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Fig. 2.4. Orientation map in the macaque. (a) Orientation preference and (b) orientation selectivity maps in a 7.5 mm × 5.5 mm area of adult macaque monkey V1, measured by optical imaging techniques. Each neuron in (a) is colored according to the orientation it prefers, using the color key on top. Nearby neurons in the map generally prefer similar orientations, forming groups of the same color called iso-orientation patches. Other qualitative features are also found. Linear zones are straight lines along which the orientations change continuously, like a rainbow; a linear zone is marked with a long white rectangle. Pinwheels are points around which orientations change continuously. They often occur in matched pairs: such a pair is circled in white. At saddle points a long patch of one orientation is nearly bisected by another; one saddle point is marked with a bowtie. Fractures are sharp transitions from one orientation to a very different one; a fracture between red and blue (without purple in between) is marked with a white square. Orientation selectivity measures how closely the input must match the neuron's preferred orientation for it to respond. As shown in (b), neurons at pinwheel centers and fractures tend to be less selective (dark areas) in the optical imaging response, whereas iso-orientation patches, linear zones and saddle points tend to be more selective (light areas). Reprinted with permission from Blasdel (1992b), copyright 1992 by the Society for Neuroscience; annotations added and brightness increased.