Brain aging starts from early development and continues to the end of life. Right from the beginning, the process serves to help the organism cope with lifelong demands arising in its physical, emotional and social environments. Since the dynamic of changes in the environment can vary widely in terms of frequency, magnitude, quality and duration, it is important that the organism be appropriately equipped to make physiological and behavioral responses in a correspondingly flexible and temporally-appropriate manner. Its adaptive responses should be available in a retrievable form for long periods such that survival becomes more of a routine than a burden.
The brain possesses different functional systems for coping with the demands of life. These functional systems also have the prerequisites for developing and acquiring the necessary adaptation processes, which are then available as flexible adaptation strategies. Like other complex biological systems, the brain is characterized by a high degree of plasticity, both in terms of its construction (morphology) and changeability through experience (learning). This plasticity is not a fixed factor, but undergoes changes in the course of life, most pronounced in (early) childhood. It also plays a special role under pathological conditions, e.g. after morphological or functional damage. With increasing age, the brain loses morphological substance, namely, nerve cells and fiber connections. This normal process is accompanied by corresponding functional changes, but interestingly and perhaps surprisingly, by only small functional losses if the brain otherwise enjoys a positive mood and the organism engages in regular mental and physical activities.
Maintaining mental and other skills, and corresponding habits, despite the loss of neurons and fiber connections, is usually attributed to the “functional reserve” of the brain. There is good evidence, that one feature of functional reserve is the ability for functional switching between cortical structures within the same functional system or supportive systems (co-activation). Another way in which skills and habits may be maintained in the aging brain that facilitate learning is by recruiting complementary and adaptive mental and behavioral programs. In both instances, systematic and repeated experience with the coping process in question at the behavioural level is required for its successful and enduring acquisition. For optimal development of the brain (and thus the individual), both the neurobiological foundations as well as the various functions at the level of learning and behaviour, it is crucial that there is a good balance between stimuli that promote plasticity and those that pose challenges.
It is important to mention here that, each functional system ages at its own quantitative and qualitative pace. This may have a positive impact on the acquisition and optimization of adaptive strategies, but may be hampered by unfavourable events in early childhood and during puberty that delay or limit the development of the neurobiological and functional, especially motivational, emotional, and cognitive foundations. Negative experiences (e.g. genetic errors, hypoxia at birth, undernourishment) during early life may also trigger pathological development of the brain and therefore, compromise adaptation capacities. Similarly, adverse events such as cerebrovascular, demyelinating and degenerative diseases or traumatic brain injury that may occur long after in utero development and birth can impose disruptive effects, changing the course of, or even transiently stopping, the development of the brain’s functional systems and its “normal” functions. Finally, mental disorders, psychiatric illnesses and psychological trauma can also have lasting negative effects on the development and functioning of the brain.
Neuroscientific research has produced many important and interesting findings about the role of functional plasticity of the brain and, in particular, on the modulatory influence of (and interactions with) environmental conditions during the lifespan as well as after brain disease. However, our knowledge about the particular formation of experience and its interaction with the individual physical, social and emotional “environmental factors” is still rather limited. For a better understanding of this issue, research strategies would be necessary, which allow the direct transfer of research results to the respective individual, with respect to his/her prerequisites and requirements for optimal adaptability in their unique ecological settings. Interestingly, studies show that patients with cognitive dysfunction, e.g. in the areas of attention or memory, can improve just as well in their familiar (natural) environment as in a specialized facility. As long as we do not know which patients may benefit more from activities in an external environment, or the cognitive strategies they use to cope with the challenges of daily living, neurorehabilitation services should be individually tailored and delivered in a complementary manner. Emphasis should be placed on mapping and encouraging self-initiative and use of functional reserves to optimize self-management of the subject’s functional disorder. Specific knowledge about the most favourable form of learning and exercise for such an adaptation and the influence of favourable and unfavourable factors would allow a tailor-made support or treatment of the affected people and ensure high effectiveness. Such an approach would require a bidirectional interaction and co-operation between “basic” and applied “research”. The question then is: how can both disciplines, and of course, their sub-disciplines, be brought together in a more holistic, problem-oriented manner? The complexity of the application of basic neuroscientific research findings to the applied domain of neurorehabilitation can be demonstrated using the examples shown in Box 1
Box 1. . Application of neurobiological principles to neurorehabilitation and vice versa.
Example 1. Suppose a right-handed patient has difficulty using his right hand after brain damage because gripping movements are impaired. The precise knowledge of the motor functions of the hand when grasping and its central nervous control mechanisms allow the direct development of appropriate, i.e. specific treatment programs. The desired treatment outcome is that the patient regains the ability to grip with the affected hand. Since the gripping movements have developed in a usage- and object-specific manner, the training of the respective hand movements must also be specific to a certain extent. For example, the patient should be able to grasp objects and manipulate them so as to be able to eat (cutlery use) or drink (moving cup or glass to mouth), etc. Treatment efficacy can be measured directly with respect to quality/accuracy and speed as well as ease of transfer of the rehabilitation outcome to the subject’s familiar environment. In this example, there is a comparatively direct path from the results of basic research to systematic application and everyday activities.
Example 2. Let us assume that a patient who suffered traumatic brain injury demonstrates a significant impairment of attention, especially the ability to concentrate on a task. Unlike the hand and its functions, attention is not an empirical entity, but rather a psychological construct for which there naturally cannot be such a clear, empirically founded definition. The individual components of attention, such as alertness, maintenance, selective or divided attention, are based on a taxonomy that is widely accepted in cognitive research. The brain structures that play a special role in attention are known from neuropsychological studies on disorders of attention and on brain activities in cognitive tasks that require “attention”. The (unsurprising) phenomenon that, on the one hand, any damage to the brain, regardless of location, can lead to a loss of attention, and on the other, that many brain regions are also active in tasks with little demand on “attention”, implies an important role for attention in all activities of the brain. However, this more general role of attention poses a special challenge for neuropsychological diagnostics and treatment. Consideration needs to be given to the fact that attention may be task-specific (gripping in Example 1), i.e. that the brain also controls the amount (time and quality) of attention dedicated to a particular task under a given condition. Despite these hurdles, cognitive psychologists have developed test procedures with good quality criteria for measuring various aspects of attentional performance. Nonetheless, the use of these tests may be somewhat restricted in patients with disorders of attention functions by the above-mentioned features of the construct “attention”. Notably, attention can only be assessed indirectly with the help of particular (visual or auditory) perceptual functions and a motor (finger or hand, or verbal) reaction. Now, if our hypothetical patient performs below average in the selective attention test, i.e. shows significantly lower accuracy (high error rate) and performance speed, it could be that s/he is repeatedly distracted and needs repeated prompts to concentrate on the task. This may be interpreted as “increased distractibility” in other rehabilitation disciplines. What is important is that, at the behavioral level, the attention problem becomes more pronounced than in the test result, since the errors alone do not reflect the quality of the attention problem. Thus, while the goal of neuropsychological treatment would be to improve concentration, behavioral therapy would be aimed at reducing distractibility using any one of several cognitive training methods. Improvements in attention (lower distractibility), measured by cognitive tests, would however not necessarily imply that distractibility will no longer represent a potential interference with the subject’s daily living and working performance. This can only be demonstrated in a neurohabilitation setting where different conditions to which the patient might be exposed are simulated. The interpretational and treatment challenges illustrated in this case differ remarkably from those portrayed in the example of impaired gripping; to meet them, we need improvements in methodology and rehabilitation services organization that also provide scientific proof of the robustness of the treatment approach as well as their ecological validity: a decrease in the disability and an increase in the capacity for independent living.
From these examples, it becomes clear that, particularly in the area of disorders of cognition, irrespective of the age of the individuals who need treatment, special research efforts are necessary in order to gain a comprehensive scientific basis (neurobiological substrates and processes) for such interventions. Admittedly, it is difficult to reliably determine changes or disorders of cognitive functions and to treat them effectively in terms of ecological validity. However, the creative interplay between cognitive and neuropsychological research should also lead to innovative application solutions in this area of rehabilitation.
With regard to treatment and rehabilitation support of children, adolescents, adults and the elderly, it is critical to know whether an individual is amenable to interventions that promote restitution of functions or whether compensatory procedures might be more appropriate. Functional losses can be compensated for by the subject's own “reserve” or by external means, e.g. through technical aids. Finally, there is also the possibility of adapting the environment to the functional problems of the subject, i.e. by changing the environmental conditions in such a way that they are no longer a particular hindrance. Unfortunately, apart from the areas of motor skills and visual perception, there are hardly any systematic studies on this. Very little is presently known about the possible approaches for restitution or compensation in individuals. With regard to development- and age-related functional difficulties, important research questions are, which forms of learning or practice are best suited for a given function, and what are the optimal environmental conditions (in the broadest sense) for successful adaptation.
There is of course an essential difference between childhood and old age. Older people usually have gained extensive experience with a plethora of lifetime challenges, and thus have developed a correspondingly large number of individual routines. In contrast, children have still to learn a large number of routines and develop their individual habits. However, the benefit of age comes with a major disadvantage. Routines and habits are difficult to change, in part because they promise (and provide) reliability and security. Flexibility is a crucial requirement for developing successful adaptation strategies and for switching from the usual (now less suitable) routines, to alternatives that are (likely more) successful. Furthermore, the environmental conditions are fundamentally different for children and for the elderly. We know from basic neuroscientific research that a so-called enriched environment is very important in both cases. However, what is an “enriched environment” from the perspective of a 2-year old child versus that of a a 70-year old? Is the individual familiar environment still appropriate, or does it appear more complex and difficult to adjust to satisfactorily? What about the effects of sensory, cognitive, emotional, and/or social deprivation in older age? Older people rightly want to keep their identity (as do children). However, what defines the identity of a person? It is probably the individual’s self-image, which is not only based on the cognitive, but possibly more on the emotional evaluation of oneself, i.e. personality, consisting of genetical determined characteristics and views developed over the years.
Further scientific investigations into the nature (and mediating substrates and mechanisms) of interactions between the functional resources of the brain and the ecological setting of individual subjects are needed to provide a solid basis for the development of programs by public and private institutions and organizations that are responsible for, or committed to, ensuring that their clients’ desire to lead independent lives (as far as this is possible) as they age is satisfied. Service providers should consider that satisfaction and happiness are ultimately powerful drivers of development and health as they serve as rewards that encourage further (sometimes arduous) lifelong learning. Greater collaboration between neuroscientists and providers of neurorehabilitative care will help strengthen the knowledge base to guide decisions about the care needed to maintain and restore age-related declines in brain function.
Biography
Josef Zihlconducted research on patients at the Max-Planck-Institute of Psychiatry in Munich (Germany) for over 40 years. He was Chair of Neuropsychology at the Ludwig-Maximilians-University in Munich from 1995 until becoming emeritus professor in 2015. Zihl continues to follow his main interests in the neuropsychology of vision and cognition, visual rehabilitation, developmental neuropsychology, mental ageing, and neuropsychiatry and still contributes to education in neurohabilitation in Germany and Italy. Zihl is co-developer of NeuroEyeCoach which helps patients with visual field disorders due to brain injury to scan their environment more efficiently.