Raymon M. Glantz - US grants

The means by which information from sensory receptors is encoded is an important general problem in neurobiology. This project is a study of a specific example of this problem, the coding of visual information in a relatively simple neuronal network, the crayfish lamina ganglion. The ganglia of the crayfish optical system are arranged in columnar fashion. Their neurons are distributed in an ordered manner such that they form a map of the retina. Each point in visual space is encoded by five ganglion neurons which form a single column. These neurons have graded responses to light. Thus, each column can encode intensity, local contrast, orientation of polarized light, and wavelength (color) at a small region of the visual field. The Principal Investigator has characterized the morphology and connections of many of the ganglion cells in this neural network, and has also determined the chemical messengers by which some of them communicate. In this project he hopes to determine how the neural circuits already identified compute the values of various aspects of the stimulus, including polarization angle and velocity of movement, and how visual behavior is influenced by their activity. The methods he is using include electrical recording from neurons that can be identified microscopically from dye injected into them at the time of recording, and using immunocytochemical and pharmacological procedures to identify their chemical messengers (neurotransmitters).

IBN-9507878 Raymon Glantz A broad spectrum of brain activities, from object recognition to postural adjustments, require that neurons and neural circuits make computations. In recent years, neurobiologists have made impressive progress in two related areas: one concerns the biophysical and molecular mechanisms of synapses and the second concerns the mathematical and logical operations which formally describe what whole systems of nerve cells do. In the study of how the visual system works, algorithms successfully model visual tracking and fixation, binocular vision, and motion detection. Future progress in understanding more about the visual system depends upon integrating the biophysical and algorithmic approaches. This will require detailed information about the structure and dynamic properties of neurons involved in particular computations. Many visual systems can compute the direction and velocity of moving targets. Recent studies indicate that directional motion detection is achieved through synaptic operations embedded in the structure of the neuron's receptive field. The same directional mechanisms were independently found in visual neurons in higher vertebrates and in the crayfish. The mechanisms depends upon spatial and temporal asymmetries of the excitation and inhibition within the receptive field. This project focuses on experimental and theoretical studies of the synaptic mechanisms which support directional motion detection. Physiological measurements will quantify the characteristics of the synapses which form the directional mechanism. The simulations will determine if the measured properties of the synapses are sufficient to support the directional mechanism.

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.