Thursday, 19 June 2008

Seeing Things? Part 4 – Retinal Fatigue & Afterimages

Stare at the white dot in the centre of the following image for 30 seconds, and then look at a blank white wall or piece of white paper. What do you see?

Well, it’s kind of obvious that it’s the American Flag, right? That much is clear from the image above! The illusion in this case, however, is that when you transfer your gaze to a blank white wall afterwards, the red, white and blue of the flag appear correctly. This optical effect is known as an afterimage and it is caused by retinal fatigue.

Our retinas contain two types of photoreceptor cells that convert light coming into our eyes into signals that are sent to and interpreted by our brain – rods are sensitive to light and dark, and cones are sensitive to red, green and blue light. Together, red, green and blue light make white light, and other colours are a combination of different mixtures of these three primary colours, as seen in the image below.

When we stare at an image for at least 30 seconds, the cones that are maximally sensitive to the colour of that image become tired and stop transmitting signals to the brain. For example, fixating on a blue spot will cause cones sensitive to blue light to experience fatigue. When we then transfer our gaze to a white wall, even though all three primary light colours are entering our eyes, no blue is transmitted to our brain. Our brain therefore only receives red and green light, which it will interpret as yellow (red and green combined). It is for this reason that afterimages always appear to be the opposite or complementary light colour to the original image. As the cones recover, the afterimage will fade.

Although our eyes are fixated on a particular point when doing exercises such as this, it doesn’t mean that our eyes are completely steady. They do move slightly and this can also cause us to start seeing an object’s complementary colour around the edges of the object, particularly if we have been staring at an object placed against a plain background. This phenomenon can occur just as easily when the “object” in question is a person, in that the colour of a person’s hair, skin and clothes can all contribute to retinal fatigue of the corresponding cones if stared at for a long period of time.

While I in no way doubt the existence of an energy field (or aura) surrounding people, it is for the abovementioned reason that I am sometimes doubtful of various claims to have seen auras, especially if the reported aura appears to be complementary in colour to the person’s clothes. I am also doubtful of the method often cited to teach people to see auras, which involves staring at a person (or yourself in a mirror) sitting against a plain white background for an extended period of time, as these are prime conditions required to induce retinal fatigue and create an afterimage.

[Image Credit: Wikimedia Commons]

This is Part 4 of 5 in a series exploring common visual experiences often mistaken for paranormal occurrences. The other posts in the series are as follows:
* Part 1 - Floaters
* Part 2 - Flashes Of Light
* Part 3 - Pareidolia
* Part 5 - Reflections & Conclusions


Jude said...

I enjoy reading your articles, but haven't left a comment up til now, but this one was very interesting for me because of the aura thing that people claim to see....Thanks

Mags | Woo-Woo Wisdom said...

Welcome, Jude, and thank you for the comment! I'm glad to hear that you've been enjoying the articles.

Diane said...

Thanks for this series. I am keenly interested in seeing and perceptions and filters. I would not want to SEE something unless it was really there. (And not just my "retinal fatigue") For many years I worked in a profession where we examined things with a microscope. We looked for patterns. Sometimes I would see something concerning and then be unable to find it again later. (An effect from my retinal fatigue?)
I look forward to reading the rest of this series. Thank you.

Tom Volkar / Delightful Work said...

Hey Mags,

Questioning your own woo-woo huh? I'm just kidding. But it is good to know that not all alternative practitioners accept everything at face value. There are a lot of wild modalities out there and as you know it's not all wisdom.

Keep at it. I'd love to see even more controversy in your writing.


Mags | Woo-Woo Wisdom said...

Diane - Your profession sounds fascinating! How long did it take to train your eyes and brain to find the patterns that you were looking for?

Like you, I also want to see the things that are really there, not get caught up in physical illusions. That's probably why I'm not a fan of stage magic!

Mags | Woo-Woo Wisdom said...

Hi Tom - Yes, I definitely have a questioning mind! I think it's when we are gullible and blindly* accept paranormal explanations for events that we give "woo-woo" a bad name :). There is a lot of wisdom in woo-woo, but unfortunately it can get lost when we don't practice discernment.

[* How's that for a pun, given that this is a series on visual experiences?!]

YogaforCynics said...

Fascinating stuff.
The people offering to take pictures of people's auras for a fee always struck me along the same lines as those writing books channneled from dead celebrities.