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Fig. 2.7. Lateral connections in the tree shrew orientation map. (a) The vertical and horizontal orientation preferences in a 8 mm × 5 mm section of V1 in the adult tree shrew, measured using optical imaging. The areas responding to vertical stimuli are plotted in black and those responding to horizontal stimuli in white. Vertical in the visual field (90^o) corresponds to a diagonal line at 135^o in this plot. The small green dot in the middle marks the site where a patch of vertical-selective neurons were injected with a tracer chemical. The neurons to which that chemical propagated through lateral connections are colored red. Short-range lateral connections target all orientations equally, but long-range connections go to neurons that have similar orientation preferences and are extended along the orientation preference of the source neurons. (b) The same information plotted on a 2.5 mm × 2 mm section of the full orientation map to the right and below the injection site. The injected neurons are colored greenish cyan (80^o), and connect to other neurons with similar preferences. Measurements in monkeys show similar patchiness, but in monkey the connections do not usually extend as far along the orientation axis of the neuron (Sincich and Blasdel 2001). These results, theoretical analysis, and computational models suggest that the lateral connections play a significant role in orientation processing (Bednar and Miikkulainen 2000b; Gilbert 1998; Sirosh 1995). Reprinted with permission from Bosking et al. (1997), copyright 1997 by the Society for Neuroscience.