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The eye contains two types of photosensitive cells, rods and cones, which convert light energy into signals that are carried to the brain by the optic nerve. Rods are very light sensitive, and give us black and white vision but they produce quite a low-resolution image. Cones, on the other hand, give very high resolution, colour images, but they are not as sensitive to light.
In the middle of the retina is a small dimpled region called the fovea. This contains only cone cells, so in ordinary light we would look straight at an object to produce the best picture. In dim lighting, the rod cells are better for imaging, and the largest concentration of these occurs near the outer edge of the eye. This means that we can see objects in low light better out of the corner of our eyes These cells also give us the best motion detection, so we can see movement best out of the corner of our eyes.
The investigator sitting in a darkened room might perceive a small point of light, maybe from an item of technical investigation equipment. It can be seen more clearly out of the corner of the eye, but no detail can be perceived. To compensate for this, the investigator attempts to look straight at the object to try and resolve it. But when we constantly focus on one point of light, we begin to lose a proper perception of it as the chemical in the cone cells, iodopsin, becomes bleached out. To try and overcome this, our eyes do what is called saccadic scanning, which basically means our eyes make small rapid movements, of which we are unaware. This saccadic movement is misinterpreted as movement of the light source, rather than movement of the eye, as there are no other objects visible to relate the movement to. This is what is called the autokinetic effect, and differs between individuals – some people see small movements, whereas others see the light source moving up to 8 inches. So a small stationery point of light has suddenly turned into a moving ball of light – is this a ghost manifestation occurring? It is important to remember also that, just as when we focus on a point of light in a darkened room the eye needs to keep moving to keep perceiving it, the same is true of the rods and our peripheral vision – we will no longer be able to perceive a stationery object at the periphery of our vision unless we keep moving our eyes.