Contrast adaptation
Pablo Artal in Handbook of Visual Optics, 2017
The initial mechanism of adaptation, perhaps partially responsible for afterimages, is light adaptation in the photoreceptors. It involves adjustment in the gain of the phototransduction cascade in the photoreceptor outer segments. It is controlled by calcium that comes into the cell through the light-controlled ion channels that close when photons are absorbed. High calcium levels increase the gain of phototransduction. The mechanism is not very fast (range of many seconds, as can be seen when we enter a dimly illuminated room, or come from a dark room into bright light). Therefore, it cannot explain “contrast gain control,” the fast adjustment of CS when we adapt to a low- or high-pass-filtered images within fractions of a second (see experiments by Webster et al.; Figure 21.11). A fast mechanism that could account for these changes involves the synapses of the photoreceptors, which interact with neighbored photoreceptors through electrical and pH-mediated mechanisms that can respond very fast.
Measurement and Analysis Techniques for Three-Dimensional Eye Movements
Michael Fetter, Thomas Haslwanter, Hubert Misslisch, Douglas Tweed in Three-Dimensional Kinematics of Eye, Head and Limb Movements, 2020
While the horizontal and vertical components of eye position determine what we look at, and can be measured and analyzed with mostly the same methods, the additional measurement of ocular torsion requires not only new measurement techniques, but also new concepts in the analysis of the data. A number of measurement techniques have been used to determine the torsional component of eye position, like afterimages (Helmholtz, 1867), methods using the blind spot (Colenbrander, 1963) or the axis of astigmatism (Kushner and Kraft, 1983), search coils, and video-images. While afterimages are still occasionally used (Kushner and Kraft, 1983; Probst-Müller et al., 1996), only the last two methods are now commonly employed. Since the mathematical concepts used in the analysis of three-dimensional eye position data have been summarized recently (Haslwanter, 1995) this article will concentrate on practical aspects of the measurement of three-dimensional eye position and -velocity with search coils or video systems, and on the analysis of 3D data.
ENTRIES A–Z
Philip Winn in Dictionary of Biological Psychology, 2003
Following exposure to a prolonged and/or intense pattern of light, the afterimage is a continuing visual sensation corresponding to the stimulated areas of the RETINA. This is believed to reflect prolonged photochemical events following pigment bleaching in the PHOTORECEPTORS, which both generate a neural signal (giving a bright sensation on a dark background) and reduce sensitivity (hence a dark sensation on a bright background). The afterimage is fixed on the retina, and so like any stabilized image, appears to fade even before the pigment has regenerated.
About the stereoscopic phenomena and Wheatstone’s attack on the theory of the identical points of the retinas: Part 2
Published in Strabismus, 2022
Ernst Wilhelm Brücke
Wheatstone further states that when he drew the stereoscopic images in color on a background of its complementary color and then looked at the stereoscopic image for some time, he would then alternately see the afterimage of one drawing and of the other with his eyes closed. However, at the moment when both appeared at the same time, he saw the afterimage of the stereoscopic image in relief. He says that in this experiment one must strictly fixate one point of the image in the stereoscope. When I do this, I see other parts of the image twice and, after closing my eyes, I see the afterimage of the part which I saw single in the stereoscope in all its energy, followed by vaguer lines, which at one moment seem to belong to one double image, then to the other. On the other hand, if I leave my visual distance [vergence] so much leeway that I can see single under all circumstances, the afterimage resembles the stereoscopic image. These are results that agree both with the theory of the identical points of the retinas, with du Tour’s experiments 5 and with the conclusions drawn by Müller 3(p80) from these experiments and from his own experiments.
Hallucinogen persisting perception disorder: A literature review and three case reports
Published in Journal of Addictive Diseases, 2018
Valentin Yurievich Skryabin, Maria Vinnikova, Anna Nenastieva, Vladislav Alekseyuk
At the age of 30, the patient regularly consumed cannabinoids (including synthetic cannabinoids) for 10 days during his vacation. Also, he ingested LSD once at that time. After the use of hallucinogen, he felt that he was “going out of his mind”, was afraid, and thought he was “going mad.” After the end of a bad trip, he noticed the appearance of perceptual distortions with the predominance of visual snow and positive afterimages. In contrast to the previous episode of perception disorder, these distortions were present constantly. In addition to these disorders, the patient noticed that he had an unusual headache almost every day, “as if the back of the head was completely numb”, during which the “visuals” were amplified, and a sense of derealization appeared. This condition initially frightened him, but over time this fear has been replaced by a constant sense of anxiety and internal tension.
Ernst Wilhelm Brücke on stereoscopic vision
Published in Strabismus, 2022
Nicholas J. Wade
Brücke’s speculations about rapid eye movements were very insightful; they predated by several decades experimental studies of eye movements, particularly by Hering23 and Javal,24 that established the saccade and fixation sequences that occur when reading. Both used afterimages as one of the methods to determine how the eye moves over text. Measuring rapid changes in convergence presented a far greater challenge as they are very small in comparison to version movements. Brücke maintained that such rapid eye movements occur all the time when observing objects in the environment. Moreover, he stated that the eye movements are unconscious. When attempting to maintain fixation on a stereopair, like the truncated cone illustrated in Wheatstone,4 Brücke reported seeing double images contrary to Wheatstone’s observation of singleness and depth. On the basis of this descriptive difference Brücke questioned Wheatstone’s ability to fixate accurately; the contrary argument could be advanced.
Related Knowledge Centers
- Cone Cell
- Eye
- Neural Adaptation
- Palinopsia
- Photoreceptor Cell
- Retina
- Rod Cell
- Illusory Palinopsia
- Closed-Eye Hallucination
- Retinal Ganglion Cell