Neurobiological Background - Hierarchical Neural Networks for Image Interpretation - page 21

Fig. 2.4. Dorsal and ventral visual streams. The dorsal stream ascends from V1 to the parietal
cortex. It is concerned with spatial aspects of vision ('where'). The ventral stream leads to
the inferotemporal cortex and serves object recognition ('what') (adapted from [117]).
tasks. A hierarchy of areas represents aspects of the visual stimuli that are increas-
ingly abstract.
As illustrated in Figure 2.5, in higher areas the complexity and diversity of the
processed image features increases, as do receptive field size and invariance to stim-
ulus contrast, size, or position. At the same time spatial resolution decreases. For
instance, area V4 neurons are broadly selective for a wide variety of stimuli: color,
light and dark, edges, bars, oriented or non-oriented, moving or stationary, square
wave and sine wave gratings of various spatial frequencies, and so on. One con-
sistent feature is that they have large center-surround receptive fields. Maximum
response is produced when the two regions are presented with different patterns
or colors. Recently, Pasupathy and Connor [176] found cells in V4 tuned to com-
plex object parts, such as combinations of concave and convex borders, coarsely
localized relative to the object center. V4 is believed to be important for object dis-
crimination and color constancy.
The higher ventral areas, such as area IT in the temporal cortex, are not neces-
sarily retinotopic any more since neurons cover most of the visual field. Neurons
in IT respond to complex stimuli. There seem to exist specialized modules for the
recognition of faces or hands, as illustrated in Figure 2.6. These stimuli deserve
specialized processing since they are very relevant for our social interaction.
Both streams do not work independently, but in constant interaction. Many re-
ciprocal connections between areas of different streams exist that may mediate the
binding of spatial and recognition aspects of an object to a single percept.