Long-term deprivation affects visual perception and cortex

Distinct effects of attention on the neural responses to form and motion processing: a SSVEP source-imaging study

We measured neural responses to local and global aspects of form and motion stimuli using frequency-tagged, steady-state visual evoked potentials (SSVEPs) combined with magnetic resonance imaging (MRI) data. Random dot stimuli were used to portray either dynamic Glass patterns (Glass, 1969) or coherent motion displays. SSVEPs were used to estimate neural activity in a set of fMRI-defined visual areas in each subject. To compare activity associated with local versus global processing, we analyzed two frequency components of the SSVEP in each visual area: the high temporal frequency at which the local dots were updated (30 Hz) and the much lower frequency corresponding to updates in the global structure (0.83 Hz). Local and global responses were evaluated in the context of two different behavioral tasks--subjects had to either direct their attention toward or away from the global coherence of the stimuli. The data show that the effect of attention on global and local responses is both stimulus and visual area dependent. When attention was directed away from stimulus coherence, both local and global responses were higher in the coherent motion than Glass pattern condition. Directing attention to coherence in Glass patterns enhanced global activity in areas LOC, hMT+, V4, V3a, and V1, while attention to global motion modulated responses by a smaller amount in a smaller set of areas: V4, hMT+, and LOC. In contrast, directing attention towards stimulus coherence weakly increased local responses to both coherent motion and Glass patterns. These results suggest that visual attention differentially modulates the activity of early visual areas at both local and global levels of structural encoding.

Sight restoration

Cases of sight onset after extended periods of congenital blindness provide windows into visual development and brain plasticity. Such cases are extremely rare in the developed world. Here, we make the argument that in meeting a public health challenge in the developing world, that of providing treatment to curably blind children, we have the opportunity to have a beneficial impact on science and society simultaneously. A recent initiative, Project Prakash, is motivated by these twin goals. We briefly describe this effort, some of its early results, and also the caveats that need to be kept in mind when interpreting the findings.

Plasticity of the dorsal "spatial" stream in visually deprived individuals

Studies on visually deprived individuals provide one of the most striking demonstrations that the brain is highly plastic and is able to rewire as a function of the sensory input it receives from the environment. In the current paper, we focus on spatial abilities that are typically related to the dorsal visual pathway (i.e., spatial/motion processing). Bringing together evidence from cataract-reversal individuals, early- and late-blind individuals and sight-recovery cases of long-standing blindness, we suggest that the dorsal “spatial” pathway is mostly plastic early in life and is then more resistant to subsequent experience once it is set, highlighting some limits of neuroplasticity.

Familiarity does not affect the unilateral field advantage for repetition detection

We have previously reported evidence that repetitions of letters, colors, sizes, and common motion paths are more rapidly detected when they are presented unilaterally (i.e., both in the same visual field) versus bilaterally (one element in each visual field; Butcher and Cavanagh (Attention, Perception, & Psychophysics 70:714-724, 2008). Here, we report evidence that this unilateral field advantage (UFA) for repetition detection does not depend on prior experience with the elements that comprise the repetition. In Experiment 1, native English, Persian, and Japanese speakers were tested on a repetition detection task involving characters from Western, Arabic, and Japanese character sets. The character sets were tested in blocks, in each of which subjects were presented with four characters for 16 ms and asked to report whether any two of the characters were identical. The subjects were faster detecting repetitions that were presented unilaterally rather than bilaterally, and there was no interaction with stimulus familiarity. A second experiment replicated this finding with native English speakers only, using a longer stimulus duration (150 ms). We had previously proposed that the UFA arises because the low-level processes that group physically identical items operate more efficiently within than across hemifields. Our data now indicate that this grouping process is insensitive to item familiarity, supporting the claim that the process is low-level.

Spatial updating depends on gaze direction even after loss of vision

Direction of gaze (eye angle + head angle) has been shown to be important for representing space for action, implying a crucial role of vision for spatial updating. However, blind people have no access to vision yet are able to perform goal-directed actions successfully. Here, we investigated the role of visual experience for localizing and updating targets as a function of intervening gaze shifts in humans. People who differed in visual experience (late blind, congenitally blind, or sighted) were briefly presented with a proprioceptive reach target while facing it. Before they reached to the target's remembered location, they turned their head toward an eccentric direction that also induced corresponding eye movements in sighted and late blind individuals. We found that reaching errors varied systematically as a function of shift in gaze direction only in participants with early visual experience (sighted and late blind). In the late blind, this effect was solely present in people with moveable eyes but not in people with at least one glass eye. Our results suggest that the effect of gaze shifts on spatial updating develops on the basis of visual experience early in life and remains even after loss of vision as long as feedback from the eyes and head is available.

On the matching of seen and felt shape by newly sighted subjects

How do we recognize identities between seen shapes and felt ones? Is this due to associative learning, or intrinsic connections these sensory modalities? We can address this question by testing the capacities of newly sighted subjects to match seen and felt shapes, but only if the subjects can see the objects well enough to form adequate visual representations of their shapes. In light of this, a recent study by R. Held and colleagues fails to demonstrate that their newly sighted subjects' inability to match seen and felt shape was due to a lack of intermodal connections rather than a purely visual deficit, as the subjects may not have been able visually to represent 3D shape in the perspective-invariant manner required for intermodal matching. However, the study could be modified in any of several ways to help avoid this problem.

Distinct mechanism for long-term contrast adaptation

To optimize perception, neurons in the visual system adapt to the current environment. What determines the durability of this plasticity? Longer exposures to an environment produce longer-lasting effects, which could be due to either (i) a single mechanism controlling adaptation that gains strength over time, or (ii) long-term mechanisms that become active after long-term exposure. Using recently developed technology, we tested adaptation durations an order of magnitude greater that those tested previously, and used a "deadaptation" procedure to reveal effects of a unique long-term mechanism in the longest adaptation periods. After 4 h of contrast adaptation, human observers were exposed to natural images for 15 min, which completely cancelled perceptual aftereffects of adaptation. Strikingly, during continued testing this deadaptation faded, and the original adaptation effects reappeared. This pattern strongly suggests that adaptation was maintained in a distinct long-term mechanism, whereas deadaptation affected a short-term mechanism.

'Visual' acuity of the congenitally blind using visual-to-auditory sensory substitution

Sensory Substitution Devices (SSDs) convey visual information through sounds or touch, thus theoretically enabling a form of visual rehabilitation in the blind. However, for clinical use, these devices must provide fine-detailed visual information which was not yet shown for this or other means of visual restoration. To test the possible functional acuity conveyed by such devices, we used the Snellen acuity test conveyed through a high-resolution visual-to-auditory SSD (The vOICe). We show that congenitally fully blind adults can exceed the World Health Organization (WHO) blindness acuity threshold using SSDs, reaching the highest acuity reported yet with any visual rehabilitation approach. This demonstrates the potential capacity of SSDs as inexpensive, non-invasive visual rehabilitation aids, alone or when supplementing visual prostheses.

Retinal prostheses: current clinical results and future needs

Degenerative diseases such as age-related macular degeneration (AMD) and retinitis pigmentosa (RP) primarily affect the photoreceptors, ultimately resulting in significant loss of vision. Retinal prostheses aim to elicit neural activity in the remaining retinal cells by detecting and converting light into electrical stimuli that can then be delivered to the retina. The concept of visual prostheses has existed for more than 50 years and recent progress shows promise, yet much remains to be understood about how the visual system will respond to artificial input after years of blindness that necessitate this type of prosthesis. This review focuses on 3 major areas: the histopathologic features of human retina affected by AMD and RP, current results from clinical trials, and challenges to overcome for continued improvement of retinal prostheses.

FINANCIAL DISCLOSURE(S)

Proprietary or commercial disclosures may be found after the references.

Sensory rehabilitation in the plastic brain

The purpose of this review is to consider new sensory rehabilitation avenues in the context of the brain's remarkable ability to reorganize itself following sensory deprivation. Here, deafness and blindness are taken as two illustrative models. Mainly, two promising rehabilitative strategies based on opposing theoretical principles will be considered: sensory substitution and neuroprostheses. Sensory substitution makes use of the remaining intact senses to provide blind or deaf individuals with coded information of the lost sensory system. This technique thus benefits from added neural resources in the processing of the remaining senses resulting from crossmodal plasticity, which is thought to be coupled with behavioral enhancements in the intact senses. On the other hand, neuroprostheses represent an invasive approach aimed at stimulating the deprived sensory system directly in order to restore, at least partially, its functioning. This technique therefore relies on the neuronal integrity of the brain areas normally dedicated to the deprived sense and is rather hindered by the compensatory reorganization observed in the deprived cortex. Here, we stress that our understanding of the neuroplastic changes that occur in sensory-deprived individuals may help guide the design and the implementation of such rehabilitative methods.

Human CRB1-associated retinal degeneration: comparison with the rd8 Crb1-mutant mouse model

PURPOSE

To investigate the human disease due to CRB1 mutations and compare results with the Crb1-mutant rd8 mouse.

METHODS

Twenty-two patients with CRB1 mutations were studied. Function was assessed with perimetry and electroretinography (ERG) and retinal structure with optical coherence tomography (OCT). Cortical structure and function were quantified with magnetic resonance imaging (MRI). Rd8 mice underwent ERG, OCT, and retinal histopathology.

RESULTS

Visual acuities ranged from 20/25 to light perception. Rod ERGs were not detectable; small cone signals were recordable. By perimetry, small central visual islands were separated by midperipheral scotomas from far temporal peripheral islands. The central islands were cone mediated, whereas the peripheral islands retained some rod function. With OCT, there were small foveal islands of thinned outer nuclear layer (ONL) surrounded by thick delaminated retina with intraretinal hyperreflective lesions. MRI showed structurally normal optic nerves and only subtle changes to occipital lobe white and gray matter. Functional MRI indicated that whole-brain responses from patients were of reduced amplitude and spatial extent compared with those of normal controls. Rd8 mice had essentially normal ERGs; OCT and histopathology showed patchy retinal disorganization with pseudorosettes more pronounced in ventral than in dorsal retina. Photoreceptor degeneration was associated with dysplastic regions.

CONCLUSIONS

CRB1 mutations lead to early-onset severe loss of vision with thickened, disorganized, nonseeing retina. Impaired peripheral vision can persist in late disease stages. Rd8 mice also have a disorganized retina, but there is sufficient photoreceptor integrity to produce largely normal retinal function. Differences between human and mouse diseases will complicate proof-of-concept studies intended to advance treatment initiatives.

Biological motion processing as a hallmark of social cognition

Visual processing of biological motion (BM) produced by living organisms is of immense value for successful daily-life activities and, in particular, for adaptive social behavior and nonverbal communication. Investigation of BM perception in neurodevelopmental disorders related to autism, preterm birth, and genetic conditions substantially contributes to our understanding of the neural mechanisms underpinning the extraordinary tuning to BM. The most prominent research outcome is that patients with daily-life deficits in social cognition are also compromised on visual body motion processing. This raises the question of whether performance on body motion perception tasks may serve a hallmark of social cognition. Overall, the findings highlight the role of structural and functional brain connectivity for proper functioning of the neural circuitry involved in BM processing and visual social cognition that share topographically and dynamically overlapping neural networks.

The human visual cortex responds to gene therapy-mediated recovery of retinal function

Leber congenital amaurosis (LCA) is a rare degenerative eye disease, linked to mutations in at least 14 genes. A recent gene therapy trial in patients with LCA2, who have mutations in RPE65, demonstrated that subretinal injection of an adeno-associated virus (AAV) carrying the normal cDNA of that gene (AAV2-hRPE65v2) could markedly improve vision. However, it remains unclear how the visual cortex responds to recovery of retinal function after prolonged sensory deprivation. Here, 3 of the gene therapy trial subjects, treated at ages 8, 9, and 35 years, underwent functional MRI within 2 years of unilateral injection of AAV2-hRPE65v2. All subjects showed increased cortical activation in response to high- and medium-contrast stimuli after exposure to the treated compared with the untreated eye. Furthermore, we observed a correlation between the visual field maps and the distribution of cortical activations for the treated eyes. These data suggest that despite severe and long-term visual impairment, treated LCA2 patients have intact and responsive visual pathways. In addition, these data suggest that gene therapy resulted in not only sustained and improved visual ability, but also enhanced contrast sensitivity.

Brain development: critical periods for cross-sensory plasticity

Recent work has shown that visual deprivation of humans during a critical period leads to motion area MT+ responding to auditory motion. This cross-sensory plasticity, an important form of brain reorganization, may be mediated by top-down brain circuits from pre-frontal cortex.

Contextual illusions reveal the limit of unconscious visual processing

The perception of even the most elementary features of the visual environment depends strongly on their spatial context. In the study reported here, we asked at what level of abstraction such effects require conscious processing of the context. We compared two visual illusions that alter subjective judgments of brightness: the simultaneous brightness contrast illusion, in which two circles of identical physical brightness appear different because of different surround luminance, and the Kanizsa triangle illusion, which occurs when the visual system extrapolates a surface without actual physical stimulation. We used a novel interocular masking technique that allowed us to selectively render only the context invisible. Simultaneous brightness contrast persisted even when the surround was masked from awareness. In contrast, participants did not experience illusory contours when the inducing context was masked. Our findings show that invisible context is resolvable by low-level processes involved in surface-brightness perception, but not by high-level processes that assign surface borders through perceptual completion.

Development of human visual function

By 1985 newly devised behavioral and electrophysiological techniques had been used to track development of infants' acuity, contrast sensitivity and binocularity, and for clinical evaluation of developing visual function. This review focus on advances in the development and assessment of infant vision in the following 25 years. Infants' visual cortical function has been studied through selectivity for orientation, directional motion and binocular disparity, and the control of subcortical oculomotor mechanisms in fixation shifts and optokinetic nystagmus, leading to a model of increasing cortical dominance over subcortical pathways. Neonatal face processing remains a challenge for this model. Recent research has focused on development of integrative processing (hyperacuity, texture segmentation, and sensitivity to global form and motion coherence) in extra-striate visual areas, including signatures of dorsal and ventral stream processing. Asynchronies in development of these two streams may be related to their differential vulnerability in both acquired and genetic disorders. New methods and approaches to clinical disorders are reviewed, in particular the increasing focus on paediatric neurology as well as ophthalmology. Visual measures in early infancy in high-risk children are allowing measures not only of existing deficits in infancy but prediction of later visual and cognitive outcome. Work with early cataract and later recovery from blinding disorders has thrown new light on the plasticity of the visual system and its limitations. The review concludes with a forward look to future opportunities provided by studies of development post infancy, new imaging and eye tracking methods, and sampling infants' visual ecology.

Building the bionic eye: an emerging reality and opportunity

Once the topic of folklore and science fiction, the notion of restoring vision to the blind is now approaching a tractable reality. Technological advances have inspired numerous multidisciplinary groups worldwide to develop visual neuroprosthetic devices that could potentially provide useful vision and improve the quality of life of profoundly blind individuals. While a variety of approaches and designs are being pursued, they all share a common principle of creating visual percepts through the stimulation of visual neural elements using appropriate patterns of electrical stimulation. Human clinical trials are now well underway and initial results have been met with a balance of excitement and cautious optimism. As remaining technical and surgical challenges continue to be solved and clinical trials move forward, we now enter a phase of development that requires careful consideration of a new set of issues. Establishing appropriate patient selection criteria, methods of evaluating long-term performance and effectiveness, and strategies to rehabilitate implanted patients will all need to be considered in order to achieve optimal outcomes and establish these devices as viable therapeutic options.

Unravelling the development of the visual cortex: implications for plasticity and repair

The visual cortex comprises over 50 areas in the human, each with a specified role and distinct physiology, connectivity and cellular morphology. How these individual areas emerge during development still remains something of a mystery and, although much attention has been paid to the initial stages of the development of the visual cortex, especially its lamination, very little is known about the mechanisms responsible for the arealization and functional organization of this region of the brain. In recent years we have started to discover that it is the interplay of intrinsic (molecular) and extrinsic (afferent connections) cues that are responsible for the maturation of individual areas, and that there is a spatiotemporal sequence in the maturation of the primary visual cortex (striate cortex, V1) and the multiple extrastriate/association areas. Studies in both humans and non-human primates have started to highlight the specific neural underpinnings responsible for the maturation of the visual cortex, and how experience-dependent plasticity and perturbations to the visual system can impact upon its normal development. Furthermore, damage to specific nuclei of the visual cortex, such as the primary visual cortex (V1), is a common occurrence as a result of a stroke, neurotrauma, disease or hypoxia in both neonates and adults alike. However, the consequences of a focal injury differ between the immature and adult brain, with the immature brain demonstrating a higher level of functional resilience. With better techniques for examining specific molecular and connectional changes, we are now starting to uncover the mechanisms responsible for the increased neural plasticity that leads to significant recovery following injury during this early phase of life. Further advances in our understanding of postnatal development/maturation and plasticity observed during early life could offer new strategies to improve outcomes by recapitulating aspects of the developmental program in the adult brain.

Vision substitution and depth perception: early blind subjects experience visual perspective through their ears

AIM

Sensory substitution (SS) represents a unique opportunity to provide congenitally blind persons with visual-like experience. Although visual experience influences the way we perceive the external world, little is known about the effects of SS experience.

PURPOSE

To investigate the effects of perceptual experience (visual versus sensory substitution) on depth perception through an SS system, object localization abilities of early blind (n = 10), and blindfolded sighted control subjects (n = 20) were assessed before and after a practicing period with a visual-to-auditory SS device.

METHOD

During the pre- and post-test, subjects had to replace, by hand, an object previously localized using the device. The practicing phase consisted of three sessions during which subjects tried to localize and grasp an object using the device. Results. At the pre-test, sighted subjects spontaneously used efficiently different pictorial depth cues to estimate object distance while the blind subjects were affected by their lack of visual experience and were significantly less accurate. Post-test showed that the brief practicing phase sufficed to enable blind subjects to acquire the rules of visual depth and to use them efficiently with the device.

CONCLUSIONS

These results suggest the possibility to compensate for some effects of early and long-lasting blindness by providing visual-like experience via SS. Theoretical implications are discussed.

Neonatal sensory deprivation and the development of cortical function: unilateral and bilateral sensory deprivation result in different functional outcomes

The normal development of sensory perception in mammals depends on appropriate sensory experience between birth and maturity. Numerous reports have shown that trimming some or all of the large mystacial vibrissa (whiskers) on one side of the face after birth has a detrimental effect on the maturation of cortical function. The objective of the present study was to understand the differences that occur after unilateral whisker trimming compared with those that occur after bilateral deprivation. Physiological deficits produced by bilateral trimming (BD) of all whiskers for 2 mo after birth were compared with the deficits produced by unilateral trimming (UD) for the same period of time using extracellular recording under urethan anesthesia from single cells in rat barrel cortex. Fast spiking (FSUs) and regular spiking (RSUs) units were separated and their properties compared in four subregions identified by histological reconstructions of the electrode penetrations, namely: layer IV barrel and septum, and layers II/III above a barrel and above a septum. UD upregulated responses in layer IV septa and in layers II/III above septa and perturbed the timing of responses to whisker stimuli. After BD, nearly all responses were decreased, and poststimulus latencies were increased. Circuit changes are proposed as an argument for how inputs arising from the spared whiskers project to the undeprived cortex and, via commissural fibers, could upregulate septal responses after UD. Following BD, more global neural deficits create a signature difference in the outcome of UD and BD in rat barrel cortex.

Leber congenital amaurosis due to RPE65 mutations and its treatment with gene therapy

Leber congenital amaurosis (LCA) is a rare hereditary retinal degeneration caused by mutations in more than a dozen genes. RPE65, one of these mutated genes, is highly expressed in the retinal pigment epithelium where it encodes the retinoid isomerase enzyme essential for the production of chromophore which forms the visual pigment in rod and cone photoreceptors of the retina. Congenital loss of chromophore production due to RPE65-deficiency together with progressive photoreceptor degeneration cause severe and progressive loss of vision. RPE65-associated LCA recently gained recognition outside of specialty ophthalmic circles due to early success achieved by three clinical trials of gene therapy using recombinant adeno-associated virus (AAV) vectors. The trials were built on multitude of basic, pre-clinical and clinical research defining the pathophysiology of the disease in human subjects and animal models, and demonstrating the proof-of-concept of gene (augmentation) therapy. Substantial gains in visual function of clinical trial participants provided evidence for physiologically relevant biological activity resulting from a newly introduced gene. This article reviews the current knowledge on retinal degeneration and visual dysfunction in animal models and human patients with RPE65 disease, and examines the consequences of gene therapy in terms of improvement of vision reported.

Cortical maps and white matter tracts following long period of visual deprivation and retinal image restoration

Abnormal visual input during development has dramatic effects on the visual system. How does the adult visual system respond when input is corrected? MM lost his left eye and became blind in the right due to corneal damage at the age of 3. At age 46, MM regained his retinal image, but his visual abilities, even seven years following the surgery, remain severely limited, and he does not rely on vision for daily life. Neuroimaging measurements reveal several differences among MM, sighted controls, sighted monocular, and early blind subjects. We speculate that these differences stem from damage during the critical period in development of retinal neurons with small, foveal receptive fields. In this case, restoration of functional vision requires more than improving retinal image contrast. In general, visual restoration will require accounting for the developmental trajectory of the individual and the consequences of the early deprivation on cortical circuitry.

Synthetic synaesthesia and sensory substitution

Visual information can be provided to blind users through sensory substitution devices that convert images into sound. Through extensive use to develop expertise, some blind users have reported visual experiences when using such a device. These blind expert users have also reported visual phenomenology to other sounds even when not using the device. The blind users acquired synthetic synaesthesia, with visual experience evoked by sounds only after gaining such expertise. Sensorimotor learning may facilitate and perhaps even be required to develop expertise in the use of multimodal information. Furthermore, other areas where expertise is acquired in dividing attention amongst cross-modal information or integrating such information might also give rise to synthetic synaesthesia.

Neural reorganization following sensory loss: the opportunity of change

There is growing evidence that sensory deprivation is associated with crossmodal neuroplastic changes in the brain. After visual or auditory deprivation, brain areas that are normally associated with the lost sense are recruited by spared sensory modalities. These changes underlie adaptive and compensatory behaviours in blind and deaf individuals. Although there are differences between these populations owing to the nature of the deprived sensory modality, there seem to be common principles regarding how the brain copes with sensory loss and the factors that influence neuroplastic changes. Here, we discuss crossmodal neuroplasticity with regards to behavioural adaptation after sensory deprivation and highlight the possibility of maladaptive consequences within the context of rehabilitation.

Visual maps: To merge or not to merge

A recent study of a child born with one cerebral hemisphere has revealed an extreme developmental reorganization of visual cortex. Self-organizing visual maps demonstrate a surprisingly flexible restructuring in response to cortical loss.

Visual parsing after recovery from blindness

How the visual system comes to bind diverse image regions into whole objects is not well understood. We recently had a unique opportunity to investigate this issue when we met three congenitally blind individuals in India. After providing them treatment, we studied the early stages of their visual skills. We found that prominent figural cues of grouping, such as good continuation and junction structure, were largely ineffective for image parsing. By contrast, motion cues were of profound significance in that they enabled intraobject integration and facilitated the development of object representations that permitted recognition in static images. Following 10 to 18 months of visual experience, the individuals' performance improved, and they were able to use the previously ineffective static figural cues to correctly parse many static scenes. These results suggest that motion information plays a fundamental role in organizing early visual experience and that parsing skills can be acquired even late in life.

See it with feeling: affective predictions during object perception

People see with feeling. We 'gaze', 'behold', 'stare', 'gape' and 'glare'. In this paper, we develop the hypothesis that the brain's ability to see in the present incorporates a representation of the affective impact of those visual sensations in the past. This representation makes up part of the brain's prediction of what the visual sensations stand for in the present, including how to act on them in the near future. The affective prediction hypothesis implies that responses signalling an object's salience, relevance or value do not occur as a separate step after the object is identified. Instead, affective responses support vision from the very moment that visual stimulation begins.

Retinotopically specific reorganization of visual cortex for tactile pattern recognition

Although previous studies have shown that Braille reading and other tactile discrimination tasks activate the visual cortex of blind and sighted people, it is not known whether this kind of crossmodal reorganization is influenced by retinotopic organization. We have addressed this question by studying "S," a visually impaired adult with the rare ability to read print visually and Braille by touch. S had normal visual development until 6 years of age, and thereafter severe acuity reduction due to corneal opacification, but no evidence of visual-field loss. Functional magnetic resonance imaging revealed that, in S's early visual areas, tactile information processing activated what would be the foveal representation for normally sighted individuals, and visual information processing activated what would be the peripheral representation. Control experiments showed that this activation pattern was not due to visual imagery. S's high-level visual areas, which correspond to shape- and object-selective areas in normally sighted individuals, were activated by both visual and tactile stimuli. The retinotopically specific reorganization in early visual areas suggests an efficient redistribution of neural resources in the visual cortex.

Visual Motion Area MT+/V5 Responds to Auditory Motion in Human Sight-Recovery Subjects

Using functional magnetic resonance imaging, we found that cortical visual motion area MT+/V5 responded to auditory motion in two rare subjects who had been blind since early childhood and whose vision was partially recovered in adulthood. Visually normal control subjects did not show similar auditory responses. These auditory responses in MT+ were specific to motion compared with other complex auditory stimuli including frequency sweeps and speech. Thus, MT+ developed motion-specific responses to nonvisual input, suggesting that cross-modal plasticity can be influenced by the normal functional specialization of a cortical region. Regarding sight recovery after early blindness, our results further demonstrate that cross-modal responses coexist with regained visual responses within the visual cortex.

Pioneers of cortical plasticity: six classic papers by Wiesel and Hubel

This essay looks at six APS classic papers published by D. H. Hubel and T. N. Wiesel that first identified a developmental critical period for environment influenced receptive field plasticity in the visual pathway. These classic papers are freely available online. These are listed here, in chronological order.

Wiesel TN, Hubel DH.

Effects of visual deprivation on morphology and physiology of cells in the cat's lateral geniculate body. J Neurophysiol 26: 978–993, 1963 ( http://jn.physiology.org/cgi/reprint/26/6/978 ).

Hubel DH, Wiesel TN.

Receptive fields of cells in striate cortex of very young, visually inexperienced kittens. J Neurophysiol 26: 994–1002, 1963 ( http://jn.physiology.org/cgi/reprint/26/6/994 ).

Wiesel TN, Hubel DH.

Single-cell responses in striate cortex of kittens deprived of vision in one eye. J Neurophysiol 26: 1003–1017, 1963 ( http://jn.physiology.org/cgi/reprint/26/6/1003 ).

Wiesel TN, Hubel DH.

Comparison of the effects of unilateral and bilateral eye closure on cortical unit responses in kittens. J Neurophysiol 28: 1029–1040, 1965 ( http://jn.physiology.org/cgi/reprint/28/6/1029 ).

Hubel DH, Wiesel TN.

Binocular interaction in striate cortex of kittens reared with artificial squint. J Neurophysiol 28: 1041–1059, 1965 ( http://jn.physiology.org/cgi/reprint/28/6/1041 ).

Wiesel TN, Hubel DH.

Extent of recovery from the effects of visual deprivation in kittens. J Neurophysiol 28: 1060–1072, 1965 ( http://jn.physiology.org/cgi/reprint/28/6/1060 ).

Binding 3-D object perception in the human visual cortex

How do visual luminance, shape, motion, and depth bind together in the brain to represent the coherent percept of a 3-D object within hundreds of milliseconds (msec)? We provide evidence from simultaneous magnetoencephalographic (MEG) and electroencephalographic (EEG) data that perception of 3-D objects defined by luminance or motion elicits sequential activity in human visual cortices within 500 msec. Following activation of the primary visual cortex around 100 msec, 3-D objects elicited sequential activity with only little overlap (dynamic 3-D shapes: MT-LO-Temp; stationary 3-D shapes: LO-Temp). A delay of 80 msec, both in MEG/EEG responses and in reaction times (RTs), was found when additional motion information was processed. We also found significant positive correlations between RT, and MEG and EEG responses in the right temporal location. After about 400 msec, long-lasting activity was observed in the parietal cortex and concurrently in previously activated regions. Novel time-frequency analyses indicate that the activity in the lateral occipital (LO) complex is associated with an increase of induced power in the gamma band, a hallmark of binding. The close correspondence of an induced gamma response with concurrent sources located in the LO in both experimental conditions at different points in time ( approximately 200 msec for luminance and approximately 300 msec for dynamic cues) strongly suggests that the LO is the key region for the assembly of object features. The assembly is fed forward to achieve coherent perception of a 3-D object within 500 msec.

The development of visual feature binding processes after visual deprivation in early infancy

Higher visual functions were investigated in patients treated for bilateral congenital cataracts in two experiments. Participants were asked to detect either real or illusory contours (Kanizsa squares in Experiment 1 or one of four different Kanizsa contours in Experiment 2) among distractor items. Compared to normally sighted participants matched for age, gender and education, cataract patients treated after the age of 5-6 months took relatively longer to detect Kanizsa figures (Experiments 1 and 2) and they had higher miss rates (Experiment 2). The present results suggest that the ability of visual feature binding depends on early visual input and is permanently impaired if patterned vision is prevented in early infancy for 5 months or more.

Canine and human visual cortex intact and responsive despite early retinal blindness from RPE65 mutation

BACKGROUND

RPE65 is an essential molecule in the retinoid-visual cycle, and RPE65 gene mutations cause the congenital human blindness known as Leber congenital amaurosis (LCA). Somatic gene therapy delivered to the retina of blind dogs with an RPE65 mutation dramatically restores retinal physiology and has sparked international interest in human treatment trials for this incurable disease. An unanswered question is how the visual cortex responds after prolonged sensory deprivation from retinal dysfunction. We therefore studied the cortex of RPE65-mutant dogs before and after retinal gene therapy. Then, we inquired whether there is visual pathway integrity and responsivity in adult humans with LCA due to RPE65 mutations (RPE65-LCA).

METHODS AND FINDINGS

RPE65-mutant dogs were studied with fMRI. Prior to therapy, retinal and subcortical responses to light were markedly diminished, and there were minimal cortical responses within the primary visual areas of the lateral gyrus (activation amplitude mean +/- standard deviation [SD] = 0.07% +/- 0.06% and volume = 1.3 +/- 0.6 cm(3)). Following therapy, retinal and subcortical response restoration was accompanied by increased amplitude (0.18% +/- 0.06%) and volume (8.2 +/- 0.8 cm(3)) of activation within the lateral gyrus (p < 0.005 for both). Cortical recovery occurred rapidly (within a month of treatment) and was persistent (as long as 2.5 y after treatment). Recovery was present even when treatment was provided as late as 1-4 y of age. Human RPE65-LCA patients (ages 18-23 y) were studied with structural magnetic resonance imaging. Optic nerve diameter (3.2 +/- 0.5 mm) was within the normal range (3.2 +/- 0.3 mm), and occipital cortical white matter density as judged by voxel-based morphometry was slightly but significantly altered (1.3 SD below control average, p = 0.005). Functional magnetic resonance imaging in human RPE65-LCA patients revealed cortical responses with a markedly diminished activation volume (8.8 +/- 1.2 cm(3)) compared to controls (29.7 +/- 8.3 cm(3), p < 0.001) when stimulated with lower intensity light. Unexpectedly, cortical response volume (41.2 +/- 11.1 cm(3)) was comparable to normal (48.8 +/- 3.1 cm(3), p = 0.2) with higher intensity light stimulation.

CONCLUSIONS

Visual cortical responses dramatically improve after retinal gene therapy in the canine model of RPE65-LCA. Human RPE65-LCA patients have preserved visual pathway anatomy and detectable cortical activation despite limited visual experience. Taken together, the results support the potential for human visual benefit from retinal therapies currently being aimed at restoring vision to the congenitally blind with genetic retinal disease.

Visual performance using a retinal prosthesis in three subjects with retinitis pigmentosa

PURPOSE

To assess visual task performance in three blind subjects implanted with epiretinal prostheses.

DESIGN

Prospective, investigational device exemption trial.

METHODS

Three subjects with light perception or no light perception vision were enrolled at a single center. All subjects had retinitis pigmentosa (RP). Main inclusion criteria: light perception or worse vision in one eye and some visual experience as an adult before blindness. Main exclusion criteria included other ophthalmic problems. A prototype retinal prosthesis was implanted in the eye with worse light sensitivity. The prosthesis had 4 x 4 array of platinum electrodes tacked to the epiretinal surface. The prosthesis was wirelessly controlled by a computer or by a head-worn video camera. Visual function testing was performed in single masked or double masked fashion. Scores from the visual task were compared to chance to determine statistical significance.

RESULTS

The subjects performed significantly better than chance in 83% of the tests. Using the video camera, subjects scored as follows on simple visual tasks: locate and count objects (77% to 100%), differentiate three objects (63% to 73%), determine the orientation of a capital L (50% to 77%), and differentiate four directions of a moving object (40% to 90%). A subset of tests compared camera settings using multipixels vs single pixels. Using multipixel settings, subjects performed better (17%) or equivalent (83%) in accuracy and better (25%) or equivalent (75%) in reaction time.

CONCLUSIONS

Three RP implant subjects used epiretinal prostheses to perform simple visual tasks. Multipixel settings proved slightly more effective than single pixel settings.

'Who is the ideal candidate?': decisions and issues relating to visual neuroprosthesis development, patient testing and neuroplasticity

Appropriate delivery of electrical stimulation to intact visual structures can evoke patterned sensations of light in individuals who have been blind for many years. This pivotal finding has lent credibility to the concept of restoring functional vision by artificial means. As numerous groups worldwide pursue human clinical testing with visual prosthetic devices, it is becoming increasingly clear that there remains a considerable gap between the challenges of prosthetic device development and the rehabilitative strategies needed to implement this new technology in patients. An important area of future work will be the development of appropriate pre- and post-implantation measures of performance and establishing candidate selection criteria in order to quantify technical advances, guide future device design and optimize therapeutic success. We propose that the selection of an 'ideal' candidate should also be considered within the context of the variable neuroplastic changes that follow vision loss. Specifically, an understanding of the adaptive and compensatory changes that occur within the brain could assist in guiding the development of post-implantation rehabilitative strategies and optimize behavioral outcomes.

Vision following extended congenital blindness

Animal studies suggest that early visual deprivation can cause permanent functional blindness. However, few human data on this issue exist. Given enough time for recovery, can a person gain visual skills after several years of congenital blindness? In India, we recently had an unusual opportunity to work with an individual whose case history sheds light on this question. S.R.D. was born blind, and remained so until age 12. She then underwent surgery for the removal of dense congenital cataracts. We evaluated her performance on an extensive battery of visual tasks 20 years after surgery. We found that although S.R.D.'s acuity is compromised, she is proficient on mid- and high-level visual tasks. These results suggest that the human brain retains an impressive capacity for visual learning well into late childhood. They have implications for current conceptions of cortical plasticity and provide an argument for treating congenital blindness even in older children.

Effects of early visual deprivation on perceptual and cognitive development

During early infancy, visual capabilities are quite limited. Nevertheless, patterned visual input during this period is necessary for the later development of normal vision for some, but not all, aspects of visual perception. The evidence comes from studies of children who missed early visual input because it was blocked by dense, central cataracts in both eyes. In this article, we review the effects of bilateral congenital cataracts on two aspects of low-level vision--acuity and contrast sensitivity, and on three aspects of higher-level processing of faces. We end by discussing the implications for understanding the developmental mechanisms underlying normal perceptual and cognitive development.