Functional excitatory laminar input to distinct pyramidal cell types in layer 6 of primate V1

Farran Briggs and Edward M. Callaway

Salk Institute

A fundamental goal in neurophysiology is to understand how various inputs give rise to specific functional properties of neurons. The first visual cortical area of primates, V1, is an ideal area in which to address how input sources contribute to cellular function because it contains distinct anatomically and physiologically defined populations of neurons separated into layers. We examined the laminar sources of functional excitatory input onto a population of excitatory pyramidal neurons in the deepest cortical layer of primate V1, layer 6, using scanning laser photostimulation. We found that different excitatory laminar input patterns correlated with each of 8 anatomically defined cell types. Class I neurons are defined by axonal arborizations selective for the magnocellular (M) recipient layer 4C alpha and/or the parvocellular (P) recipient layer 4C beta. Class I neurons whose axonal arbors targeted layer 4C alpha received functional input from M-dominated layer 4B while Class I neurons whose axonal arbors targeted layer 4C beta received functional input from P-dominated layer 2/3. All Class I cells received inputs of similar strengths directly from layers 4C alpha and 4C beta. Class II neurons tend to lack extensive axonal arborizations within layer 4C. Each of the three anatomically defined types of Class II neuron tended to receive excitatory functional input from the same layers targeted by their local axons. These results indicated a combination of segregation and integration of M- and P-specific information as it travels through V1. Distinct functional input patterns for each pyramidal cell type implies each layer 6 excitatory neuron may perform a unique functional role.