Laying in bed trying to get to sleep after watching a DVD the other night, my mind was fairly active and as such my eyes were open and focusing on random things in the dark room. However, a flashing object on the bed-side table snapped me out of my deep thought. I picked up the DVD remote for my Xbox and stared at it, thinking that the glow-in-the-dark buttons were to blame, before I realised that (a) the glow on the buttons was barely noticeable and (b) something that glows in the dark would not flicker on and off.
After putting the remote back on the table and lying down again, I noticed the buttons glowing brighter out of the corner of my eye. I bolted upright, thinking ‘What the hell is going on here?’ I picked up and looked directly at the remote once more and it was back to lifelessness, as though it were some kind of reject from Toy Story. It then dawned on me through some optician-style eye movements that the buttons were bright only when I didn’t look directly at the thing. If I moved my eyes to the carpet or the ceiling or the window, then the buttons would cheekily glow at me from the edges of my vision, yet the millisecond my focus was on the object itself, it was dark again.
The initial flashing must then have been due to my eyes darting around the room, causing an ‘on-off-on-off’ flicker. I am puzzled as to why something would be brighter outside of my focus when you’d expect visual acuity to be weaker. I have since also discovered this phenomenon after turning off the TV (in the dark) - there was a small white dot in the centre of the screen that only revealed itself when I didn’t look directly at the TV.
So, physicists, opticians and general know-it-alls, what is the explanation behind this? Does the blame lie with my focus hiding something or is my peripheral vision creating something that isn’t there, or both? Is the darkness, and thus my reliance on my rod cells, a factor?
There is a “dead zone” in the center of your vision, where the retina resides. Looking slightly off-center will let you see dim/small things that you otherwise could not.
D’oh! I forgot about the old ‘blind spot’. I mean, I’d heard about it (people sometimes use it as an example of the inefficiencies of the human eye in Intelligent Design debate) and have performed that ‘disappearing dot’ illusion before, I guess I wasn’t astute enough to make the connection with my glowing remote experience.
I suppose it’s never much of an issue in normal light and that when it happened to me lying in the dark I was quite alarmed because it was really that noticeable. Not a subtle ‘I’m imagining things’ moment but a ‘blinking LED’ effect. Infact, I’d first thought it was the battery indicator on the laptop because it was that conspicuous.
While your blind spot may have been partly to blame, it’s more likely that it was the anatomy of the fovea that was to blame. While the blind spot of the eye is slightly off your normal central visual axis, the fovea is the portion of the retina that covers the center of your field of vision.
There are two light-sensitive cell types in the retina: rods and cones. The simple explanation is that cones give you color vision and require higher levels of light to see, while rods are responsible for night vision (see later for the complex explanation). The distribution of rods and cones over the retina is not uniform: there are no rods in the fovea, and there are no cones in the periphery of the retina. The cones concentrated in the fovea give you the high-resolution vision needed for distinguishing fine detail; like reading, threading a needle, etc. When the available light level drops below a threshhold value, the cones stop firing and the rods take over, giving you the lower-resolution, ‘grainy’ night vision. Because there are no rods in the fovea, you have a central ‘blind spot’ for night vision, which is different from the true blind spot of the eye (no rods OR cones in the true blind spot). Looking slightly ‘away’ from a dim light source at night focuses its light away from the fovea and onto more rods, resulting in an increase in its apparent brightness.
You can observe the same effect on a starry night. Look at a star out of the corner of your eye, and then directly. You will observe a measureable difference in apparent brightness.
I’ve noticed a similar effect on CRT televisions late at night. In that case, the glowing is due to the phosphor of the screen glowing very faintly for a while after the TV is turned off. I can only see it out of the corner of my eyes because… I’ve been watching TV and the center of my vision is less sensitive to light than before.
I actually tested if it was due to the rod/cone distribution brossa mentions by having the TV on for a while but not watching it, then turning it off - I could see it in the centre of my vision again. Tomorrow, try not watching TV, but make the remote turn on regardless, and see if the same effect occurs.
I’ve used that trick often while stargazing - when someone just can’t see a star, I tell them to look a little to one side. This is invariably followed by “Oh! There it is!”.
Right, amateur astronomers refer to this technique as “averted vision”. Which is not to be confused with “averted imagination”: The former is often needed to see the Ring Nebula through a small scope. The latter is needed to see the writing along the inner edge of the Ring.
