Developmental Science 4:4 (2001), pp 463± 471
REPORT
Going around transparent and grid-like barriers: detour ability
as a perception ± action skill
Jeffrey J. Lockman1 and Christina D. Adams2
1. Tulane University, USA
2. West Virginia University, USA
Abstract
Early detour ability may not generalize immediately across similar problems in different perception±action systems, but
instead may reveal a pattern of developmental onset that is more domain-specific. To investigate this possibility, we
examined how 10-month-old (n = 24) and 12-month-old (n = 24) infants performed detours via different action modes
and around barriers that differed in transparency. Infants made reaching and locomotor detours to retrieve an object
located behind either an upright transparent barrier or an upright transparent barrier overlaid with a grid pattern. The
results indicated that infants were more likely to make reaching than locomotor detours and explored the transparent
and grid barriers differently. Additionally, younger infants more often attempted to contact the object through the
entirely transparent barrier than did older infants, especially when making a reaching detour. The results suggest that
during detour development, infants learn to coordinate relevant perceptual information with emerging actions.
As infants gain increasing control of their limbs, they
become more and more able to transform relations
between themselves and their environments. For instance, through reaching, infants displace their arms to
obtain objects. Likewise, through locomotion, infants
displace their entire bodies to gain desired goals.
Objects or goals cannot always be attained directly,
however. Everyday environments are cluttered. Barriers
or obstacles often prevent direct paths to goals,
necessitating some type of detour. Clearly, the ability
to make detours is adaptive, but the development of this
ability has been conceptualized in different ways.
One such way was offered by Piaget (1954) as part of
his constructivist account of sensorimotor development.
According to Piaget, detour knowledge is an achievement which represents a new way of thinking about
spatial relations in the world. More specifically, Piaget
contended that during the fifth stage of sensorimotor
development, infants begin to recognize that they can
get back and forth between locations by using routes
other than direct ones. Even so, Stage 5 infants only
perform detours by relying on direct perception or by
following an object's disappearance path (Piaget, 1954).
With the onset of representational functioning in Stage
6, these limitations are overcome. Infants are now able
to plan and navigate indirect routes that go beyond
direct perception and=or do not merely duplicate an
object's prior disappearance trajectory.
Although Piaget based his proposals on observations
of his own children and just their locomotor behaviors,
Bruner (1970) offered an alternative account, based on
observations of manual skill. Following Bernstein
(1967), Bruner suggested that, like other motor skills,
detour reaching involves reducing the many degrees of
freedom involved in the various subroutines or components that compose this motor act. As these subroutines
become more automatic, infants begin to assemble them
in a more flexible, goal-directed manner. Bruner's (1970)
empirical observations suggest that by the end of the
first year infants are generally capable of awkward
detour reaches.
Diamond also has focused on developmental differences in infant detour reaching strategies. Diamond
(1990, 1991a, 1991b) has suggested that the development
of detour reaching is linked to advances in neuromotor
and cognitive development. Specifically, maturational
Address for correspondence: Jeffrey J. Lockman, Department of Psychology, Tulane University, New Orleans, LA 70118, USA; e-mail:
[email protected]
# Blackwell Publishers Ltd. 2001, 108 Cowley Road, Oxford OX4 1JF, UK and 350 Main Street, Malden, MA 02148, USA.
464
Jeffrey J. Lockman and Christina D. Adams
changes in the dorsolateral prefrontal cortex enable
infants to inhibit direct reaches and plan indirect ones,
requiring the ability to relate information over spatial
and=or temporal separations. Diamond's (1990, 1991a,
1991b) research indicates that infants begin to use
detour reaches to retrieve objects placed in a transparent
box with an opening to one side in the latter part of the
first year.
One limitation with the preceding diverse treatments
of detour behavior is that particular efforts have been
based largely on observations of one form of motor
behavior (reaching or locomotion) in conjunction with
presentation of only certain types of tasks or obstacles
(e.g. opaque or transparent barriers). Other work on
detour behavior and perception ± action development,
however, suggests a more unified developmental account
of detour behavior, based on a joint consideration of
motor, task and developmental factors. The underpinnings of this new approach can be found in the
perception ± action literature, in which perception and
action are viewed as reciprocal and complementary,
functioning in an integrated manner in a given perception ±action system (Reed, 1982; Gibson, 1988; Thelen
Smith, 1994; Goldfield, 1995; Bertenthal & Clifton,
1998). From a developmental standpoint, this view
suggests that, as new motor systems come on-line, young
children may need to discover through active exploration how a particular perception ±action system functions, including the relevant information that can be
used to plan or guide behavior (Gibson, 1988; Adolph,
Eppler & Gibson, 1993a; Gibson & Pick, 2000). Applied
here, a more unified account of detour development
requires consideration of how young children learn
about detours in particular perception ±action systems
and, in a reciprocal vein, how the properties of
particular perception ± action systems influence detour
behavior within that system.
Some previous work on detour ability is consistent
with this framework. In a longitudinal study, Lockman
(1984) found that infants made reaching detours before
corresponding locomotor ones and were less likely to
make detours around transparent than opaque barriers
(see also Diamond, 1990, 1991a, 1991b). A follow-up
longitudinal study indicated that the reaching ± locomotor difference was unlikely to be due to the different
size barriers that were used in the two tasks. In short, the
absence of immediate generalization from reaching to
locomotion, even though infants possess the requisite
motor responses, suggests that infants may need to learn
and explore the properties of perception± action systems
with reference to particular detour tasks. Conceptually
related results have been reported recently by Adolph
(1997, 2000) who found that infants' knowledge about
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affordances associated with surfaces did not generalize
immediately from crawling to walking or from sitting to
crawling.
A perception ±action framework for understanding
early detour ability also requires consideration of how
infants use information in real time while executing a
detour. To determine that direct access to a goal is
blocked, infants as young as 9 months rely on visual
(Bruner, 1970; Lockman, 1984; Diamond, 1990, 1991a,
1991b) as well as auditory information (Rieser, Doxsey,
McCarrell & Brooks, 1982). Additionally, while navigating a detour, infants need to steer clear of the
obstacles that prevent direct access to the goal. For this
purpose, patterns of optical flow information may be
especially relevant. Schmuckler and Gibson (1989)
found that, under conditions of imposed optical flow
associated with a moving room (see Lee & Lishman,
1975), toddlers were more likely to lose their balance
while walking in a cluttered than a non-cluttered room.
These results suggest that, while walking, toddlers are
just beginning to coordinate optical information for
steering around obstacles with that for maintaining
balance.
The major aim of the present study was to provide
further support for a perception± action framework for
understanding early detour behavior. One goal was to
provide a more detailed account of the exploratory
behaviors that infants use when presented with barriers
blocking direct access to a goal. In other studies of
affordance learning, infants' exploratory attempts with
objects or surfaces have provided insights into how they
discover object affordances for manual action (Ruff,
1984; Lockman & McHale, 1989; Palmer, 1989a) or
surface affordances for locomotion (Gibson et al., 1987;
Adolph, Eppler & Gibson, 1993b).
Another goal was to extend the range of barriers that
have been investigated in prior investigations of detour
behavior. In the present study, transparent and grid-like
barriers that nevertheless were largely transparent were
employed. These types of barriers were used in part to
help understand a puzzling finding in the detour
literature. As noted, infants experience difficulty with
transparent barriers relative to opaque ones (Lockman,
1984; Diamond, 1990, 1991a, 1991b) and may underestimate a transparent barrier's spatial extent when
attempting to go over it (Schmuckler, 1996). If infants
explore the transparent and grid-like barriers differently
even though the goal object is visible behind the grid
barrier, this would suggest that infants are registering
something about the barrier's overall surface properties
rather than a property of local transparency. The socalled visual capture phenomenon that has been found
previously with transparent barriers may not be simply a
Detour ability 465
function of being able to see the object through the
barrier, but rather of the barrier's overall surface
characteristics.
Finally, infants were tested on both reaching and
locomotor versions of these barrier tasks. As noted, in
past work with opaque and transparent barriers, infants
have been shown initially to be more successful making
a reaching detour than a corresponding locomotor one
(Lockman, 1984). If a similar reaching± locomotor
difference were also found with other types of barriers,
this would provide further support for the idea that
detour development and possibly other related abilities
should be conceptualized as developing at least partially
within particular perception± action systems (Reed,
1982; Adolph, 1997; Gibson & Pick, 2000).
Method
Participants
The participants included 48 infants, 24 at 10 months
(M = 10.2 months, range 9.67 ± 10.93 months) and 24 at
12 months (M = 12.36 months, range 11.4 ± 13.03
months). Seven additional infants were eliminated from
the study, either because they became upset (n = 4) or
because of experimental error (n = 3). Infants were
primarily from middle-class European-American families. All infants were able to reach and locomote.
Families were located through birth rosters of area
hospitals, records from a child-birth class and a
preschool waiting list.
Design
The study was organized as a 2 (age) 2 (barrier:
transparent or grid) 2 (action mode) 3 (trial) design,
with repeated measures on the last two factors. Twelve
infants at each age level had to reach and locomote
around either a completely transparent barrier or a
transparent barrier with a grid pattern on the surface.
An infant was given three trials each of the reaching and
corresponding locomotor task for a total of six trials.
Within each barrier condition, half of the infants at each
age level were presented the reaching version of the task
first.
Apparatus
Reaching detour
Infants were seated at a rectangular wooden table
(42.3 cm 23 cm) in a booster chair whose seat was
# Blackwell Publishers Ltd. 2001
10 cm from the floor. An upright, Plexiglas barrier was
placed, facing the infant, in a slot 3.2 cm away from the
edge of the table nearest the infant. Infants were
positioned so that the center of the barrier was aligned
with the midline of their bodies. Because the barrier
(30.5 cm 14 cm) was too high for a seated infant to
reach over, infants had to reach around the barrier to
obtain an object located behind it. The surface of the
barrier was either completely transparent or transparent
with a grid pattern. The grid pattern was constructed
with black tape (7 mm wide), forming 2.5 cm 2.5 cm
squares of transparency.
Locomotor detour
Infants were seated on the floor facing the barrier
(approximately 45 cm from the barrier), with the midline of the their bodies aligned with the barrier's center.
The two Plexiglas barriers (69.5 cm 74 cm) had surface patterns that corresponded to those of the reaching
barriers.
Procedure
To initiate a trial, the experimenter, who was seated
behind the infant, gave the infant an object. Objects used
included keys, a rattle or the infant's own small toy.
Once the infant displayed interest in the object, the
experimenter took the object from the child and moved
it up and over the center of the barrier. Having placed
the object on the other side of the barrier, the
experimenter brought her arm back along the same
path. By reversing the object's disappearance trajectory,
the experimenter did not provide the infant with
information about how to make the detour. The infant
was given approximately 1 min to begin to retrieve the
object by reaching or locomoting around the barrier. If
the infant failed to do so within that time, the
experimenter retrieved the object by reversing the
object's initial path over the barrier. The infant was
then allowed a small amount of time to play with the
object before the experimenter began the next trial. This
procedure was repeated so that three consecutive trials
were presented in each action mode. During all testing,
the caregiver was seated near the child.
All trials were videotaped with a camera, concealed
mostly by curtains, located in the corner of the room
facing the child. From the videotapes, successful
contact, latency to contact and exploratory behaviors
of the infants were coded. Success was defined as
retrieving or contacting the object behind the barrier.
Exploratory behaviors included touching the barrier
directly in front of the object, banging the barrier and
466
Jeffrey J. Lockman and Christina D. Adams
shaking the barrier. Latency to contact the object was
defined as the number of seconds from the time the
experimenter had brought her hand back after placement of the object behind the barrier until the infant
retrieved the object or until a maximum of 60 s had
elapsed. Inter-observer reliability was established for
nine subjects. Reliability was calculated by dividing the
number of agreements by the total number of agreements and disagreements for each dependent measure.
Reliabilities ranged from 0.92 to 1.00 for success and
exploratory behaviors. For latency to contact, the
Pearson r was 0.99.
Results
Preliminary analyses indicated no effects associated with
order. Order was therefore not included as a factor in
the following analyses. Analyses of success and latency
are presented first and specific exploratory behaviors
subsequently.
Success
For each of the six trials, infants were given a score of 1
each time they retrieved an object from behind the
barrier. Analysis of success scores in a 2 (age) 2
(barrier) 2 (action mode) 3 (trial) analysis of
variance (ANOVA) yielded significant main effects
associated with action mode (F(1, 44) = 15.36, p < 0.01)
and trial (F(2, 88) = 11.52, p < 0.01), but these effects
were qualified by a significant action mode trial
interaction (F(2, 88) = 3.11, p < 0.05). Inspection of
the means involved in this interaction indicated that,
whereas infants performed at a uniformly high level
across trials on the reaching detour tasks, performance
was initially low but improved somewhat on the
locomotor detour tasks (see Figure 1). Newman ± Keuls
analyses (p < 0.05) indicated that within the reaching
task there was no difference in rates of success across
trials, but within the locomotor task infants succeeded
more often on trials 2 and 3 relative to trial 1.
Nevertheless, within each corresponding trial in the
sequence, infants succeeded more often on the reaching
task than the locomotor one (p < 0.05). In short,
although success rates improved across trials on the
locomotor detour task, infants still succeeded more
often on a reaching detour trial relative to the
corresponding locomotor one.
Besides these significant findings, there were trends
linked to age and barrier. Younger infants tended to
succeed less often than older infants (M = 0.64,
0.81 for 10- and 12-month-olds respectively,
# Blackwell Publishers Ltd. 2001
Figure 1 Mean frequency of success as a function of action
mode and trial number.
F(1, 44) = 3.79, p < 0.06) and infants tended to retrieve
the object more often around the grid than the
transparent barrier (M = 0.81, 0.64 respectively,
F(1, 44) = 3.79, p < 0.06).
To explore further the relation between success on the
reaching and corresponding locomotor versions of the
tasks, we considered the degree to which individual
infants performed at relatively similar levels across
reaching and locomotor conditions. Correlations based
on success scores computed at each age level indicated
some relation at 10 months (r = 0.47, p < 0.05) but none
at 12 months (r = 0.16, ns).
Latency
Latency means were analyzed in a 2 (age) 2
(barrier) 2 (action mode) 3 (trial) ANOVA. This
analysis yielded main effects associated with age,
barrier, action mode and trial. Older infants required
less time (M = 20.87 s) on the tasks than did younger
infants (M = 30.28 s) (F(1, 44) = 4.83, p < 0.05). Importantly, infants required less time to retrieve an
object located behind a grid barrier (M = 20.64 s) than
a transparent one (M = 30.51 s) (F(1, 44) = 5.32,
p < 0.05). Not surprisingly, infants retrieved the object
more quickly when reaching (M = 16.49 s) than when
locomoting (M = 34.67 s) (F(1, 44) = 36.93, p < 0.01).
Finally, infants became quicker over trials
(F(2, 88) = 26.92, p < 0.01). For this last effect, Newman ± Keuls tests indicated that infants were significantly slower on trial 1 (M = 32.41 s) relative to trial 2
(M = 23.5 s) or trial 3 (M = 20.82 s). None of the
interactions was significant.
Detour ability 467
Exploratory behaviors
Infants' exploratory attempts with the barriers may
provide insights into how infants register the barriers'
affordances and the types of affordances that they detect.
Specific exploratory behaviors across reaching and
locomotor tasks were examined in separate 2 (age) 2
(barrier) 2 (action mode) 3 (trial) ANOVAs.
Attempts to touch object through the barrier
Infants' attempts to touch the object through the barrier
decreased over trials (F(2, 88) = 12.84, p < 0.01), with
barrier contact decreasing significantly from the first to
last trial (M = 1.89, 1.39, 0.83 for trials 1, 2 and 3
respectively, Newman ±Keuls tests, p < 0.05). Additionally, a three-way interaction involving age, surface and
action mode (F(1, 44) = 5.18, p < 0.05) qualified main
effects on each of these factors (F(1, 44) = 4.78,
p < 0.05). The three-way interaction is depicted in
Figure 2. Tests of simple effects conducted in each
barrier condition revealed that the two-way interaction
involving age and action mode was significant in
the transparent barrier condition (F(1, 22) = 12.17,
p < 0.01), but only the age effect was significant in the
grid condition (F(1, 22) = 4.31, p < 0.05).
Considering the transparent barrier condition first,
the two-way interaction was due largely to the younger
infants' repeated attempts to contact the object through
the barrier in the reaching task. Newman ± Keuls tests
indicated that, across trials, the younger infants tried to
contact the object through the barrier more (M = 11.42)
than did the older infants in the reaching task
(M = 3.25) or either age group in the locomotor task
(M = 4.67, 5.67 for the 10- and 12-month-olds respectively). (Means are summed across trials.) No other
differences in the interaction were significant.
In the grid barrier condition, the significant age effect
indicated that across trials younger infants attempted to
touch the object through the barrier more (M = 2.88)
than did older infants (M = 1.0). Despite the significant
age effect, it is important to note that attempts to touch
the object through the grid barrier even at 10 months
were relatively infrequent (see Figure 2(b)).
As implied by the above analyses, direct comparisons
across barrier tasks within each action mode support the
idea that infants explored the corresponding transparent
and grid barriers differently. For the reaching condition,
a 2 (age) 2 (surface) ANOVA yielded main effects of
age (F(1, 44) = 10.65, p < 0.01) and surface (F(1, 44) =
8.17, p < 0.01). Across trials, younger infants attempted
to contact the object through the barrier more
(M = 7.67) than did older infants (M = 2.29) and
# Blackwell Publishers Ltd. 2001
Figure 2 Number of attempts to contact object through the
barrier as a function of age, action mode and barrier surface: (a)
transparent barrier; (b) grid barrier.
infants attempted to touch the object through the
transparent barrier (M = 7.33) more than through the
grid one (M = 2.62). In the locomotor condition,
the only effect that was significant in the 2 (age) 2
(surface) ANOVA was surface (F(1, 44) = 7.99,
p < 0.01). During the locomotor task, infants attempted
to contact the object more in the transparent barrier
condition (M = 5.17) than in the grid one (M = 1.25).
Thus in both the reaching and locomotor tasks, infants
attempted to contact the object more by touching the
transparent than the grid barrier.
Banging=pushing
On a given trial, younger infants banged the barrier
more (M = 1.743) than did older infants (M = 0.556)
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Jeffrey J. Lockman and Christina D. Adams
(F(1, 44) = 9.43, p < 0.05). Additionally, infants were
more likely to bang the barrier when locomoting
(M = 1.736) than when reaching (M = 0.563),
(F(1, 44) = 7.58, p < 0.05). In the locomotor condition,
infants appeared to be trying to bang or push the barrier
out of the way.
Shaking
Shaking the barrier was a relatively low frequency
behavior, but nevertheless varied according to the
barrier's surface characteristics. On a given trial, infants
were more likely to shake the transparent barrier
(M = 0.27) than the grid one (M = 0.04), (F(1, 44) =
5.68, p < 0.05).
Relation between successful reaching detours and
attempts to touch the object through the barrier
Finally, to determine whether successful reaching
detours supplant or follow attempts to contact the
object through the barrier on individual trials, we
examined the relation between the two patterns of
responding in the reaching condition (see Table 1). At
each age level on the first trial in the series with each
barrier surface, roughly equal numbers of infants
evidenced both response patterns: first attempting to
contact the object directly and then making a detour or
making a detour without first attempting to contact the
object directly. By the third trial, however, different
patterns emerged for the grid and transparent barriers.
This difference in response patterns to the grid and
transparent barriers was marginally significant at 10
months (Fisher's exact test, p < 0.07) and significant at
12 months (Fisher's exact test, p < 0.05). Specifically
with the grid barrier, more infants at each age level made
Table 1 Reaching strategy as a function of age, trial and barrier
for the reaching barrier task
Trial number
Trial 1
Trial 3
Reach
through,
then
around
Reach
around
only
Reach
through,
then
around
Reach
around
only
(a) 10-month-olds
Grid
Transparent
7
5
4
2
3
7
7
2
(b) 12-month-olds
Grid
Transparent
5
5
5
5
2
8
10
4
Barrier
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a detour without first attempting to contact the object
directly. In contrast, with the transparent barrier, more
infants at each age level attempted to contact the object
directly through the barrier before making a successful
detour reach. These results suggest that the likelihood
of infants displaying both direct and detour reaching
strategies in the reaching condition varies as a function
of barrier surface and trial: infants late in the first year
display both strategies on initial trials with a grid barrier
and over a series of trials with a transparent barrier.1
Discussion
Taken together, the present results indicate that detour
ability undergoes an extended period of development
near the end of the first year. Infants are more likely to
make reaching detours than corresponding locomotor
ones and they exhibit different patterns of exploration
with transparent barriers and transparent barriers overlaid with a grid pattern. What significance do these
findings hold for theories that have been used to explain
the early development of detour ability?
The present findings argue most directly against
cognitive-structural accounts of spatial development, in
general, and detour ability, in particular. In his theory of
infant spatial cognition, Piaget (1954) contended that
detour problems that share the same underlying
structure or formal properties are solved by infants at
the same point in development. In the present work,
both the reaching and locomotor versions of the task
can be described as sharing the same underlying spatial
structure, yet infants were more likely to reach than
locomote around similar barriers.
By the same token, the current results indicate that the
entirely transparent barriers posed greater difficulty for
1
We also examined the relation between attempting to contact the
object through the barrier and successful detours in the locomotor
condition. Because infants succeeded on the locomotor trials less often,
cell frequencies were necessarily reduced. For the 10-month-old group
in the grid barrier condition, on trial 1, one infant first attempted to
contact the object directly before locomoting around the barrier
whereas five infants made a locomotor detour without first attempting
to contact the object directly. In contrast, this pattern was reversed for
the 10-month-old group on trial 1 in the transparent barrier condition:
four infants first attempted to contact the object directly before
locomoting around the barrier, whereas none made a locomotor
detour without first attempting to contact the object (p < 0.05, Fisher's
exact test). By trial 3, the difference in response patterns to the grid and
transparent barriers was not significant. Likewise, at 12 months, on
both trials 1 and 3, the difference in response patterns to the grid and
transparent barriers was not significant. Although the frequencies are
small, the findings suggest that direct attempts to contact the object
and locomotor detour strategies vary as a function of barrier surface
and trial at 10 months.
Detour ability 469
infants than did grid barriers, despite the same underlying spatial structure of the detour tasks. Infants took
a longer amount of time to solve or attempt to solve
the detour problem involving the transparent barrier
(especially in the reaching condition), displaying greater
amounts of manual exploration with the transparent
barrier than with the corresponding grid one. Particularly noteworthy, infants more often attempted direct
contact of the object through the transparent barrier,
especially in the reaching version of the task. With
infants slightly younger than those tested in the present
work, differences in success rates and performance
across the transparent and grid barriers may be even
more pronounced. Viewed together, the present results
offer little support for predictions associated with a
cognitive-structural account of early spatial development, like that of Piaget (1954).
The present findings are also relevant for accounts of
detour development in which successful performance is
linked to inhibition of prepotent direct reaching
responses and the integration of information over
separations in space (Diamond, 1990, 1991a, 1991b).
The findings reported here indicate, along with
previous ones (Lockman, 1984), that the likelihood of
inhibiting a direct motor response in detour situations
is not necessarily equivalent across action modes. In the
current study, success rates varied systematically within
individuals, based on whether infants were required to
reach or locomote around either transparent or grid
barriers. It may be that the integration of spatial
information across larger separations (see Diamond,
1990, 1991a, 1991b) contributed to infants' relative
difficulties in the locomotor version of the detour task
used here. However, prior work indicates that, within
the locomotor mode, barrier width does not appreciably influence when infants initially make detours
(Lockman, 1984). In short, relating information across
increasingly larger spatial separations may not be the
only factor that underlies the differential motor results
reported here.
Additionally, for infants late in the first year, shortterm experience appears also to influence the likelihood
of whether infants make a detour (see also Diamond,
1991b). In the present study, success in the locomotor
condition and efficient performance on the reaching and
locomotor tasks improved across trials. Further, the
results regarding the relation between direct and detour
reaching strategies in the reaching task indicate that
infants inhibit direct reaching responses over the course
of only several trials, at least in the grid barrier
condition. Taken together, the present findings suggest
that the relative strengths of direct and detour retrieval
strategies appear to vary jointly as a function of action
# Blackwell Publishers Ltd. 2001
mode, the barrier's surface properties and infants'
experience within a trial block.
While Diamond's (1990, 1991a, 1991b) proposals
about maturation of the dorsolateral prefrontal cortex
and cognitive development might be used to account for
some of the present findings, we suggest that the results
are also compatible with a perception ± action interpretation of early detour behavior. On this view, detour
behavior is the result of infants' efforts to relate their
actions to the environment. More specifically, in the case
of barriers preventing direct access to a goal, infants
must discover the information that indicates that a
direct route is blocked as well as how to execute a detour
in a given action mode.
In this connection, consider first the matter of
discovering the information that indicates that a direct
route is blocked. Barriers vary in terms of their surface
characteristics, especially their optical qualities. As an
infant directly approaches an opaque barrier through
movements of the head, trunk or legs, the edges of the
barrier progressively occlude more and more of the
background scene, indicating that direct access through
the barrier is blocked (Gibson, 1979; Adolph et al.,
1993b). In contrast, with apertures and potential
passageways, the background expands as the aperture
is approached, indicating that direct access through the
aperture to the background scene is possible (Gibson,
1979; Palmer, 1989b; Schmuckler & Li, 1998).
Applied here, the analysis suggests that, in the case of
our entirely transparent barriers, infants were obtaining
visual information that was more consistent with the
presence of an aperture and this led to less immediate
detour solutions and increased instances of haptic
exploration of the barrier. In contrast, in the case of
the transparent barrier overlaid with a grid, the addition
of this pattern, even though relatively minor in overall
area, conveyed important information about the nature
of the surface in front of them and what it afforded for
action in terms of access to the object and the
background scene. Alternatively, the transparent portions of the grid surface may also have indicated the
presence of apertures, but ones too small to reach
through. Either way, even though the goal object was
visible to infants through both surfaces, the differential
transparent and grid barrier results suggest that infants
are registering the affordance of the surface and are not
just visually captured by the object behind the barrier.
Further, infants appeared to benefit and learn from
their exploration of the barriers. Short-term experience
resulted in certain forms of exploratory behavior
decreasing over trials and an overall reduction in the
latency to solve these detour problems. Additionally, on
many individual reaching trials in the current study,
470
Jeffrey J. Lockman and Christina D. Adams
infants displayed variability in their response strategies:
they attempted to reach through the barrier and they
detoured around it as well (see also Diamond, 1990,
1991a, 1991b). Such within-subject variability, in which
individual children employ multiple strategies over a
short period of time, may indicate that children are
exploring or actively contrasting potential approaches
or solutions to problems (Thelen & Smith, 1994; Siegler,
1996). Viewed from this perspective, the present findings
cast infants' unsuccessful attempts to reach directly
through a transparent or partially transparent barrier in
a more positive light, and not just as a failure of
inhibition. More broadly, the results suggest that near
the end of the first year infants explore different types of
surfaces and, through these exploratory efforts, learn to
differentiate information that indicates the presence of
an obstacle and the need for a detour.
Consider next the issue of executing a detour, given
that the obstacle and its meaning in terms of lack of
direct access have been detected. The present findings
indicate that making detours in one action mode
(reaching) does not necessarily guarantee that a detour
will be made in another action mode (locomotion),
although the younger but not older age group displayed
a modest relation in relative success rates across the two
action modes. Viewed as a whole, these results are
consistent with those of Lockman (1984) who reported a
reaching ± locomotor difference in longitudinal work on
detour ability with opaque and transparent barriers. The
present findings thus extend the range of barrier surfaces
for which a reaching ±locomotor detour difference has
been found. Because reaching is a more practised skill
relative to locomotion, infants may be able to use
relevant perceptual information to guide, control and
execute reaching detours before and=or more easily than
corresponding locomotor ones. Further, the types of
information that individuals must differentiate while
making a locomotor detour may be more complex than
when making a reaching detour, due to the demands of
coordinating the limbs, maintaining postural stability
and steering clear of an obstacle (see Schmuckler &
Gibson, 1989).
The reaching ± locomotor difference found in work on
detour ability is also conceptually related to recent
findings concerning infants' perception of slopes'
affordances. Infants' knowledge about whether a slope
is risky or safe does not immediately transfer from a
practised (crawling) to a new (walking) locomotor skill
(Adolph et al., 1993b; Adolph,1997). Viewed together,
these and the current detour findings suggest that a
central representation may not initially underlie formally similar behaviors that are nevertheless realized
through different action modes. As new modes of action
# Blackwell Publishers Ltd. 2001
emerge in infancy, the coupling of perception with these
actions may not occur immediately, but instead may
depend on infants' active experiences that result in
discovering and exploring the reciprocal relation between the two (Bertenthal, 1996; Gibson & Pick, 2000).
With reference to detour ability, the present results
suggest that infants are still engaging in this dynamic
coupling process near the end of the first year.
Acknowledgements
We wish to thank the families who participated in this
study.
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Received: 8 October 1998
Accepted: 31 January 2001