Functional outcomes following lesions in visual cortex: Implications for plasticity of high-level vision

Holistic processing and face expertise after pediatric resection of occipitotemporal cortex

The nature and extent of hemispheric lateralization and its potential for reorganization continues to be debated, although there is general agreement that there is a right hemisphere (RH) advantage for face processing in human adults. Here, we examined face processing and its lateralization in individuals with a single preserved occipitotemporal cortex (OTC), either in the RH or left hemisphere (LH), following early childhood resection for the management of drug-resistant epilepsy. The matched controls and those with a lesion outside of OTC evinced the standard superiority in processing upright over inverted faces and the reverse sensitivity to a nonface category (bicycles). In contrast, the LH and the RH patient groups were significantly less accurate than the controls and showed mild orientation sensitivities at best (and not always in the predicted directions). For the two patient groups, the accuracies of face and bicycle processing did not differ from each other and were not obviously related to performance on intermediate level global form tasks with, again, poorer thresholds for both patient groups than controls and no difference between the patient groups. These findings shed light on the complexity of hemispheric lateralization and face and nonface object processing in individuals following surgical resection of OTC. Overall, this study highlights the unique dynamics and potential for plasticity in those with childhood cortical resection.

Differential functional reorganization of ventral and dorsal visual pathways following childhood hemispherectomy

Hemispherectomy is a surgical procedure in which an entire hemisphere of a patient's brain is resected or functionally disconnected to manage seizures in individuals with drug-resistant epilepsy. Despite the extensive loss of both ventral and dorsal visual pathways in one hemisphere, pediatric patients who have undergone hemispherectomy show a remarkably high degree of perceptual function across many domains. In the current study, we sought to understand the extent to which functions of the ventral and dorsal visual pathways reorganize to the contralateral hemisphere following childhood hemispherectomy. To this end, we collected fMRI data from an equal number of left and right hemispherectomy patients who completed tasks that typically elicit lateralized responses from the ventral or the dorsal pathway, namely, word (left ventral), face (right ventral), tool (left dorsal), and global form (right dorsal) perception. Overall, there was greater evidence of functional reorganization in the ventral pathway than in the dorsal pathway. Importantly, because ventral and dorsal reorganization was tested within the very same patients, these results cannot be explained by idiosyncratic factors such as disease etiology, age at the time of surgery, or age at testing. These findings suggest that because the dorsal pathway may mature earlier, it may have a shorter developmental window of plasticity than the ventral pathway and, hence, be less malleable after perturbation.

Differential functional reorganization of ventral and dorsal visual pathways following childhood hemispherectomy

Hemispherectomy is a surgical procedure in which an entire hemisphere of a patient's brain is resected or functionally disconnected to manage seizures in individuals with drug-resistant epilepsy. Despite the extensive loss of input from both ventral and dorsal visual pathways of one hemisphere, pediatric patients who have undergone hemispherectomy show a remarkably high degree of perceptual function across many domains. In the current study, we sought to understand the extent to which functions of the ventral and dorsal visual pathways reorganize to the contralateral hemisphere following childhood hemispherectomy. To this end, we collected fMRI data from an equal number of left and right hemispherectomy patients who completed tasks that typically elicit lateralized responses from the ventral or the dorsal pathway, namely, word (left ventral), face (right ventral), tool (left dorsal), and global form (right dorsal) perception. Overall, there was greater evidence of functional reorganization in the ventral pathway than in the dorsal pathway. Importantly, because ventral and dorsal reorganization was tested in the very same patients, these results cannot be explained by idiosyncratic factors such as disease etiology, age at the time of surgery, or age at testing. These findings suggest that because the dorsal pathway may mature earlier, it may have a shorter developmental window of plasticity than the ventral pathway and, hence, be less malleable.

Peripheral visual field loss and activities of daily living

PURPOSE OF REVIEW

Peripheral visual field (VF) loss affects 13% of the population over 65. Its effect on activities of daily living and higher order visual processing is as important as it is inadequately understood. The purpose of this review is to summarize available literature on the impact of peripheral vision loss on driving, reading, face recognition, scene recognition and scene navigation.

RECENT FINDINGS

In this review, glaucoma and retrochiasmal cortical damage are utilized as examples of peripheral field loss which typically spare central vision and have patterns respecting the horizontal and vertical meridians, respectively. In both glaucoma and retrochiasmal damage, peripheral field loss causes driving difficulty - especially with lane maintenance - leading to driving cessation, loss of independence, and depression. Likewise, peripheral field loss can lead to slower reading speeds and decreased enjoyment from reading, and anxiety. In glaucoma and retrochiasmal field loss, face processing is impaired which impacts social functioning. Finally, scene recognition and navigation are also adversely affected, impacting wayfinding and hazard detection leading to decreased independence as well as more frequent injury.

SUMMARY

Peripheral VF loss is an under-recognized cause of patient distress and disability. All peripheral field loss is not the same, differential patterns of loss affect parameters of activities of daily living (ADL) and visual processing in particular ways. Future research should aim to further characterize patterns of deranged ADL and visual processing, their correlation with types of field loss, and associated mechanisms.

The resilience of the developing reading system: multi-modal evidence of incident and recovery after a pediatric stroke

Decades of neuroscientific findings have elucidated the highly specialized brain areas involved in reading, especially along the ventral occipitotemporal stream where the critical step of recognizing words occurs. We report on a 14-year-old female with temporary dyslexia after a left ventral occipitotemporal ischemic stroke. Our longitudinal multimodal findings show that the resolution of the reading impairment was associated with heightened activity in the left posterior superior and inferior temporal gyri. Our findings highlight the role of the left inferior temporal gyrus in reading and the importance of perilesional and ipsilateral cortical areas for functional recovery after childhood stroke.

Neuroregeneration and plasticity: a review of the physiological mechanisms for achieving functional recovery postinjury

Neuronal networks, especially those in the central nervous system (CNS), evolved to support extensive functional capabilities while ensuring stability. Several physiological "brakes" that maintain the stability of the neuronal networks in a healthy state quickly become a hinderance postinjury. These "brakes" include inhibition from the extracellular environment, intrinsic factors of neurons and the control of neuronal plasticity. There are distinct differences between the neuronal networks in the peripheral nervous system (PNS) and the CNS. Underpinning these differences is the trade-off between reduced functional capabilities with increased adaptability through the formation of new connections and new neurons. The PNS has "facilitators" that stimulate neuroregeneration and plasticity, while the CNS has "brakes" that limit them. By studying how these "facilitators" and "brakes" work and identifying the key processes and molecules involved, we can attempt to apply these theories to the neuronal networks of the CNS to increase its adaptability. The difference in adaptability between the CNS and PNS leads to a difference in neuroregenerative properties and plasticity. Plasticity ensures quick functional recovery of abilities in the short and medium term. Neuroregeneration involves synthesizing new neurons and connections, providing extra resources in the long term to replace those damaged by the injury, and achieving a lasting functional recovery. Therefore, by understanding the factors that affect neuroregeneration and plasticity, we can combine their advantages and develop rehabilitation techniques. Rehabilitation training methods, coordinated with pharmacological interventions and/or electrical stimulation, contributes to a precise, holistic treatment plan that achieves functional recovery from nervous system injuries. Furthermore, these techniques are not limited to limb movement, as other functions lost as a result of brain injury, such as speech, can also be recovered with an appropriate training program.

Successful Reorganization of Category-Selective Visual Cortex following Occipito-temporal Lobectomy in Childhood

Investigations of functional (re)organization in children who have undergone large cortical resections offer a unique opportunity to elucidate the nature and extent of cortical plasticity. We report findings from a 3-year investigation of a child, U.D., who underwent surgical removal of the right occipital and posterior temporal lobes at age 6 years 9 months. Relative to controls, post-surgically, U.D. showed age-appropriate intellectual performance and visuoperceptual face and object recognition skills. Using fMRI at five different time points, we observed a persistent hemianopia and no visual field remapping. In category-selective visual cortices, however, object- and scene-selective regions in the intact left hemisphere were stable early on, but regions subserving face and word recognition emerged later and evinced competition for cortical representation. These findings reveal alterations in the selectivity and topography of category-selective regions when confined to a single hemisphere and provide insights into dynamic functional changes in extrastriate cortical architecture.

Perceptual Function and Category-Selective Neural Organization in Children with Resections of Visual Cortex

The consequences of cortical resection, a treatment for humans with pharmaco-resistant epilepsy, provide a unique opportunity to advance our understanding of the nature and extent of cortical (re)organization. Despite the importance of visual processing in daily life, the neural and perceptual sequellae of occipitotemporal resections remain largely unexplored. Using psychophysical and fMRI investigations, we compared the neural and visuoperceptual profiles of 10 children or adolescents following unilateral cortical resections and their age- and gender-matched controls. Dramatically, with the exception of two individuals, both of whom had relatively greater cortical alterations, all patients showed normal perceptual performance on tasks of intermediate- and high-level vision, including face and object recognition. Consistently, again with the exception of the same two individuals, both univariate and multivariate fMRI analyses revealed normal selectivity and representational structure of category-selective regions. Furthermore, the spatial organization of category-selective regions obeyed the typical medial-to-lateral topographic organization albeit unilaterally in the structurally preserved hemisphere rather than bilaterally. These findings offer novel insights into the malleability of cortex in the pediatric population and suggest that, although experience may be necessary for the emergence of neural category-selectivity, this emergence is not necessarily contingent on the integrity of particular cortical structures.SIGNIFICANCE STATEMENT One approach to reduce seizure activity in patients with pharmaco-resistant epilepsy involves the resection of the epileptogenic focus. The impact of these resections on the perceptual behaviors and organization of visual cortex remain largely unexplored. Here, we characterized the visuoperceptual and neural profiles of ventral visual cortex in a relatively large sample of post-resection pediatric patients. Two major findings emerged. First, most patients exhibited preserved visuoperceptual performance across a wide-range of visual behaviors. Second, normal topography, magnitude, and representational structure of category-selective organization were uncovered in the spared hemisphere. These comprehensive imaging and behavioral investigations uncovered novel evidence concerning the neural representations and visual functions in children who have undergone cortical resection, and have implications for cortical plasticity more generally.

Over time, the right results will emerge

Scientific research involves going beyond the well-trodden and well-tested ideas and theories that form the core of scientific knowledge. During the time scientists are working things out, some results will be right, and others will be wrong. Over time, the right results will emerge. Lisa Randall (Frank B. Baird, Jr. Professor of Science, Physics Department, Harvard University) We are grateful to all the commentators for the important and thoughtful comments raised in response to the Geskin and Behrmann (G & B) literature survey. The issues raised in the introduction to this Special Issue and in these commentaries not only address and challenge aspects of the G & B literature review, but contribute perspectives and extensions that go well beyond the scope of the review. As is evident from G & B and from the 13 commentaries, many aspects of congenital prosopagnosia (CP) remain controversial. Adopting the language of the quote above, the intention of the G & B survey, along with the commentaries and this response, is to establish a collaborative process from which the right results (and right theory) will emerge in time. We are grateful to the editor of this Special Issue, Dr. Brad Mahon, for his support and for facilitating this collaborative exchange in Cognitive Neuropsychology.