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Abstract:
Patient DF is unaware of visual orientation, yet can use
visual contours to guide motor responses. We tested her ability to
utilise contours defined by Gestalt grouping principles. Single
cell studies indicate that early visual areas (V1 and V2) process
the orientation of illusory contours defined by the 'continuity' of
line terminations. We studied patterns of concentric circles where
the orientation of a central displaced rectangular area was defined
exclusively by continuity. Contrary to previous reports, we found
that DF could use the continuity in these patterns to guide motor
responses. DF was unable to detect 'inside-outside' relations or
'connectedness': she could not discriminate elements bounded by a
common contour from elements that were not, or discriminate
elements that were connected by a contour from those that were not.
DF has close to normal colour vision and can readily identify red
and green colour spots. She was, however, unable to group the spots
by 'similarity' in their colour when the spots were arranged as a
regular pattern of red and green columns. Grouping by similarity
can thus fail even when colour perception of isolated elements is
intact. The disruption of most grouping principles but selective
sparing of continuity for DF indicates that different grouping
principles are implemented in different cortical areas.
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