Stimulus size and the magnitude of the visual illusion of extent

Psychophysical experiments have been performed to study the dependence of the magnitude of the illusion of extent on the size of the referential part of the stimulus for different lengths of the imaginary wings of a modified Brentano figure. The experimental data are explained in terms of a model based on the concept of biases of the centroids of excitatory patterns evoked by stimulus terminating elements. A good fit of experimental data to calculated values confirms the model’s assumption that the areas of perceptual influence increase depend on their eccentricity in the visual field. An estimation of the model’s parameters also confirms the assumption that distortion of perceived information on the relative positioning of the centers of masses of the stimulus terminators is one of the main factors determining the magnitude of the illusion of extent and supports the hypothesis on the relationship of the phenomenon of this illusion with characteristics of the mechanism responsible for the perception of three-dimensional coordinates of visual objects.     

Increased visual after-effects in migraine following pattern adaptation extend to simultaneous tilt illusion

Much previous research into visual processing in migraine supports a model of abnormal cortical processing, in between the headache attacks, that is characterised by hyperexcitability, heightened responsiveness, a lack of habituation and/or a lack of intra-cortical inhibition. Shepherd (2001) reported two adaptation studies that challenged this view, one using the tilt after-effect, the second using the motion after-effect. Models of cortical function in migraine based on hyperexcitability and a lack of inhibition lead to specific predictions in an adaptation study: there should have been smaller after-effects in people with migraine than in people without. Both experiments, however, revealed larger after-effects in the migraine group than in the control group. Here, these results are extended to the simultaneous tilt illusion and an identical pattern of results was obtained: there were consistently larger effects in the migraine group than in the control group. The results from the three experiments are not consistent with a lack of inhibition in migraine. The results are discussed in terms of alternative models of cortical function, including a lack of excitation and reduced central energy reserves.     

Lipid pathway alterations in Parkinson's disease primary visual cortex

Background

We present a lipidomics analysis of human Parkinson''s disease tissues. We have focused on the primary visual cortex, a region that is devoid of pathological changes and Lewy bodies; and two additional regions, the amygdala and anterior cingulate cortex which contain Lewy bodies at different disease stages but do not have as severe degeneration as the substantia nigra.

Methodology/Principal Findings

Using liquid chromatography mass spectrometry lipidomics techniques for an initial screen of 200 lipid species, significant changes in 79 sphingolipid, glycerophospholipid and cholesterol species were detected in the visual cortex of Parkinson''s disease patients (n = 10) compared to controls (n = 10) as assessed by two-sided unpaired t-test (p-value <0.05). False discovery rate analysis confirmed that 73 of these 79 lipid species were significantly changed in the visual cortex (q-value <0.05). By contrast, changes in 17 and 12 lipid species were identified in the Parkinson''s disease amygdala and anterior cingulate cortex, respectively, compared to controls; none of which remained significant after false discovery rate analysis. Using gas chromatography mass spectrometry techniques, 6 out of 7 oxysterols analysed from both non-enzymatic and enzymatic pathways were also selectively increased in the Parkinson''s disease visual cortex. Many of these changes in visual cortex lipids were correlated with relevant changes in the expression of genes involved in lipid metabolism and an oxidative stress response as determined by quantitative polymerase chain reaction techniques.

Conclusions/Significance

The data indicate that changes in lipid metabolism occur in the Parkinson''s disease visual cortex in the absence of obvious pathology. This suggests that normalization of lipid metabolism and/or oxidative stress status in the visual cortex may represent a novel route for treatment of non-motor symptoms, such as visual hallucinations, that are experienced by a majority of Parkinson''s disease patients.     

The extent of visual control in the courtship tracking of D. melanogaster

The courtship tracking behaviour of male D. melanogaster was filmed in a range of visual mutants and in wild type flies under white and infra-red light. The absence of visual input severely restricts the velocity at which the female may be tracked but does not remove the male's ability to discriminate the female's abdomen from her head. Degradation of visual processing, such as occurs in certain visual mutants, leads mainly to disruption of the control of orientation in tracking. The gain control of translation may only be meaningfully assessed when vision is enabled. Courtship tracking does not depend upon the presence of a binocular field of vision; one eyed flies court with a lateral displacement of the body, and loose visual contact with the female readily when her image moves from the sighted towards the blind eye. The selection of the left or right wing for extension follows a distinct pattern with the male's position and orientation around the female; this may be controlled collateral to locomotor output and after interlateral comparison of sensory input.     

Grey matter volume in early human visual cortex predicts proneness to the sound-induced flash illusion

Visual perception can be modulated by sounds. A drastic example of this is the sound-induced flash illusion: when a single flash is accompanied by two bleeps, it is sometimes perceived in an illusory fashion as two consecutive flashes. However, there are strong individual differences in proneness to this illusion. Some participants experience the illusion on almost every trial, whereas others almost never do. We investigated whether such individual differences in proneness to the sound-induced flash illusion were reflected in structural differences in brain regions whose activity is modulated by the illusion. We found that individual differences in proneness to the illusion were strongly and significantly correlated with local grey matter volume in early retinotopic visual cortex. Participants with smaller early visual cortices were more prone to the illusion. We propose that strength of auditory influences on visual perception is determined by individual differences in recurrent connections, cross-modal attention and/or optimal weighting of sensory channels.     

Experiment and illusion in reproductive medicine

The diffusion of medical technology is largely determined by the marketplace demands supported by national and historical contexts. Using the cases of cesarean delivery and newborn intensive care in the United States, this article presents the argument that the interaction of four factors accounts for the rapid diffusion of untested technologies. These factors are economic expansion in an unrestricted market, the vulnerability of the patient population, a social disposition towards emergency medicine, and the vested interest of medical specialists. Jeanne Guillemin is a professor of sociology at Boston College. She served on the National Institutes of Health Consensus Task Force on Cesarean Birth and authored the section of the task force’s 1981 report on international data on cesarean rates. She is also the coauthor, with Lynda Lytle Holmstrom, ofMixed Blessings: Intensive Care for Newborns, which analyzes the national and international development of this hospital service, as well as authoring numerous articles on medical policy.     

Emmert's Law and the moon illusion

A cognitive account is offered of puzzling, though well known phenomena, including increased size of afterimages with greater distance (Emmert's Law) and increased size of the moon near the horizon (the Moon Illusion). Various classical distortion illusions are explained by Size Scaling when inappropriate to distance, 'flipping' depth ambiguity being used to separate botton-up and top-down visual scaling. Helmholtz's general Principle is discussed with simpler wording - that retinal images are attributed to objects - for object recognition and spatial vision.     

Perceptual distance and the moon illusion

The elevated moon usually appears smaller than the horizon moon of equal angular size. This is the moon illusion. Distance cues may enable the perceptual system to place the horizon moon at an effectively greater distance than the elevated moon, thus making it appear as larger. This explanation is related to the size-distance invariance hypothesis. However, the larger horizon moon is usually judged as closer than the smaller zenith moon. A bias to expect an apparently large object to be closer than a smaller object may account for this conflict. We designed experiments to determine if unbiased sensitivity to illusory differences in the size and distance of the moon (as measured by d') is consistent with SDIH. A moon above a 'terrain' was compared in both distance and size to an infinitely distant moon in empty space (the reduction moon). At a short distance the terrain moon was adjudged as both closer and smaller than the reduction moon. But these differences could not be detected at somewhat greater distances. At still greater distances the terrain moon was perceived as both more distant and larger than the reduction moon. The distances at which these transitions occurred were essentially the same for both distance and size discrimination tasks, thus supporting SDIH.     

Phase channels and the square-wave illusion

We have utilized a phase-sensitive effect, the square-wave illusion, to investigate the presence and tuning of phase-sensitive mechanisms. Subjects viewed a 1 c/deg triangular-wave grating and reported the presence or absence of the illusion, before and after adaptation to square-wave gratings of various spatial frequencies. Illusion strength declined with adaptation and was a function of spatial frequency. This function may reflect the spatial frequency tuning of a 'phase channel'. We have also measured by adaptation a contrast sensitivity function, maximally sensitive to a frequency of 1 c/deg. When normalized to the same maximal depression from baseline sensitivity, the phase channel and spatial-frequency channel shared the same low frequency tuning, but the phase channel was broader, with extended sensitivity to higher frequencies. These results can be understood if the phase channel is constructed from combinations of phase sensitive spatial-frequency channels.     

Knowledge in perception and illusion.

Following Hermann von Helmholtz, who described visual perceptions as unconscious inferences from sensory data and knowledge derived from the past, perceptions are regarded as similar to predictive hypotheses of science, but are psychologically projected into external space and accepted as our most immediate reality. There are increasing discrepancies between perceptions and conceptions with science''s advances, which makes it hard to define ''illusion''. Visual illusions can provide evidence of object knowledge and working rules for vision, but only when the phenomena are explained and classified. A tentative classification is presented, in terms of appearances and kinds of causes. The large contribution of knowledge from the past for vision raises the issue: how do we recognize the present, without confusion from the past. This danger is generally avoided as the present is signalled by real-time sensory inputs-perhaps flagged by qualia of consciousness.     

Histological alterations of the visual organ following high grade barometric depression limited to the cranium

The Authors describe the histologic report of the eye and the first part of optic nerve of young goats and rabbits exposed to high grade barometric depression (850/cm2) limited to cranium.     

Perceptual illusion and the real-time control of action

Participants were cued by an auditory tone to grasp a target object from within a size-contrast display. The peak grip aperture was unaffected by the perceptual size illusion when the target array was visible between the response cue and movement onset (vision trials). The grasp was sensitive to the illusion, however, when the target array was occluded from view when the response was cued (occlusion trials). This was true when the occlusion occurred 2.5 s before the response cue (delay), but also when the occlusion coincided with the response cue (no-delay). Unlike previous experiments, vision and occlusion trials were presented in random sequence. The results suggest that dedicated, real-time visuomotor mechanisms are engaged for the control of action only after the response is cued, and only if the target is visible. These visuomotor mechanisms compute the absolute metrics of the target object and therefore resist size-contrast illusions. In other situations (e.g. prior to the response cue, or if the target is no longer visible), a perceptual representation of the target object can be used for action planning. Unlike the real-time visuomotor mechanisms, perception-based movement planning makes use of relational metrics, and is therefore sensitive to size-contrast illusions.     

The consultation and the therapeutic illusion.

At 45 general-practice surgery sessions 200 patients in whom no definite diagnosis could be made were randomly selected for one of two procedures. Either they were given a symptomatic diagnosis and medications, or they were told that they had no evidence of disease and therefore they required no treatment. No difference in outcome was found between these two methods as judged by the return or not of the patient within one month and his statement that he did or did not get better.     

The Thatcher illusion in humans and monkeys

Primates possess the remarkable ability to differentiate faces of group members and to extract relevant information about the individual directly from the face. Recognition of conspecific faces is achieved by means of holistic processing, i.e. the processing of the face as an unparsed, perceptual whole, rather than as the collection of independent features (part-based processing). The most striking example of holistic processing is the Thatcher illusion. Local changes in facial features are hardly noticeable when the whole face is inverted (rotated 180°), but strikingly grotesque when the face is upright. This effect can be explained by a lack of processing capabilities for locally rotated facial features when the face is turned upside down. Recently, a Thatcher illusion was described in the macaque monkey analogous to that known from human investigations. Using a habituation paradigm combined with eye tracking, we address the critical follow-up questions raised in the aforementioned study to show the Thatcher illusion as a function of the observer''s species (humans and macaques), the stimulus'' species (humans and macaques) and the level of perceptual expertise (novice, expert).