Dynamical Systems Approach to Infant Motor

Development: Implications for Epigenetic Robotics

Eugene C. Goldfield

Childrens Hospital Boston and Harvard Medical School

A dynamical systems approach to motor develop-ment is presented. It highlights the challenges facedby human infants as they discover the potential ofthe body to perform different goal-directed actions.Some of the same challenges may face robotic sys-tems that are modeled on epigenetic processes. Thedifferent categories of goal-directed actions, such aslocomoting and ingesting foodstuffs, may emerge asa consequence of energy transactions between thebrain, the biomechanical properties of the body, anda highly structured environment. Each dynamicalsystem for performing certain goal-directed actions,called an action system, is conceptualized as a tem-porary coupling of biological components for doingwork.

Different action systems of animals and humans– such as orienting, locomoting, and appetitive be-havior (eating and drinking) – are assembled frommicroscopic components. Complex biological sys-tems, including social insects such as termites as wellas vertebrates with internal organs, may rapidly re-configure themselves to form particular structural ar-rangements, a process called self-organization. Selforganizing systems are characterized by multiple lev-els in which interactions at one level result in theemergence of new properties at another that cannotbe understood as a simple addition of their individualcontributions. For example, the cellular interactionsinvolved in neuromuscular communications are thebasis for the respiratory pumping mechanism, butthe functional properties of the organs of breathingemerge at the level of organ systems.

The human nervous system may have evolved asynergetic strategy similar to that apparent in earlymulti-cellular life: establishing temporary couplingsbetween dynamical systems in order to harness avail-able energy resources. The coupled dynamical sys-tems underlying human motor behavior are assem-bled from different biological devices comprising theneural, circulatory, musculoskeletal and other sys-tems. These devices include skeletal levers, mus-cular engines, elastic storage systems, and positivepressure pumps. How do human infants – and, byimplication, epigenetic robots – develop the abilityto use the devices inherent in the organs of the bodyfor performing different categories of action?

By performing rhythmical behaviors, such as kick-ing or sucking, with a body that has a particularanatomical configuration, the self-organizing proper-ties of these rhythms assemble into useful devices.Each of the assembled devices is a collection of dy-namical systems coupled to each other with varyingdegrees of strength. These dynamical systems aredescribed by equations of motion with tunable pa-rameters. Infants may explore the effects of theirown goal-directed efforts on what the body actuallydoes, and the nervous system may tune system pa-rameters to make the assembled devices stable andenergetically efficient.

To illustrate the assembly and tuning of action sys-tems during infancy, this talk focuses on two motorskills: locomotion and appetition (sucking, swallow-ing, and breathing). Data from my research on in-fant bouncing as well as learning to crawl are usedto illustrate the process by which self-produced ac-tions are transformed into a device for locomotion,a coupled spring-pendulum system. First, I considerdata from longitudinal studies of infants learning tobounce while supported upright by a harness andspring. Effortful kicking assembles a mass-spring sys-tem with limit cycle dynamical properties. Explo-ration of the relation between kicking and bouncingover longitudinal sessions results in a change in infantbehavior: infants become more likely to kick at themoment in the cycle of bouncing when potential en-ergy is highest, its resonant frequency. At a “peak”session, successive bounces increase dramatically inlength, and kicking force decreases, indicating thatthe system is at resonance.

A second aspect of locomotor development in-volves inserting opportunities for postural supportwith the ground surface into an ongoing limit cycleoscillatory process. For example, during brief periodsof rocking while in a prone posture, all of the limbsare oscillating in phase. During rocking, the infantmay discover that kicking keeps them “trapped” at asingle location on the support surface. The bilateralasymmetry of hand use during this period may allowthe infants efforts to reach forward with one handwhile supporting the body with the other to breakfree of this biomechanical trap. In dynamical terms,the hand preference is a symmetry breaking process,

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Berthouze, L., Kaplan, F., Kozima, H., Yano, H., Konczak, J., Metta, G., Nadel, J., Sandini, G., Stojanov, G. and Balkenius, C. (Eds.)Proceedings of the Fifth International Workshop on Epigenetic Robotics: Modeling Cognitive Development in Robotic Systems

Lund University Cognitive Studies, 123. ISBN 91-974741-4-2

and allows the system to alternate postural supporton the right and left sides of the body, as is evidentin crawling.

The discussion of a second action system, appeti-tion, focuses on how coupling of the actions of thetongue, lips, jaw and pharynx to the respiratory cy-cle of inspiration and expiration assembles a biolog-ical positive displacement pump for moving air tothe lungs and foodstuffs to the digestive tract. Aparticular challenge for newborn infants is that theoropharynx is a shared anatomical pathway. Howdoes the experience of early breastfeeding and bottlefeeding make it possible for the CNS to discover thecouplings of the component rhythms that result inswallowing without taking fluid into the lungs? In or-der to insert safe swallows into the respiratory cycle,the CNS may explore the stable patterns of relativephase between sucking and breathing for “windows”during which swallows do not occur as air is enter-ing the lungs. Data is presented on newborn infantscoordination of sucking, swallowing, and breathingduring breastfeeding. Swallows are not randomlydistributed during each complete cycle of respiration,but rather are attracted to a relative phase of either 0degrees or 180 degrees. This suggests that the intrin-sic dynamics of ongoing respiratory activity createphase windows during which swallows are safe. Thebistable dynamics of coupled oscillators may providea means for rapidly switching the organization of thepump so that swallowing and breathing do not occursimultaneously.

The discussion of appetition concludes with pre-sentation of a computer-controlled system for regu-lating milk flow among premature infants and otherclinical populations who have difficulty coordinatingsucking, swallowing, and breathing.

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