This research aims to understand how the auditory system, and other time-based senses, encode and recognize complex features of natural stimuli. In the auditory system, temporal coding is important for localization and recognition of speech and other sounds. In electroreception in fishes, independently evolved neural pathways have converged upon similar anatomical and physiological solutions to temporal coding requirements. This project focuses on electroreception in mormyrid fishes because these fish offer several significant research opportunities: an unusual but potentially important mechanism of temporal comparison involving timed inhibition; a system with short, simple signals that invite neurobiological analysis; and a conveniently exposed anatomy that permits observation of neural activity by optical imaging, and electrophysiological exploration by single cell analysis with microelectrodes. The researchers, based at Cornell University will test four related hypotheses about temporal coding in electroreception. To test the hypothesis that recognition of species-specific signals is based on temporal analysis, the researchers will conduct playback experiments designed to identify key features of stimuli eliciting approach and avoidance. A quantitative multi-factorial survey of species and sex differences in Electric Organ Discharge (EOD) waveforms will be undertaken to examine the importance of temporal codes in signal diversity. To test the hypothesis that neural pathways sharpen the timing of spikes responding to EODs, the researchers will record spike jitter at different levels of a sensory hierarchy from periphery to midbrain. To test the hypothesis that temporal analysis is based on a delay-line anticoincidence detector in the midbrain, the researchers will record evoked potentials in response to different stimulus geometries, and also employ optical imaging of calcium-activated fluorescence to monitor activity of time-comparison cells which are too small to record from physiologically. The researchers will also test with optical imaging the presence or absence of a map of stimulus duration in the midbrain predicted to be absent. Finally, the researchers will test the hypothesis that multidimensional stimulus features including stimulus location, waveform duration, and stimulus repetition rate are preserved and analyzed by higher centers in the time-coding pathway. ? ?
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