Response properties of electrosensory neurons in the lateral mesencephalic nucleus of the paddlefish

Many fishes and amphibians are able to sense weak electric fields from prey animals or other sources. The response properties of primary afferent fibers innervating the electroreceptors and information processing at the level of the hindbrain is well investigated in a number of taxa. However, there are only a few studies in higher brain areas. We recorded from electrosensory neurons in the lateral mesencephalic nucleus (LMN) and from neurons in the dorsal octavolateral nucleus (DON) of the paddlefish. We stimulated with sine wave stimuli of different amplitudes and frequencies and with moving DC stimuli. During sinusoidal stimulation, DON units increased their firing rate during the negative cycle of the sine wave and decreased their firing rate to the positive cycle. Lateral mesencephalic nucleus units increased their rate for both half cycles of the sine wave. Lateral mesencephalic nucleus units are more sensitive than DON units, especially to small moving dipoles. Dorsal octavolateral nucleus units respond to a moving DC dipole with an increase followed by a decrease in spike rate or vice versa, depending on movement direction and dipole orientation. Lateral mesencephalic nucleus units, in contrast, increased their discharge rate for all stimuli. Any change in discharge rate of DON units is converted in the LMN to a discharge rate increase. Lateral mesencephalic nucleus units therefore appear to code the presence of a stimulus regardless of orientation and motion direction.