Connectivity and Functional Organization in the Mammalian Visual Cortex

A remarkable aspect of the early phases of visual processing is the elaboration of receptive field properties along the visual pathway, from the center-surround cells of the retina and lateral geniculate nucleus (LGN), to the orientation and directionally selective simple and complex cells of the mammalian striate cortex. Yet the underlying neuronal circuitry responsible for the construction of new receptive field properties at each stage is poorly understood. Anatomical techniques have revealed several types of neuronal connections that are likely to participate in this process, but the actual functional role of these connections is, in most cases, not known. The present studies focus on the horizontal connections that are intrinsic to the cortex and run laterally, parallel to the surface of the cortex. Anatomical studies have shown that these connections extend for up to several millimeters and the clustered nature of the axonal collaterals suggests a specificity of these connections related to the columnar organization of receptive field properties. The first series of experiments, performed in the cat striate cortex, explore the relationship between receptive field properties of single cells and the participation of these cells in the long-range horizontal connections. These connections were detected and measured physiologically by electrical recording and cross-correlation analysis, a statistical method that provides a sensitive measure of the strength and type of connections. These experiments showed that, among oriented cells, cells having a particular optimal orientation only interacted with other cells having a similar optimal orientation, up to distances of several millimeters in the cortex.