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