No subject

[P.vandenHurk]

Hello,

Each time I use the 'read_fcdc_event' function, I get the following error:

??? Error using ==> read_event
type field not defined for each event

Error in ==> read_fcdc_event at 51
[event] = read_event(varargin{:});

Can you tell me how I can fix this error?

Thanks,

Best regards,
Paul

Robert Oostenveld schreef:
> Hi Sameer,
>
> One small comment to start with: please use a subject in your emails
> to the list. That makes it easier to keep track of discussions and
> threads.
>
> On 28 Feb 2007, at 21:59, Sameer Walawalkar wrote:
>> I am currently experimenting with beamformers and was playing with
>> http://www2.ru.nl/fcdonders/fieldtrip/doku.php?id=fieldtrip:documentation:use_your_own_forward_leadfield_model_in_an_inverse_beamformer_computation
>>
>> to generate the grid.
>
> You have 306ch Neuromag data, and I don't recall exactly whether that
> has been fully tested for forward computations. Some background: at
> each of the 102 locations of each "chip" you have 3 channels: two of
> those have 2 coils, and one is a magnetometer with only one coil.
>
> Please have a look at
> http://www2.ru.nl/fcdonders/fieldtrip/doku.php?id=fieldtrip:documentation:frequently_asked_questions#how_are_electrodes_magnetometers_or_gradiometers_described
>
> for the description of "grad". I would expect in your case that
>  grad.pnt and ori   102*2+102*2+102= 510 rows long, each with 3 columns
>  grad.tra           306 x 510
>
> The gradiometer coils are shifted w.r.t. the center of the "chip" by a
> small amount. I think that currently the baseline (i.e. the distance
> between the coils) is hardcoded. Joachim Gross once gave me code for
> that, it is included in feldtrip/private/fif2grad. Please look at it
> and see whether it applies to your 306ch gradiometer design as well.
>
>>
>> I did the following
>>
>>
>> vol   = [];
>> vol.r = 12 * [0.88 0.92 1.00]; % radii of spheres, the head radius is
>> 12 cm
>> vol.c = [1 1/80 1];            % conductivity
>> vol.o = [0 0 0];              % center of sphere
>
> this indicates a 4-shell concentric-sphere EEG model. It does not make
> sense for MEG, since in MEG teh conductivity of the different layers
> in the spherical model does not affect the field distribution.
>
>> % compute the leadfields that can be used for the beamforming
>> cfg            = [];
>> cfg.vol        = vol;
>> cfg.resolution = 2;  % same unit as above, i.e. in cm
>> grid = prepare_leadfield(cfg, dataPre);
>>
>> (I expect prepare_leadfield to pick up grad definitions from the data)
>>
>> ??? Error using ==> fieldtrip/private/compute_leadfield
>> different number of spheres for the radius and origin
>
> FT works with MEG either with a single sphere or multisphere models, i.e.
>   vol.r = 12
>   vol.o = [0 0 5]
> This origin is a better guess for the center of the sphere, remember
> that in the MEG case you do not move the sensors towards the head, but
> the position of the sensors (coils) is expressed with respect to the
> head coordinate system. In case of EEG, it is more common to use
> standard electrodes that are moved towards the head. You can use
> HEADMODELPLOT to look at the geometry of the sensors and head model.
>
> A multisphere model contains a sinsgle sphere for each sensor.
> Actually, for the forward computation it should contain a single
> sphere for each coil. Multisphere models currently are not implemented
> for Neuromag data.
>
> The reason for the error is that FT is confused about it being not a
> single-sphere, but also not a multi-sphere model. Also your later
> errors are explained by that. Please use a single sphere model. Or
> alternatively you can use a realistic single-shell (Nolte) model,
> which is more accurate than the multi-sphere models, but I suggest
> first to try around with a single sphere. The most important thing for
> you to check is whether the grad structure is geometrically correct.
> Lauri Parkonen will probably be able to help you with checking that.
>
> best regards,
> Robert
>
>
>> Error in ==> prepare_leadfield at 209
>>   grid.leadfield{dipindx} = compute_leadfield(grid.pos(dipindx,:),
>> sens, vol, 'reducerank', cfg.reducerank, 'normalize', cfg.normalize);
>>
>> Error in ==> SomeMaster at 15
>> grid = prepare_leadfield(cfg, dataPre);
>>
>>
>>
>> To get around this error, I change vol to
>>
>> vol.o = [0 0 0 ; 0 0 0 ; 0 0 0  ];
>>
>> ??? Error using ==> fieldtrip/private/compute_leadfield
>> number of spheres is not equal to the number of coils
>>
>> Error in ==> prepare_leadfield at 209
>>   grid.leadfield{dipindx} = compute_leadfield(grid.pos(dipindx,:),
>> sens, vol, 'reducerank', cfg.reducerank, 'normalize', cfg.normalize);
>>
>> Error in ==> SomeMaster at 15
>> grid = prepare_leadfield(cfg, dataPre);
>>
>>
>> I thought this might be due to the 4 HPI coils, so I changed vol to
>> vol.r = 12 * [0.88 0.92 0.96 1.00];
>> vol.o = [0 0 0 ; 0 0 0 ; 0 0 0 ; 0 0 0  ];
>>
>> but it was of no use.
>>
>> Any suggestions?
>>
>> Thanks,
>> sameer
>>

--

-----------------------------------------------------------
Paul van den Hurk

Joint Principal Investigator

Mindfulness Project

Inserm U280, Lyon, France,

Radboud University Nijmegen:
Department of computer sciences and
Department of Medical Physics and Biophysics/126
Geert Grooteplein 21 6525 EZ Nijmegen

Email: P.vandenHurk at cs.ru.nl

Tel: 024-3613326