respectively. Red, yellow corresponds to

Comparing results

Unfortunately, DPB do not provide a plot of their averaged time series in single channels in the Letter (c.f. our Fig. 2). Therefore, the first figure that can be compared is the plot of phases, their Fig. 2 and our Fig. 4. It is surprising that they find very clean transitions in so many sensors. Even more puzzling is where these sensors are located, i.e. in occipital and frontal areas and in the left and right boundaries of the sensor array. Over the contralateral motor area where the signal

Discussing the differences

Comparing the figures in DPB and those shown here it is quite obvious that there are a number of discrepancies even beyond what Daffertshofer et al. describe. A key may lie in the strong occipital activity that can be seen in Fig. 5 of DPB. In their experiment the subject had her eyes closed (a quite unusual setup because of the lengthy trail runs in this type of experiment) which is well known to trigger strong alpha activity. Even though the data were filtered in a range 0.1–7 Hz the only

Acknowledgements

Research supported by NIMH (Neurosciences Research Branch) Grants MH42900, KO5 MH01386, and the Human Frontier Science Program.

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