|
Visual form analysis in the ventral
stream can be described by a hierarchical processing model. In this model
cells in every stage of visual processing integrate information from a group
of cells in the preceding stages and hence respond to a more complex feature
than cells in the previous stages. We used Glass pattern to explore this
hierarchical model in the human visual cortex. A Glass pattern consists of
randomly distributed dot pairs (dipoles) whose orientations are determined
by a geometric transform. To perceive the structure in a Glass pattern, an
observer needs to perform local grouping to find dipoles and global grouping
across dipoles to get an overall shape. We expected cortical mechanisms for
local and global grouping located in different brain areas. Each dipole
contained two 5.4’ square dots separated by 27’. The Glass patterns
contained randomly distributed dipoles covering 2% of image. The coherence
of a Glass pattern was defined as the proportion of dipoles oriented tangent
to a concentric global form. To eliminate grouping at the dipole level, we
also used line element textures which had bars (5.4’x27’) in place of
dipoles. The BOLD activation (N=6) was collected on a Bruker 3T magnet (EPI,
TR=3s, TE=60ms, flip angle=90o). Each 252s block design run had two test
conditions alternating with each other in a 36s period. Compared with a zero
coherent pattern, the 100% coherent Glass patterns activated the lateral
occipital complex (LOC) and the inferior temporal (IT). The Glass and the
texture pattern of the same global structure showed differential activation
in V2 and V3 which was posterior to the areas responding to Glass patterns.
A rotating wedge of coherent dipoles showed that the dorsal portion of the
LOC had a retinotopic property for Glass patterns. A rotating concentric
pattern produced only weak retinotopic activation at the ventral LOC. The
contrast between line texture and Glass patterns suggests that local
grouping occurred in V2 and V3. The rotating wedge suggests the dorsal LOC
is for analyzing curvatures while the rotating circle hints that the ventral
LOC is responsible for global forms. |
