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The term “acquired brain injury” refers to any type of brain damage that occurs after birth. Two main types of acquired brain injury are stroke and traumatic brain injury (TBI). A stroke occurs when there is a blockage or bleed in the vascular system of the brain, while a TBI results from an external force to the head. Older adults are at a higher risk of both stroke and TBI; thus, overall incidence is increasing as the proportion of older adults in the population is growing. Stroke and TBI result in immediate and long-term cognitive changes. Impairments in the domains of language, attention, memory, executive functions, perception, and social cognition have been documented following stroke and TBI. However, strokes tend to cause focal or selective cognitive disorders, while cognitive deficits following TBI are widespread and can be generalized. Individuals who have suffered a stroke or TBI may also experience psychosocial changes; for example, symptoms of depression and anxiety are common. Functional outcomes, including independence in activities, are varied and are associated with a range of factors including age, injury severity, cognitive disorders, and psychosocial factors. To achieve optimal outcomes for individuals following stroke and TBI, and to reduce the impact of the injury on everyday functioning, a multidisciplinary rehabilitation process is recommended.

Autism spectrum disorder (ASD), or autism, is a neurodevelopmental disorder that is typically recognized and diagnosed in childhood. There is no established biological marker for autism; rather, the diagnosis is made based on observation of behavioral traits, including (a) persistent deficits in social interaction and communication, and (b) restricted, repetitive patterns of behavior, interests, or activities. Because autism is a spectrum disorder, autistic individuals are a highly heterogeneous group and differ widely in the presentation and severity of their symptoms. The established prevalence of ASD is approximately 1% of the population. Information about autism in adulthood is limited; most of the literature examines childhood and adolescence. While the term “later life” has traditionally been associated with those over the age of 65, a dire lack of understanding exists for those on the autism spectrum beyond early adulthood. Individuals remain on the spectrum into later life, though some mild improvements in symptoms are observed over time. Autistic adults experience high levels of physical and mental health comorbidities. Rates of participation in employment and education are also lower than that of the general population. Quality of life is reportedly poorer for autistic adults than for nonautistic peers, though this is not affected by age. More robust studies of the health, well-being, and needs of autistic adults are needed, especially qualitative investigations of adulthood and aging and longitudinal studies of development over the lifespan.

The sensation of vision arises from the detection of photons of light at the eye, but in order to produce the percept of the world, extensive regions of the brain are required to process the visual information. The majority of information entering the brain via the optic nerve from the eye projects via the lateral geniculate nucleus (LGN) of the thalamus to the primary visual cortex, the largest visual area, having been reorganized such that one side of the brain represents one side of the world. Damage to the primary visual cortex in one hemisphere therefore leads to a loss of conscious vision on the opposite side of the world, known as hemianopia. Despite this cortical blindness, many patients are still able to detect visual stimuli that are presented in the blind region if forced to guess whether a stimulus is present or absent. This is known as “blindsight.” For patients to gain any information (conscious or unconscious) about the visual world, the input from the eye must be processed by the brain. Indeed, there is considerable evidence from functional brain imaging that several visual areas continue to respond to visual stimuli presented within the blind region, even when the patient is unaware of the stimulus. Furthermore, the use of diffusion imaging allows the microstructure of white matter pathways within the visual system to be examined to see whether they are damaged or intact. By comparing patients who have hemianopia with and without blindsight it is possible to determine the pathways that are linked to blindsight function. Through understanding the brain areas and pathways that underlie blindsight in humans and non-human primates, the aim is to use modern neuroscience to guide rehabilitation programs for use after stroke.

The process of brain development begins shortly after conception and in humans takes decades to complete. Indeed, it has been argued that brain development occurs over the lifespan. A complex genetic blueprint provides the intricate details of the process of brain construction. Additional operational instructions that control gene and protein expression are derived from experience, and these operational instructions allow an individual to meet and uniquely adapt to the environmental demands they face. The science of epigenetics provides an explanation of how an individual’s experience adds a layer of instruction to the existing DNA that ultimately controls the phenotypic expression of that individual and can contribute to gene and protein expression in their children, grandchildren, and ensuing generations. Experiences that contribute to alterations in gene expression include gonadal hormones, diet, toxic stress, microbiota, and positive nurturing relationships, to name but a few. There are seven phases of brain development and each phase is defined by timing and purpose. As the brain proceeds through these genetically predetermined steps, various experiences have the potential to alter its final form and behavioral output. Brain plasticity refers to the brain’s ability to change in response to environmental cues or demands. Sensitive periods in brain development are times during which a part of the brain is particularly malleable and dependent on the occurrence of specific experiences in order for the brain to tune its connections and optimize its function. These periods open at different time points for various brain regions and the closing of a sensitive period is dependent on the development of inhibitory circuitry. Some experiences have negative consequences for brain development, whereas other experiences promote positive outcomes. It is the accumulation of these experiences that shape the brain and determine the behavioral outcomes for an individual.

Clinicians conduct capacity evaluations to determine an older adult’s ability to make and execute a decision within key domains of functioning. Questions of capacity often arise when an older adult experiences a decline in cognitive functioning due to Alzheimer’s disease, stroke, or severe psychiatric illness, for example. Capacity is related to legal competency, and a lack of capacity may be proved by providing evidence that an older adult is unable to understand the act or decision in question; appreciate the context and consequences of the decision or act; reason about the potential harms and benefits; or express a choice. Capacity is domain-specific, time-specific, and decision-specific. Domains include financial capacity, medical treatment and research consent capacity, driving capacity, sexual consent capacity, and voting capacity. Each capacity domain encompasses activities that may vary in complexity or risk, and thus require different levels of capacity. For example, within the medical treatment consent capacity domain, an older adult may lack the capacity to consent to a complicated and risky surgical procedure while retaining the capacity to consent to a routine blood draw. Clinicians determine capacity by using a combination of tools including capacity assessment instruments, task-specific functional evaluations, interviews with the patient and family members, measures of cognitive functioning, and consideration of social, physical, and mental health factors. Extensive research has been conducted to determine the reliability and validity of a variety of capacity assessment instruments for many domains. These instruments generally assess the patient’s responses to vignettes pertaining to the domain in question, information gleaned from structured and semi-structured interviews, functional ability, or a combination of these methods. Although there is still need for more research, especially in emerging domains, capacity assessments help to protect vulnerable older adults from harm while allowing them to retain the highest possible level of autonomy.

Cerebral palsy (CP) is defined as non-progressive damage to the brain at or around birth, which leads to varying symptoms depending on the extent and location of damage. The leading symptom is sensory-motor impairment of varying expression, but additional perceptual, cognitive, and socio-emotional symptoms are common. CP can be divided into four types, with bilateral spastic being by far the most frequent, followed by the unilateral spastic, the dyskinetic, and the ataxic variants. The intellectual, linguistic, and cognitive profile of CP is extremely variant, but all qualities correlate more or less with CP type and motor impairment. Early diagnosis is important since early intervention may promote all developmental dimensions. Generally, individuals with unilateral spastic CP have the best (almost normal) intellectual, linguistic, and cognitive outcomes, while those with bilateral spastic CP fare the worst. Language perception is often an individual strength, while language expression, and particularly speech, may be heavily impaired. Attention and executive functions are often impaired as compared to typically developing controls, even in those children with normal intellectual functioning. The same holds true for visual perceptual functions, which are impaired in almost half of all children and adolescents with CP. The potential neuropsychological dysfunctions are a risk factor for arithmetic functions and literacy. Obstacles to participate in society are high for individuals with CP and heavily dependent on their motor, language, intellectual, and cognitive functions. However, quality of life is good for most children and adolescents, and they develop a sound self-concept. On the other side, bully experience is more common than amongst typically developing children and is associated with behavior problems and executive dysfunction. The development of children and adolescents with CP is determined by a complex interplay between physical, intellectual, and neuropsychological functions.

Research on the interplay of cognition and mobility in old age is inherently multidisciplinary, informed by findings from life span developmental psychology, kinesiology, cognitive neuroscience, and rehabilitation sciences. Early observational work revealed strong connections between sensory and sensorimotor performance with measures of intellectual functioning. Subsequent work has revealed more specific links between measures of cognitive control and gait quality. Convergent evidence for the interdependence of cognition and mobility is seen in patient studies, wherein cognitive impairment is associated with increased frequency and risk of falling. Even in cross-sectional studies involving healthy young and older adults, the effects of aging on postural control and gait are commonly exacerbated when participants perform a motor task with a concurrent cognitive load. This motor-cognitive dual-task method assumes that cognitive and motor domains compete for common capacity, and that older adults recruit more cognitive capacity than young adults to support gait and posture. Neuroimaging techniques such as magnetic resonance imaging (MRI) have revealed associations between measures of mobility (e.g., gait velocity and postural control) and measures of brain health (e.g., gray matter volumes, cortical thickness, white matter integrity, and functional connectivity). The brain regions most often associated with aging and mobility also appear to subserve high-level cognitive functions such as executive control, attention, and working memory (e.g., dorsolateral prefrontal cortex, anterior cingulate). Portable functional neuroimaging has allowed for the examination of neural functioning during real-time walking, often in conjunction with detailed spatiotemporal measures of gait. A more recent strategy that addresses the interdependence of cognitive and motor processes in old age is cognitive remediation. Cognitive training has yielded promising improvements in balance, walking, and overall mobility status in healthy older adults, and those with age-related neurodegenerative conditions such as Parkinson’s Disease.

Individuals with mild cognitive impairment (MCI) experience cognitive difficulties and many find themselves in a transitional stage between aging and dementia, making this population a suitable target for cognitive intervention. In MCI, not all cognitive functions are impaired and preserved functions can thus be recruited to compensate for the impact of cognitive impairment. Improving cognition may have a tremendous impact on quality of life and help delay the loss of autonomy that comes with dementia. Several studies have reported evidence of cognitive benefits following cognitive intervention in individuals with MCI. Studies that relied on training memory and attentional control have provided the most consistent evidence for cognitive gains. A few studies have investigated the neurophysiological processes by which these training effects occur. More research is needed to draw clear conclusions on the type of brain processes that are engaged in cognitive training and there are insufficient findings regarding transfer to activities of daily life. Results from recent studies using new technologies such as virtual reality provide encouraging evidence of transfer effects to real-life situations.

The number of people living with dementia is growing, and with limited pharmacological treatment options the importance of psychosocial interventions is increasingly recognized. Cognitive rehabilitation is particularly well placed to address the needs of people living with mild and moderate dementia and their family supporters, as it offers a range of tools to tackle the complexity of the condition. It utilizes powerful approaches of problem solving and goal setting combined with evidence-based rehabilitative techniques for managing cognitive impairments. It also incorporates strategies to address emotional and motivational aspects of dementia that may affect a person’s well-being. It is provided on an individual basis, usually in people’s homes, making it directly applicable to everyday life. It is also genuinely person-centered and flexible as the therapy goals are agreed in a collaborative process between the therapist, person with dementia, and family members. Cognitive rehabilitation does not claim to address underlying pathology, but instead focuses on a person’s functional ability and enjoyment of life. Evidence for effectiveness of cognitive rehabilitation in the context of mild and moderate dementia, mostly Alzheimer’s disease (AD), is gradually accumulating with a number of randomized control trials demonstrating that people with mild and moderate dementia can significantly improve their functioning in targeted areas. For example, the GREAT trial with 475 people with mild to moderate Alzheimer’s, vascular, and mixed dementia completed in 2017 in the United Kingdom demonstrated that cognitive rehabilitation improves everyday functioning in relation to individual therapy goals. There is a growing interest in cognitive rehabilitation and the focus shifts to extending evidence to less-common forms of dementia, particularly in people with non-amnestic presentation. Future efforts need to concentrate on promoting the approach and optimizing application in real-life settings with the aim of maximizing benefits for people living with dementia and their families.

Concussions affect millions of athletes of all ages each year in a variety of sports. Athletes in certain sports such as American football, ice hockey, rugby, soccer, and combative sports like boxing are at higher risk for concussion. Direct or indirect mechanical forces acting on the skull and brain cause a concussion, which is a milder form of brain injury. Conventional neuroimaging (e.g., computerized tomography [CT], magnetic resonance imaging [MRI]) for concussion is typically negative. Concussions involve both neurometabolic and subtle structural damage to the brain that results in signs (e.g., loss of consciousness [LOC], amnesia, confusion), symptoms (e.g., headache, dizziness, nausea), and functional impairment (e.g., cognitive, balance, vestibular, oculomotor). Symptoms, impairment, and recovery time following concussion can last from a few days to weeks or months, based on a variety of risk factors, including younger age, female sex, history of concussion, and history of migraine. Following a concussion, athletes may experience one or more clinical profiles, including cognitive fatigue, vestibular, oculomotor, post-traumatic migraine (PTM), mood/anxiety, and/or cervical. The heterogeneous nature of concussion warrants a comprehensive approach to assessment, including a thorough clinical examination and interview; symptom inventories; and cognitive, balance, vestibular, oculomotor, and exertion-based evaluations. Targeted treatment and rehabilitation strategies including behavior management, vestibular, vision, and exertion therapies, and in some cases medication can be effective in treating the various concussion clinical profiles. Some athletes experience persistent post-concussion symptoms (PCS) and/or psychological issues (e.g., depression, anxiety) following concussion. Following appropriate treatment and rehabilitation strategies, determination of safe return to play is predicated on being symptom-free and back to normal levels of function at rest and following exertion. Certain populations, including youth athletes, may be at a higher risk for worse impairment and prolonged recovery following concussion. It has been suggested that some athletes experience long-term effects associated with concussion including chronic traumatic encephalopathy (CTE). However, additional empirical studies on the role of concussion on CTE are needed, as CTE may have multiple causes that are unrelated to sport participation and concussion.

The major theories of consciousness that distinguish conscious from unconscious states can be grouped into two main classes, either higher-order or integration theories. There is evidence that different types of mental states can be unconscious, though that conclusion depends on the theory of consciousness assumed. Unconscious memory (in the sense of the influence of a prior event not recollected) can shape perception and liking and control our behavior. Subliminal perception can produce semantic priming and guide attention and decision making; and optical variables that a person describes incorrectly can guide action. Implicit learning can shape judgments and choices in complex environments. Unconscious intentions can allow people to respond appropriately in goal-directed ways while the person experiences the actions as involuntary. Unconscious attitudes are no more or less plausible than any other mental state being unconscious, but it has been hard to obtain evidence for unconscious attitudes as distinct from gut reactions one does not agree with.

Dementia, also now known as major neurocognitive disorder, is a syndrome involving decline in two or more areas of cognitive function sufficient to disrupt a person’s daily function. Mild cognitive impairment (MCI), also known as minor neurocognitive disorder, represents a syndrome on the continuum of cognitive decline that is a stage prior to development of functional deficits. It involves decline in one or more areas of cognitive function with independence in instrumental activities of daily living, even though they may require greater effort or compensation on the part of the individual. Neuropsychological assessment of cognition and behavior provides the most powerful biomarkers for MCI and dementia syndromes associated with neurodegenerative diseases. Discrete cognitive and behavioral patterns that occur early in the course of cognitive decline aids in differential clinical diagnosis. Additionally, all diagnostic schemes for dementia syndromes include criteria that require the appraisal of functional status, which tests an individual’s capacity to engage in decision making and carry out activities of daily living independently. Methods for assessing functional status have historically had poor reliability and validity. Nevertheless, in a clinical setting, neuropsychologists rely on a combination of self-report, collateral informants, caregiver questionnaires, and objective performance-based measures to better assess functional status. Revisions to clinical criteria for dementia reflect the adoption of new research diagnostic criteria for neurodegenerative diseases, largely driven by the National Institutes of Aging (NIA) and the Alzheimer’s Association 2011 research criteria for Alzheimer’s disease (AD). The new approach differentiates the syndromic presentations common to most neurodegenerative diseases from the etiologies (AD, LBD, VaD, etc.) based on biomarkers. In the preclinical stage, biomarker abnormalities are present years before clinical symptom manifestation. In mild cognitive impairment stage, there is a report/concern for cognitive change by the patient, informant, or clinician. There is objective cognitive decline from estimated premorbid functioning and preserved independence in functional abilities. In the dementia stage, in the context of impaired functional status, there may be prominent cognitive and behavioral symptoms that may involve impairment in memory, executive function, visuospatial functioning, and language, as well as changes in personality and behavior. The most common dementias are AD, dementia with Lewy bodies (DLB), frontotemporal dementia (FTD), and vascular dementia (VaD). All can follow a trajectory of cognitive decline similar to the aforementioned stages and are associated with neuropathogenic mechanisms that may or may not be distinctive for a particular syndrome. Briefly, Alzheimer’s dementia is associated with accumulation of amyloid plaques and tau neurofibrillary tangles. Lewy body dementias (i.e., Parkinson’s disease dementia and DLB) are characterized by Lewy bodies (alpha-synuclein aggregates) and Lewy neurites in the brainstem, limbic system, and cortical regions; DLB is also associated with diffuse amyloid plaques. Frontotemporal dementia is a conglomerate of syndromes that may overlap and include behavioral variant FTD, semantic dementia, and primary progressive aphasia (PPA). FTD dementia syndromes are marked by frontotemporal lobar degeneration (FTLD) caused by pathophysiological processes involving FTLD-tau, FTLD-TDP, FTLD-FUS, or their combination, as well as beta amyloid. Lastly, vascular dementia is associated with cerebrovascular disease that can include large artery occlusions, microinfarcts, brain hemorrhages, and silent brain infarcts; comorbid AD pathology may lower the threshold for dementia conversion. There is an emerging shift in the field toward exploring prevention strategies for dementia. Given the lack of precision in our language regarding the distinction between dementia syndromes and etiologies, we can reallocate some of our efforts to preventing dementia more broadly rather than intervening on a certain pathology. Research already supports that many individuals have biomarker evidence of brain pathology without showing cognitive impairment or even sufficient levels of pathology in the brain to warrant a diagnosis without ever displaying the clinical syndrome of dementia. That said, building cognitive reserve or resilience through lifestyle and behavioral factors may slow the rate of cognitive decline and prevent the risk of a future dementia epidemic.

Voluntary eye movements and visual perception are closely intertwined in humans and nonhuman primates because of the limitation of high-acuity vision to a very small, specialized area at the center of the retina, the fovea. Only when the image of an object is projected on the foveal region by eye and head movements it is possible, to perceive fine visual details such as letters during reading. In order to improve visual perception and to benefit from high-resolution foveal vision, rapid saccadic eye movements frequently change the direction of both eyes to selected peripheral locations. Continuous sequences of these voluntary saccades and fixations determine what humans see and in how much detail they perceive objects and their visual surroundings. Where, when, and how humans move their eyes depends not only on the visual properties of the target object but also on their intentions and prevailing tasks. Accordingly, target locations for saccades differ depending on the things people do—whether they just look around, actively search for something, read, or do sports. Instead of the classical dichotomy of bottom-up and top-down processes, recent research on gaze behavior has focused on the dynamic interplay of factors such as task demands, rewards, scene content, temporal sequences, and individual and historical differences. Besides saccadic eye movements, humans are also able to rotate their eyes continuously when they pursue moving objects of interest. Smooth pursuit eye movements stabilize the image of a moving object on the foveal region and prevent degradation of the retinal target image resulting from motion smear. The use of pursuit eye movements also improves the prediction of future target movement. Pursuit initiation is often combined with interceptive saccades that direct the fovea to the moving target, and catch-up saccades that correct for small mismatches concerning eye and target position, speed, and/or direction. Because each eye movement alters retinal input, compensations for retinal displacements are needed to maintain a stable representation of the environment. Overall, both saccadic and smooth pursuit eye movements provide optimal uptake of visual information for perception and guidance of actions.

Faces carry a wide variety of socially important information, including invariant personal properties that can be used to recognize someone’s identity and highly changeable characteristics that assist in interpreting their feelings and intentions. These relatively invariant and more changeable aspects of face perception present differing everyday demands that, together with genetic and cultural influences, help determine the organization of processes involved in human face perception.

Oscillatory activity present in brain signals reflects the underlying time-varying electrical discharges within and between ensembles of neurons. Among the variety of non-invasive techniques available for measuring of the brain’s oscillatory activity, magnetoencephalography (MEG) presents a remarkable combination of spatial and temporal resolution, and can be used in resting-state or task-based studies, depending on the goals of the experiment. Two important kinds of analysis can be carried out with the MEG signal: spectral a. and functional connectivity (FC) a. While the former provides information on the distribution of the frequency content within distinct brain areas, FC tells us about the dependence or interaction between the signals stemming from two (or among many) different brain areas. The large frequency range combined with the good resolution offered by MEG makes MEG-based spectral and FC analyses able to highlight distinct patterns of neurophysiological alterations during the aging process in both healthy and pathological conditions. Since disruption in spectral content and functional interactions between brain areas could be accounted for by early neuropathological changes, MEG could represent a useful tool to unveil neurobiological mechanisms related to the cognitive decline observed during aging, particularly suitable for the detection of functional alterations, and then for the discovery of potential biomarkers in case of pathology. The aging process is characterized by alterations in the spectral content across the brain. At the network level, FC studies reveal that older adults experience a series of changes that make them more vulnerable to cognitive interferences. While special attention has been dedicated to the study of pathological conditions (in particular, mild cognitive impairment and Alzheimer’s disease), the lack of studies addressing the features of FC in healthy aging is noteworthy. This area of research calls for future attention because it is able to set the baseline from which to draw comparisons with different pathological conditions.

Since the first cluster of people with HIV was identified in 1981, significant biomedical advances, most notably the development of antiretroviral therapy (ART), have led to considerably increased life expectancy as well as a reduction in the morbidity and mortality associated with HIV/AIDS. As a result, HIV/AIDS is no longer considered a terminal illness, but rather a chronic illness, and many persons living with HIV/AIDS are beginning to enter or have already reached later life. In fact, Americans ages 50 years and older comprise approximately half of all individuals with HIV/AIDS and represent the most rapidly growing subpopulation of persons living with HIV/AIDS in the United States. Despite significant advances in HIV/AIDS treatment and prognosis, older adults living with HIV (OALH) face a number of unique challenges and circumstances that can lead to exacerbated symptoms and poorer outcomes, despite demonstrating generally better ART adherence than their younger counterparts. These detrimental outcomes are due to both chronological aging and cohort effects as well as social and behavioral factors and long-term ART use. For instance, neurocognitive deficits and neuropsychiatric symptoms, including depression, anxiety, apathy, and fatigue, are often observed among OALH, which can result in feelings of loneliness, social isolation, and reduced social support. Taken together, these factors can lead to elevated levels of problems with everyday functioning (e.g., activities of daily living) among OALH. In addition, sociocultural factors such as race/ethnicity, ageism, sexism, homophobia, transphobia, geographic region, socioeconomic status and financial well-being, systemic barriers and disparities, and cultural values and beliefs play an influential role in determining outcomes. Notwithstanding the challenges associated with living with HIV/AIDS in later life, many persons living with HIV/AIDS are aging successfully. HIV/AIDS survivor and community mobilization efforts, as well as integrated care models, have resulted in some significant improvements in overall HIV/AIDS patient care. In addition, interventions aimed at improving successful aging outcomes among OALH are being developed in an attempt to effectively reduce the psychological and physical morbidity associated with HIV disease.

Intraindividual variability in the present context refers to the moment-to-moment variation in attentional or executive engagement over a given time period. Typically, it is measured using the response latencies collected across the trials of a behavioral neurocognitive task. In aging research, the measure has received a lot of recent interest as it may provide important insights into age-related cognitive decline and neuropathology as well as having potential as a neurocognitive assessment tool in healthcare settings. In the present chapter, we begin by reviewing the key empirical findings relating to age and intraindividual variability. Here, research shows that intraindividual variability increases with age and predicts a range of age-related outcomes including gait impairment, falls and errors more broadly, mild cognitive impairment, dementia, and mortality. Brain imaging research suggests that greater variability is associated with age-related or neuropathological changes to a frontal–cingulate–parietal network and that white matter compromise and dopamine depletion may be key underlying mechanisms. We then consider the cognitive and neurobiological theoretical underpinnings of the construct before providing a description of the various methods and metrics that have been used to compute measures of variability – reaction time cut-offs, raw and residualized intraindividual standard deviations, coefficient of variation, ex-Gaussian curve and fast Fourier transformation. A further section considers the range of neurocognitive tasks that have been used to assess intraindividual variability. Broadly, these tasks can be classified on a continuum of cognitive demands as psychomotor, executive control or higher-order cognitive tasks (e.g., episodic memory). Finally, we provide some pointers concerning the pressing issues that future research needs to address in the area. We conclude that the existing body of theoretical and empirical work underlines the potential of intraindividual reaction time variability measures as additions to the neuropsychological test batteries that are used in the early detection of a range of age-related neurocognitive disorders in healthcare settings.

The left and right hemispheres of the brain process sensory information in different ways and function differently in controlling behavior. This lateralization of brain function, originally thought to be unique to humans, is now known to occur in a broad range of non-human vertebrates and even in invertebrates, indicating that it is an essential feature of both large and small brains. Some evidence indicates that lateralization of brain function improves cognitive capacity of the brain. Many, often unrelated, brain functions are lateralized. In humans, these include differential specialization of the hemispheres to process language and produce speech, express emotions, respond to faces, attend to spatial information, and control hand use. As now clear for handedness, both genetic expression and environmental influences are involved in complex ways. Since lateral division of function in the brains of non-human animals has sufficient similarity to that of humans, it is probable that at least some asymmetries were evolutionary precursors to language and speech. It is notable that lateralization is in no species totally in a single direction. Some evidence from studies of animal species provides support for the hypothesis that alignment of laterality in most individuals in a species occurs only for control of behavior that requires one lateralized individual to interact with another lateralized individual (i.e., in social interaction). In humans, individuals with non-right-handedness (left-handed and mixed-handed) are more prone to a number of psychiatric conditions or other behavioral conditions, including autism, schizophrenia, and dyslexia.

Early in life, the brain has a substantial capacity for change, often referred to as neuroplasticity. Disrupted visual input to the brain during an early “critical” or “sensitive period” of heightened neuroplasticity induces structural and functional changes within neural systems and causes amblyopia, a sensory disorder associated with abnormal development of the brain areas involved in perception. Amblyopia impairs a broad range of visual, multisensory, and motor functions, and recovery from amblyopia requires a substantial change in visual information processing within the brain. Therefore, not only is amblyopia caused by an interaction between visual experience and heightened neuroplasticity, recovery from amblyopia also requires significant neuroplastic change within the brain. A number of evidence-based treatments are available for young children with amblyopia whose brains are still rapidly developing and have a correspondingly high level of neuroplasticity. However, adults with amblyopia are often left untreated because of the idea that the adult brain no longer has sufficient neuroplasticity to relearn how to process visual information. In the early 21st century, it became clear that this idea was not correct. A number of interventions that can enhance neuroplasticity in the mature visual cortex have been identified using animal models of amblyopia and are now being translated into human studies. Other promising techniques for enhancing visual cortex neuroplasticity have emerged from studies of adult humans with amblyopia. Examples of interventions that may improve vision in adult amblyopia include refractive correction, patching of the amblyopic eye (reverse patching), monocular and binocular perceptual learning, noninvasive brain stimulation, systemic drugs, and exercise. The next important stage of research within this field will be to conduct fully controlled randomized clinical trials to assess which, if any, of these interventions can be translated into a mainstream treatment for amblyopia in adulthood.