Visual responses of nucleus isthmi in a teleost fish (Lepomis macrochirus).

Extracellular electrical activity was recorded from the nucleus isthmi of the bluegill sunfish (Lepomis macrochirus) in response to brief flashes produced by red light emitting diodes, and other visual stimuli. Metal microelectrodes detected positive spikes outside the nucleus, and negative spikes inside. Spikes of a continuous range of amplitudes up to 1 mV occurred in bursts, spontaneously and visually triggered. The highest amplitude spikes were triggered by the appearance or movement of stimuli throughout the visual field of the contralateral eye. Smaller spikes were triggered by stimuli throughout both visual fields. However, all spiking activity habituated with repeated stimulation in one region of the field. Stimulating at 12 widely spaced positions within the visual field of one eye yielded no consistent differences in the numbers of large spikes evoked. Different penetrations within and around the nucleus also gave uniform distributions of spike numbers. Thus no visuotopic organization was evident. The large spike response evoked by contralateral field stimulation was partially inhibited by a competing stimulus presented to the ipsilateral eye.