Visual Afterimages Research Articles

Alpha rhythm during generation of visual afterimages

Alpha rhythm during generation of visual afterimages

The Time Course of Visual Afterimages: Data and Theory

Sequential viewing of two orthogonally related patterns produces an afterimage of the first pattern (Vidyasagar et al, 1999 Nature 399 422-423; Francis and Rothmayer, 2003 Perception and Psychophysics 65 508-522). We investigated how the timing between the first stimulus (a vertical bar grating) and the second stimulus (a horizontal bar grating) affected the visibility of the afterimage (a perceived vertical grating). As the duration from offset of the first stimulus increased, reports of afterimages decreased. Holding fixed the total time from offset of the first stimulus and increasing the duration from offset of the second stimulus while decreasing the time between the first and second stimuli, caused a decrease in afterimage reports. We interpret this finding in terms of Grossberg's BCS - FCS (boundary contour system--feature contour system) theory. In this theory, the afterimage percept is the result of color complement after-responses in the FCS system interacting with orientation after-responses in the BCS system. The two types of after-responses interact at a stage of neural filling-in to produce the afterimage percept. As the duration between the stimuli increases, the color after-responses weaken so that visible filling-in is less likely to occur. A similar effect occurs for the orientation after-responses but at a faster time scale. Simulations of the model match the experimental data.

Preservation of Emmert's Law in a Visual Form Agnosic

Size constancy was investigated in DF, a patient with visual form agnosia, using a technique based on Emmert's law of visual after-images. DF was first given a task in which she was asked to indicate the distance of a vertical surface and a task where she had to estimate the width of a series of squares (widths ranging from 5 cm to 35 cm) placed at varying distances and having a constant visual angle. In the distance estimation task, DF greatly overestimated the distance of the vertical surface placed in front of her. DF also had great difficulty performing the size estimation task. DF then performed a task in which she stared at a bright 5 cm square for a brief period of time at a distance of 30 cm followed by the presentation of a vertical surface which varied in distance and was asked to indicate the width of the after-image either verbally or manually. DF's after-images conformed to the size-distance relationship predicted by Emmert's law—as the distance of the vertical surface increased her perception of the size of the after-images also increased. These data demonstrate that although DF is rather impaired in tasks that require explicit estimates of size and distance, at some level, DF must have relatively intact size constancy mechanisms given that her estimates of the width of the after-image conform to Emmert's law. Thus, the processes underlying explicit judgements of size and distance appear to differ from those underlying the size and distance scaling of after-images. I thank Mel Goodale for allowing me to conduct this research in his laboratory at the University of Western Ontario. This work was supported by CIHR (MG) and NSERC (PS) grants. This work was also supported by the Canada Research Chairs program (PS).

Brain potentials associated with conscious aftereffects induced by unseen stimuli in a blindsight subject.

The study is of brain activity in a blindsight subject (D.B.), who reports conscious visual afterimages of stimuli of which he is unaware when they are presented. This contrast offered a unique opportunity to study event-related potential recordings of conscious versus unconscious visual phenomena generated by the very same stimulus in the identical locus of the visual field. The behavioral results confirmed the reliability of the difference in the subject's report for inducing stimuli versus their aftereffects. The rationale of the event-related potential analysis was to subtract "on" signals from "off" signals, the latter associated with the onset of conscious events and the former for events that remained unconscious. Because there are inherent differences in on and off potentials, the subtractive resultants for the blind hemifield were compared with the same subtractions for the good hemifield when the subject was aware both of the stimuli and their afterimages. A differential pattern in subtractive resultants emerged with a strong anterior left frontal focus for the blind field and a posterior focus for the intact field. The results are compared with other studies suggesting an anterior focus for conscious visual events.

Display System by Light Emitter Embedding Windmill Blade

This paper proposes the new display system by windmill blade. The proposed method is a system by the visual afterimage phenomenon, using windmill blade in which the light emitter has been embedded. This system shows new effectiveness of the wind power generator system.

El empleo de las postimágenes en el diagnóstico de la enfermedad de Alzheimer

For the first time visual after-images have been used in patients with Alzheimer's disease (AD). This is a simple, non-invasive system for analysis of visual pathway function, disorders of which it is hoped may be related to the patient's condition. To obtain a method for early diagnosis which may also be used for follow-up and evaluation of the disease over a the course of time. We wished to ascertain whether patients with AD have alterations in the perception of visual after-images, and ascertain whether use of after-images is effective. We used a portable perimeter for post-images in 20 patients with Alzheimer's disease and in 20 normal persons. The results of two different tests were analysed, measuring the time taken for the after-image to disappear from the visual field of those being examined and trying to detect whether there was a relationship between the state of the person and the results of the test. It was shown statistically that the time taken for the after-image to disappear was less in the patients than in healthy persons, and this rapid rate of disappearance is directly related to the severity of the disease. We consider that the use of after-images is simple, effective and useful for early diagnosis and confirmation of patients with EA and their follow-up.

Emmert's law in the dark: active and passive proprioceptive effects on positive visual afterimages.

The relationship between apparent size and apparent distance is given by Emmert's law, which states that a retinal image is proportional in size to the distance of the surface it is projected upon. This principle also applies to retinal afterimages in that they, too, will change in apparent size if distance cues suggest that the location of the object onto the retinal image has been altered. It has also been known for some time that non-retinal cues can produce quantitative and qualitative effects on an afterimage when it is viewed in the dark. In the present two studies, positive afterimages of an observer's hand, as well as objects held by that hand, were used as targets to investigate the effects on size-constancy scaling of moving the hand to and fro along the line of sight for different distances in the dark. Results show that, when observers focus on a held object, the changes in size predicted by Emmert's law occur in response to both active and passive proprioceptive or haptic cues. The most intriguing result consisted of the finding that, when only the hand is the target, there appears to be a limit to the decrease in apparent hand size. It appears that the visual system 'refuses' to size-scale the hand below a limit it accepts as representative or acceptable of 'its' hand.

A computational model for human visual motion perception system

Reichardt's elementary motion detectors are employed for detecting motion. Moving image-stationary background discrimination is realized by correlating the detected motion information with properly delayed visual afterimages. Fuzzy-set theory is used in a nonlinear integration algorithm for obtaining a moving object direction. Both hypothetical neural coding schemes of temporal coding and firing rate coding are used for representation of visual motion information. A biologically plausible electronic neural network setup has been built for real-time examination of proposed model. The simulation results could well explain the phenomenon of visual anti-camouflage. The proposed model may erect a bridge between natural vision and robot vision.

Auditory afterimage: tonotopic representation in the auditory cortex.

The auditory afterimage is a sensation which occurs for several seconds after the exciting acoustic signal has been switched off, and which roughly corresponds to the inverse of the spectrum of the exciting signal. In contrast to the well-known visual afterimage, the physiological mechanism generating the auditory afterimage has been questionable so far. Neuromagnetic source imaging revealed that the source of cortical neural activity which coincides with the sensation of the afterimage is located in the auditory cortex and exhibits a tonotopic organization similar to that of the sustained response which occurs during continuous presentation of an acoustic stimulus. It is concluded that the neural processes leading to the generation of the two phenomena -sustained response and auditory afterimage - are similar.

Visual scotoma and visual afterimages: some evidence that the perceived visual afterimage may not be a purely retinal phenomenon: a single case study.

Observations of the differential appearance and behaviour of a deliberately induced visual scotoma and a patterned visual afterimage are reported. Although a retinal scotoma behaves like a visual afterimage in some ways, there are sufficient differences to suggest that the perceived visual afterimage may not be just the simple consequence of prolonged retinal stimulation.

Duration of visual afterimages on modulated backgrounds: Postreceptoral processes

A foveal afterimage produced by a small photoflash increases in duration when the luminance of a 5.8° diameter background on which it is seen is temporally modulated. At a modulation frequency of 1 Hz and a depth of modulation of 52%, the duration of the afterimage is prolonged by 335% compared to the duration obtained on a steady background. This increase has been attributed to the functional border resulting from the difference in excitability between bleached and unbleached photoreceptors. Afterimage duration is also prolonged, although only by 20%, when the luminance of the background is kept constant, while the luminance of an annular surround is modulated. This finding suggests a weak effect of neural lateral interaction (via area contrast). If the background luminance is modulated only in the contralateral eye (dichoptic presentation), afterimage duration increases by as much as 54% compared to the unmodulated state. This result indicates that afterimages, in part, are sustained by processes mediated by the visual cortex.

Semantic generalization investigated using second-order conditioned visual afterimages.

A series of experiments is reported during which three subjects were initially classically conditioned to produce visual percepts to a sound only, second-order conditioned to transfer that response to an animal name, and finally taught a "foreign language". The results suggest that semantic generalization occurs whether the semantic relationships are formed either before or after the conditioned response is established.

Information Processed from Brief Visual Displays by Learning-Disabled Boys

The spans of apprehension of learning-disabled and normal boys were compared by means of a forced-choice letter-recognition task involving tachistoscopic exposures of letter displays. This task provides an estimate of the span that is relatively insensitive to memory or motivational influences. In Experiment 1, the spans of learning-disabled and normal boys were compared under 4 information conditions: with no noise letters present or with 2, 4, or 8 noise letters present. Experiments 2-4 examined 3 possible interpretations of the deficit in the span of apprehension observed for learning-disabled boys in Experiment 1. In Experiment 2, the possibility that the noise letters were more distracting for learning-disabled boys was examined by varying the amount of physical similarity and redundancy between the signal and noise letters. In Experiment 3, the possibility that the visual afterimage of the letter displays faded more rapidly for the learning-disabled boys was examined. In Experiment 4, the possibility that the learning-disabled boys were slower to pick up information from the decaying afterimages was examined. The results of these three experiments indicated that the decreased spans of apprehension observed for the learning-disabled boys in Experiment 1 resulted either from greater distractiveness or from the slower pickup of information, or both. The implications for visual training are discussed.

Prolongation of a visual afterimage with systematic alternation of room illumination

The present study investigated the effect of changing room illumination on perceived afterimage duration. Subjects observed a light pulse and reported upon their perception of a resultant afterimage. This afterimage was first produced and observed in the dark and was continually viewed in this environment until it was reported to have disappeared. The room lights were then turned on, and the subjects reported if the afterimage reappeared and, if it did, on its persistence. Upon the afterimage’s disappearance in the lighted environment, the room lights were turned off and, again, subjects reported upon the afterimage. This systematic alternation between dark and light room environment continued until subjects no longer reported perceiving an afterimage. It was found that such alternation prolonged afterimage duration, with four illumination changes appearing to be the maximal number for producing the effect.

The role of a cognitive factor in the prolongation of an induced visual afterimage

Induced visual afterimages endure for very short periods of time (about 15 sec-2 min). In an attempt to prolong the perceived duration of afterimages, an experiment was conducted to examine the roles of stimulus codability (the degree of intersubject agreement on the name of an inducing stimulus object) and stimulus complexity (the number of elements in the inducing object) on afterimage duration. We found that stimulus codability affected afterimage duration: A highly codable stimulus produced the most enduring afterimage. Stimulus complexity, on the other hand, did not affect afterimage duration. We conclude that, in addition to physiological factors that affect the production and prolongation of an afterimage, cognitive factors may play a role as well. One such factor may be stimulus codability.

The neurophysiological correlates of the visual after-images

A short review of neurophysiological correlates of human visual after-images is presented in the article. Some temporal parameters of bright and dark phases of human visual after-images are compared with the corresponding parameters of a single unit activity of contralateral neurons. By way of illustration, our own investigations of the single unit activity of the lateral geniculate body of rabbits evoked by a brief (10 ms duration) retinal light illumination have been used. The necessity to investigate not only monocular (contralateral), but also binocular (ipsilateral and contralateral) recovery cycles of single units of various visual nervous system structures is emphasized. An attempt is made to consider the hypotheses which suggest an explanation of nature of inhibition in the visual system. The experimental evidences provided by other authors and achieved on the basis of the extracellular and intracellular investigations have been used in the present article, with the view of explaining the inhibition by means of postsynaptic hyperpolarization if the stimuli presented to the retina are brief and diffuse. Some considerations on the biological role of inhibition have been discussed.

Binocular pattern stabilization by afterimage technique: reliability of children's time-dependent judgments of chromatic features.

To investigate visual afterimage technique as a means of producing retinally stabilized pattern, children's judgments of time-dependent shifts in chromatic appearance of random-dot configurations were twice examined within a 3-wk. interval. 13 subjects, aged 7 to 12 yr., were very consistent (r = .93) in their test-retest response times. This supports the belief that after image procedures can be developed as a reliable means of studying developmental aspects of stabilized pattern perception.

Emmert’s imaginal law

The apparent size of a visual afterimage increases in proportion with the distance at which that image is viewed; this is Emmert’s law. Contrary to Emmert’s law for afterimages, this paper reports that the apparent size of a mentally imaged object decreases monotonically with increases in the distance of the viewing surface.

Psychopathology in childhood and adolescence as reflected in projected afterimage serials.

A serial visual afterimage (AI) test was administered to 136 normal and 75 anxiety‐ridden children 4–16 years of age. The scoring of the AI serials involved signs of primitive functioning (size‐constant and positive AIs), anxiety (large and dark AIs together with primitive AIs in older children) and process discontinuity (intermittent regressions to small, positive, or physiognomic AIs). Differences between normal and clinical children were generally highly significant in these respects. Special attention was directed to the difference between subjects with intermittent regressions to primitive functioning and subjects where regression represented a lasting functional alternative. The former subjects were more often characterized by paroxysmal or primary anxiety.

EEG correlates of the afterimage of visual stimulation.

ABSTRACTOccipital EEG alpha activity following visual stimuli of varying intensity was studied with particular reference to the visual afterimage. Subjects each received three stimuli of differing intensity under two conditions: with and without reporting of afterimage onset and offset required. A third condition controlled for possible reporting effects on the EEG. Findings indicate that visual afterimages are a significant determinant of the duration of EEG alpha desynchronization following visual stimulation and that where visual stimuli are employed the duration of EEG alpha blocking is unsuitable as an indicator of the OR to such stimulation.