Contributions of individual layer 6 pyramidal neurons to local circuitry in macaque primary visual cortex.

We have studied the contributions of individual layer 6 pyramidal neurons in macaque primary visual cortex to local cortical circuitry by intracellular labeling and analysis of the morphologies of 58 neurons. These neurons are separated based on the laminar specificity of axonal and dendritic arbors into two classes, class I and class II, and into several types within these classes. Class I neurons project axons heavily and predominantly to layer 4C, whereas class II neurons have axonal projections primarily to layers other than 4C. Only 16 of the 58 neurons in our sample (28 percent) project to the white matter. Class I projection neurons are found at the top and bottom of layer 6, suggesting that they project to the lateral geniculate nucleus, whereas class II projection neurons are located in the middle of layer 6, suggesting that they project to the claustrum. The different types of class I neurons are distinguished from one another based on the sublaminar specificity of their axonal and dendritic arbors within layer 4, where they are biased toward compartments dominated by input from different functional streams. They are also distinct in their distributions within the depth of layer 6. The distinctive characteristics of the neuronal types we have identified suggest that each receives input from different sources and projects to a set of targets that is functionally appropriate. Thus, each type is likely to contribute uniquely to computations within V1 and extrinsically.