The "thin man" phenomenon: a sign of cortical plasticity following inferior homonymous paracentral scotomas

The Invisibility of Scotomas I: The Carving Hypothesis

SIGNIFICANCE

Veridical depictions of scene appearance with scotomas allow better understanding of the impact of field loss and may improve the development and implementation of rehabilitation. Explanation and depiction of the invisibility of scotoma may lead to patients' understanding and thus better compliance with related treatments.

PURPOSE

Simulations of perception with scotomas guide training, patient education, and rehabilitation research. Most simulations incorrectly depict scotomas as black patches, although the scotomas and the missing contents are usually invisible to patients. We present a novel approach to capture the reported appearance of scenes with scotomas.

METHODS

We applied a content-aware image resizing algorithm to carve out the content elided under the scotomas. With video sequences, we show how and why eye movements fail to increase the visibility of the carved scotomas.

RESULTS

Numerous effects, reported by patients, emerge naturally from the scotoma carving. Carving-eliminated scotomas over natural images are barely visible, despite causing substantial distortions. Low resolution and contrast sensitivity at farther eccentricities and saccadic blur reduce the visibility of the distortions. In a walking scenario, static objects moving smoothly to the periphery disappear into and then reemerge out of peripheral scotomas, invisibly.

CONCLUSIONS

Scotoma carving provides a viable hypothetical simulation of vision with scotomas due to loss of neurons at the retinal ganglion cell level and higher. As a hypothesis, it generates predictions that lend themselves to future clinical testing. The different effects of scotomas due to loss of photoreceptors are left for follow-up work.

Face distortions in prosopometamorphopsia provide new insights into the organization of face perception

Prosopometamorphopsia (PMO) is a striking condition of visual perception in which facial features appear distorted, for example drooping, swelling, or twisting. Although numerous cases have been reported, few of those investigations have carried out formal testing motivated by theories of face perception. However, because PMO involves conscious visual distortions to faces which participants can report, it can be used to probe fundamental questions about face representations. Here we review cases of PMO that address theoretical questions in visual neuroscience including face specificity, inverted face processing, the importance of the vertical midline, dissociable representations for each half of the face, hemispheric specialization, the relationship between face recognition and conscious face perception, and the reference frames that face representations are embedded within. Finally, we list and touch upon eighteen open questions that make clear how much is left to learn about PMO and the potential it has to provide important advances in face perception.

Decreased spontaneous brain activity in the dorsal visual pathway in age-related macular degeneration patients revealed by fractional amplitude of low-frequency fluctuation

BACKGROUND

Age-related macular degeneration (AMD) is the leading cause of visual loss in the developed world and damages the central retina. Growing evidences demonstrated that AMD patients were associated with brain structure changes in visual pathway. However, it remains unknown whether alterations of spontaneous brain activity changes occur in AMD patients.

PURPOSE

The purpose of this study was to investigate the effect of central vision loss on spontaneous brain activity in AMD patients.

MATERIAL AND METHODS

Seventeen AMD patients and 17 healthy controls (HCs) underwent resting-state MRI scans. The fractional amplitude of low-frequency fluctuations (fALFFs) was applied to investigate the spontaneous brain activity changes in AMD patients.

RESULTS

Compared with HC group, AMD patients showed significant decreased fALFF values in the right calcarine/cuneus (brodmann area 17,8) and right superior parietal lobule (brodmann area 7).

CONCLUSION

Our results showed that AMD patients had decreased brain activities in the dorsal visual pathway, which offer important insights into the neural mechanisms of central visual field defect in AMD patients.

A century of prosopometamorphopsia studies

Prosopometamorphopsia is an extremely rare disorder of visual perception characterised by facial distortions. We here review 81 cases (eight new ones and 73 cases published over the past century) to shed light on the perception of face gestalts. Our analysis indicates that the brain systems underlying the perception of face gestalts have genuine network properties, in the sense that they are widely disseminated and built such that spatially normal perception of faces can be maintained even when large parts of the network are compromised. We found that bilateral facial distortions were primarily associated with right-sided and bilateral occipital lesions, and unilateral facial distortions with lesions ipsilateral to the distorted hemifield and with the splenium of the corpus callosum. We also found tentative evidence for the involvement of the left frontal regions in the fusing of vertical hemi-images of faces, and of right parietal regions in the fusing of horizontal hemi-images. Evidence supporting the remarkable adaptability of the network comes from the relatively high recovery rates that we found, from the ipsilateral hemifield predominance of hemi-prosopometamorphopsia, and from a phenomenon called cerebral asthenopia (heightened visual fatigability) which points to the dynamic nature of compensatory mechanisms maintaining normal face perception, even in chronic cases of prosopometamorphopsia. Finally, our analysis suggests that specialised networks for the representation of face gestalts in familiar-versus-unfamiliar faces and for own-versus-other face may be present, although this is in need of further study.

Visual brain plasticity induced by central and peripheral visual field loss

Disorders that specifically affect central and peripheral vision constitute invaluable models to study how the human brain adapts to visual deafferentation. We explored cortical changes after the loss of central or peripheral vision. Cortical thickness (CoTks) and resting-state cortical entropy (rs-CoEn), as a surrogate for neural and synaptic complexity, were extracted in 12 Stargardt macular dystrophy, 12 retinitis pigmentosa (tunnel vision stage), and 14 normally sighted subjects. When compared to controls, both groups with visual loss exhibited decreased CoTks in dorsal area V3d. Peripheral visual field loss also showed a specific CoTks decrease in early visual cortex and ventral area V4, while central visual field loss in dorsal area V3A. Only central visual field loss exhibited increased CoEn in LO-2 area and FG1. Current results revealed biomarkers of brain plasticity within the dorsal and the ventral visual streams following central and peripheral visual field defects.

Learning to see again: biological constraints on cortical plasticity and the implications for sight restoration technologies

The 'bionic eye'-so long a dream of the future-is finally becoming a reality with retinal prostheses available to patients in both the US and Europe. However, clinical experience with these implants has made it apparent that the visual information provided by these devices differs substantially from normal sight. Consequently, the ability of patients to learn to make use of this abnormal retinal input plays a critical role in whether or not some functional vision is successfully regained. The goal of the present review is to summarize the vast basic science literature on developmental and adult cortical plasticity with an emphasis on how this literature might relate to the field of prosthetic vision. We begin with describing the distortion and information loss likely to be experienced by visual prosthesis users. We then define cortical plasticity and perceptual learning, and describe what is known, and what is unknown, about visual plasticity across the hierarchy of brain regions involved in visual processing, and across different stages of life. We close by discussing what is known about brain plasticity in sight restoration patients and discuss biological mechanisms that might eventually be harnessed to improve visual learning in these patients.

Hyper-vision of mirror symmetry in patients with macular degenerationreflects parafoveal cortical reorganization

PURPOSE

This study aims at comparing participants with juvenile macular degeneration (MD) and normally sighted observers in their sensitivity to mirror and translational symmetry.

METHODS

We measured in 25 normal sighted and 9 MD participants sensitivity (d') to detect the symmetry of two dot patterns presented at the opposite sides of their central scotoma.

RESULTS

At a large dot patterns separation (13.3 deg), at which detection failed in normally sighted observers, MD patients had high sensitivity to mirror symmetry, whereas translational symmetry was undetected.

CONCLUSIONS

The mirror-translational dissociation is not predicted by the well-known phenomenon of shrinking the location of images surrounding the scotoma. Our results indicate higher capacity of MD with respect to normally sighted observers to organize mirror symmetric dot patterns far apart into a unique percept. Our results suggest that MD have acquired the capability to use information only present in mirror symmetry, i.e., the co-aligned position of the centre of low-frequency filters connecting symmetric dot pairs on opposite sides of the scotoma. This relevant functional change in vision of MD patients may find its explanation in a functionally acquired high-level cortical representation of visual input.

Facial dysmorphopsia: a notable variant of the "thin man" phenomenon?

BACKGROUND

The aim of this work is to investigate the facial distortion (dysmorphopsia) experienced by patients with homonymous paracentral scotomas and to analyze the interrelationship with the previously described "thin man" phenomenon.

METHODS

Routine neuro-ophthalmological examination and brain MRI in three patients who suffered from small homonymous paracentral scotomas due to infarction or arteriovenous malformations of the occipital lobe. They all complained of distortion and shrinkage of their interlocutor's face contralateral to the brain lesion. The phenomenon appeared some seconds after steady fixation on the interlocutor's nose and was evident with both left and right homonymous scotomas. The patients did not notice a gap in the area corresponding to the scotoma and objects other than faces were perceived normally.

RESULTS

Homonymous paracentral scotomas can lead to focal displacement of facial features towards the center of the field defect with resulting distortion of the face on the affected side. This so-called "dysmorphopsia" makes faces appear regionally narrower than they are in reality and may be induced even by visual field defects that remain undetected by conventional perimetry using 6° × 6° grids. Predilection for faces is probably associated with the superior location of scotomas or specific impairment of face processing abilities related to the lesion site.

CONCLUSIONS

Facial dysmorphopsia is most probably associated with cortical "filling-in" and spatial distortion, and can hence be regarded as a special entity of the "thin man" phenomenon.

Illusory contours over pathological retinal scotomas

Our visual percepts are not fully determined by physical stimulus inputs. Thus, in visual illusions such as the Kanizsa figure, inducers presented at the corners allow one to perceive the bounding contours of the figure in the absence of luminance-defined borders. We examined the discrimination of the curvature of these illusory contours that pass across retinal scotomas caused by macular degeneration. In contrast with previous studies with normal-sighted subjects that showed no perception of these illusory contours in the region of physiological scotomas at the optic nerve head, we demonstrated perfect discrimination of the curvature of the illusory contours over the pathological retinal scotoma. The illusion occurred despite the large scar around the macular lesion, strongly reducing discrimination of whether the inducer openings were acute or obtuse and suggesting that the coarse information in the inducers (low spatial frequency) sufficed. The result that subjective contours can pass through the pathological retinal scotoma suggests that the visual cortex, despite the loss of bottom-up input, can use low-spatial frequency information from the inducers to form a neural representation of new complex geometrical shapes inside the scotoma.

Perceptual distortion in homonymous paracentral scotomas

BACKGROUND

Cortical remapping after peripheral or central visual deafferentation alters visual perception, but it is unclear whether such a phenomenon impinges on areas remote from a scotoma. To investigate this question, we studied variations of perceptual spatial distortion in the visual field of patients with homonymous paracentral scotoma.

METHODS

Two patients with right inferior homonymous paracentral scotoma were asked to describe their perception of a series of figures showing two isometric vertical lines symmetrically located on either side of a fixation point. In each figure, the fixation point varied by steps of 2 degrees along a hypothetical vertical line equidistant between the test lines. The lines subtended 20 degrees of visual angle, and the right line passed through the scotoma in both cases. Time for spatial distortion to manifest was recorded.

RESULTS

Both subjects reported that the right line was perceived as shorter than the left one. The line shortening varied in magnitude with the distance of the fixation point from the end of the line and was more pronounced when the distance increased. Moreover, perceptual line shortening appeared 5-10 seconds after steady fixation, but values of shortening varied during the following 10 seconds. In addition, the right line appeared uninterrupted or slightly blurred in the scotoma region.

CONCLUSIONS

These observations reflect long-range cortical reorganization after brain damage. Larger receptive fields in the periphery of the visual map could explain why perceptual shortening is more pronounced with increased eccentricity.

Emotional Disturbances after Stroke

Impairment after stroke may have acute and long-lasting psychological implications. Additionally, organic brain dysfunction also appears to play an important role in poststroke affective modifications. Emotional state is multidetermined and can be specifically modified by alteration of some brain networks. This article illustrates a certain number of acute and more chronic emotional disturbances after stroke, such as mood disorders, emotional dyscontrol, and modification of emotional experiences. Some neural mechanisms implicated in these modifications are discussed. The main modifications described are depression anxiety, psychosis, modification of emotional experience, and fatigue.

[Adaptation to central scotoma. Part II. Perceptual filling-in phenomenon]

The visual cortex may reorganize after occurrence of a scotoma. Different experimental studies have shown that there is an attempt to minimize the impact of the scotoma, the missing information being filled-in by surrounding information. The clinical consequences of this filling-in phenomenon have been extensively studied by Safran and co-workers. The present review summarizes the current literature on this phenomenon and its clinical consequences. Furthermore, the authors present their own experience with the filling-in phenomenon in patients with age-related macular degeneration. Their study shows that, with few exceptions, the phenomenon only occurs in patients with bilateral central scotoma, in their better eye.

Hyper-vision in a patient with central and paracentral vision loss reflects cortical reorganization

SM, a 21-year-old female, presents an extensive central scotoma (30 deg) with dense absolute scotoma (visual acuity = 10/100) in the macular area (10 deg) due to Stargardt's disease. We provide behavioral evidence of cortical plastic reorganization since the patient could perform several visual tasks with her poor-vision eyes better than controls, although high spatial frequency sensitivity and visual acuity are severely impaired. Between 2.5-deg and 12-deg eccentricities, SM presented (1) normal acuity for crowded letters, provided stimulus size is above acuity thresholds for single letters; (2) a two-fold sensitivity increase (d-prime) with respect to controls in a simple search task; and (3) largely above-threshold performance in a lexical decision task carried out randomly by controls. SM's hyper-vision may reflect a long-term sensory gain specific for unimpaired low spatial-frequency mechanisms, which may result from modifications in response properties due to practice-dependent changes in excitatory/inhibitory intracortical connections.

Filling-in phenomenon in patients with age-related macular degeneration: differences regarding uni- or bilaterality of central scotoma

PURPOSE

The purpose of this study was to explore the presence of the filling-in phenomenon in patients with uni- or bilateral central scotoma (CS) resulting from natural history or laser photocoagulation of choroidal neovascularization in age-related macular degeneration (AMD).

METHODS

Sixteen consecutive patients with unilateral CS and 14 patients with bilateral CS were assessed (44 eyes) with a scanning laser ophthalmoscope (SLO). Scotoma was delineated by scotometry with a point (1 degree x1 degree) moving radially from the periphery to the center of the lesion. In addition, patients underwent a line test, consisting of a horizontal line moving vertically and a vertical line moving horizontally, from the periphery to the center. The lines were longer than the macular lesion and were projected onto the retina. Patients were asked to indicate when the lines seemed interrupted. The perceptual filling-in phenomenon was considered to be present when limits of the perceived scotoma, determined by the line test, were smaller than those assessed by scotometry. In patients with bilateral CS, the results were analyzed to distinguish the less or more severely affected eye.

RESULTS

In all eyes, the limits of the scotoma obtained with the scotometry test corresponded to the anatomic edges of the macular lesion. In patients with bilateral CS, the filling-in phenomenon was observed in 12 out of 14 (85%) less severely affected eyes, but only in one (7%) of their more severely affected eyes. In patients with unilateral CS, the phenomenon was observed in only one out of 16 (6%) eyes.

CONCLUSION

These results suggest that the filling-in phenomenon mostly occurs in patients with bilateral central scotoma, and almost always in their less affected eye. Thus, it did usually not occur in an eye if the fellow eye was better.

Visual consciousness in health and disease

Conscious experience is an essential part of normal human life and interaction with the environment. Yet the nature of consciousness and conscious perception remains a mystery. Because of its subjective nature, consciousness has been difficult to investigate scientifically, but clues have been gained through studies involving patients with cortical lesions. During the past decade, the development of event-related fMRI has provided insights into aspects of conscious perception in control subjects and patients with cortical lesions by correlating awareness and performance with neural activity during visual tasks. This article reviews how recent research has advanced understanding of conscious perception, its relationship to neural activity and visual performance, and how this relationship can be altered by visual dysfunction. It also presents recent research about how conscious awareness of vision might be represented at a neural level in the central nervous system.

Horizontal space misrepresentation in unilateral brain damage. II. Eye-head centered modulation of visual misrepresentation in hemianopia without neglect

We used a visual distance reproduction task (endpoint task) to evaluate horizontal space representation in two left brain damaged (LBD) and three right brain damaged (RBD) patients with contralateral homonymous hemianopia and no neglect. All patients were examined in the chronic phase of the stroke and were aware of their visual field defect. Along with contralesional deviation in the line bisection task, all patients estimated size (Landmark task) and distances in the contralesional space as being longer than equivalent size and distances located in the ipsilesional space. Misreproduction of distances was abolished or reduced when the task was performed in the ipsilesional head-centred space with the head turned contralesionally. This finding points out that misrepresentation of horizontal space linked to hemianopia can be modulated by combined proprioceptive input from eye and neck muscles. The pattern of misrepresentation found in chronic hemianopic patients is opposite to the one described in chronic neglect patients with concomitant hemianopia. These different patterns of space misrepresentation are the likely consequence of the presence, in hemianopics, and the absence, in neglect patients with hemianopia, of compensatory strategies based on the non-retinotopic and multimodal coding of spatial positions falling in the retinotopically organised blind field.