Nervous systems represent external events by trains of impulses in sensory interneurons, but the exact mapping between stimuli and response features is generally unknown. The central thesis of this proposal is that any mapping between stimuli and response functions is generally unknown. The central thesis of this proposal is that any biologically useful neural information code must represent external signals and be decoded (i.e. read) by other neurons or effectors. Elucidating the neural code requires mapping the relationship between the stimulus and the neural response and between that neural response and post-synaptic (or effector) action which results in measurable behavior. Practical problems in deciphering a neural code are mitigated by studying relatively simple systems with known connectivity and simple behaviors. The neuronal pathway subserving the crayfish dorsal light reflex is one such system, consisting of an afferent ensemble of 14 identified sustaining fibers which monosynaptically excite a set of identified optomotor neurons. We propose a four part study of the coding-decoding issues raised above. i) Behavioral Studies- to determine the steady-state and dynamic stimulus parameters that result in movement ii) An analysis of the sustaining fiber encoder mechanism to quantify the information transfer in the light to sustaining fiber EPSP and from the EPSP to spike trains (i.e. the encoder mechanism). Using new information-theoretical data analysis techniques can quantify the efficacy of neural information processing as represented by analog signals (EPSPs), and by impulse trains of single neurons or neural populations. Pulse trains elicited with extrinsic current injection will provide an independent characterization of the encoder mechanism. iii) Analysis of the sustaining fiber to motoneuron functional connections to determine how sustaining fiber impulse train features control motoneuron excitability. The decoding features of the synapse will be examined with information-theoretic analysis of simultaneously recorded sustaining fiber and motoneuron impulse trains, by conditional cross- correlations and by subjecting the synapse to artificial pulse trains generated by current injections into the sustaining fibers. iv) Analysis of the relation between motoneuron response and behavior. This simple system will thus be examined from an information-theoretic perspective, using both empirical and modeling techniques. The goal is to both decipher the code and elucidate how the code is formed. The problem of neural coding has important implications for the design of prosthetic devices and the linkage between such devices and the nervous system.