Biological Bases of Backpropagation

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Experimental and theoretical investigations of possible mechanisms of backflow of information in neurons are proposed. Such retrograde flow of information is a crucial aspect of learning and plasticity of neural networks involving backpropagation. The experimental preparations to be used are hippocampal slices of the rat brain. Evidence of retrograde pathways of informational flow other than membrane electrical potentials will be sought. One stimulus will be mechanical compression of efferent axons or axon hillocks. Changes in synaptic response at the dendritic and axonal levels will be monitored as evidence of backpropagation. The time scale of interest will be milliseconds as opposed to slower processes of axonal cytoplasmic flow in either direction. If functional plasticity is demonstrated, electron- microscopic examination of synapses will be made to seek cytoskeletal or vesicle alterations that correlate with the functional changes. Theoretical models will consist of cytoskeletal components of actin-myosin gel, microtubular structure and a viscoelastic matrix representing cytoplasm. An active tension will be assumed in the actin-myosin gel. Possible modes of wave propagation in the composite structure will be investigated. The significance of the proposed research will be very important of correlation of biological counterparts to theoretical models of neural networks.