Blind people's eyes sense light




He seems to be completely unreceptive
The tests I gave him show no sense at all
His eyes react to light; the dials detect it
He hears but cannot answer to your call

There is no chance, no untried operation
All hope lies with him and none with me
Imagine though the shock from isolation
When he suddenly can hear and speak and see

His eyes can see, his ears can hear, his lips speak
All the time the needles flick and rock
No machine can give the kind of stimulation
Needed to remove his inner block

  • "Go to the Mirror Boy", Tommy, The Who


Doesn't it depend on the type of blindness?


I remember a Derren Brown trick where he was getting blind people to tell him how many fingers he was holding up. I saw how he was doing it immediately (before he told people how he had done it), but it might have looked impressive if you didn't know.

I think the blind people in the article have zero vision though.


Yeah, that was my instant thought. There are many causes of blindness out there, including not having eyes. or having a severed optic nerve.


The article does cite a previous study that drew the conclusion that photoreceptors in "blind" eyes can detect light, but only after an exposure of 30 minutes. This study, in contrast, used much shorter durations of light and concluded that:

"...the brain can detect light in the absence of working vision. They also suggest that light can quickly alter brain activity through pathways unrelated to sight. The researchers posit that this nonvisual light sensing may aid in regulating many aspects of human brain function, including sleep/wake cycles and threat detection."

Since they did not specify what types of blindness (which can happen via a number of mechanisms, including complete photocereptor death and atrophy) the test subjects were afflicted with, we are left to assume that the test was performed on a wise enough range of subjects that they were able to determine the type of blindness did not matter, and the "sensing" of the light was not connected to any kind of stimulation of the eye at all. Remember, there are types of blindness where the photoreceptors are completely absent.

Instead I believe this study may be backing up the idea that our bodies as a whole can react to light, much like how exposure to certain wavelengths have been shown to help people who are afflicted with a Seasonal Affective Disorder. So the question should be: Is this study confirming that we can detect light via our skin, however slightly?


I recall a story about a study that suggested that a person's circadian wake/sleep cycle could be influenced by exposing a spot of skin to light (I think that they used the back of the knees for some reason).

I'm guessing that they are referring more to what I remember as blindsight (or second-sight)... I think that I read about it in one of Roger Penrose's "consciousness" books.


Imma bit peeved with the SciAm blogger for not linking to the original study he summarises. I suspect it has to do with the melanopsin ganglion cells, which detect light levels for circadian rhythm purposes and for the pupillary response. So someone can have lost all their image-forming rod-cell and cone-cell photodetectors, but the melanopsin cells keep working.
Unfortunately these cells are sparsely distributed in the retina, and they do not seem to send any information to the visual cortex.

Afterthought -- melanopsin is most sensitive to blue light, so the reference in the SciAm post to "less than a minute of blue light exposure" supports this interpretation.

Afterthought #2: other science bloggers evidently received the same press release from Vanderwalle, but thoughtfully chose to include more of the salient details, such as a link to the original study. It turns out that the SciAm blogger, in the process of dumbing things down, left out the most interesting findings about melanopsin's effect on alertness and cognitive function.


The light on skin idea is probably something to do with vitamin D production. Darkness on your eyes causes your body to produce melatonin, which makes you sleepy. Probably two important parts of the circadian rhythm. Vitamin D uptick probably signals "daytime" to other body systems?


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