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Methods
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Then, the next control points are considered. If there has been an improvement when the
previous control point was tested, the new deformed atlas (deformed with the best estimation
of the displacement fields at that time) is used to re-calculate the derivatives. After all relevant
control points have been tested, the next iteration starts (if the number of iterations specified
by the user is greater than one).
5.2.6.2 Wedge and Ring Visual Stimuli –“Grad. (2 Directions)”
The algorithm described above works very well for matching phase data for the rotating
wedge visual stimulus case. However, since the direction, in which pixel values are displaced,
is the direction of the gradient in the phase-wedge atlas (angular direction), this algorithm is
not ideal for the ring-stimulus case. The reason for this is that the direction of displacement is
expected to be in direction of the gradient in the phase-ring image, which is estimated to be
perpendicular to the search direction for the wedge-stimulus case.
In case, the user wants to use the ring atlas in the matching process, the “Grad. (2 Direc-
tions)” algorithm should be selected from the options menu. At first, an algorithm was imple-
mented where pixel values are displaced separately in horizontal and vertical directions. How-
ever, the algorithm was improved so that the displacements are no longer done in horizontal
and vertical but in radial and angular directions. More specifically, the radial and angular di-
rections in the visual field are taken, not the ones in the flat map. The directions are estimated
based on the best-deformed wedge atlas.
In more detail, the new algorithm, like described before, orders the control points accord-
ing to coherence values. The range of displacement values, which is tested for each control
point, is determined individually for angular direction (direction of the gradient in the phase-
wedge atlas image) and radial direction (perpendicular to the former direction). The directions
are calculated based on the best-deformed wedge atlas as explained before. The number of
displacement values is specified as an option. For a certain control point, test displacement
values in angular direction, which are determined by the range and the number of control
points in that range, are tested. Every time a new displacement value has been assigned to the
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