NEURONAL STRUCTURE AND FUNCTION IN A SENSORY PROCESSOR

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The goal is to understand how neural networks process biologically relevant sensory information and orchestrate adaptive behavioral responses. The electric sense of fish has been chosen as a model system since, in spite of its relative simplicity, it shares basic design principles with hearing and vision in higher animals. We want to continue intracellular studies of neurons in the midbrain region of weakly electric fish in order to relate physiological response properties to morphological features. Electric fish evaluate modulations in the amplitude and phase of the electric signal that results from the interference of their own electric organ discharges (EODs) with those of neighbors. Similar, but spatially varying modulations in phase and amplitude are caused by moving objects, and the detection of such objects may thus be impaired by interfering EODs. Much as in the auditory system of the owl, phase and amplitude information are coded and processed by separate neuronal channels up to the level of the midbrain. Both forms of information are then combined for the control of behavioral responses. Physiologically identified neurons are labelled by intracellular injection of either HRP or Lucifer Yellow and studied under the light microscope in order to determine their morphological features, such as laminar location of somata, dendritic organization and targets of projection. We will attempt to correlate differences in dendritic morphology with differences in functional properties, such as sensitivity to either phase or amplitude modulations, sensitivity to spatio-temporal modulations of phase and amplitude that characterize the effects of moving objects, and vulnerability of object detection to jamming. Of particular interest are: 1) Neurons of laminae 5 and 7 of the mesencephalic torus that appear to receive phase information only to the extent that their dendrites invade lamina 6, where phase information arrives from the hindbrain and where differences in the phase of signals relayed from different parts of the body surface are computed. 2) Laminar location and morphology of total neurons that detect moving objects but also are sensitive to jamming signals with particular frequency relations to the animal's own EOD.