Grayscale median (GSM) post-processing, posterizing, and color mapping for carotid ultrasound.

Factors related to atherosclerotic plaques may indicate instability, such as ulcerations, intraplaque hemorrhages, lipid core, thin or irregular fibrous cap, and inflammation. The grayscale median (GSM) value is one of the most widespread methods of studying atherosclerotic plaques and it is therefore important to comprehensively standardize image post-processing. Post-processing was performed using Photoshop 23.1.1.202. Images were standardized by adjusting the grayscale histogram curves, setting the darkest point of the vascular lumen (blood) to zero and the distal adventitia to 190. Posterization and color mapping were performed. A methodology that presents the current state of the art in an accessible and illustrative way should contribute to the dissemination of GSM analysis. This article describes and illustrates the process step by step.

After-image formation by adaptation to dynamic color gradients.

The eye's retinotopic exposure to an adapter typically produces an after-image. For example, an observer who fixates a red adapter on a gray background will see an illusory cyan after-image after removing the adapter. The after-image's content, like its color or intensity, gives insight into mechanisms responsible for adaptation and processing of a specific feature. To facilitate adaptation, vision scientists traditionally present stable, unchanging adapters for prolonged durations. How adaptation affects perception when features (e.g., color) dynamically change over time is not understood. To investigate adaptation to a dynamically changing feature, participants viewed a colored patch that changed from a color to gray, following either a direct or curved path through the (roughly) equiluminant color plane of CIE LAB space. We varied the speed and curvature of color changes across trials and experiments. Results showed that dynamic adapters produce after-images, vivid enough to be reported by the majority of participants. An after-image consisted of a color complementary to the average of the adapter's colors with a small bias towards more recent rather than initial adapter colors. The modelling of the reported after-image colors further confirmed that adaptation rapidly instigates and gradually dissipates. A second experiment replicated these results and further showed that the probability of observing an after-image diminishes only slightly when the adapter displays transient (stepwise, abrupt) color transitions. We conclude from the results that the visual system can adapt to dynamic colors, to a degree that is robust to the potential interference of transient changes in adapter content.

Pós-processamento da mediana de escala de cinza (GSM), posterização e remapeamento de cores para ultrassonografia de carótidas / Grayscale median (GSM) post-processing, posterizing, and color mapping for carotid ultrasound

Resumo Fatores relativos à placa aterosclerótica podem indicar instabilidade como ulcerações, hemorragias intraplaca, núcleo lipídico, capa fibrosa delgada ou irregular e inflamação. A mediana de escala de cinza (GSM, de greyscale median) da placa é um dos métodos mais difundidos de estudo da placa aterosclerótica; nesse sentido, é importante criar uma padronização da pós-processamento de forma compreensível. O pós-processamento foi realizado no software Photoshop 23.1.1. A padronização da imagem foi alcançada com o ajuste de curvas do histograma de escalas de cinza definindo o ponto mais escuro do lúmen vascular (sangue) para zero e a adventícia distal para 190. A posterização e o remapeamento de cores foram realizados. Um método que apresenta o atual estado da arte da técnica de forma acessível e ilustrativa pode contribuir para disseminação da análise de GSM. Neste artigo, esse processo é demonstrado passo a passo.

Abstract Factors related to atherosclerotic plaques may indicate instability, such as ulcerations, intraplaque hemorrhages, lipid core, thin or irregular fibrous cap, and inflammation. The grayscale median (GSM) value is one of the most widespread methods of studying atherosclerotic plaques and it is therefore important to comprehensively standardize image post-processing. Post-processing was performed using Photoshop 23.1.1.202. Images were standardized by adjusting the grayscale histogram curves, setting the darkest point of the vascular lumen (blood) to zero and the distal adventitia to 190. Posterization and color mapping were performed. A methodology that presents the current state of the art in an accessible and illustrative way should contribute to the dissemination of GSM analysis. This article describes and illustrates the process step by step.

Ptolemy's Optics, double-vision, and the technological afterimage.

In contrast to earlier theorists within the Greek optical tradition, who relied almost exclusively on geometrical diagrams to articulate and explain vision, Ptolemy employed several material instruments in his investigation of sight. These included rulers, glass cylinders, mirrors, and a bronze plaque designed to measure angles of incidence and reflection. These devices allowed Ptolemy to expand the operational definition of vision far beyond that of his predecessors, as he explicated several previously unexamined visual behaviors, including binocular vision, diplopia, and refraction. This article argues that these tools did more than make new phenomena visible; they also set the parameters for what these phenomena looked like-sometimes to such a degree that features of these instruments merged with the visual behaviors that they rendered visible. In some cases, this occurred as a type of "double-exposure," where the investigative tool became layered over top of the process of sight, such as when Ptolemy's "ruler" for investigating binocular vision became a template for imagining the mechanism of spatial perception employed by the eyes. In other cases, this merging occurred as a type of "technological afterimage," where the instrument provided an implicit model for phenomena it was not directly investigating. Ptolemy's bronze plaque stands as an example of this second type, insofar as it inspired his account of ocular geometry and facilitated novel assertions about the eye's operations, even though it did not directly inspect these features. In general, this article thus outlines how the technologies of investigation can structure patterns of thought and naturalize certain physical arguments, whether for the phenomena that they directly articulate or for those indirectly associated with their particular use cases.

The Riemannian Geometry Theory of Visually-Guided Movement Accounts for Afterimage Illusions and Size Constancy.

This discussion paper supplements our two theoretical contributions previously published in this journal on the geometric nature of visual space. We first show here how our Riemannian formulation explains the recent experimental finding (published in this special issue on size constancy) that, contrary to conclusions from past work, vergence does not affect perceived size. We then turn to afterimage experiments connected to that work. Beginning with the Taylor illusion, we explore how our proposed Riemannian visual-somatosensory-hippocampal association memory network accounts in the following way for perceptions that occur when afterimages are viewed in conjunction with body movement. The Riemannian metric incorporated in the association memory network accurately emulates the warping of 3D visual space that is intrinsically introduced by the eye. The network thus accurately anticipates the change in size of retinal images of objects with a change in Euclidean distance between the egocentre and the object. An object will only be perceived to change in size when there is a difference between the actual size of its image on the retina and the anticipated size of that image provided by the network. This provides a central mechanism for size constancy. If the retinal image is the afterimage of a body part, typically a hand, and that hand moves relative to the egocentre, the afterimage remains constant but the proprioceptive signals change to give the new hand position. When the network gives the anticipated size of the hand at its new position this no longer matches the fixed afterimage, hence a size-change illusion occurs.

From Receptive to Perceptive Fields: Size-Dependent Asymmetries in Both Negative Afterimages and Subcortical On and Off Post-Stimulus Responses.

Negative afterimages are perceptual phenomena that occur after physical stimuli disappear from sight. Their origin is linked to transient post-stimulus responses of visual neurons. The receptive fields (RFs) of these subcortical ON- and OFF-center neurons exhibit antagonistic interactions between central and surrounding visual space, resulting in selectivity for stimulus polarity and size. These two features are closely intertwined, yet their relationship to negative afterimage perception remains unknown. Here we tested whether size differentially affects the perception of bright and dark negative afterimages in humans of both sexes, and how this correlates with neural mechanisms in subcortical ON and OFF cells. Psychophysically, we found a size-dependent asymmetry whereby dark disks produce stronger and longer-lasting negative afterimages than bright disks of equal contrast at sizes >0.8°. Neurophysiological recordings from retinal and relay cells in female cat dorsal lateral geniculate nucleus showed that subcortical ON cells exhibited stronger sustained post-stimulus responses to dark disks, than OFF cells to bright disks, at sizes >1°. These sizes agree with the emergence of center-surround antagonism, revealing stronger suppression to opposite-polarity stimuli for OFF versus ON cells, particularly in dorsal lateral geniculate nucleus. Using a network-based retino-geniculate model, we confirmed stronger antagonism and temporal transience for OFF-cell post-stimulus rebound responses. A V1 population model demonstrated that both strength and duration asymmetries can be propagated to downstream cortical areas. Our results demonstrate how size-dependent antagonism impacts both the neuronal post-stimulus response and the resulting afterimage percepts, thereby supporting the idea of perceptual RFs reflecting the underlying neuronal RF organization of single cells.SIGNIFICANCE STATEMENT Visual illusions occur when sensory inputs and perceptual outcomes do not match, and provide a valuable tool to understand transformations from neural to perceptual responses. A classic example are negative afterimages that remain visible after a stimulus is removed from view. Such perceptions are linked to responses in early visual neurons, yet the details remain poorly understood. Combining human psychophysics, neurophysiological recordings in cats and retino-thalamo-cortical computational modeling, our study reveals how stimulus size and the receptive-field structure of subcortical ON and OFF cells contributes to the parallel asymmetries between neural and perceptual responses to bright versus dark afterimages. Thus, this work provides a deeper link from the underlying neural mechanisms to the resultant perceptual outcomes.

Hues of Color Afterimages.

We studied the relationship between color afterimages and complementary colors. The hues of afterimages of 24 inducer hues, uniformly distributed over the rgb color circle, were measured by an iterative method of adjustment. The judgment of equality of hue of the afterimage and a synthesized patch was effectively judged at the moment immediately after the switch-off of the inducer, when the synthesized patch went through any number of iterative adjustments. The two patches-both phenomenally present, but only one optically presented-appeared to the left and right of a fixation mark that was fixated throughout the whole procedure. Thus, both patches were present in eccentric vision. The hues of afterimages were found to be quite different from the hue of the complementary of the inducer. Almost one half of the color circle (orange to chartreuse) leads to afterimage hues in a narrow region of purples. This implies that color circles based on diametrically opposed inducer-afterimage hues are necessarily inconsistent. Yet, perhaps surprisingly, the relation between primary and afterimage hues is still approximately an involution (they are reciprocally related).

Social contingency modulates the perceived distance between self and other.

Although social contingency, namely contingent reactions of other to one's own actions, critically affects attachment formation, can it also modulate the perceived distance between self and other? Previous studies have suggested a positive answer. However, these studies are criticized for not showing true top-down effects on perception because of pitfalls such as task demands. We show that social contingency reduced the perceived distance between self and other while avoiding pitfalls. According to Emmert's law, the perceived size of an afterimage increases with perceived distance. Thus, if social contingency modulates the perceived distance, the perceived size of afterimage should inevitably reflect it. The results showed that the size of the afterimages of a face that contingently responded to participants' actions was perceived as smaller than those of non-contingent and unresponsive faces. This effect was more salient with increasing viewing distances. Thus, prior knowledge of interaction with environment modulates online perceptual processing in size constancy, probably through its influence on perceived distance.

Vocal tract constancy in birds and humans.

Humans perceive speech as being relatively stable despite acoustic variation caused by vocal tract (VT) differences between speakers. Humans use perceptual 'vocal tract normalisation' (VTN) and other processes to achieve this stability. Similarity in vocal apparatus/acoustics between birds and humans means that birds might also experience VT variation. This has the potential to impede bird communication. No known studies have explicitly examined this, but a number of studies show perceptual stability or 'perceptual constancy' in birds similar to that seen in humans when dealing with VT variation. This review explores similarities between birds and humans and concludes that birds show sufficient evidence of perceptual constancy to warrant further research in this area. Future work should 1) quantify the multiple sources of variation in bird vocalisations, including, but not limited to VT variations, 2) determine whether vocalisations are perniciously disrupted by any of these and 3) investigate how birds reduce variation to maintain perceptual constancy and perceptual efficiency.

A Model for a Filling-in Process Triggered by Edges Predicts "Conflicting" Afterimage Effects.

The goal of our research was to develop a compound computational model that predicts the "opposite" effects of the alternating aftereffects stimuli, such as the "color dove illusion" (Barkan and Spitzer, 2017), and the "filling in the afterimage after the image" (van Lier et al., 2009). The model is based on a filling-in mechanism, through a diffusion equation where the color and intensity of the perceived surface are obtained through a diffusion process of color from the stimulus edges. The model solves the diffusion equation with boundary conditions that takes the locations of the chromatic edges of the chromatic inducer (chromatic stimulus) and the achromatic remaining contours into account. These contours (edges) trigger the diffusion process. The same calculations are done for both types of afterimage effects, with the only difference related to the location of the remaining contour. While a gradient toward the inducing color produces a perception of the complementary color, an opposite gradient yields the perception of the same color as that of the chromatic inducer. Furthermore, we show that the same computational model can also predict new alternating aftereffects stimuli, such as the spiral stimulus, and the averaging of colors in alternating afterimage stimuli described by Anstis et al. (2012). The suggested model is able to predict most of the additional properties related to the "conflicting" phenomena that have been recently described in the literature, and thus supports the idea that a shared visual mechanism is responsible for both the positive and the negative effects.

Treatment of vestibular disorders with weak asymmetric base-in prisms: An hypothesis with a focus on Ménière's disease.

BACKGROUND: Regular treatments of Ménière's disease (MD) vary largely, and no single satisfactory treatment exists. A complementary treatment popular among Dutch and Belgian patients involves eyeglasses with weak asymmetric base-in prisms, with a perceived high success rate. An explanatory mechanism is, however, lacking. OBJECTIVE: To speculate on a working mechanism explaining an effectiveness of weak asymmetric base-in prims in MD, based on available knowledge. METHODS: After describing the way these prisms are prescribed using a walking test and its effect reported on, we give an explanation of its underlying mechanism, based on the literature. RESULTS: The presumed effect can be explained by considering the typical star-like walking pattern in MD, induced by a drifting after-image comparable to the oculogyral illusion. Weak asymmetric base-in prisms can furthermore eliminate the conflict between a net vestibular angular velocity bias in the efferent signal controlling the VOR, and a net re-afferent ocular signal. CONCLUSIONS: The positive findings with these glasses reported on, the fact that the treatment itself is simple, low-cost, and socially acceptable, and the fact that an explanation is at hand, speak in favour of elaborating further on this treatment.

Negative Afterimages From Flicker-Augmented Colors.

A patch that alternates between two hues such as dark green and light blue looks greenish on a light gray surround and bluish on a dark gray surround ("flicker-augmented contrast"). Thus, when an edge alternates between two hues in the same location, the visual system selects the more salient hue-the one with the higher Michelson contrast. However, the afterimage is the same pink, driven by the time integral of the physical, not the perceptual, adapting hues and regardless of the surround luminance. So the process of edge biasing does not transfer to the mechanism that creates afterimages.

Asymmetries of Dark and Bright Negative Afterimages Are Paralleled by Subcortical ON and OFF Poststimulus Responses.

Humans are more sensitive to luminance decrements than increments, as evidenced by lower thresholds and shorter latencies for dark stimuli. This asymmetry is consistent with results of neurophysiological recordings in dorsal lateral geniculate nucleus (dLGN) and primary visual cortex (V1) of cat and monkey. Specifically, V1 population responses demonstrate that darks elicit higher levels of activation than brights, and the latency of OFF responses in dLGN and V1 is shorter than that of ON responses. The removal of a dark or bright disc often generates the perception of a negative afterimage, and here we ask whether there also exist asymmetries for negative afterimages elicited by dark and bright discs. If so, do the poststimulus responses of subcortical ON and OFF cells parallel such afterimage asymmetries? To test these hypotheses, we performed psychophysical experiments in humans and single-cell/S-potential recordings in cat dLGN. Psychophysically, we found that bright afterimages elicited by luminance decrements are stronger and last longer than dark afterimages elicited by luminance increments of equal sizes. Neurophysiologically, we found that ON cells responded to the removal of a dark disc with higher firing rates that were maintained for longer than OFF cells to the removal of a bright disc. The ON and OFF cell asymmetry was most pronounced at long stimulus durations in the dLGN. We conclude that subcortical response strength differences between ON and OFF channels parallel the asymmetries between bright and dark negative afterimages, further supporting a subcortical origin of bright and dark afterimage perception.SIGNIFICANCE STATEMENT Afterimages are physiological aftereffects following stimulation of the eye, the study of which helps us to understand how our visual brain generates visual perception in the absence of physical stimuli. We report, for the first time to our knowledge, asymmetries between bright and dark negative afterimages elicited by luminance decrements and increments, respectively. Bright afterimages are stronger and last longer than dark afterimages. Subcortical neuronal recordings of poststimulus responses of ON and OFF cells reveal similar asymmetries with respect to response strength and duration. Our results suggest that subcortical differences between ON and OFF channels help explain intensity and duration asymmetries between bright and dark afterimages, supporting the notion of a subcortical origin of bright and dark afterimages.

Self-Grounded Vision: Hand Ownership Modulates Visual Location through Cortical ß and γ Oscillations.

Vision is known to be shaped by context, defined by environmental and bodily signals. In the Taylor illusion, the size of an afterimage projected on one's hand changes according to proprioceptive signals conveying hand position. Here, we assessed whether the Taylor illusion does not just depend on the physical hand position, but also on bodily self-consciousness as quantified through illusory hand ownership. Relying on the somatic rubber hand illusion, we manipulated hand ownership, such that participants embodied a rubber hand placed next to their own hand. We found that an afterimage projected on the participant's hand drifted depending on illusory ownership between the participants' two hands, showing an implication of self-representation during the Taylor illusion. Oscillatory power analysis of electroencephalographic signals showed that illusory hand ownership was stronger in participants with stronger α suppression over left sensorimotor cortex, whereas the Taylor illusion correlated with higher ß/γ power over frontotemporal regions. Higher γ connectivity between left sensorimotor and inferior parietal cortex was also found during illusory hand ownership. These data show that afterimage drifts in the Taylor illusion do not only depend on the physical hand position but also on subjective ownership, which itself is based on the synchrony of somatosensory signals from the two hands. The effect of ownership on afterimage drifts is associated with ß/γ power and γ connectivity between frontoparietal regions and the visual cortex. Together, our results suggest that visual percepts are not only influenced by bodily context but are self-grounded, mapped on a self-referential frame. SIGNIFICANCE STATEMENT: Vision is influenced by the body: in the Taylor illusion, the size of an afterimage projected on one's hand changes according to tactile and proprioceptive signals conveying hand position. Here, we report a new phenomenon revealing that the perception of afterimages depends not only on bodily signals, but also on the sense of self. Relying on the rubber hand illusion, we manipulated hand ownership, so that participants embodied a rubber hand placed next to their own hand. We found that visual afterimages projected on the participant's hand drifted laterally, only when the rubber hand was embodied. Electroencephalography revealed spectral dissociations between somatic and visual effects, and higher γ connectivity along the dorsal visual pathways when the rubber hand was embodied.

Size Constancy is Preserved but Afterimages are Prolonged in Typical Individuals with Higher Degrees of Self-Reported Autistic Traits.

Deficits in perceptual constancies from early infancy have been proposed to contribute to autism and exacerbate its symptoms (Hellendoorn et al., Frontiers in Psychology 6:1-16, 2015). Here, we examined size constancy in adults from the general population (N = 106) with different levels of self-reported autistic traits using an approach based on negative afterimages. The afterimage strength, as indexed by duration and vividness, was also quantified. In opposition to the Hellendoorn and colleagues' model, we were unable to demonstrate any kind of relationship between abilities in size constancy and autistic traits. However, our results demonstrated that individuals with higher degrees of autistic traits experienced more persistent afterimages. We discuss possible retinal and post-retinal explanations for prolonged afterimages in people with higher levels of autistic traits.

How lateral inhibition and fast retinogeniculo-cortical oscillations create vision: A new hypothesis.

The role of the physiological processes involved in human vision escapes clarification in current literature. Many unanswered questions about vision include: 1) whether there is more to lateral inhibition than previously proposed, 2) the role of the discs in rods and cones, 3) how inverted images on the retina are converted to erect images for visual perception, 4) what portion of the image formed on the retina is actually processed in the brain, 5) the reason we have an after-image with antagonistic colors, and 6) how we remember space. This theoretical article attempts to clarify some of the physiological processes involved with human vision. The global integration of visual information is conceptual; therefore, we include illustrations to present our theory. Universally, the eyeball is 2.4cm and works together with membrane potential, correspondingly representing the retinal layers, photoreceptors, and cortex. Images formed within the photoreceptors must first be converted into chemical signals on the photoreceptors' individual discs and the signals at each disc are transduced from light photons into electrical signals. We contend that the discs code the electrical signals into accurate distances and are shown in our figures. The pre-existing oscillations among the various cortices including the striate and parietal cortex, and the retina work in unison to create an infrastructure of visual space that functionally "places" the objects within this "neural" space. The horizontal layers integrate all discs accurately to create a retina that is pre-coded for distance. Our theory suggests image inversion never takes place on the retina, but rather images fall onto the retina as compressed and coiled, then amplified through lateral inhibition through intensification and amplification on the OFF-center cones. The intensified and amplified images are decompressed and expanded in the brain, which become the images we perceive as external vision. SUMMARY: This is a theoretical article presenting a novel hypothesis about the physiological processes in vision, and expounds upon the visual aspect of two of our previously published articles, "A unified 3D default space consciousness model combining neurological and physiological processes that underlie conscious experience", and "Functional representation of vision within the mind: A visual consciousness model based in 3D default space." Currently, neuroscience teaches that visual images are initially inverted on the retina, processed in the brain, and then conscious perception of vision happens in the visual cortex. Here, we propose that inversion of visual images never takes place because images enter the retina as coiled and compressed graded potentials that are intensified and amplified in OFF-center photoreceptors. Once they reach the brain, they are decompressed and expanded to the original size of the image, which is perceived by the brain as the external image. We adduce that pre-existing oscillations (alpha, beta, and gamma) among the various cortices in the brain (including the striate and parietal cortex) and the retina, work together in unison to create an infrastructure of visual space thatfunctionally "places" the objects within a "neural" space. These fast oscillations "bring" the faculties of the cortical activity to the retina, creating the infrastructure of the space within the eye where visual information can be immediately recognized by the brain. By this we mean that the visual (striate) cortex synchronizes the information with the photoreceptors in the retina, and the brain instantaneously receives the already processed visual image, thereby relinquishing the eye from being required to send the information to the brain to be interpreted before it can rise to consciousness. The visual system is a heavily studied area of neuroscience yet very little is known about how vision occurs. We believe that our novel hypothesis provides new insights into how vision becomes part of consciousness, helps to reconcile various previously proposed models, and further elucidates current questions in vision based on our unified 3D default space model. Illustrations are provided to aid in explaining our theory.

Do Zwicker Tones Evoke a Musical Pitch?

It has been argued that musical pitch, i.e. pitch in its strictest sense, requires phase locking at the level of the auditory nerve. The aim of the present study was to assess whether a musical pitch can be heard in the absence of peripheral phase locking, using Zwicker tones (ZTs). A ZT is a faint, decaying tonal percept that arises after listening to a band-stop (notched) broadband noise. The pitch is within the frequency range of the notch. Several findings indicate that ZTs are unlikely to be produced mechanically at the level of the cochlea and, therefore, there is unlikely to be phase locking to ZTs in the auditory periphery. In stage I of the experiment, musically trained subjects adjusted the frequency, level, and decay time of an exponentially decaying sinusoid so that it sounded similar to the ZT they perceived following a broadband noise, for various notch positions. In stage II, subjects adjusted the frequency of a sinusoid so that its pitch was a specified musical interval below that of either a preceding ZT or a preceding sinusoid (as determined in stage I). Subjects selected appropriate frequency ratios for ZTs, although the standard deviations of the adjustments were larger for the ZTs than for the equally salient sinusoids by a factor of 1.1-2.2. The results suggest that a musical pitch may exist in the absence of peripheral phase locking.

Is palinopsia in migraineurs an enhanced physiological phenomenon?

Background We report the occurrence of palinopsia in patients with migraine and its correlation with migraine characteristics, triggers and allodynia. Methods This study included 153 consecutive patients with migraine and recorded their clinical details, including allodynia and migraine triggers and characteristics. Palinopsia was evaluated in migraineurs and 101 controls by using a questionnaire and a novel method. Results According to the questionnaire assessment, 9.8% migraineurs had palinopsia. According to the novel method, 57.5% of migraineurs and 12% of controls had palinopsia. Migraineurs most frequently had palinopsia to red color (51.6%), followed by yellow (49.7%), blue (47.7%), green (46.4%) and the least to white (30.7%). A similar pattern with a lesser frequency was noted in controls. The duration of palinopsia was longer in migraineurs than in controls (32.68 ± 20.24 vs. 5.92 ± 4.55 seconds; p < 0.001). Migraineurs with palinopsia differed from those without in terms of noise as a migraine trigger ( p < 0.001) and allodynia as a migraine-associated phenomenon ( p = 0.03). In multivariable analysis, predictors of palinopsia were the frequency ( p = 0.003) and severity ( p = 0.04) of headache and the presence of headache during examination ( p = 0.0001). Conclusion Migraineurs had a pattern of palinopsia to different colors that was similar to the controls, but the palinopsia of migraineurs was more frequent and of longer duration, especially during headaches.

Afterimages are biased by top-down information.

The afterimage illusion refers to a complementary colored image continuing to appear in the observer's vision after the exposure to the original image has ceased. It is assumed to be a phenomenon of the primary visual pathway, caused by overstimulation of photoreceptors of the retina. The aim of the present study was to investigate the nature of afterimage perceptions; mainly whether it is a mere physical, that is, low-level effect or whether it can be modulated by top-down processes, that is, high-level processes. Participants were first exposed to five either strongly female or male faces (Experiment 1), objects highly associated with female or male gender (Experiment 2) or female versus male names (Experiment 3), followed by a negativated image of a gender-neutral face which had to be fixated for 20s to elicit an afterimage. Participants had to rate their afterimages according to sexual dimorphism, showing that the afterimage of the gender-neutral face was perceived as significantly more female in the female priming condition compared with the male priming condition, independently of the priming quality (faces, objects, and names). Our results documented, in addition to previously presumed bottom-up mechanisms, a prominent influence of top-down processing on the perception of afterimages via priming mechanisms (female primes led to more female afterimage perception).

Transformation of the Visual Afterimage Under Subject's Eye and Body Movements and the Visual Field Constancy Mechanisms.

Two types of positive afterimages differing in their structural complexity--called poor and rich--were used to investigate the visual field constancy mechanisms during eye and head movements. In the case of a poor afterimage, consistent with Emmert's law, every eye and head movement caused the afterimage to appear moving (in exactly the same way), unlike a real object, which appeared to remain stationary during those same eye and head movements (although its retinal image moved opposite to the eye movement). However, in the case of a rich afterimage, the afterimage appeared stationary during eye movements, while a small stationary test light in the real space appeared to move, violating Emmert's law. It is suggested that, in these two cases, the different apparent transformations reflected functioning of different constancy mechanisms. Both mechanisms implement projection of retinal images upon a hypothetical constant visual screen in strict accordance with the subject's movements but in two different ways. The experiments have indicated that, during binocular fusion, the visual afferent system is able to use information from the structural organization of the visual flow to implement the visual field stability and to calculate gaze direction independently of proprioceptive signals.