[Mne_analysis] Maxfilter error: ill conditioned matrix

[Sophie Herbst]

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Dear Eric,
it seems like we have a problem with one HPI coil-set.
The weird thing is that during the recording, when I measured the HPI
before each run, it claims that 3/4 coils are fine.

For maxfilter, head_pos is None, but I am using another run as reference:
destination =
<Transform  |  MEG device->head>
[[ 0.71960711  0.63715118 -0.27605024 -0.07788484]
 [-0.67182261  0.73935592 -0.04479922 -0.01268383]
 [ 0.17555553  0.21769466  0.96009851  0.06360817]
 [ 0.          0.          0.          1.        ]]

Here is my call to maxfilter:
sss = maxwell_filter(imp,verbose=False, destination=destination,
 bad_condition='error',st_duration = 30,
calibration=cal_path, cross_talk=ct_path)

info['dev_head_t']['trans'][:3, 3] for imp (my raw data) is:
array([-0.07870771, -0.01272191, 0.06554133]).

I realized that the outcome of info['dev_head_t'] differs strongly between
a run for which maxfilter runs fine, and one for which I get the error, all
from the same participant and recording.

Run 1 with the error:
info['dev_head_t']:
<Transform  |  MEG device->head>
[[ 0.71646905  0.64524961 -0.26518956 -0.07870771]
 [-0.67314613  0.73924029 -0.0199627  -0.01272191]
 [ 0.18315783  0.19281393  0.96398985  0.06554133]
 [ 0.          0.          0.          1.        ]]

Run 3 with no error:
Out[76]:
<Transform  |  MEG device->head>
[[ 0.17830184 -0.8871333   0.42567968  0.04394912]
 [ 0.86059254  0.35034028  0.36965176 -0.01058153]
 [-0.47706306  0.30042711  0.82592654  0.06920396]
 [ 0.          0.          0.          1.        ]]

Do you have any idea where the problem could come from?
Thank you!
Sophie

Eric Larson <larson.eric.d at gmail.com> schrieb am Mi., 18. Apr. 2018 um
18:44 Uhr:

> Hey Sophie,
>
> What is the head position change/distance (`destination` and the
> `info['dev_head_t']['trans'][:3, 3])?
>
> Are you using regularization? Time-varying head positions (head_pos is not
> None)?
>
> The "bad condition" suggests that something might be amiss, and if nothing
> is, that the resulting data reconstruction could amplify noise. Usually
> using regularization prevents this problem, though.
>
> Eric
>
>
> On Wed, Apr 18, 2018 at 9:32 AM, Sophie Herbst <ksherbst at googlemail.com>
> wrote:
>
>> Dear all,
>> I am using maxfilter from within mne_python, and I am getting the
>> following error for several participants:
>> Matrix is badly conditioned: 22221 >= 1000
>> Head position change is over 25mm
>>
>> At first I thought it was because the HPI wasn't fit properly (I had one
>> participant for whom 2/4 coils had a too large distance), but I am also
>> getting the matrix-badly-condition error with other participants for whom
>> the HPI was fine.
>>
>> I know that I can set the function to give a warning instead of an error,
>> but I still would like to understand what happens.
>> Do you have any recommendation on how to deal with this?
>>
>> Thank you,
>> Sophie
>>
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