The researchers also observed that flash intensity showed a sigmoidal relationship with phase shifting, with a half-maximal shift observed at 8 lux and 90% of the maximal shift occurring after exposure to flashes as dim as 50 lux. None of the flash sequences caused acute suppression of melatonin.
“We did not anticipate the invariance, that anything from 10 microseconds to 10 seconds gives us no difference [in phase shifting],” Dr. Zeitzer said. “That was surprising. I thought that more light would be slightly less effective in terms of photons but still give a bigger [phase] shift, but that didn’t happen. In the intensity response, we see things are more sensitive to light flashes than they are to continuous light, which is also surprising. It implies that a different part of the eye is responding to light flashes than it is to continuous light. It provides more information about how to minimize the amount of light we’re using and maximize the amount of shift.”
Which photoreceptors underlie the responses remains unclear, he continued, “but given the characteristics of photoreceptors, our hypothesis is that flashes are being mediated through a cone cell response, while the response to continuous light is being primarily mediated through a melanopsin response. A future question we plan to investigate is, can selective sequential simultaneous activation of different photoreceptors create enhanced phase shifts?”
The study was supported by the Department of Defense. Dr. Zeitzer reported having no financial disclosures.