Esther Thelen

Development as a dynamic system.

Development is about creating something more from something less, for example, a walking and talking toddler from a helpless infant. One current theoretical framework views the developmental process as a change within a complex dynamic system. Development is seen as the emergent product of many decentralized and local interactions that occur in real time. We examine how studying the multicausality of real-time processes could be the key to understanding change over developmental time. We specifically consider recent research and theory on perseverative reaching by infants as a case study that demonstrates this approach.

Rhythmical stereotypies in normal human infants

Naturalistic, longitudinal observations of 20 normal infants biweekly during their first year showed that they performed a great quantity and variety of rhythmical and highly stereotyped behaviours. Forty-seven movement patterns are described involving the legs and feet; the head and face; the arms, hands, and fingers; and the whole torso in various postures. These behaviours showed developmental regularities as well as constancy of form and distribution. Groups of stereotypies involving particular parts of the body or postures had characteristic ages of onset, peak performance, and decline. The onset of particular stereotypy groups was highly correlated with motor development. It is proposed that rhythmical stereotypies are manifestations of incomplete cortical control of endogenous patterning in maturing neuromuscular pathways.

Self-organizing systems and infant motor development

Abstract A dynamical systems approach is used to characterize early motor development. Contemporary theories of motor performance emphasize the self-organizing and autonomous properties of muscle synergies as an example of the dynamical behavior of complex, nonlinear systems. The organization of leg movements in infants is consistent with the dynamical approach. Using this perspective, we view skill development as a multidimensional, emergent phenomenon. The developmental principles derived from motor behavior may apply to other domains.

The organization of spontaneous leg movements in newborn infants.

Spontaneous, supine kicking in newborn (2- and 4-week-old) infants is described in terms of its temporal structure, interjoint coordination, and muscle activation characteristics as measured by surface electromyography. Phasic kick movements shoed a constrained temporal organization in the movement, but not the pause phases. Hip, knee, and ankle joints moved in temporal and spatial synchrony, and all three joints showed a rhythmical or periodic organization over time. EMGs revealed antagonist coactivation at the initiation of the flexor movement, but little or not extensor activity. The dorsal muscles, the gastrocnemius and hamstrings, showed less activity than the ventral pair, tibialis anterior and quadriceps. Burst and onset-to-peak durations were also constrained. As a result of neural mechanisms and biomechanical forces, newborn leg movements are structured muscle synergies. This organization has implications both for newborn functioning and for later development.

The relationship between physical growth and a newborn reflex

Abstract Young infants both grow at a very rapid rate and show dramatic changes in body composition. The behavioral consequences of somatic growth have been little explored. Here we report three studies on the relationship between body-build changes and a newborn reflex, stepping. Study 1 compared the number of steps and several body-build measures in 40 infants seen at 2, 4, and 6 weeks. At each age, overall arousal was the best predictor of steps. At 4 weeks, however, those infants who gained weight and became “chubby” most rapidly stepped less. Infants gained weight and added fat at the most rapid rate between 2 and 4 weeks. In Study 2, we manipulated leg mass by adding small weights to the legs. Infants stepped less and with weaker flexion movements when the legs were weighted. In Study 3, we reduced the effects of leg mass by submerging infants’ legs in water. Stepping rate increased and joint flexions were greater. We proposed that muscle strength development may not be synchronous with mass increase and that peripheral as well as central nervous system factors contribute to infant behavioral development. The disappearance of stepping is better explained by asynchronous physical growth than by previous hypotheses.

Dynamic Systems Theory and the Complexity of Change

The central thesis of this paper is that grand theories of development are alive and well and should be paramount to those interested in behavioral intervention. Why? Because how we think about development affects how we approach treatment. Here I discuss the central concepts of a new theory of development—dynamic systems theory—to highlight the way in which a theory can dramatically alter views of what intervention is all about. Rather than focusing on one root of maladaptive behavior such as biological predispositions, environmental causes, or motivational states, dynamic systems theory presents a flexible, time-dependent, and emergent view of behavioral change. I illustrate this new view with a case study on how infants develop the motivation to reach for objects. This example highlights the complex day-by-day and week-by-week emergence of new skills. Although such complexity presents daunting challenges for intervention, it also offers hope by emphasizing that there are multiple pathways toward change.

Postural Control During Reaching in Young Infants: A Dynamic Systems Approach

We conceptualize the coordinated development of posture and reaching within Schöners (Ecological Psychology, 7:291-314, 1995) dynamic model of coupled levels of control: load, timing, and goal. In particular, the goal of postural stability must be maintained during a reach. Using longitudinal data from four infants followed from 3 weeks to 1 year, we show that coordination of the head with upper and lower arm activity is critical for successful reaching. First, infants acquire stable head control several weeks before reaching onset. Furthermore, reaching onset is characterized by a reorganization of muscle patterns to include more trapezius and deltoid activity, serving to stabilize the head and shoulder and provide a stable base from which to reach. We argue that initially, the system is working on postural stability and reaching as goals. Infants secondarily select appropriate muscle patterns to achieve those goals depending, in part, on their individual body sizes, body proportions and energy levels. Motor development proceeds as a continual dialogue between the nervous system, body, and environment.

Understanding movement control in infants through the analysis of limb intersegmental dynamics

One important component in the understanding of the control of limb movements is the way in which the central nervous system accounts for joint forces and torques that may be generated not only by muscle actions but by gravity and by passive reactions related to the movements of limb segments. In this study, we asked how the neuromotor system of young infants controls a range of active and passive forces to produce a stereotypic, nonintentional movement. We specifically analyzed limb intersegmental dynamics in spontaneous, cyclic leg movements (kicking) of varying intensity in supine 3-month-old human infants. Using inverse dynamics, we calculated the contributions of active (muscular) and passive (motion-dependent and gravitational) torque components at the hip, knee, and ankle joints from three-dimensional limb kinematics. To calculate joint torques, accurate estimates were needed of the limbs anthropometric parameters, which we determined using a model of the human body. Our analysis of limb intersegmental dynamics explicitly quantified the complex interplay of active and passive forces producing the simple, involuntary kicking movements commonly seen in 3-month-old infants. our results revealed that in nonvigorous kicks, hip joint reversal was the result of an extensor torque due to gravity, opposed by the combined flexor effect of the muscle torque and the total motion-dependent torque. The total motion-dependent torque increased as a hip flexor torque in more vigorous kicks; an extensor muscle torque was necessary to counteract the flexor influences of the total motion-dependent torque and, in the case of large ranges of motion, a flexor gravity torque as well. Thus, with changing passive torque influences due to motions of the linked segments, the muscle torques were adjusted to produce a net torque to reverse the kicking motion. As a consequence, despite considerable heterogeneity in the intensity, range of motion, coordination, and movement context of each kick, smooth trajectories resulted from the muscle torque, counteracting and complementing not only gravity but also the motion-dependent torques generated by movement of the linked segments.

Motor development as foundation and future of developmental psychology

The study of how infants and children come to control their bodies is perhaps the oldest topic in scientific developmental psychology. Yet, for many years the study of motor development lay dormant. In the last two decades, however, there has been an enormous resurgence of interest. As at the time of the very beginnings of our field, the contemporary study of motor development is contributing both empirically and theoretically to the larger questions in development and especially to our understanding of developmental change. In this essay, I trace the course of the changing fortunes of motor development, evaluate where we have been, what we are doing, and speculate on some critical issues for the future. The purpose of this essay is to comment on the general themes and influences that have been a part of motor development’s “rise-fall-and-rise-again” history. For a more comprehensive review of substantive topic areas in motor development, readers are referred to the authoritative treatment recently published by Bertenthal and Clifton (1998) and to the excellent monograph by Goldfield (1995).

Kicking, rocking, and waving: Contextual analysis of rhythmical stereotypies in normal human infants

I analysed the triggering contexts of over 14,000 bouts of rhythmical stereotypies of the legs, arms, and whole torso observed in 20 normal infants during their first year. Infant stereotypies were elicited by a wide variety of contexts including non-alert states, interactions with the caregiver and other persons, feeding situations, object interest, and kinaesthetic changes. The frequency of stereotypy in all contexts was a function of age. Non-alert states and interactions with the caregiver were disproportionately associated with leg and torso stereotypies. The relationship between these human stereotypies and the function and control of stereotyped behavior in other species is discussed.