The early receptor potential (ERP) in the barnacle photoreceptor is shown by intracellular recording to exhibit a strong dependence on the color of the stimulus and of the preceding adaptation. The adaptation effects appear to be stable for at least 3 h in the dark. Most strikingly, the ERP is positive after red adaptation and mainly negative after blue adaptation. The simplest hypothesis which accounts for these observations is that two thermally stable pigment states with different absorption spectra contribute to the ERP. All ERP responses appear to be consistent with the sums of different ratios of the ERP's of the two pure states. The relative populations of the two states are shown to vary reciprocally, suggesting that the two are states of the same closed pigment cycle. Both states have approximately Dartnall nomogram-shaped absorption spectra, one peaked near 495 nm, and the other near 532 nm.

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