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Flat Maps
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It is useful to create a flattened representation of the cortical surface (for reasons why this is
useful please refer to section 3.6). The temporal phase data can then be color coded and over-
laid to create a phase map. A phase map is therefore a map of temporal phases over cortex lo-
cation. In the case of the ring experiment, the phases encode locations in the visual field along
the dimension of eccentricity.
Another visual stimulus that is often used is a flickering rotating wedge (Figure 13 B). It
rotates slowly about the fixation point. For this stimulus, the neural activity spreads from the
lower to the upper lip of the calcarine [Wandell, 1999 #16], i.e., in the angular dimension. The
waves of neural activity travel in opposite directions in V1 and V2, because there is a reversal
of the angular retinotopic organization in the visual areas. Reversals also occur at the bounda-
ries between V2 and V3. It is important to note that the activity measurements can be used to
segregate the visual areas V1, V2, and V3 if the visual stimulus is a rotating wedge.
The activity measurements used to segregate the visual areas can be represented as phase
maps. As for the ring stimulus, the phases are color-coded. Therefore, different colors repre-
sent different angular positions in the visual field. Because the traveling waves travel in oppo-
site directions in V1 and V2 and in V2 and V3, the direction of increasing phase values also
reverses. These phase reversals at boundaries between retinotopically organized visual areas
can be used to segregate retinotopic visual areas, i.e., to find the boundaries between those
areas.
An important property of phase maps from rotating wedge stimuli (Figure 15 bottom), is
explained next. As mentioned earlier, the left visual field is represented in the right cortical
hemisphere and vice versa. Therefore, if a rotating wedge visual stimulus is applied and the
subject fixates at the rotation center, the left hemisphere will theoretically see only half of the
phases. These phases correspond to the location of the wedge in the right visual field. In prac-
tice, phase images of the left hemisphere also show phase values that should only be found in
the right hemisphere. This is due to noise, not ideal fixating, and other factors. In contrast, if
an expanding or contracting ring visual stimulus is used (Figure 15 top), all phases can be
measured in each hemisphere.
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