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Jamie M. Zeitzer, Raymond P. Najjar; Temporal Integration of Light in a Human Non-visual Circuit. Journal of Vision 2016;16(4):46-47. doi: 10.1167/16.4.49.
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© 2017 Association for Research in Vision and Ophthalmology.
Much of the circuitry of the human image-forming visual system is optimized to detect changes over very short time scales. It is only when individual photopic images are separated by gaps under 66 msec (15 Hz) can we consciously perceive these images as continuous. Retinal circuitry underlying non-image forming (NIF) photoreception is interspersed, yet distinct from that used for image formation. The NIF photoreceptive system appears, however, to be optimized to respond to a gestalt of the visual environment (specifically overall environmental illuminance levels rather than specific features. We tested whether the NIF system differed in its ability to integrate light over time by examining the NIF responses of circadian phase shifting, melatonin suppression, and improvement in alertness. Under controlled conditions of a 35-hour protocol, light was presented as a sequence of 2-msec flashes over an hour (hours 2–3 after habitual bed time) at frequencies ranging from 0.004 to 0.4 Hz. Circadian phase shifting displayed non-linear integration of light flashes. In fact, when given in a sequence every 10 seconds for an hour (0.72 seconds of light), flashes were at least two-fold more effective in phase delaying the circadian system as compared with an equiluminous continuous light exposure 5000-times the duration. The flash sequences did not, however, suppress melatonin concentrations or change alertness in a dose-dependent manner. The physiology of the retinohypothalamic circuitry underlying circadian phase shifting, therefore, has the capacity to integrate light over much longer time scales than is observed with image forming vision.
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