Abstract
The threshold for evoking nerve impulses from single ommatidia was measured in the darkadapted, lateral eye of Limulus in situ. Our results are consistent with the notion that under dim illumination the eccentric cell fires a nerve impulse for every absorbed photon. However, the threshold is probably not constant and may vary as a result of the spontaneous activity in the eye. This variability could produce deceptively low estimates for the threshold. Measurements of the incident light intensity and corneal attenuation, together with an estimate of the photopigment absorption, suggest that the quantum efficiency for eliciting a nerve impulse is between 0.05 and 0.2. Perhaps the large, regenerativelike potential fluctuations we have recorded from the Limulus photoreceptor cells in situ amplify the effect of absorbed photons, and enable the eye to respond to single quanta.
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