Neural Plasticity Across the Lifespan

How the brain can change

Gianfranco Denes

• Publisher: Routledge
• Year: 2015
• ISBN:
  9781317909941
• Edition: 1
• Topic: Psychology
• Subtopic: History & Theory in Psychology
• Index: Psychology

Neural Plasticity Across the Lifespan reviews the recent scientific developments which are transforming our understanding of the human brain. For many years it was thought that modifications to the structural and functional organization of the brain were limited to a short early period of life, "the critical period", and, in adults, to the memory system. Recent research suggests that on the contrary we should see the human brain as a flexible structure, which adapts and modifies in response to learning, sensory experience, age and disease.

The book provides an integrated overview of contemporary research on neural plasticity - the process by which the brain can change in structure and function to cope with new experiences and react to the effects of acquired damage or sensory deprivation. It reviews data on plasticity in the developing brain, looking at both typical and atypical development, alongside clinical and observational research on the adult population. It covers a number of key topics, including:

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• different forms of neural plasticity

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• factors affecting neural plasticity (ageing and gender),

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• neural plasticity in language acquisition, memory and bodily self-consciousness

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• mechanisms of repair – plasticity following sensory deprivation and acquired brain damage.

This is an accessible overview of an emerging field of research which has fundamental implications for how we perceive our potential to change throughout our lives. It will be essential reading for all students of cognitive development, cognitive neuroscience and lifespan development.

Section 1
Introducing plasticity

1
Plasticity: A Tentative Definition

Biological existence is an iterative reciprocal process that takes place among genes, individuals and the environment. Genes provide a menu of developmental possibilities, but the phenotypic outcome results from the expression of the genes of an organism, as well as from the influence of environmental factors to which the organism is exposed during its life and random events (stochasticism) – for example, the process of mutation or the genetic rearrangement associated with the development of the immune system (Cashmore, 2010).

The awareness of phenotypic plasticity began to emerge in antiquity, when the notion that a person’s physical performance could be improved by physical training became widespread: exercised muscles responded to the stimulation and remodeled to improve performance. In the intervening years, scientists have characterized many physiological aspects of this phenomenon across a range of tissues and biological systems. With the introduction of advanced technical tools, ranging from molecular biology to neuroimaging, it has proved possible to uncover some of the mechanisms that underpin postnatal functional and anatomical changes as well as the factors responsible for such changes.

According to Bateson and Gluckman (2011), every organism is the product of two apparently separate processes: robustness and plasticity. Robustness can be defined as the process that leads to an invariant outcome, making a phenotype insensitive or resistant to genetic variations or environmental perturbations (the constitutive trait). This mechanism can work at different levels, from molecular to behavioral, ensuring that all members of the same species share the same traits (Masel and Siegal, 2009). This trait might have evolved as an adaptation mechanism to reduce the effect of mutations, or as a process to avoid environmental variation, or perhaps as an intrinsic property of biological systems. Different mechanisms work together to maintain the constancy of the phenotype; these include the lack of detection of environmental changes, elasticity (see below in this chapter), and the presence of repair mechanisms. Robustness is not, however, an all-or-nothing phenomenon that acts in the same way in all the life stages: a trait that is robust at one stage may lose its property at a different stage. Bateson and Martin (1999), for example, have shown that some behavioral traits, such as smiling, present in the first months of life, are greatly modified in the subsequent months following social interaction.

In contrast, phenotypic plasticity (the set of inducible traits) refers to the ability of species or single individual phenotypes to exhibit alternative morphological, behavioral and physiological characteristics in response to unpredictable environmental conditions (West-Eberhard, 1989; Garland Jr. and Kelly, 2006) or to developmental or acquired body damage. Such phenotype plasticity, or malleability, is universal among living things: well-known examples of plastic development are seasonal polyphenism (the phenomenon where two or more distinct phenotypes are expressed by the same gene) in butterflies, as well as phenotypic changes like acclimation, learning and immune system adaptation (for a review, see Gilbert and Epel, 2009).

The distinction between robustness and plasticity-induced changes can reflect the distinction between gene-induced developmental processes and the ability of a single individual to change its performance in response to environmental changes.

Plasticity and robustness are not, however, mutually excluding processes. Several lines of evidence lead us to view them as interdependent components of the process that generates individual variation. As stressed by Minelli and Fusco (2010), there is no biological criterion for such distinction because the evolution of alternative developmental pathways for distinct environmental settings may share significant similarities with the evolution of physiological adaptations (Arenas-Mena, 2010).

Plasticity must be distinguished from elasticity, a property in bodies by which they recover their former shape or dimensions after the removal of external pressure or of an altering force, as happens with rubber bands. Examples include the elasticity of the skin, the process of repairing a wound, or catch-up growth after starvation.

Plasticity phenomena can be classified in different ways, based on the nature of the interested trait (morphological, physiological, behavioral), the nature of the environmental cue (e.g. diet, specific learning), the phase of sensitivity of the organism to the environment (early on in development or in adult life) or a change of strategy in processing information. A striking example of these latter mechanisms comes from the studies comparing face-processing strategies between typically developed subjects (TD) and subjects affected by Autism Spectrum Disorders (ASD). ASD is characterized by severe social and communication difficulties. A striking qualitative difference between ASD and TD lies in the ways in which ASD and TD observers process faces. Whereas TD individuals rely primarily on information around eyes and eyebrows for face identification, several studies reported that ASD subjects used different strategies, relying mostly on information around the mouth/lower face region (Rutherford et al., 2007). At a closer view, however, other important differences were recently found between the two groups by Nagai et al. (2013): while all TD observers showed the identical strategy in face processing, using the eye/brow regions, the strategies used by ASD observers were not uniform: some of them used a typical mouth/lower face recognition strategy and others used the same strategy used by TD observers, relying on information concentrated in the forehead region. This intergroup difference could be interpreted as an effect of some ASD subjects compensating for the innate atypical strategy used in childhood by learning to discriminate faces relying on the same type of information used by TD children.

On the other hand, plastic changes can involve anatomical modifications of specific anatomical systems over the lifespan and allow cortical processing to adjust and adapt to experience or to the effects of sensory deprivation or acquired brain damage and provide a mechanism for improving functioning in an adaptive manner. The adjustment processes can extend to different fields; it is possible, for example, to adjust to different temperatures or to cope with the effects of a body part injury due to a mutation or acquired postnatal damage. For example, blind people can learn to use their hands and fingers for reading, through the Braille system; similarly, pre-verbal deaf children can develop an efficient way of expressing language through hand signs (see chapter 7). Another example is the immune system, within which antibodies are formed in response to foreign proteins that have not previously been encountered by the individual.

Several factors can enhance or limit an organism’s plasticity: the time scale over which a plastic response is expressed can be almost immediate, as in the case of some physiological or behavioral responses in animals (West-Eberhard, 2003). Alternatively, plastic responses can be comparatively slow, such as the morphological alterations exhibited by animals in response to diet (Wainwright et al., 1991; Price, 2006).

The organism, however, does not possess equipotentiality, nor do its structures conform to the same plasticity principles and constraints. Striated muscle tissue demonstrates a remarkable malleability and can adjust its metabolic and contractile make-up in response to alterations in functional demands, such as following prolonged training. In contrast, it has long been thought that neural plasticity would be possible only in early life and be restricted to specific regions (see chapter 3).

One of the most important variables in determining the degree of plasticity is the time period; there is often a critical or sensitive period during which an organism has heightened sensitivity in order to acquire information from exogenous stimuli, to develop normally, to acquire a particular skill or to be particularly sensitive to noxious stimuli. Exposure to toxins or drugs during specific stages of development can have dramatic effects: the assumption by pregnant women of thalidomide, a mild sedative, caused an enormous risk of limb abnormalities i(phocomelia) in the newborns.

Conversely, a common trait of aging is a reduction in an organism’s plasticity. The gathering of crucial information in the sensitive period can influence later learning or, as detailed in the following chapters, the impact of anomalous sensory experiences may constrain the organism’s future plasticity. Visual deprivation in early life, for instance, can impair the development of the visual cortex (Hubel and Wiesel, 1970; Hubel, Wiesel and Levay, 1977).

Although it is generally acknowledged that phenotypic plasticity is an important property of all biological systems, allowing the organism to cope with environmental unpredictability and/or heterogeneity, the effects of the changes are not always beneficial. When, for example, lowland animals are exposed to high altitude, an environment where the oxygen concentration is low, they develop an increase of hematocrit (Ht, the proportion of blood that is expressed as a percentage of the red blood cells [erythrocytes] to the rest of the blood constituent) and hemoglobin (Hb, the protein in red blood cells that carries oxygen). However, it is not clear whether the acclimatization response to environmental hypoxia can be regarded generally as a beneficial mechanism of adaptive phenotypic plasticity, or whether it might sometimes represent a misdirected response that acts as a hindrance to adaptation; the excessive number of red blood cells limits exercise performance and pulmonary function, leading the process of adaptation away from the apparent phenotypic optimum under chronic hypoxia. In the following chapters, examples of neural maladaptive plasticity will be reviewed.

In conclusion, increasing our understanding of phenotype plasticity, from molecules to behavior, can help us improve and repair human skills at different levels of anatomical, physiological and cognitive complexity.¹

Note

1 Excusatio non petita: Although, plasticity is a widespread phenomenon across all biological systems, this review will focus only on the nervous system, and almost exclusively on the human brain cortex, the most useful tool in trying to adapt to our ever-changing environment.

Bibliography

Arenas-Mena C. (2010), Indirect development, transdifferentiation and the macroregulatory evolution metazoans. Philosophical Transactions of the Royal Society B, Biological Sciences, 365: 653–669.

Bateson P. and Gluckman P. (2011), Plasticity, Robustness, Development and Evolution, Cambridge (MA), Cambridge University Press.

Bateson P. and Martin P. (1999), Design for a Life: How Behaviour Develops, London, Jonathan Cape.

Cashmore A. R. (2010), The Lucretian swerve: The biological basis of human behavior and the criminal justice system. Proceedings of the National Academy of Sciences USA, 107: 4499–4504.

Garland T. Jr. and Kelly S. A. (2006), Phenotypic plasticity and experimental evolution. Journal of Experimental Biology, 209: 2344–2361.

Gilbert S. F. and Epel D. (2009), Ecological Developmental Biology: Integrating Epigenetics, Medicine, and Evolution, Sunderland (MA), Sinauer Associates.

Hubel D. H. and Wiesel T. N. (1970), The period of susceptibility to the physiological effects of unilateral eye closure in kittens. Journal of Physiology, 206(2): 419–436.

Hubel D. H., Wiesel T. N., Levay S. (1977), Plasticity of ocular dominance columns in monkey striate cortex. Philosophical Transactions of the Royal Society B, Biological Sciences, 278(961): 377–409.

Masel J. and Siegal M. L. (2009), Robustness: Mechanism and consequences. Trends in Genetics, 25(9): 395–403.

Minelli A. and Fusco G. (2010), Developmental plasticity and the evolution of animal complex life cycles. Philosophical Transactions of the Royal Society B, Biological Sciences, 365: 631–640.

Nagai M., Bennett P. J., Rutherford M. D., Gaspar C. M., Kumada T., Sekuler A. B. (2013), Comparing face processing strategies between typically-developed observers and observers with autism using sub-sampled-pixels presentation in response classification technique. Vision Research, 79: 27–35.

Price T. D. (2006), Phenotypic plasticity, sexual selection, and the evolution of colour patterns. Journal of Experimental Biology, 209: 2368–2376.

Rutherford M. D., Clements K. A., Sekuler A. B. (2007), Differences in discrimination of eye and mouth displacement in autism spectrum disorders. Vision Research, 47: 2099–2110.

Wainwright P.C., Osenberg C.W., Mittelbach G.G. (1991), Trophic polymorphism in the pumpkinseed sunfish (Lepomis gibbosus Linnaeus): effects of environment on ontogeny. Functional Ecology, 5: 40–55.

West-Eberhard M. J. (1989), Phenotypic plasticity and the origins of diversity. Annual Review of Ecology and Systematics, 20: 249–278.

West-Eberhard M. J. (2003), Developmental Plasticity and Evolution, New York, Oxford University Press.

2
Neural plasticity

General Principles

The search for the nature of the knowledge that characterizes the human species, allowing it to achieve its outstanding level of cognition, can be traced back to classical Greek philosophy. Since the fifth century BC, there have been two opposite views: the first emphasizing the innate structure of knowledge (Plato) and the second supporting the idea that human knowledge is a consequence of postnatal processes of learning and memory (Aristotle). The dialogue between nativists and empiricists, or on the nature-versus-nurture¹ origin of the human mind, continued and was enriched in the following centuries, although it was mostly considered a theoretical subject, more pertinent to philosophy than science.

It was only in the last two centuries, when the notion that the nature of cognitive skills and their development throughout the lifespan could be experimentally tackled, that the debate between empiricists and nativists acquired a scientific status (for a short review, see Cashmore, 2010).

In more recent times, the massive entrance of biology into the field of cognition entirely reshaped the research field. Genetics, comparative and developmental neuroanatomy and the advent of neuroimaging techniques took over the discipline and, at the same time, enriched the philosophical debate with opportunities for mind-enriching, interdisciplinary collaborations.

At first, the meaning of the terms nature and nurture did not seem quite adequate...

Table of contents

• Cover Page
• Half Title Page
• Title Page
• copyright Page
• Dedication Page
• Contents
• Foreword
• Preface
• SECTION 1 Introducing plasticity
• Section 2 The role of specific factors on neural plasticity
• Section 3 Specific skills
• Section 4 The mechanisms of repair