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Raymond P. Najjar, H. Craig Heller, Jamie M. Zeitzer; Temporal integration of non-image forming light. Journal of Vision 2013;13(15):P5. doi: 10.1167/13.15.40.
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Beyond image-forming (IF) effects, light can evoke changes in subcortical functions including circadian timing and sleep drive. Non-image forming (NIF) functions of light are subserved by a network of rods, cones, and intrinsically photosensitive retinal ganglion cells. The physiology of NIF photoreception remains incompletely understood. NIF functions have specific intensity and timing responses that are distinct from IF. We have shown recently that 60 ultra-short (2-ms) flashes successively administered over an hour can elicit a 45-minute delay in circadian timing. Herein, we present data on ultra-short light flashes and possible models of temporal integration of NIF light.
Twenty one (24.7±4.8 y.o) healthy participants were empanelled in a 16-day protocol and were exposed on the night of Day-15 to a 60-minute stimulus of bright, ultra-short flashes of varying frequency (inter-stimulus intervals, ISI= 2.5 to 240-seconds). Circadian phase shift was calculated as the difference in dim light melatonin onset between Days 15 and 16.
A maximum change in circadian timing was observed after a 5-second ISI. Light-evoked changes dropped as ISI increased and changes in circadian timing were related to ISI following a non-linear 2-parameter exponential decay curve. Subjective sleepiness was significantly decreased during light exposure; this was not dependent, however, on the ISI of the flashes.
The circadian system appears to integrate ultra-short flashes of light over time in a non-linear fashion. This temporal integration is such that 1.44-seconds of light delivered as discreet flashes and evenly distributed over an hour is nearly equipotent to continuous light 2500 times the duration.
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