Robert C. Eaton
Audrey L. Guzik
Janet L. Casagr
Ctr. for Neurosci., EPO Box 334, Univ. of Colorado, Boulder, CO 80309
In response to sudden sound, many fishes rapidly accelerate away from the stimulus. This complex behavior, or C-start, is mediated by a network of brain-stem neurons that receive acoustic input and connect to motoneurons in the spinal cord. In the brain-stem network, the bilateral pair of Mauthner cells (M-cells) play the major role in determining the initial direction of the C-start. Each M-cell axon crosses the brain and connects to motoneurons on the opposite side of the body, so that the animal turns away from the side of the activated M-cell. M-cells receive primary acoustic afferents and inhibitory input from a network of ``PHP'' cells. PHP cells have a very short latency response to sound and operate in a feedforward mode to regulate M-cell firing threshold. These studies suggest that the PHP cells receive specific combinations of pressure- and displacement-sensitive auditory afferents that inhibit the M-cell to sounds coming from the side of the body opposite the stimulus. Thus only the correct M-cell fires, while its opposite counterpart is inhibited by PHP cells. Present studies involve an electrophysiological, behavioral, and neurocomputational analysis of this hypothesis. [Work supported by NIH and ONR.]