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Příspěvekod ATA » pon, 17. čer 2013, 23:35

The Stabilized Retinal Image
If an image is made stationary on the retina it will disappear in seconds. Figure 33 illustrates what is perceived by a subject presented with a stabilize image of a red/green boundary based on the research of the Russian physiologist Yarbus (1967) and others (Ditchburn, 1973). Initially the subject sees the red/green scene but in several seconds it fades into a featureless field. If a pale blue light is added to both sides only a blue field is seen without the red/green scene. This also fades within seconds. If the pale blue field is removed the subject sees a faint red/green scene but this fades again. This indicates that we only see in transients determined by change in the retinal image.
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Troxler’s Effect
The effect is enhanced if the stimulus is small, of low contrast, or blurred. It works best the further the stimulus is away from the fixation point.
Troxler’s fading was first discovered by Ignaz Paul Vital Troxler, back in 1804. It is part of the general principle in sensory systems that an unvarying stimulus soon disappears from our awareness. Here’s a fun experiment you can try yourself: place a small piece of paper on the inside of your forearm. You can feel it for a few seconds, but then the sensation is no longer present. This is because the tactile neurons have adapted. Now, if you jiggle your arm up and down, giving varying stimulation, you will continue to feel the paper until it falls off! Similar fading can be seen of a fixated stimulus when its retinal image is made stationary on the retina, a stabilized retinal image. One can induce an afterimage, usually by an intense, brief flash, such as when one is photographed using a photographic flash.

This causes an image to be bleached onto the retina by the strong adaptation of the rods and cones. In all these cases, the stimulus fades away after a short time and disappears. Troxler’s fading can occur without any extraordinary stabilization of the retinal image in peripheral vision because the neurons in the visual system beyond the rods and cones have large receptive fields. This means that the small, involuntary eye movements made when fixating something fail to move the stimulus onto a new cell’s receptive field, giving unvarying stimulation.
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