Circadian rhythm of optic nerve impulses recorded in darkness from isolated eye of Aplysia.

The isolated eye of the sea hare Aplysia califomica shows a circadian rhythm of optic nerve impulses when kept in total darkness. Peak activity on the first day of isolation occurs during the projected " dawn" of the light-dark cycle to which the whole animal had previously been entrained. Eyes from animals previously exposed to constant light show a free-running rhythm. This simple photoreceptor provides a quantized output with an ideal control (the other eye) for studies on rhythms.