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Abstract:
The perception of visual scenes can be anchored with 3 main
frames of reference (RF); viewer-based, object-based, and
environment-based. We sought to distinguish the neural circuits
underlying each of these RFs using fMRI at 1.5T. Subjects made
left/right judgments while attending to each RF. Each visual
display, viewed under low-level illumination, consisted of an
asymmetrical cross (object) and a vertical bar (environment)
presented in two of four possible positions. Subjects determined
whether the object was either:
- on the subject's left (L) or right (R) visual field
(viewer-based)
- to the L/R of the vertical bar (environment-based) or
- whether the object was pointing to the L or R
(object-based)
A control condition consisted in pressing the same button for every
stimulus presentation. Ten subjects each performed 8 runs (each
consisting of two 24-secs blocks of the 4 conditions) during the
scanning session. Composite analysis reveal that, when compared to
control, the 3 RF conditions showed overlapping regions of
activation in the frontal (L pre-central gyrus), parietal (L+R
superior parietal, supramarginal and angular gyri) and occipital
(superior occipital gyrus) lobes. To examine RF-specific regions,
we isolated pixels that survived the subtractions with both of the
other conditions. Some of the regions activated for each RF were:
- Object-based RF: lateral occipital and fusiform gyri and
medial superior frontal gyrus
- Environment-based RF: medial dorsal frontal gyrus
- Viewer-based RF: right parieto-occipital junction and
pre-cuneus.
These results suggest that the three principal visual frames of
reference consist of both distinct and partially overlapping neural
circuits.
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