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The perception of Moiré fringes is studied with two different tests for binocular and monocular conditions. The results are negative for binocular conditions, and positive for the monocular case when afterimages are used.
An attempt is made to list the visual phenomena exploited in op art. These include moire frinlude moiré fringes, afterimages, Hermann grid effects, Gestalt grouping principles, blurring and movement due to astigmatic fluctuations in accommodation, scintillation and streaming possibly due to eye movements, and visual persistence. The historical origins of these phenomena are also noted.
After prolonged fixation of coloured gratings of low spatial frequency, images of the gratings can be elicited up to 90 min thereafter when the colour of a spatially homogeneous test field is suddenly changed. Only adapting gratings with luminance contrast induce clear aftereffects. Control experiments rule out afterimages as an explanation of the aftereffects.
The indication of subjective states during steady visual stimulation by means of key depression has been assumed to be free of systematic bias. However, it is shown that the reported visibility of simple stimuli may be determined by the level of categorisation employed before the initiation of an overt response. In particular, the duration of unitary disappearance of a stimulus is found to be greater when this is the only category of change to be reported than when the same measure is derived from concurrent reports of the disappearance of its parts. The correspondence of findings both with afterimages and with steady fixation shows that peripheral factors such as eye movements cannot provide an adequate explanation. Experiments comparing part/whole categorisation of the visibility of single lines suggest caution in the acceptance of previous studies designed to indicate underlying neurophysiological mechanisms in subjective disappearances.
Following an earlier report, the article offers additional points concerning visual images observed after enucleation. These concern: (i) the relationship between field-cut due to the lesion before surgery and corresponding blank areas within the post-enucleation images; (ii) similarities in motion between the foveal images and foveally-fixated afterimages; (iii) possible correspondence of time course of the images to transneuronal atrophy and cell loss in human and monkey studies.
Participants adjusted the saturation of one rectangular color stimulus so that it appeared to match the saturation of another rectangular color stimulus for stimulus pairs of different relative areas. In Experiment 1, in which stimuli of complementary colors were used, higher saturation stimuli were consistently chosen for large areas to match lower saturation stimuli in smaller areas. In Experiment 2, better saturation matches were obtained if participants fixated and the stimuli were presented for very brief durations. In Experiment 3, larger color contrast effects were obtained with hue pairs that were nearer to complementary than with hue pairs that were more nearly the same. These findings agree with the interpretation that biases in relative saturation selected to match stimuli of different relative size are largely the result of negative afterimages of the larger stimulus having greater impact on the apparent saturation of the smaller stimulus.
Patients with intermittent exotropia (strabismus) can either fixate normally or allow one eye to deviate outward by as much as 60 degrees. Two such patients (D.N. and K.C.) were studied and it was found that during eye deviation, binocular correspondence is maintained by completely 'remapping' egocentric space for the deviating eye alone using extraretinal signals from that eye. Also, by using foveal afterimages we showed that binocular rivalry occurs at a site earlier than this egocentric remapping, probably in area 17 itself. And finally, consistent with the neural remapping hypothesis, patient K.C. also experienced monocular diplopia; objects appeared double when viewed with the deviating eye.
The value of flight of colours (FOC), i.e. the succession of coloured afterimages following light stimulation, in diagnosing and following visual impairment was evaluated in 65 patients with multiple sclerosis (MS). Previous optimistic reports of the use of a pocket flashlight (PFL) method could not be confirmed, and are ascribed to inadequate methods. An electronic flashlight (EFL) method developed by us, proved a significantly better diagnostic tool (61% abnormal versus 18% abnormal). The EFL method was diagnostically less sensitive than pattern reversal visual evoked potential (PR-VEP) (80% abnormal). It did, however, add to PR-VEP by diagnosing 16 eyes with normal PR-VEP latency. The EFL method reflected the degree of degenerative changes in the central visual pathways and may be of value in following the disease activity in MS. The potential of the EFL method may be further improved by proposed modifications.
Thirty neurotic outpatients suffering from pronounced anxiety were studied and after a therapeutic period of 1.5 to 2 years. The patients were described by means of rating scales and two perceptgenetic methods, the Meta-Contrast Technique and the Afterimage Experiment. It was possible to associate original status with improvement or non-improvement of manifest anxiety. The most positive prognostic sign was sensitivity to marginal (inner and outer) cues, whereas the most negative signs were pre-psychotic discontinuities, projection, and lack of defence mechanisms.
In the dark-adapted salamander retina, spikes could be elicited from rods under normal physiological conditions. Spike activity was observed in rods during the recovery phase of the response to saturating light. These action potentials were calcium spikes, blocked by cadmium and L-type calcium channel blockers. In response to light stimuli that saturate the rod peak response, calcium action potentials occurred with a delay that depended on light intensity, with stronger light increasing spike latency. Therefore, these spikes encode rod visual information at light intensities beyond rod saturation. Postsynaptic currents of similar time course were observed in second and third order neurones. Since rods exposed to brighter light stimuli produced more delayed spike activity, these signals might contribute to negative afterimages.
One of our approaches to teaching a course in anatomy and physiology is to stress the fundamental, systems-level concepts. One successful strategy we use is to continually highlight the relationships among anatomy, physiology, and behavior. In this article, we describe a set of classroom demonstrations that stress these links while fostering critical thinking. These demonstrations, on the topic of sensory system structure and function, rely on two perceptual consequences of neural adaptation in the visual system: afterimages and aftereffects. Viewing specific visual stimuli under binocular or monocular conditions with interocular transfer permits several concepts to be observed and discussed, including neural adaptation, anatomical and functional segregation of visual system pathways, and the relationship among visual system structure, function, and perception. This article discusses how to produce and present the required visual stimuli, suggests a set of questions to stimulate critical thinking, and presents student evaluation of this activity.
The preservation or recurrence of images in the visual field after removal of the real stimulus is called 'palinopsia' or 'visual perseveration' which can be split up into three different types according to the latency between the real external stimulus and its apparent recurrence. For a long time it was subject of discussion whether the delusion is a sign of lacking cortical inhibition or of a cortical afterimage or part of an epileptic seizure. If there are different reasons for the three sorts of visual perseveration has not been answered yet. In our case report, however, it is possible to demonstrate a patient's posttraumatic lateralized long-latency visual perseveration for the first time as a clear-cut focal epileptic seizure which is approved by synchronous temporo-parietal epileptic discharges in the electroencephalogram.
Two percept-genetic methods, the Meta-Contrast Technique (MCT) and the Serial Afterimage Test (AI), were used to study adaptive aspects of perception in a group of 37 subjects suffering from manifest anxiety. The perceptual processes laid bare by these methods reflected not only anxiety but also defensive formations. MCT and AI correlated with each other in these respects and, when combined, correlated with rated symptoms. In accordance with the theory behind percept-genetic methods, anxiety signs appearing late in the test serials were more likely to correspond to manifest symptoms than signs which appeared early. The AI test scores seemed to reflect a difference between anxiety experienced as an inherent quality of life and anxiety as a foreign force struggling for dominance.
J.E. Purkynĕ (1787-1869) was one of the leading physiologists in the first half of the 19th century, became the founder of the "exact subjectivism", and remained during his entire life a very careful observer in psychophysical experiments. He made moreover many important discoveries in anatomy, neuroanatomy, embryology and pharmacology. Despite his outstanding contributions to science, he remained in his general philosophy and feelings deeply attached to the Romantic spirit of the age. After Purkynĕ returned to Prague in 1849 he played an important role in his nation's search for a national identity and became a leading figure in the cultural and social life of Prague. Purkynĕ's contributions to the physiology of vision, the oculomotor and the vestibular systems are reviewed. In vision Purkynĕ discovered the Purkynĕ-tree, flicker patterns seen in diffuse light, moving light nebula in the Eigengrau, spatially patterned structures in pressure phosphenes, the antagonistic properties of Galvanic phosphenes with respect to current polarity, periodic afterimages, the Troxler effect for colour perception with the peripheral retina and the principle of the ophthalmoscope. The Purkynĕ-shift in chromatic sensitivity when the adaptation level shifts from scotopic to photopic vision was rediscovered by Purkynĕ. Purkynĕ was the first to describe the "Purkynĕ images" of the anterior and posterior surfaces of the cornea and the lens and the changes in the third Purkynĕ image when the eye accommodates. He described optokinetic nystagmus and optokinetic after-nystagmus and explained movement aftereffects as a habituation of the gaze motor system. Important contributions were made by Purkynĕ to the physiology of the vestibular system, but he did not propose the existence of peripheral receptors for this system. He believed that direct mechanical effects on certain parts of the brain (especially the cerebellum) are responsible for the mechanisms of vertigo. Purkynĕ's five categories of vertigo are still valid today (vertigo of motion, Galvanic vertigo, vertigo caused by general brain ischemia, height vertigo and vertigo caused by alcohol and other narcotics). Purkynĕ's contribution to sensory physiology can be considered as one of the essential foundations on which modern sensory physiology and psychophysics are built.
In examination and treatment of patients with strabismus one often encounters abnormal sensory adaptations such as eccentric fixation and anomalous retinal correspondence. Prognosis and treatment of strabismus are dependent on the presence or absence of these conditions. When eccentric fixation is present it becomes difficult to interpret the results of many tests used to determine retinal correspondence. This determination is further complicated when the patient is unable to give accurate responses. A technique utilizing visuoscopy and an afterimage has been performed on somewhat less cooperative patients. The results compare favorably to amblyoscopic measurements of the angle of anomaly.
Cyclodeviations are rotations of the eye about an anterioposterior axis. Cyclorotations are either manifest deviations (cyclotropia) or latent (cyclophoria). Perhaps 5% of patients have symptoms related to uncompensated cyclophoria. Diagnostic techniques include fixation disparity evaluation combined with binocular refraction at distance and near. Biofeedback with afterimages, cyclovergence training, surgery, and multiple eyewear are some possible treatment methods.
PURPOSE: An early study claimed that ethanol ingestion can attenuate binocular rivalry and, in some cases, may produce the phenomenal fusion of normally rivalrous stimuli. The current study examined those claims in an experiment incorporating key controls that were lacking in the earlier study. METHODS: There were two conditions, one in which observers consumed ethanol and the other in which they consumed a placebo. Observers reported the course of binocular rivalry for several hours after drink consumption using two types of rivalry stimuli, one consisting of a continuously illuminated set of stimuli, the other consisting of dichoptic afterimages. RESULTS: Ingestion of ethanol resulted in a decrease in rivalry alternation rates, although there was no evidence of phenomenal fusion. Results were essentially identical for both stimulus types. CONCLUSION: It is suggested that this effect is not caused by ethanol-induced changes in oculomotor mechanisms but may be caused by ethanol-induced decreases in contrast sensitivity.
The infrared sensory membranes of the pit organs of pit vipers have an extremely rich capillary vasculature, which has been noted passim in the literature, but never illustrated or studied in detail. We rendered the pit vasculature visible in various ways, namely, by microinjection of India ink, by a combination of ink and succinate dehydrogenase staining, and by making resin casts for scanning electron microscope study. We also used transmission electron microscopy for identifying the types (arterioles, venules, capillaries) of blood vessels. Then we compared the pit vasculature with that of the retina and the dermis. Good visualization of the vasculature was obtained with both ink and resin injection. Arterioles, venules, and capillaries could be distinguished with all methods used. The monolayer vasculature was denser in the pit membrane than in the retina or skin. Each loop of the network enclosed a small number of infrared receptors so that all receptors were in contact with a capillary on at least one side. The forward-looking areas of the pit had a denser network than side-looking areas. Since infrared rays cause nerve impulses by raising the temperature of individual receptors, the capillary network functions not only as a supplier of energy but also as a cooling mechanism to reduce afterimages. Thus the denser network in the forward-looking areas causes these areas to be more sensitive and have better image resolution than the rest of the membrane.