Sorry, one additional question: per your suggestion, I can handle adding/averaging the real and imaginary component masks, but what syntax can I use in paint-sess to convert those composite masks into fieldsign maps?
Thanks, Jeff On Mon, May 11, 2009 at 12:40 PM, Jeff Phillips < jeffrey.s.phill...@gmail.com> wrote: > Hi Don, > > Thanks for your comments. I will try operating on the eccentricity and > polar angle maps first--however, if I calculate a fieldsign from average > maps, I presume that the fieldsign map will not be similarly graded, but > rather binary. I'll have to think about how to adapt that to our purposes. > > > I'm confident that the stimulus order was not reversed between sessions. > One difference which did exist was that the first session involved > unidirectional stimulation (i.e., only clockwise wedges and only expanding > rings), while the second session involved bidirectional stimulation in > alternating runs (i.e., both clockwise/counterclockwise and > expanding/contracting rings). Would this difference create any problems? > > More generally, I'm still a bit confused about the calculation of the > fieldsign. The wiki notes that "positive" and "negative" are arbitrarily > defined--in other words, these terms don't bear any relation to > clockwise/counterclockwise stimulation. It also appears (but please correct > me if I'm wrong) that the meanings of these terms are not linked across the > polar angle and eccentricity manipulations--that is, I haven't read of any > rule that says if you call clockwise wedges positive, then expanding rings > must also be termed positive. I thought that knowing this directionality > would be important to interpreting the cross-product. > > I have been unable to find a document, wiki page, etc. which explains some > of these details--if you happen to know of one, I would appreciate the > reference. > > Thanks, > > Jeff > > > On Fri, May 8, 2009 at 3:32 PM, Don Hagler <dhagle...@hotmail.com> wrote: > >> I think you should average your polar angle and eccentricity maps across >> session and then calculate fieldsign from that. You do a complex average (a >> separate average for real and imaginary components). >> >> By the way, the sign of the fieldsign measure should have a fixed meaning; >> the orientation of the cross product of the gradients of polar angle and >> eccenctricity, relative to the cortical surface. Are you sure you didn't >> reverse the stimulus order or something? Was the projector's image upside >> down or flipped left/right? >> >> ------------------------------ >> Date: Thu, 7 May 2009 18:47:32 -0400 >> From: jeffrey.s.phill...@gmail.com >> To: freesurfer@nmr.mgh.harvard.edu >> Subject: [Freesurfer] Can fieldsigns be added across sessions? >> >> >> Hi all, >> >> I would like to add fieldsigns across multiple retinotopy sessions in the >> same subject, in order to get a metric of the reliability of the mapping. >> However, when I tried to do this in one subject, I found that fieldsigns >> from two sessions largely canceled one another out. This led to a >> discussion in our lab of whether the fieldsign 1) has a consistent meaning >> across sessions and subjects, or 2) whether the fieldsign in a given region, >> say V1, may switch from positive to negative due to noise or between-subject >> differences. For example, a colleague speculated that paint-sess could >> start with +1 at an arbitrary starting point on the edge of an occipital >> patch, then flip the sign whenever the polar angle/eccentricity gradients >> reversed themselves. Thus, V5 might be +1, V4 = -1, V3 = +1, V2 = -1, and >> V1 would be +1. However, if noise in a given session resulted in a failure >> to detect V4, then V1 would end up being the opposite fieldsign, -1. Is >> this correct? If so, then I might be shooting myself in the foot by adding >> fieldsign maps from different sessions. I would really appreciate any >> insight about how the fieldsign is assigned, and whether circumstances like >> the ones I describe could cause it to flip for a given functional region. >> >> Thanks, >> >> Jeff Phillips >> >> ------------------------------ >> Windows Live™: Keep your life in sync. Check it >> out.<http://windowslive.com/explore?ocid=TXT_TAGLM_BR_life_in_synch_052009> >> > >
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