Neurobiological Background - Hierarchical Neural Networks for Image Interpretation - page 23

Fig. 2.7. Hypercolumn in V1. Within 1mm 2 of cortex all features of a small region of the vi-
sual field are represented. Orientation columns are arranged around pinwheels. Ocular dom-
inance stripes from the ipsilateral (I) and the contralateral (C) eye alternate. Blobs represent
color contrast (adapted from [117]).
disparity. A third regular structure in V1 is the blob system. Neurons in the blobs are
insensitive to orientation, but respond to color contrast. Their receptive fields have
a center-surround shape, mostly with double color opponency.
Similar substructures exist in the next higher area, V2. Here, not columns, but
thin stripes, thick stripes, and interstripes alternate. The stripes are oriented orthog-
onally to the border between V1 and V2. A V2 'hyperstripe' covers a larger part of
the visual field than a V1 hypercolumn and represents different aspects of the stimuli
present in that region. As illustrated in Figure 2.4, the blobs in V1 send color infor-
mation primarily to the thin stripes in V2, while the orientation sensitive interblobs
in V1 connect to interstripes in V2. Both thin and interstripes project to separate
substructures in V4. Layer 4B of V1 that contains cells sensitive to the magnocellu-
lar (M) information projects to the thick stripes in V2 and to area MT. Thick stripes
also project to MT. Hence, they also belong to the M pathway.
These structured maps are not present at birth, but depend for their development
on visual experience. For example, ocular dominance stripes in V1 are reduced in
size if during a critical period of development input from one eye is deprived. The