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Flat Maps
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The curvature can then be color-coded, e.g., different gray values can be used for different
ranges of curvature. mrFindBorders only uses functional flat maps; it does not use anatomical
flat maps.
When showing a visual stimulus, instead of measuring the amplitude of the responses in
visual areas, phase data and coherence data of the responses can be derived from the fMRI
data. Different functional flat maps are known. Here, only the functional flat maps that
mrFindBorders uses are explained. Especially phase data is important because the pattern in
phase data can be used to separate visual areas.
3.8.1 Phase Maps
As explained before, the areas V1, V2, and V3 are retinotopically organized. Because of this,
it is possible to control the location of the neural activity by choosing appropriate visual stim-
uli [Wandell, 1999 #16]. When presenting a subject with a flickering contracting (Figure 13
A) or expanding (Figure 14 A) ring visual stimulus, the signal at each location within the vis-
ual field changes periodically. The contrast reversal rate (flicker frequency) is, for example,
8Hz. The contrast reversal is used to improve the response during the activation periods. The
stimulus alternation frequency is, for example, 1/36 Hz. When measuring the neural responses
at different locations in a retinotopic visual area, a flickering expanding ring visual stimulus
with a constant velocity (from the fovea to the periphery) causes all signals (from the different
locations in the visual cortex) to vary at the same frequency. However, the temporal phase of
the signals varies depending on the location in cortex in eccentricity direction (see section 3.4
about fMRI). The phase values can be color-coded (Figure 14 D). The color ring shows the
mapping between eccentricity and colors. The stimulus shown in Figure 14 A creates a travel-
ing wave (Figure 14 C) [Wandell, 1999 #16]. Therefore, to create a wave of neural activity
that travels across cortex in the eccentricity dimension, a contracting or expanding flickering
ring visual stimulus (Figure 13 A) can be used. The neural activity can be measured in differ-
ent visual areas, including V1, V2, and V3.
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