Sensory processing and the network mechanisms far reading neuronal population codes

In many neuronal systems, information appears to be represented in the acti vity of populations of neurons. Such neuronal population codes must also be read out, or interpreted, by downstream networks. Recent studies in both v ertebrate and invertebrate systems have begun to elucidate some of the gene ral mechanisms underlying these processes. Directed behaviors, that involve a directional response to a directional sensory input, have been a particu larly useful context for these studies because, among other things, their i nput-output relationship is easily defined and experimentally controlled. W e have recently shown that the neuronal network underlying a directed behav ior in the medicinal leech utilizes a specific population coding scheme bas ed on a neuronal population vector. A population vector of mechanosensory n euron activity correlates well with behavioral output and the connectivity of the downstream network is well suited for accurately reading out this po pulation code.