Circadian periodicity in termination of photorefractoriness in the yellow-throated sparrow

Abstract

Photorefractory male yellow-throated sparrows (Gymnorhis xanthocollis) were subjected to six resonance light cycles, consisting of 6 h basic photophase coupled with scotophases of longer durations (in cycles of 12 (6L:6D) h, 24 (6L:18D) h, 36 (6L:30D) h, 48 (6L:42D) h, 60 (6L:54D) h and 72 (6L:66D) h). Termination of photorefractoriness was evident in cycles of 24, 48 and 72 h, but not in 12, 36 and 60 h. The results are consistent with the 'external-coincidence' model (Bünning hypothesis) and suggest that endogenous circadian rhythmicity is involved in the termination of photorefractoriness. © 1985 Birkhäuser Verlag Basel.