External Email - Use Caution To add to the explanation from Bruce:
It's easy to remember the sign of 'sulc' if you know that it is an abbreviation for 'sulcal depth' -- in that case it makes sense that it is negative for gyri. Regarding curvature, if you do not have a background in math/physics, I would suggest to read the excellent Wikipedia articles on that (https://secure-web.cisco.com/1eM5ABlxLFULXpIV8y-bbSb1GhHvFGo1EsI-SHSsD8aFOLx16YoQsik0fRiFAbIE1JuKc9l3jty3MghbKKkwmbmQfElTgni2mARtASEccWhuu89N_woUQLU0vSJSfYE4jfICLCv45Fw3TBR-rAL3kXJjmu0t-7Rp3Evfg3SOA8LoetDPiv4u5pEm7Cjp6aEh5_Fik8-1gLocoQrbbouNpkje-HjGVzDJkUhaWcfu6epvIQwsXVuar_kJD7Gz1kQTpH5jlPficYtOMPCCMCKxMaA/https%3A%2F%2Fen.wikipedia.org%2Fwiki%2FCurvature). Also read the related ones (for mean curvature, Gaussian curvature, and the one about principle curvatures). Afterwards it will be very clear. Tim -- Dr. Tim Schäfer Postdoc Computational Neuroimaging Department of Child and Adolescent Psychiatry, Psychosomatics and Psychotherapy University Hospital Frankfurt, Goethe University Frankfurt am Main, Germany > On 06/15/2021 4:53 AM Fischl, Bruce <bfis...@mgh.harvard.edu> wrote: > > > Maybe a better way to think about curvature is that it indicates whether the > surface is locally above (positive) or below (negative) the tangent plane > > On Jun 14, 2021, at 10:29 PM, Fischl, Bruce <bfis...@mgh.harvard.edu> wrote: > > > Sulc is the integrated dot product of the movement vector with the (outwards > pointing) surface normal during inflation. In gyral regions the movement > vector is consistently inwards and has a negative dot product with the > surface normal, which is why it is negative. > > As for your second question, I’m not sure I understand. The mean curvature is > the average of the two principle curvatures. If you think of the surface > locally as a height function over the tangent plane, then the two curvatures > are the eigenvalues of the Hessian of that function (if that helps). > > Cheers > Bruce > > From: freesurfer-boun...@nmr.mgh.harvard.edu > <freesurfer-boun...@nmr.mgh.harvard.edu> On Behalf Of 1248742467 > Sent: Monday, June 14, 2021 9:23 PM > To: freesurfer <freesurfer@nmr.mgh.harvard.edu> > Subject: [Freesurfer] In Freesurfer, the sulc value of the gyrus is NOT > positive, on the contrary, they are negative? > > > External Email - Use Caution > Hello, Fs > In Freesurfer, the sulc value of the gyrus is NOT positive, on the contrary, > they are negative？ > by the way, how the mean curvature reflects the degree of fold? Since the > mean curvature is a local measurement, does the mean curvature map reflects > the fold degree? > _______________________________________________ > Freesurfer mailing list > Freesurfer@nmr.mgh.harvard.edu > https://secure-web.cisco.com/1Eb4xq4CX3GZx_5NdnHleKfrS7lKbe8yMipvOqbtWwkLQY2YiE4dxiPxrRCoL22Lx8WfScI0iSLB5a-fsoBTkO7-wWyli-CZp8G7_dZ_30vBLswWRlqO79dKoXf2MJLRwlkpc-wMQLrlhJ1EM3-Y6HEVtnbjOrVdfhcj7ManEFKeik0I0qoQPwh3kkO-o4AARZhDhZw15NGZ5uwAlI-zhcS_-h-C26VRnYTL3_cetzJRhSqWjD12tiznkcO1__4VX4ZM1tHsMLq5fmTcPonhxGQ/https%3A%2F%2Fmail.nmr.mgh.harvard.edu%2Fmailman%2Flistinfo%2Ffreesurfer > The information in this e-mail is intended only for the person to whom it is > addressed. If you believe this e-mail was sent to you in error and the e-mail > contains patient information, please contact the Mass General Brigham > Compliance HelpLine at > http://secure-web.cisco.com/1DZiNFukpBCOy4Kpraothv0_gN5bq4-rcWy4Q58wgEj_-cH0ywMTxFO6exOrM57gtlitPpvjZtsHrARGkuZyl11HL_I1NrrrtmyL1NWwSq0QTqm3VQ2Z9xUQKk2OmkP-gd-C12bKD73Ver11X0nU2tU3grmwhkcT7lJoxcZTLluUWXZr7hzcDhub79g-fiDuFxfq_lzHRLMGvynKEBptex840aQEdivtffpW26QfYt1MllACyCTMk9dOVTqThhAlQI-cVl-BqU1jWKy2-gOZAOw/http%3A%2F%2Fwww.massgeneralbrigham.org%2Fcomplianceline > . If the e-mail was sent to you in error but does not contain patient > information, please contact the sender and properly dispose of the e-mail. > Please note that this e-mail is not secure (encrypted). If you do not wish > to continue communication over unencrypted e-mail, please notify the sender > of this message immediately. Continuing to send or respond to e-mail after > receiving this message means you understand and accept this risk and wish to > continue to communicate over unencrypted e-mail. _______________________________________________ Freesurfer mailing list Freesurfer@nmr.mgh.harvard.edu https://mail.nmr.mgh.harvard.edu/mailman/listinfo/freesurfer
