[Les the Scientist (OP)]

How come, if you rub your eyes hard enough and long enough, you get this weird kaleidescope effect even though your eyes are shut ?

[MayoFlyFarmer]

This is because your rod and cone cells sense that they are being stimulated even though it is not the type of stimulation that tehy are designed for. (manual stimulation instead of photons).  So they send a confused signal to your brain.

Are YOUR mice nude? []

[Les the Scientist (OP)]

Now my next question is..... would blind people experience the same ?  Or would that depend on what has caused their blindness ?  Sure I'd read somewhere that blind people experience colours during dreams, even those who have been born blind.

[chris]

This is a really interesting question, mainly because I didn't immediately know the answer and hence had to go away and think about it ! When that failed, I called a friend who is a consultant opthalmologist to ask his opinion !

These lights are a so-called endoptic phenomenon. In other words, they originate from within the eye itself. The most likely explanation for the 'lights' or sparkles that you see when you rub your eyes is temporary interruption of the blood flow into the eye.

The patterns produced are very similar to those that you see when you stand up too quickly from a hot bath, or if you jump out of bed too fast. In these instances a 'postural drop' in blood pressure temporarily leads to reduced retinal perfusion and consequent inappropriate retinal activity, producing strange light patterns.

The retina has the highest metabolic rate of any tissue in the body and hence is the most energy and oxygen-hungry. Any interruption to its blood flow therefore causes rapid symptoms.

When you rub your eyes you are applying pressure directly onto the eyeball, raising the intraocular pressure. This can affect blood flow through intra-optic capillaries feeding the choroid (the network of blood vessels at the back of the eye that feed the retina), temporarily dimishing the supply to certain parts of the retina. According to my opthamology colleague, when performing eye surgery (and hence watching the retinal vessels beneath an operating microscope) it is frequently possible to cut off blood flow through the retinal artery, just by pressing on the eye.

It's also possible that some of the effect is down to sheer stress applied to the photoreceptors, but the blood flow model seems much more plausible.

To address Les's second question, regarding whether blind people experience the same phenomenon, as I have mentioned above, this is an endoptic effect (generated within the eyeball). Therefore, anyone who is blind due to a brain lesion, or a damaged optic nerve, would not be expected to experience this effect. Someone with retinal destruction similarly would not be expected to experience the effect.

Blind people do 'see' under certain circumstances, however. 'Blind sight', when patients perform much better on visual tasks than could be expected by chance alone, and despite claiming not to be able to see what they are doing, occurs when the primary visual part of the brain is damaged, but the eyes and other brain visual processing areas remain intact. Although the visual message fails to get through to the patients consciousness, it nonetheless is made available to other brain areas that can use the information - such as the motor system enabling patients to unwittingly avoid obstacles.

Another example of blind people seeing is when they dream, although this phenomenon is restricted to individuals who have gone blind after a period of being able to see. When we dream the regions of the brain that are concerned with consciously experiencing different senses we are awake - sight, hearing, touch and so on - switch on to create dreams. As one blind man said to me "I love going to sleep because I can see again. It's also especially useful in helping to remember what colours look like".

Chris

"I never forget a face, but in your case I'll make an exception"
 - Groucho Marx

[Quantum cat]

That also leads to the question ... in dreams the sense seems to be mostly visual things with a little bit of sound (I can remember dreams where I've been speaking in french) I wonder what blind people's dreams are like? Are they mostly audio and a little bit of touch? I've never experienced "feeling" something in a dream, or at least it's featured so little that I don't remember. Does anyone remember "feeling" something in a dream? If so do tell!! I wish there was a blind person here we could talk to, but I imagine it'd be hard for them to read the forum's messages :-P I wonder if you can get feely computer screens???

And talking of reduced blood to the eyes, I hate it when you stand up suddenly, everything is pearly white, then as the picture slowly returns you get a huge massive headache.

[qazibasit]

well very  intresting question quantum same question with me but i have felt things in my dreams.