Specificity and randomness in the visual cortex

Abstract

Research on the functional anatomy of visual cortical circuits has recently zoomed in from the macroscopic level to the microscopic. High-resolution functional imaging has revealed that the functional architecture of orientation maps in higher mammals is built with single-cell precision. By contrast, orientation selectivity in rodents is dispersed on visual cortex in a salt-and-pepper fashion, despite highly tuned visual responses. Recent studies of synaptic physiology indicate that there are disjoint subnetworks of interconnected cells in the rodent visual cortex. These intermingled subnetworks, described in vitro, may relate to the intermingled ensembles of cells tuned to different orientations, described in vivo. This hypothesis may soon be tested with new anatomic techniques that promise to reveal the detailed wiring diagram of cortical circuits.

Figure 1

Single-cell resolution orientation maps from (a) a pinwheel in cat visual cortex [42], and (b) rat visual cortex [38**]. Cells are colored according to their preferred orientation. In (a), ~1,000 cells from nine different depths are overlaid. Cells are arranged up to the very center of the pinwheel. In (b), cells in one depth are displayed. Even neighboring cells are tuned to different orientations. (c) In rat visual cortex, relatively independent subnetworks are embedded in larger-scale functional architecture [31*]. Excitatory connections from layer 4 to layer 2/3 and within layer 2/3 define subnetworks of selectively interconnected neurons (red or blue). The excitation from layer 5 (gray triangles) and inhibition from layers 2/3 and 4 adaptive interneurons (IN, gray ovals) does not respect the subnetworks.