Why does the picture of a light source stick on the mind?

Question

after looking away from it?

And please if you know the answer explain theoretically and with evidences.

Thank you !

Answer 1

I think you are talking about the after image you see when you turn off a light after having looked at the bulb.  I can't give you the source but I learned in college that when light hits the nerve cells in the eye it casuses an electriic signal to be sent to the brain through the optic nerve.  This is called "firing".  At the same time a chemical is release that temporarily prevents that nerve cell and, those next to it, from signalling again.  This is called bilateral inhibition.  

So when your eyes are exposed to any light it makes a photo negative of sorts on the inside back of your eye.  Any nerve cell that didnt recieve enough light to "fire" is now free to do so;  If you focus for a moment on an lit  light bulb when you look away you'll see the shape of it as if floating in front of you only not bright but dark  Then the bilateral inhibition wears off and the image fades..

Hope this helps.

I got the info from a class lecture in 1979,

Answer 2

Afterimages are a class of visual phenomena in which some aspect of an image persists after the visual stimulus for the image ceases. In simpler words, we continue to see something that is not there anymore. When the afterimages are strong and persistent enough to be considered a medical problem, they are called palinopsia. There are basically two types of true afterimage, negative and positive.

The positive afterimage is like a positive photograph; it has the same coloration as the original image. The familiar example is when you happen to be staring at a flashbulb when it goes off and the intense white image remains even when you close your eyes or look away. Positive afterimages are usually very brief and often not even noticed. As for all afterimages, how long the effect lasts depends on the strength of the light and the length of exposure. Positive afterimages are caused by the slowness of the chemical reaction in the cells of the retina that allows those cells to sense light (see rhodopsin), a kind of hysteresis effect. Because of that property, the effect of light stimulation lingers ever so briefly after the stimulus has been removed.

The negative afterimage is like a negative photograph, where the complementary colors are reversed so that red becomes green and green becomes red, blue becomes yellow and yellow becomes blue, black becomes white, and white becomes black. In the flashbulb example, the positive afterimage of a white spot will be quickly replaced by a negative afterimage of a black spot. The negative afterimage usually persists longer than the fleeting positive afterimage, but it also depends on the intensity and length of the stimulus light.

The explanation for negative afterimages is somewhat complex, but the simplest and most direct factor is a desensitization of the cells of the retina. What makes those cells sensitive to light is a chemical called rhodopsin. A cell's supply of rhodopsin is gradually depleted by chemical reaction during exposure to light. The brighter the light, the faster the chemical is used up. So if a certain part of your retina is exposed to bright light, by staring at a white disk on a black background for example, that part of the retina will become less sensitive to light. If you then move your gaze to a white surface, you will see a negative afterimage in the form of a relatively dark disk. That happens because the cells within the disk image on the retina are now less sensitive to light than those outside the disk image, and so they don't respond as well to the same level of light and you see the dark disk, even though the white background is uniform in brightness

Answer 3

It is called a retina after image.

A photon causes the cones and rods in the back of the eye to activate a photopigment is activated and this sends a signal to the brain, the photopigment is depleted and only restored through a chemical process that takes a short bit of time.  When the rod or cone fires it keeps a signal going until it is either replaced by another signal or it is chemically recharged.  The rod or cone is discharging and waiting for a change in light levels to signal a new image.  So there is a bit of an after image when you close your eyes and keep them closed right after exposing your eye to a bright source.

We are not sure exactly what his happening to the image after it hits a rod or cone.  We know that some visual processing goes on in the retina itself and that image is sent via the optic nerve to the brain so the latent image you are talking about would be happening here.

Then their is the nature of image processing in the brain itself which keeps an image up until it is replaced by another, with the retina signal still being sent, and the same signal being sent, the brain will retain an after image of what you saw.  When brightness levels don't change and the chemical in the eye recharges itself the image vanishes.

According to Wikipedia:  http://en.wikipedia.org/wiki/Retina

"When light falls on a receptor it sends a proportional response synaptically to bipolar cells which in turn signal the retinal ganglion cells. The receptors are also 'cross-linked' by horizontal cells and amacrine cells, which modify the synaptic signal before the ganglion cells. Rod and cone signals are intermixed and combine, although rods are mostly active in very poorly lit conditions and saturate in broad daylight, while cones function in brighter lighting because they are not sensitive enough to work at very low light levels.

Despite the fact that all are nerve cells, only the retinal ganglion cells and few amacrine cells create action potentials. In the photoreceptors, exposure to light hyperpolarizes the membrane in a series of graded shifts. The outer cell segment contains a photopigment. Inside the cell the normal levels of cyclic guanosine monophosphate (cGMP) keep the Na+ channel open and thus in the resting state the cell is depolarised. The photon causes the retinal bound to the receptor protein to isomerise to trans-retinal. This causes receptor to activate multiple G-proteins. This in turn causes the Ga-subunit of the protein to bind and degrade cGMP inside the cell which then cannot bind to the Na+ cyclic nucleotide-gated ion channels (CNGs). Thus the cell is hyperpolarised. The amount of neurotransmitter released is reduced in bright light and increases as light levels fall. The actual photopigment is bleached away in bright light and only replaced as a chemical process, so in a transition from bright light to darkness the eye can take up to thirty minutes to reach full sensitivity (see Adaptation (eye)).

In the retinal ganglion cells there are two types of response, depending on the receptive field of the cell. The receptive fields of retinal ganglion cells comprise a central approximately circular area, where light has one effect on the firing of the cell, and an annular surround, where light has the opposite effect on the firing of the cell. In ON cells, an increment in light intensity in the centre of the receptive field causes the firing rate to increase. In OFF cells, it makes it decrease."

According to Wikipedia:  

"..then there will be less need for pre-processing in the retina, though it is well known that some important pre-processing does take place there, and that at least some of the communication is via action-potentials.."

Basically, there is a difference between the difference between chemical and electrical processing and that introduces a slight delay in the processing.  Normally it is not enough to cause a problem since the chemical processing cells contains enough of a reservoir to continue transmitting there isn't a delay when new photons hit the receptive cell.  When there is no new signal the cell isn't keyed to send another signal so it keeps sending out the same signal until it chemically recharges.