An Epigenetic Perspective on the Development of Self-Produced Locomotion and ItsConsequencesAuthor(s): Bennett I. Bertenthal, Joseph J. Campos, Rosanne KermoianSource: Current Directions in Psychological Science, Vol. 3, No. 5 (Oct., 1994), pp. 140-145Published by: Blackwell Publishing on behalf of Association for Psychological ScienceStable URL: http://www.jstor.org/stable/20182292Accessed: 24/11/2009 12:12

Your use of the JSTOR archive indicates your acceptance of JSTOR's Terms and Conditions of Use, available athttp://www.jstor.org/page/info/about/policies/terms.jsp. JSTOR's Terms and Conditions of Use provides, in part, that unlessyou have obtained prior permission, you may not download an entire issue of a journal or multiple copies of articles, and youmay use content in the JSTOR archive only for your personal, non-commercial use.

Please contact the publisher regarding any further use of this work. Publisher contact information may be obtained athttp://www.jstor.org/action/showPublisher?publisherCode=assocpsychsci.

Each copy of any part of a JSTOR transmission must contain the same copyright notice that appears on the screen or printedpage of such transmission.

JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range ofcontent in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new formsof scholarship. For more information about JSTOR, please contact support@jstor.org.

Association for Psychological Science and Blackwell Publishing are collaborating with JSTOR to digitize,preserve and extend access to Current Directions in Psychological Science.

http://www.jstor.org

140 VOLUME 3, NUMBER 5, OCTOBER 1994

events. Pilot data collected in our

laboratory suggest that young infants

expect a moving object to stop when

it encounters a tall, thin box but not

a short, wide box, even when the

latter is considerably larger in vol

ume than the former. We suspect that infants are led by the dominant

vertical axis of the tall box to per ceive it as a wall-like, immovable

object, and hence categorize the

event as an instance of a barrier phe nomenon; in contrast, infants tend

to view the wide box as a movable

object, and hence categorize the

event as an instance of a collision

phenomenon, resulting in incorrect

predictions. The foregoing discussion high

lighted several types of developmen tal sequences that would be antici

pated in an innate-mechanisms view

but not (without considerable elab

oration) in an innate-principles view. To gain further insight into the

nature and origins of these develop mental sequences, we have adopted a dual research strategy. First, we

are examining the development of

infants' understanding of additional

physical phenomena (e.g., gap,

containment, and occlusion phe nomena) to determine how easily these developments can be captured

in terms of the patterns described in

the model and to compare more

closely the acquisition time lines of

phenomena that are superficially distinct but deeply related. Second, as was alluded to earlier, we are at

tempting to teach infants initial con

cepts and variables to uncover what

kinds of observations, and how

many observations, are required for

learning. We hope that the pursuit of

these two strategies will eventually allow us to specify the nature of the

learning mechanisms that infants

bring to the task of learning about

the physical world.

Acknowledgments?This research was

supported by grants from the Guggenheim Foundation, the University of Illinois Cen

ter for Advanced Study, and the National

Institute of Child Health and Human De

velopment (HD-21104). I would like to

thank Jerry Dejong, for his support and

insight, and Susan Carey, Noam Chom

sky, Judy DeLoache, Cindy Fischer, John

Flavell, Laura Kotovsky, Brian Ross, and

Bob Wyer, for many helpful comments

and suggestions.

Notes

1. J. Piaget, The Construction of Reality in the Child (Basic Books, New York, 1954).

2. E.S. Spelke, Preferential looking methods as tools for the study of cognition in infancy, in Mea surement of Audition and Vision in the First Year of

Postnatal Life, G. Gottlieb and N. Krasnegor, Eds.

(Ablex, Norwood, Nj, 1985). 3. R. Baillargeon, The object concept revisited:

New directions in the investigation of infants' phys ical knowledge, in Visual Perception and Cognition in Infancy, CE. Granrud, Ed. (Erlbaum, Hillsdale,

NJ, 1993). 4. E.S. Spelke, K. Breinlinger, J. Macomber, and

K. Jacobson, Origins of knowledge, Psychological Review, 99, 605-632 (1992).

5. R. Baillargeon, L. Kotovsky, and A. Need

ham, The acquisition of physical knowledge in in

fancy, in Causal Understandings in Cognition and

Culture, G. Lewis, D. Premack, and D. Sperber, Eds. (Oxford University Press, Oxford, in press).

6. R. Baillargeon, A model of physical reasoning in infancy, in Advances in Infancy Research, Vol. 9,

C. Rovee-Collier and L. Lipsitt, Eds. (Ablex, Nor

wood, NJ, in press). 7. R. Baillargeon, Physical reasoning in infants,

in The Cognitive Neurosciences, M.S. Gazzaniga, Ed. (MIT Press, Cambridge, MA, in press).

8. E.S. Spelke, Physical knowledge in infancy: Reflections on Piaget's theory, in The Epig?nesis of

Mind: Essays on Biology and Cognition, S. Carey and R. Gelman, Eds. (Erlbaum, Hillsdale, NJ, 1991).

9. A.M. Leslie, ToMM, ToBy, and Agency: Core architecture and domain specificity, in Causal Un

derstandings in Cognition and Culture, G. Lewis, D.

Premack, and D. Sperber, Eds. (Oxford University Press, Oxford, in press).

10. P. Rochat and A. Bullinger, Posture and functional action in infancy, in Francophone Per

spectives on Structure and Process in Mental Devel

opment, A. Vyt, H. Bloch, and M. Bornstein, Eds.

(Erlbaum, Hillsdale, NJ, in press). 11. K.D. Forbus, Qualitative process theory, Ar

tificial Intelligence, 24, 85-168 (1984). 12. This example focused exclusively on the

size of the cylinder, but what of the distance trav eled by the bug in each event? It seems likely that infants encode this information not in quantitative terms (e.g., "the bug traveled x as opposed to y distance"), but rather in qualitative terms, using as their point of reference the track itself (e.g., "the bug rolled to the middle of the track"), their own spatial position (e.g., "the bug stopped in front of me"), or the brightly decorated back wall of the apparatus (e.g., "the bug stopped in front of such-and-such section of the back wall").

An Epigenetic Perspective on the

Development of Self-Produced Locomotion and Its Consequences Bennett I. Bertenthal, Joseph J. Campos, and Rosanne Kermoian

One of the most striking charac

teristics of early human develop ment is its consistency and stability.

Infants show considerable unifor

mity in the nature and timing of new

behaviors. A principal contributor to

this early uniformity is the develop

ment of species-typical behaviors, such as vocalization, locomotion, and reaching. These behaviors ensure a common set of experiences

with far-reaching consequences for

development. In this article, we

review a specific example of this

epigenetic process involving the

development of self-produced locomotion.

EXTERNAL VERSUS SELF-PRODUCED FORMS

OF EXPERIENCE

The importance of self-produced experiences is often overlooked by researchers, and, indeed, most stud ies investigating early development focus on the effects of stimulation

from the environment. A paradig matic example is the study of infants'

Published by Cambridge University Press

CURRENT DIRECTIONS IN PSYCHOLOGICAL SCIENCE 141

responses to the social stimulation of

their caretakers. The infant is viewed

as a passive recipient of environ

mental stimulation, and responses are typically assumed to be contin

gent on the actions of the caretaker.

The alternative view is that infants

are active participants in learning about self and environment, and

they provide through their own ac

tions at least some of the experi ences necessary for further growth and development. In contrast to ex

ternally produced forms of stimula

tion, these new experiences are

available to all infants regardless of

their rearing environments.

Our own research focuses on

how the onset of crawling, the emer

gence of independent mobility, in

fluences subsequent development. In most infants, crawling emerges

between 6 and 9 months of age, and

coincides with numerous changes in

sensorimotor intelligence, including new ways of coding spatial rela

tions, new concepts about objects, new forms of social communication, a burgeoning of fear, and the further

differentiation of other emotions.1 Is

it possible that experiences provided

by the emergence of crawling are

functionally related to some of these

other major developmental changes

occurring at the same time? During the past decade, research conducted

in our respective labs has shed some

light on the answer to this provoca tive question.

Bennett I. Bertenthal is Professor of Psychology at the University of

Virginia. Joseph J, Campos is Pro fessor of Psychology and Director of the Institute of Human Develop ment at the University of California at Berkeley. Rosanne Kermoian is Assistant Research Psychologist at

the University of California at

Berkeley. Address correspondence to Bennett I. Bertenthal, Depart

ment of Psychology, Gilmer Hall, University of Virginia, Charlottes

ville, VA 22903-2477.

Fig. 1. Photograph of infant crawling on the visual cliff.

FEAR OF HEIGHTS

During the third quarter of the 1 st

year, infants show a dramatic in

crease in the intensity and probabil

ity with which they express fear.

These changes are so abrupt and so

adaptive for survival that it is often

assumed that neuromaturational fac

tors are the principal cause of this

developmental shift. An especially

compelling case is made for fear of

heights because it is such a biologi

cally significant behavior. Although we do not dispute a contribution by neuromaturation, our research sug

gests that the development of fear of

heights is considerably more com

plex, and is based on the interplay between neuromaturation and other

factors, especially self-produced lo

comotor experience.

Wariness of heights is often stud

ied using a "visual cliff." Figure 1

shows a picture of this apparatus, which consists of a large sheet of

glass (8x4 ft) suspended almost 4 ft

above the floor. A narrow board is

placed across the middle, dividing the sheet into two sides. On one side

(referred to as the shallow side), a

textured checkerboard pattern is

placed directly under the glass, so

that it appears as a rigid and support able surface. On the other side (re ferred to as the deep side), the tex

tured checkerboard is placed 4 ft

below the glass, so that this side ap

pears as a cliff, or as an apparent

drop-off. In most studies, infants are

placed on the centerboard and en

couraged to cross to the mother, who stands alternately across the

deep or shallow sides of the cliff.

Our early observations testing in

fants on the visual cliff revealed that,

contrary to the predictions of other

researchers, they did not avoid the

deep side (i.e., the apparent drop off) immediately following the onset

of crawling. Instead, it was generally 6 to 8 weeks following the onset of

crawling that avoidance was first ob served. This observation was subse

quently replicated and extended in a

series of experiments.2 In one study, crawling and pre

crawling infants were tested on the

visual cliff at the same age (7.3

months). Infants were held 3 ft

above the glass surface of the cliff by an experimenter, and slowly low

ered onto the surface. During this

descent, their heart rate was mea

sured and compared with their heart

Copyright ? 1994 American Psychological Society

142 VOLUME 3, NUMBER 5, OCTOBER 1994

rate during a baseline period. In gen- I

eral, heart rate decelerates in states

of orienting and attentiveness, and

accelerates in states of defensiveness or fearfulness.3 Precrawling infants

showed no significant cardiac

changes as they were lowered onto

either side of the visual cliff (see Fig. 2). By contrast, crawling infants

showed significant cardiac accelera

tion when lowered onto the deep

side, and no cardiac change when

lowered onto the shallow side.

Although these results suggested that crawling experience affected

fear of heights, they were by no

means definitive. Logically, crawl

ing experience could covary with

any number of other developmental variables that might contribute to the

development of fear of heights. The

challenge of subsequent research

was to show that the relation be

tween crawling experience and vi

sual-cliff performance was causal

and not merely correlative. To do

so entailed a series of converging

experiments. The most convincing experiment

was one in which locomotor experi ence was manipulated in a quasi

experimental manner. A group of

precrawling infants were given 40 hr

of locomotor experience in their

homes by their caregivers, who

placed them in infant walkers for

some period of time each day. These

walkers enable infants to locomote

by supporting them in a small seat

that is attached to a frame on

wheels. Infants with walker experi ence were tested with the descent

paradigm on the visual cliff. An age matched control group of precrawl

ing infants without walker experi ence was also tested on the visual

cliff. The results provided firm sup

port for the conclusion that experi ences with self-produced locomo

tion contribute to the development of fear of heights. Specifically, the

group of precrawling infants with

walker experience showed heart rate

acceleration when lowered onto the

deep side of the visual cliff; the age matched control group showed only a slight deceleratory shift.

Another study confirmed that the

relation between locomotor experi ence and fear of heights was not

task-specific. In this experiment, wariness of heights was tested by en

couraging infants to locomote across

the deep and shallow sides of the

cliff. To assess independently the ef

fects of age of onset of crawling and

crawling experience, infants who

began to crawl at 6, 7, or 8 months

of age were tested after 11 or 41

days of locomotor experience. The

probability of an infant crossing the

cliff, as well as the latency (or

elapsed time) to begin crossing, were significantly related to crawl

ing experience, but not to the age of

onset of crawling (see Fig. 3). Taken together, these results offer

compelling evidence that crawling

experience contributes significantly to the development of fear of

heights. The explanation for this re

lation is still not completely under

stood, but we do know that falling

experiences alone are not sufficient

to account for this developmental shift on the visual cliff. A more plau sible interpretation for this shift is

that active control of locomotion, unlike passive locomotion, demands

continuous updating of one's orien

tation relative to the spatial layout. This information is provided through multimodal sources, such as visual and vestibular coding of angular ac

celeration. With locomotor experi ence, changes in angular accelera tion detected by the visual system are mapped onto analogous changes detected by the vestibular system. Fear or avoidance ensues when the

expected mapping between visual

and vestibular information is vio

lated. This violation occurs when in

fants are placed on the deep side of

the visual cliff, because angular ac

celeration on the retina is scaled to

the distance of the nearest visible

texture, whereas no such scaling oc

curs for the vestibular system.4 As a

consequence, visual and vestibular

specification of self-motion become

discrepant, and produce an aver

sive, vertiginous response by infants,

CL SI LU (D

z <

o

< ce I rr

< LU

X

3 H

O? loe deep

#? loe shallow

-B? preloc deep - ?

preloc shallow

-i i i i i r

0.5 1.5 2 2.5

SECONDS OF DESCENT

3.5

Fig. 2. Mean heart rate changes in 7.3-month-old infants as a function of locomotor

experience. Data are plotted to show second-by-second heart rate changes during

descent onto deep and shallow sides of the visual cliff. Heart rate change is calculated

using beats per minute (BPM). Loc = crawling infants; preloc

= precrawling infants.

Published by Cambridge University Press

CURRENT DIRECTIONS IN PSYCHOLOGICAL SCIENCE 143

I LU

O z LU ?C LU

5

I Z < LU

2

LU ?_

100

80 i

? 60 1

40 i

20

-#? 41 days loe. exp.

-0? 11 days loe. exp.

6.5 7 7.5 8 8.5 9

AGE AT TESTING (MONTHS)

9.5 10

100

LU

i ?o ?. LU LU ? Z Q O

60 H

40

20

- 41 days loe. exp.

-0? 11 days loe. exp.

6.5 7.5 8.5

AGE AT TESTING (MONTHS)

9.5

Fig. 3. Results from a visual-cliff experiment testing infants who began to crawl at 6, 7, or 8 months of age. The top panel shows the mean latency differences between moving onto the deep and shallow sides of the visual cliff as a function of both locomotor

experience (loc. exp.) and age at testing. The bottom panel shows the percentage of

infants not crossing the deep side of the cliff as a function of locomotor experience and

age at testing.

not unlike what happens to adults

when looking down from a very high

skyscraper.5

SPATIAL SEARCH

The search for hidden objects is

another skill that is linked to crawl

ing experience. It is fairly well estab

lished that young infants are rarely successful in searching for a hidden

object following a displacement of

themselves or the object. Perhaps the best known example of this def

icit is the A-not-B error shown by infants on Piaget's object-perma

nence test. In this test, infants con

tinue to search in a previously

successful location even after they observe the toy hidden somewhere

else. This situation changes around

8 to 9 months of age, when infants

begin to show correct search for a

displaced object. The temporal cor

respondence between this develop mental shift in search behavior and

the onset of crawling led a number

of investigators to suggest a causal

connection between crawling expe rience and the development of new

search skills.

Two recent experiments support this predicted relation. The first

study tested hands-and-knees

crawling infants, precrawling in

fants, and precrawling infants with

walker experience (mean age = 8.5

months) on a series of hiding tasks

corresponding to Piaget's object permanence scale.6 Hands-and

knees-crawling and walker-assisted

precrawling infants passed more

items on this scale than did pre

crawling infants. Infants with the

most crawling experience (9 weeks)

passed tasks that involved searching for objects hidden in a new, spatially discriminable location, whereas pre

crawling infants were not able to

pass even a task involving a single

hiding location if a second (unused)

hiding location was present. The second study involved a dif

ferent task, but the results were sim

ilar.7 Hands-and-knees-crawling,

belly-crawling, and precrawling in

fants (mean age = 7.5 months) were

tested on trials in which a toy was

hidden in one of two differently col

ored containers placed in front of the

infant. After the hiding was com

pleted, either the infant or the table was rotated 180? so that the correct

left-versus-right location of the hid

den toy was reversed. The results re

vealed that crawling experience was

systematically related to search per formance when the infant was

moved, but not when the table was

rotated (see Table 1). When the in

fant was rotated, hands-and-knees

crawling infants searched the correct

container more often than predicted

Copyright ? 1994 American Psychological Society

144 VOLUME 3, NUMBER 5, OCTOBER 1994

Table 1. Number of infants showing correct and incorrect search for hidden

toy on first trial

Search

Infant group Correct Incorrect

Infant-displacement condition

Precrawling 5 15

Belly crawling 3 7 Hands-and-knees crawling 13 5

Table-displacement condition

Precrawling 11 9

Belly crawling 3 7 Hands-and-knees crawling 10 8

by chance, precrawling infants

searched the incorrect container

more often than predicted by

chance, and belly-crawling infants

showed a mixed response. How does crawling experience

contribute to infants' searches for

hidden objects? Prior to the onset of

crawling, infants remain in one lo

cation for extended periods of time.

They are thus fairly successful in

coding the location of an object with a body-centered frame of reference.

Following the onset of crawling, in

fants are moving much more often, and it becomes inefficient for them

to code the location of an object us

ing a body-centered frame of refer

ence. During the initial stages of this

transition, infants are likely to show

much greater variability in perfor mance, which is exactly what we

observed with belly-crawling in

fants. From our perspective, it is the

greater variability in performance that occurs during this time that

drives the system to a new level of

organization. The newly emergent search strategy no longer relies on a

body-centered frame of reference, but instead uses landmarks or some

other strategy, such as visual track

ing, for continually updating the lo

cation of an object. It is noteworthy that search per

formance was related to locomotor

experience only in the infant

rotation condition. This condition,

compared with the table-rotation

condition, more closely approxi

mates the experience of infants fol

lowing the onset of crawling. Infants

learn from this experience that they must update their spatial code for

the location of an object following a

self-displacement. Apparently, the new search response that emerges

with crawling experience does not

generalize immediately to other

conditions in which this response would also improve performance.

THE ORGANIZATION OF CRAWLING BEHAVIOR

A complementary theme high

lighted by the preceding research is

that all forms of locomotor experi ence are not equivalent, and that the

transition from belly crawling to

hands-and-knees crawling is espe

cially important. This developmen tal transition shares much in com

mon with the other shifts reviewed.

Indeed, the development of hands

and-knees crawling offers one of the

clearest examples of how the selec

tion of new behaviors by infants rep resents an emergent process based

on the confluence of multiple organ ismic and environmental factors.

In a recently completed study, we

assessed longitudinally the develop ment of crawling in six infants.8 Ki

nematic analyses of interlimb coor

dination revealed that the most

stable pattern of movement corre

sponds to a diagonal coupling of the

limbs, in which diagonally opposite limbs, such as the right arm and left

leg, move simultaneously, and 180? out of phase with the other pair of

limbs. The development of this diag onal pattern is an emergent process fueled by the initial experience of

moving on hands and knees. Limb movements corresponding to a diag onal pattern are rarely observed

prior to the time when the infant de

velops sufficient strength to support the torso off the ground. By contrast, a diagonal pattern is observed a little more than 50% of the time just 1 to

2 weeks after the torso is supported off the ground (see Fig. 4).

Our interpretation for this finding is that hands-and-knees crawling re

quires that the torso remain sup

ported and balanced during forward

progression; these requirements are

irrelevant during the preceding stage of belly crawling and creeping. Once infants develop sufficient

strength in their arms and legs to

support the torso, they begin to ex

plore the various interlimb patterns available for movement, such as

moving only one limb at a time or

moving both limbs on the same side

of the body at the same time. Fol

lowing a relatively brief opportunity to explore the various interlimb pat terns available to them, infants con

verge on the same diagonal pattern because it provides greater effi

Fig. 4. Mean percentage of time that limbs are diagonally coupled during

crawling. Data are plotted from 2 weeks

prior (-2) to the onset of hands-and

knees crawling through 6 weeks follow

ing ( + 6) the onset of hands-and-knees

crawling.

Published by Cambridge University Press

CURRENT DIRECTIONS IN PSYCHOLOGICAL SCIENCE 145

ciency and stability than any of the

alternatives.

Two points should be empha sized about this developmental shift.

First, the transition to hands-and

knees crawling begins with a period

during which success in producing forward prone progression is quite variable. This experience informs

the infant that the previous interlimb

organization is no longer adequate for the new task. Gradually, the in

fant converges on a new form of in

terlimb patterning representing the

most dynamically stable organiza tion given the task at hand. Second, these developmental changes are

neither prescriptive nor obligatory, but rather represent the natural out

come of a system governed by trying to achieve the most dynamically ef

ficient solution. It is noteworthy that

this sequence of increased variabil

ity followed by greater stability and

greater generalizability is a common

theme that cuts across many differ

ent developmental theories.9

GENERALIZATIONS ABOUT DEVELOPMENT

At a specific level, the findings from this research program provide

compelling evidence that locomotor

experience is functionally related to

the development of a number of new

behavioral forms. At a more general level, these findings lend support to

three important generalizations about the developmental process.

First and foremost, our research

program shows that some of the most significant early experiences are those produced by the infant's own actions. This finding represents a radical departure from the tradi

tional perspective of viewing these

actions as merely products and not

processes of development. More

over, it confirms our contention that

some of the relevant factors contrib

uting to the uniformity of early be

havioral development are indeed ex

periential. An important goal for

future research is to assess the gen

eralizability of our finding to deter

mine whether other forms of self

generated experiences, such as

those produced by sitting or reach

ing, also contribute to the develop mental process.

Second, our findings underscore

the limitations invoked by assuming a linear model of development. Such a model makes the simplistic

assumption that performance con

tinues to improve at a constant rate

as experience increases. Logically, this prediction is problematic be cause most behaviors reach an as

ymptotic level of performance and

then show no further improvement. Moreover, the results from many studies are not consistent with the

expectation that change occurs at a

constant rate. For instance, the de

velopment of a diagonal gait pattern

following the onset of hands-and

knees crawling corresponds to an

abrupt nonlinear change rather than a monotonically increasing func

tion. Our experience is that the ap

plication of linear models, such as

linear regression, to the study of the

effects of experience sometimes

produces misleading or incorrect

results.10

Finally, this research underscores

the importance of conceptualizing

development from a systems per

spective. Developmental change is

rarely attributable to a single cause.

For example, self-produced locomo tor experience contributes to search

for hidden objects, but so do im

proved trunk control, ability to se

quence actions, and increased vi

suai attentiveness.6 It is essential that

investigators conceptualize early de

velopment as responsive to multiple factors that interrelate and subsume

organism-environment coactions.

From this perspective, behavioral

development is multidetermined, re

lational, and emergent.

Acknowledgments?The research de

scribed in this review was supported by National Institutes of Health Grants

HD16195 and HD23144, and also by a grant from the John D. and Catherine T.

MacArthur Foundation. We thank Jean nine Pinto for her helpful comments.

Notes

1. A comprehensive review of these develop mental changes is presented in B.I. Bertenthal, J.J. Campos, and K.C. Barrett, Self-produced locomo tion: An organizer of emotional, cognitive, and so cial development in infancy, in Continuities and Discontinuities in Development, R.N. Emde and R.J. Harmon, Eds. (Plenum, New York, 1984).

2. J.J. Campos, B.I. Bertenthal, and R. Ker

moian, Early experience and emotional develop ment: The emergence of wariness of heights, Psy chological Science, 3, 61-64 (1992).

3. J.J. Campos, Heart rate: A sensitive tool for the study of emotional development, in Develop

mental Psychobiology: The Significance of Infancy, L.P. Lipsitt, Ed. (Erlbaum, Hillsdale, NJ, 1976); L.A. Sroufe and E. Waters, Heart rate as a convergent measure in clinical and developmental research, Merrill-Palmer Quarterly, 23, 3-27 (1977).

4. T. Brandt, W. Bles, F. Arnold, and T.S.

Kapteyn, Height vertigo and human posture, Ad vances in Oto-Rhino-Otolaryngology, 89, 88-92 (1979).

5. For more details, see B.I. Bertenthal and J.J. Campos, A systems approach to the organizing ef fects of self-produced locomotion during infancy, in

Advances in Infancy Research, Vol. 6, C. Rovee Collier and L. Lipsitt, Eds. (Ablex, Norwood, NJ, 1990).

6. R. Kermoian and J.J. Campos, Locomotor ex

perience: A facilitator of spatial cognitive develop ment, Child Development, 59, 908-917 (1988).

7. D.L. Bai and B.I. Bertenthal, Locomotor sta tus and the development of spatial search skills,

Child Development, 63, 215-226 (1992). 8. R.L. Freedland and B.I. Bertenthal, Develop

mental changes in interlimb coordination: Transi tion to hands-and-knees crawling, Psychological Science, 5, 26-32 (1994).

9. G. Edelman, Neural Darwinism (Basic Books, New York, 1987); E. Thelen, Evolving and

dissolving synergies in the development of leg co

ordination, in Perspectives on the Coordination of Movement, S. Wallace, Ed. (Elsevier, Amsterdam,

1989). 10. B.I. Bertenthal and J.J. Campos, A reexam

inaron of fear and its determinants on the visual

cliff, Psychophysiology,21, 413-417 (1984).

Copyright ? 1994 American Psychological Society