A neural mechanism of hyperaccurate detection of phase and delay in the jamming avoidance response of weakly electric fish

The weakly electric fish Eigenmannia can detect the phase difference betwee n a jamming signal and its own signal down to 1 mus. To clarify the neurona l mechanism of this hyperaccurate detection of phase difference, we present a neural network model of the torus of the midbrain which plays an essenti al role in the detection of phase advances and delays. The small-cell model functions as a coincidence detector and can discriminate a time difference of more than 100 mus. The torus model consists of laminae 6 and 8. The mod el of lamina 6 is made with multiple encoding units, each of which consists of a single linear array of small cells and a single giant cell. The encod ing unit encodes the phase difference into its spatio-temporal firing patte rn. The spatially random distribution of small cells in each encoding unit improves the encoding ability of phase modulation. The neurons in lamina 8 can discriminate the phase advance and delay of jamming electric organ disc harges (EODs) compared with the phase of the fish's own EOD by integrating simultaneously the outputs from multiple encoding units in lamina 6. The di scrimination accuracy of the feature-detection neurons is of the order of 1 mus. The neuronal mechanism generating this hyperacuity arises from the sp atial feature of the system that the innervation sites of small cells in di fferent encoding units are distributed randomly and differently on the dend rites of single feature-detection neurons. The mechanism is similar to that of noise-enhanced information transmission.