1. Fieldsign is supposed to be -1 or 1, without values in between. It is the
sign of the cross product. I don't see why you would want to average the
fieldsign. You will get the best estimate of the fieldsign by calculating it
from the average of all your available data.
2. Reversing the stimulus direction would definitely affect the fieldsign
calculation. And you will have to take it into account if you average across
scans or sessions. By that I mean you need to reverse the phase (by setting
the imaginary component negative) of one direction before adding to the other.
I also subtract a few seconds worth of phase to account for hemodynamic delay.
3. The sign may be arbitrary, but it is not random. The fieldsign depends on
whether you consider clockwise a positive or negative rotation and expansion
positive or negative. If you change either of those conventions, the fieldsign
will flip. Also, the color wheel in tksurfer depends on those conventions.
Red is supposed to be upper field and green is suppoed to be lower field. That
too is an arbitrary assignment, but it only holds true for clockwise rotations.
For counterclockwise, you need to reverse the phase for the colors to look
right.
4. Someone else may know if the fsfast tools can handle this type of situation.
Date: Mon, 11 May 2009 12:40:41 -0400
Subject: Re: [Freesurfer] Can fieldsigns be added across sessions?
From: jeffrey.s.phill...@gmail.com
To: dhagle...@hotmail.com
CC: freesurfer@nmr.mgh.harvard.edu
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
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