Plasticity may change inputs as well as processes, structures, and responses
Introduction
Development during infancy is characterized by a series of changes—the acquisition of new abilities, the refinement of old abilities, the integration of processes, and the reorganization of systems. The first postnatal year, for example, is characterized by the acquisition of independent sitting (e.g., Adolph & Robinson, 2015), a shift from processing faces in a piecemeal fashion to processing faces holistically (e.g., Schwarzer, Zauner, & Jovanovic, 2007), the coordination of visual exploration and reaching (e.g., von Hofsten, 2004), and the emergence of brain responses specific to face processing (e.g., de Haan, Johnson, & Halit, 2003). All these developmental changes reflect, to some extent, neuroplasticity. That is, the neuroanatomical structures and connections that support these abilities have developed and adapted in response to infants’ experiences.
Developmental outcomes reflect a cascade of events, however. Any given milestone or achievement reflects the specific experiences—biological or environmental—that have shaped physical, motor, and cognitive abilities at various points in developmental time. These changes in abilities then lead to different opportunities for new experiences (i.e., different inputs), that then further change the child’s developmental trajectory (see Masten & Cicchetti, 2010). Moreover, multiple different factors contribute to the achievement of a given milestone (see Thelen & Smith, 1994). Consider as an example an infant whose first spoken word is “dog.” Clearly, the infant’s exposure to English contributed to this being the child’s first spoken word. However, children’s first spoken words also reflect their developing abilities to perceive speech sounds in their “native” language, to articulate specific speech-related sounds, to learn associations between specific objects and specific word forms, as well as their experience with a particular language. Therefore, many experiences through the first year contribute to this milestone. For example, infants’ daily exposure to one or more language shapes their developing processing of speech sounds (Dietrich, Swingley, & Werker, 2007; Kuhl, Tsao, & Liu, 2003; Werker & Desjardins, 1995; Werker & Tees, 1984). In addition, infants’ early actions on objects—and the resulting influences on their visual object perception—may contribute to their learning of object names (Smith, 2013).
In this paper, I focus on how plasticity changes the inputs that an organism experiences. At least since the time of Dewey, there has been a recognition that input is an integral part of the experience of the world (Dewey, 1896). In the context of development, psychologists have long recognized the importance of input for development as well as how inputs change over development (Gibson, 1982, Gibson, 1988, Piaget, 1954). However, despite the importance of input for understanding cognitive development, much of the work on cognitive development focuses on the outputs or products of development—changes in brain organization, strategies, skills, cognitive structures. When constructing programs of research to understand cognitive development, however, we should also consider how the input itself changes over development, and how those changes in the input contribute in important ways to subsequent development. These ideas have much in common with cascade approaches to understanding development (Bornstein, Hahn, & Wolke, 2013; Masten & Cicchetti, 2010), and one goal of this paper is to encourage researchers to think about developmental cascades broadly across domains, timeframes, and areas of development.
In this paper I will consider how differenct developmental outcomes (e.g., structures, processes, responses) translate to differences in the input, and those differences in the input further influence future developmental outcomes. Thus, here I will examine how differences in what information serves as input derive from variations in experience. This paper is organized in three sections. First, I discuss how different levels of variation in the input create different levels of variation in the products of development—i.e., plasticity. This discussion sets the stage for the second section, in which I provide two examples from research findings in two domains of how plasticity is revealed in the input. Finally, I conclude by providing a framework for understanding different ways in which input contributes to plasticity, and posit some goals for future research.
Section snippets
Levels of variation in the input and plasticity
There has long been a recognition between input and plasticity. Recognizing that there is considerably variation in the amount of overlap in the input across individuals, Greenough, Black, and Wallace (1987) distinguished between Experience Expectant Plasticity and Experience Dependent Plasticity. Some inputs are essentially universal—except under very extreme conditions, every human child experiences gravity, a caregiver, exposure to variations in temperature, and so on. These universal inputs
Plasticity changes the input
The main point here, however, is that plasticity actually creates change in the input itself. The experience-plasticity relations often focus on input as static. In other words, given a specific input (e.g., a word from a specific language), plasticity is observed in changes in the organism’s output (e.g., the ability to make a phonemic distinction). As a result, there is a unidirectional nature to the way the input-plasticity relation is often described—differences in input (experience) cause
A framework for understanding how the input contributes to plasticity
In the preceding section, I described two examples illustrating how differences in experience shape the input received by developing systems. These examples show that the representations, processes, and structures that develop differently as a result of experience influencing future input. That is, plasticity influences the input. We not only see that brain organization is plastic and responsive to difference in experience, but also that the processes and strategies that are developed for
Conclusion and suggestions for future directions
Plasticity is obviously understood in the context of experience, and how differences in experience correspond to difference in the input to developing systems. The brain is plastic and adapts in the absence of typical experience, such as coordinated input from the two eyes. Similarly, the brain adapts and creates different organization in response to more subtle differences in experience, yielding structures and organizations that differ but not as dramatically as when a typical or expected
Acknowledgements
Preparation of this manuscript was made possible by NIH grants R01 EY022525, R03 HD070651 and NSF grant BCS 0951580. I thank Karen Adolph, Steve Luck, and David Rakison for helpful comments on drafts of this paper and stimulating discussions about these issues.
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