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.phillips@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
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