Subjects had no active disease processes they were non-smokers and had normal hearing. Seven healthy adults (aged 18–48 years 6 male, 1 female) participated in a pair of two-day in-laboratory sessions that were separated by at least two weeks. Subjects signed informed consent forms prior to any procedures. This study and all related procedures described herein were reviewed and approved by the Stanford University Institutional Review Board and conform to the principles expressed in the Declaration of Helsinki. As such, we examined the capacity of the human hypothalamus to respond to millisecond flashes of light. Behavioral data from nocturnal rodents – suggest, however, that the human hypothalamus might be able to respond to light that is thousands of times shorter. To examine light-induced changes in human hypothalamic function, typical protocols use hours or, occasionally, minutes of bright light exposure. This network also projects to other hypothalamic regions. Transduction of light signals from the retina to the central circadian clock, located in the hypothalamic suprachiasmatic nucleus (SCN), is mediated through a network of retinal cones, rods, and melanopsin-expressing intrinsically photosensitive retinal ganglion cells (ipRGC). The effects of light are dependent on both the intensity, and timing of light exposure. Exposure to bright light at night has multiple effects on the human hypothalamus, including phase shifting circadian rhythms, altering hormone production, and enhancing alertness.
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