Hi Donna,

The algorithm I am proposing is the following:

Xcog = (Xcogt' * At') / sum(At);

where:

Xcog = surface center of gravity
Xcogt = tiles' center of gravity (t-by-3 matrix)
At = tiles' area (t-length vector)

Best regards,
Mateus

Donna Dierker wrote:

Hi Donald and Mateus,

I'm guessing so, but this is way oustide the sphere of anything I could implement. It sounds like something John might be able to do, and the overall strategy seems sensible to me, although I confess some of the details are fuzzy to me. I understand you're weighting by area -- not node density, but this part isn't 100% clear to me:

calculate the COG of the whole surface summing the COG of each tile multiplied by its own area and divided by the total surface area

But what matters is whether it's clear to John; whether he can do it; and how many things he needs to do before he can get to it.

On 01/30/2007 09:51 PM, DG MCLAREN wrote:

Donna/John,

Is it possible to apply the distortion correction to individual tiles?

Best Regards, Donald McLaren
=====================
D.G. McLaren
University of Wisconsin - Madison
Neuroscience Training Program
Tel: (773) 406 2464
=====================
This e-mail contains CONFIDENTIAL INFORMATION which may contain PROTECTED HEALTHCARE INFORMATION and may also be LEGALLY PRIVILEGED and which is intended only for the use of the individual or entity named above. If the reader of the e-mail is not the intended recipient or the employee or agent responsible for delivering it to the intended recipient, you are hereby notified that you are in possession of confidential and privileged information. Any unauthorized use, disclosure, copying or the taking of any action in reliance on the contents of this information is strictly prohibited and may be unlawful. If you have received this e-mail unintentionally, please immediately notify the sender via telephone at (773) 406 2464 or email.

----- Original Message -----
From: Mateus Joffily <[EMAIL PROTECTED]>
Date: Tuesday, January 30, 2007 12:29 pm
Subject: Re: [caret-users] ROI center of gravity
To: "Caret, SureFit, and SuMS software users" <caret-users@brainvis.wustl.edu>


Hi Donald,

Thanks for your comments.

I would like to propose a 'new' method for calculating the surface's COG. To avoid the nodes density problem, I think it would be reasonable to calculate the COG of every tile on the surface and, after, to calculate the COG of the whole surface summing the COG of each tile multiplied by its own area and divided by the total surface area. I have done some tests using this method and compared the results with Caret's method.

On the attached capture (cog_m1.jpg), you can see a flat map of the primary motor cortex. Depending on the method used (caret vs. 'new' method) the COG can be shifted of up to 10mm. The 'new' method shows a
COG more superior, consistent with the lower concentration of nodes at
this region of the surface. Capture 'cog_m1_zoom.jpg' is just a zoom in on the tiles' CoG (green points).

I would appreciate any comment on this method.

I couldn't find the paper you cited. I guess it hasn't been published
yet. Do you know when it will be available for download?

Best regards,
Mateus

DG MCLAREN wrote:

Mateus,

You raise a good point.
In practice, it seems that you should keep the uneven distribution of

nodes. The reason is as follows: If you resample the nodes to a more sparse uniform distribution, then you will distort the surface to some degree. The surface distortion will inevitable shift the center of gravity. I had thought about using the border points of a cluster to calculate the region; however, it seems that approach may unequally weight some parts of the cluster if there is an odd shape. It might be interesting to do a comparison between these methods.
In the end, it probably doesn't matter how you do the calculation for

COG, as they should be similar, as long as the method is reported. I should also note, a recently accepted article: Cortical network for vibrotactile attention: A fMRI study in Human Brain Mapping describes the method and uses the traditional method; so there is an established standard that one could rely on and cite in future work.
Best Regards, Donald McLaren
=====================
D.G. McLaren
University of Wisconsin - Madison
Neuroscience Training Program
Tel: (773) 406 2464
=====================
This e-mail contains CONFIDENTIAL INFORMATION which may contain

PROTECTED HEALTHCARE INFORMATION and may also be LEGALLY PRIVILEGED and which is intended only for the use of the individual or entity named above. If the reader of the e-mail is not the intended recipient or the employee or agent responsible for delivering it to the intended recipient, you are hereby notified that you are in possession of confidential and privileged information. Any unauthorized use, disclosure, copying or the taking of any action in reliance on the contents of this information is strictly prohibited and may be unlawful. If you have received this e-mail unintentionally, please immediately notify the sender via telephone at (773) 406 2464 or email.
----- Original Message -----
From: Mateus Joffily <[EMAIL PROTECTED]>
Date: Friday, January 26, 2007 12:12 pm
Subject: [caret-users] ROI center of gravity
To: "Caret, SureFit, and SuMS software users" <caret-users@brainvis.wustl.edu>


Hi,

I would like to calculate the center of gravity (COG) of a surface ROI. I see that 'Surface Region of Interest':'Statistical Report' gives

me
that information. However, if the nodes distribution over the

surface
is not uniform, the COG is biased toward the more dense regions.
I understand that, if someone wants to calculate the COG, taking

into
account the nodes-density, this is the right calculation. But, if I suppose that the surface density is uniform, is there a way of calculating the COG avoiding the nodes distribution effect?

Thanks,
Mateus
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