Neurobiological Background - Hierarchical Neural Networks for Image Interpretation - page 20

(a) (b)
Fig. 2.3. Hierarchical structure of the visual system. (a) Felleman and Van Essen's [65] flat
map of the Macaque brain with marked visual areas; (b) wiring diagram of the visual areas.
corners, or crossings, for instance. It is believed that V2 neurons play a crucial role
in perceptual grouping and line completion since they have been shown to respond
to illusory contours.
V1 and V2 are only the first two of more than 30 areas that process visual infor-
mation in the cortex. A cortical map illustrates their arrangement in Figure 2.3(a).
Part (b) of the figure shows a wiring diagram. It can be seen that these areas are
highly interconnected. The existence of about 40% of all possible connections has
been verified. Most of these connections are bidirectional, as they carry information
forward, towards the higher areas of the cortex, and backwards, from higher areas
to lower ones.
The visual areas are commonly grouped into a dorsal stream that leads to the
parietal cortex, and a ventral stream that leads to the inferotemporal cortex [39], as
shown in Figure 2.4. Both pathways process different aspects of visual information.
The dorsal or 'where' stream focuses on the fast processing of peripheral stim-
uli to extract motion, spatial aspects of the scene, and stereoscopic depth informa-
tion. Stimuli are represented in different frames of reference, e.g. body-centered and
hand-centered. It works with low resolution and serves real-time visuomotor behav-
iors, such as eye movements, reaching and grasping. For instance, neurons in the
middle temporal area MT were found to be directionally sensitive when stimulated
with random dot patterns. There is a wide range of speed selectivity and also selec-
tivity for disparity. These representations allow higher parietal areas, such as MST,
to compute structure from motion or structure from stereopsis. Also, ego-motion,
caused by head and eye movements, is distinguished from object motion.
In contrast, the ventral or 'what' stream focuses on foveal stimuli that are pro-
cessed relatively slowly. It is involved in form perception and object recognition