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MEG headphones

Dear List,

I got some responses on my query concerning piezo headphones for MEG
studies. I submit a first summary to the list. This mail will also
clarify my needs (I suppose I was not too explicit in my first mail).

Some contributors (Didier A Depireux, Biao Tian) doubted my statement
  > Usual sound application is via tubes from speakers outside of the
  > magneto-shielding booth, with really bad sound transmission values.
Especially tubes by Etymotic Research have been recommended (Didier A
Depireux, Jont Allen, http://www.etymotic.com/html/index.html ).

I should have stated that what I need is _really good_ transmission
values: A factor of 100 between low and high cutoff (e.g. 120 to 12,000
Hz). The tubes I have seen transmit 500-4,000 Hz. This can be improved,
but is it realistic to think of tubes that transmit 120-12,000 Hz?

Some contributors mailed info on headphones used in fMRI studies:

Robert J. Zatorre pointed to phones by Koss (http://www.koss.com
but I could not find the nonferromagnetic headphone on their web-site
yet, mainly because they sell so many products; waiting for e-mail
feedback from Adam Miklas, Koss corporation, amiklas@koss.com
414-967-1575.) These headphones cost around $1K U.S.

Alan R. Palmer (Alan@ihr.mrc.ac.uk) transmitted a 8-page instruction of
how to modify sennheiser HE60 electrostatic headphones so that they can
be used in MRI environment. This goes beyond my technical capabilities,
but it might be of interest to those with sufficient technical skills.

My main problem is that I am not sure about the exact requirements for
MEG studies and how they differ from fMRI studies. I contacted MEG
people and they told me that the constraints are much more strict for
MEG than for fMRI. With fMRI, high magnetic fields are applied and these
should not be disturbed by the headphones. With MEG, the best
precondition is the absence of any kind of magnetic field. I was told
that nonferromagnetic EEG equipment that works fine for fMRI produces
strong artifacts if used in the MEG booth. On the plus side, one would
not need the active noise reduction that comes with some fMRI

I would have highly welcomed any mail pointing to someone actually using
headphones in MEG studies but it seems this is not common practice. May
be this is because it is impossible... (hoping this is not true)