Weight Distribution on Tool Use Training 1
Running head: WEIGHT DISTRIBUTION ON TOOL USE TRAINING
The Effect of Previous Experience and Weight Distribution on Tool Use Training in Infancy
Ariel Borten
Thesis completed in partial fulfillment of the requirements of the Honors Program in
Psychological Sciences under the direction of Prof. Amy Needham
Vanderbilt University
April, 2011
Weight Distribution on Tool Use Training 2
Abstract
Many different factors play a role in the development of an infant’s ability to use tools. A
previous version of the current study examined active versus observational learning on an
infant’s ability to be trained to use a novel tool to succeed in a test task. The results showed that
both types of learning appeared to be equally effective. This follow-up study added a second
factor to examine the effects of training when there is a characteristic about the tool that cannot
be determined through visual observation. This time the novel tool had an unexpected weight
distribution making the straight handle of the tool much heavier than the round handle. This
study showed that the uneven weight distribution of the tool actually resulted in infants having a
higher overall success rates during test than in the previous version of the study. This suggests
that the uneven weight distribution may have focused infant’s attention on how they were
holding the tool therefore increasing their ability to success on the test task.
Weight Distribution on Tool Use Training 3
Introduction
The ability to use and manipulate tools is an important skill that develops throughout
infancy. This area of research encompasses a number of interesting developmental domains
including cognition, perception and action (Creem & Proffit, 2001 in Barrett, Davis, &
Needham, 2007). Tool use is defined as, “a purposeful, goal-directed form of complex object
manipulation that involves the manipulation of the tool to change the position, condition, or
action of another object” (Connolly and Dalgleish, 1989). This study will examine the influence
of two factors, weight distribution and active versus observation learning, on the development of
tool use in infancy. These two elements have never been studied together before and should
therefore produce new insights into the role of previous experience on tool learning
development. There are studies, on the effects of these individual factors on tool use, which
were considered when planning this study.
The apparent physical characteristics of a tool, such as weight, balance, shape or handle
design, may cause an observer to initiate a certain grasp when reaching for the object (Baber,
2003; McCarty, Clifton & Collard, 1999, 2001). There is evidence that infants are able to plan
their grasp based on the noticeable weight distribution of an object. During the first year of life,
around eleven to thirteen months, infants are able to judge the center of mass of an object, based
on its shape, in order to plan their grasp placement. Barrett and Needham (2008) found that
when infants reached for a symmetrical or an asymmetrical object, they would grasp near the
center of mass for the particular shape of that object. Therefore infants reaching for the
asymmetrical object will grasp further from the center of the shape and closer to where the center
of mass is located. Although all infants planned their grasps based on the difference in shape,
older infants were more successful at this than younger infants. The younger infants often had to
Weight Distribution on Tool Use Training 4
try alternate strategies, such as adjusting their hand placement after their initial grasp, in order to
grasp the center of mass. This shows that infants are able to use visual information about the
physical properties of an object in order to decide an appropriate response, and that this ability
improves with age (Barrett & Needham, 2008). One limitation in this area of research is how an
unobservable physical characteristic of an object would affect infants’ grasps. The current study
will address this limitation by examining how infants react when the tool’s actual weight
distribution does not match the expectation associated with its physical shape.
Another factor that may affect the way infants learn to use tools is their previous
experience with that object. There is evidence to suggest that an infant’s prior experiences with
a tool will affect how they use the tool in the future (Barrett et al., 2007; Davis, 2011). To test
this, Barrett, Davis and Needham (2007) examined an infant’s ability to use a tool they are
familiar with, a spoon, to complete a different goal. This new task required infants to use the
tool in a novel way, grasping the spoon by its bowl rather than its straight handle. To succeed in
the light box test task infants had to grasp the spoon’s bowl and insert the straight handle into a
hole in the side of a box to activate the light display inside. Since the hole was too small to fit
the bowl end of the spoon, infants could only succeed at the task if they used the tool in the new
manner. Interestingly, infants between twelve and fourteen months were unable to use the spoon
in this novel way and did not succeed in the task. However, a second group of infants this age
were introduced to a novel tool (Figure 1), with similar physical properties to a spoon, but with
which they had no experience. This time infants were able to grasp the tool by its round handle
and succeed at the task by inserting the straight handle into the hole in the light box.
This provides evidence that tool use skills are greatly affected by previous experience.
By twelve to fourteen months, infants have had both observational and active experiences using
Weight Distribution on Tool Use Training 5
a spoon (Connolly & Dalgleish, 1989). They have been exposed to grasping the spoon by its
straight handle in order to achieve the goal of eating. When the goal for the tool is changed, the
infants have a hard time changing the way they grasp the tool. However, when given a tool with
similar physical properties as a spoon, infants are able to grasp the tool in the correct way in
order to complete the new task. This shows that it is not the physical properties of a tool alone
that dictate how the infants will grasp and use the tool, but also their previous experiences with
the tool.
In order to test this effect further, Barrett et al. (2007) examined whether or not shortterm training experience would impact the way infants use a tool. The researchers wanted to see
if training experience with the novel tool could produce the bias in grasping observed with the
spoon. Infants were placed in one of three conditions and trained to regard either the round end
of the tool as the handle, the straight end of the tool as the handle, or were given training on both
ends of the tool on alternating days. After two weeks of training at home with the tool, the
infants were tested with the same light box task. The results indicated that there was an effect of
training on success at solving the light box. Those infants trained to regard the round end of the
novel tool as the handle, were more likely to succeed during test than those in the control since
the light box task required the infant to grasp the tool by the round end. They did not find a
difference between the straight handle training condition and control most likely because both
groups had such a low success rate. These results are consistent with the previous study on spoon
use: prior experience with a tool, even short-term, affects how infants grasp the tool later on.
This study also provides evidence that infants acquire grasp specific information from the
training with tools rather than functional information. Grasp specific information is how and
where to grasp a tool, whereas functional information is the different ways a tool can be used.
Weight Distribution on Tool Use Training 6
For example with a spoon, grasp specific information would be to grasp the straight handle,
whereas functional information would be how to use it to stir or activating the light box.
Previous research has shown that the brain is wired to automatically retrieve information about
how to grasp a tool (Tucker & Ellis, 1998, 2001). The low success rate of the infants in the
straight handle training condition can be explained by infants learning grasp specific information
during training. When they got to the test they retrieved that grasp specific information, grasp
the tool by the straight handle, and therefore could not use the tool for the correct function during
test. This evidence shows that infants around one year of age encode grasp specific information
before functional information about a tool.
Due to the effect of previous experience on tool use development, researchers have
examined whether or not the type of prior experience, active or observational, is important.
Gibson believed that active learning was essential in tool use development because interacting
with a tool provides feedback (Gibson, 1996 in Baber, 2003). This theory has support from
studies showing that the feedback an infant receives while grasping an object will changes his or
her subsequent grasps on the object (Barrett & Needham, 2008; Karmiloff-Smith & Inhelder,
1975). There is also evidence that active learning is beneficial over observational learning in the
perception of tool use events in infancy. Sommerville, Hildebrand and Crane (2008) found that
only active training with a tool, not observational, resulted in an increase in the infant’s ability to
identify the goal of a tool use event. Active training helped improve the infant’s perceptual
understanding of the purpose intended for the tool. Being able to perceptually understand how to
use a tool plays a major role in being able to use the tool correctly (Baber, 2003). Therefore
active learning may be beneficial over observational learning in increasing young infants
understanding of tools.
Weight Distribution on Tool Use Training 7
However, there is conflicting evidence to show that active learning is no more beneficial
than observational learning on tool use ability. Davis (2011) looked at the effect of hands-on
versus observational learning on tool use. To test this Davis, designed the following study. One
group of infants received hands-on training tasks with the novel tool and the other group
observed an experimenter using the novel tool. Then both groups participated in the light box
test task. The results indicated that there was no significant effect of type of training, hands-on
or observational, on the infant’s ability to produce the desired behavior during test. Infants in the
observational condition did not have significantly different success rates at solving the light box
than infants in the hands-on condition. This evidence conflicts with Piaget’s (1955) framework.
He believed that infants, during the first two years of life (the sensorimotor period) need active
learning in order to understand how objects relate to one another. The results of this study
showed that infants were able to learn how to use the tool equally effectively from observing
someone use the tool as having hands-on experience with the tool.
Because Davis (2011) did not find a significant difference between active or
observational learning, the goal of this current study is to examine whether or not a difference
could be produced. In order to examine this question, the novel tool has an unexpected physical
characteristic that could not be determined through visual observation. By making the straight
end of the novel tool metal and the round end from light wood, the tool has an unexpected
weight distribution.
The straight handle is much heavier than the round handle, but this cannot
be seen when visually observing the tool. This study aims to fill in gaps in previous research by
examining the effects of two factors, type of previous experience, active or observational, and an
unexpected weight distribution, on tool use learning in infancy.
Weight Distribution on Tool Use Training 8
Using this heavy tool, infants will be trained as in previous studies to regard one end of
the tool, round or straight, as the handle. Half of the infants in each of the groups will receive
only observational experience; watching the experimenter complete the training tasks. The other
half in each group will have hands-on experience using the tool to attempt the training tasks.
This means that only infants in the active experience group will be aware of the uneven weight
of the tool. All infants will be tested with the light box task. We hypothesize to replicate the
effect of handle preference training, straight or round, on success; infants who receive round
handle training being more likely to succeed during test. In addition, we hypothesize that infants
in the active training condition, who have experience with the unexpected weight difference, will
demonstrate greater effects of handle training during test than infants in the observational
condition. Lastly, we hypothesize that there will be a lower success rates for all conditions than
in the previous study due to the difficulty of compensating for the uneven weight distribution of
the tool.
Method
Participants
This study included thirty three participants ranging in age from 12 months, 15 days to 17
months, 20 days. The subjects were randomly assigned to one of four conditions: round handle
hands-on training (8 infants, age M=15 months, 12 days; 3 females), straight handle hands-on
training (9 infants, age M=14 months, 18 days; 4 females), round handle observational training
(9 infants, age: M=15 months, 0 days; 5 females), and straight handle observational training (9
infants, age: M=15 months, 8 days; 3 females). 100% of the infants were Caucasian. The data
from 8 babies was collected and excluded: 1 age below 12 months 15 days, 2 due to
experimental error, and 5 because of failure to participate or complete training methods.
Weight Distribution on Tool Use Training 9
Apparatus and Stimuli
Infants were tested individually while sitting on the parent’s lap at a semi-circle shaped
table. There was a half circle cut out on the infant’s side, which allowed the infant to be totally
surrounded by the table. The infant sat on his or her parent’s lap in a chair across the table from
the experimenter. Four video cameras filmed the procedure from different angles; one from
above the table, one from in front of the table, one from the right of the table, and one from the
left of the table. These cameras were able to capture the infant’s face and hand motions from the
front, above and both sides.
The hand preference task was done individually while the infant was sitting on the
parent’s lap on a couch outside of the testing room. This was to ensure the task was the same in
both the observational and hands on condition. The objects used for the hand preference task
were two plastic animals; a brown monkey and a pink pig.
The heavy tool was created out of the round handle of the novel tool. The round end was
made of wood measuring 7 cm in length and 4 cm at the widest area. It was 0.3 cm thick. This
round handle was attached with superglue to a straight metal rod 7 cm long and 1 cm thick. This
difference in material created the unexpected weight difference; the round handle weighing 0.15
oz and the straight handle weighing 1.50 oz. The whole tool was then wrapped in 1 cm wide
strips of electrical tape so it appeared as one complete unit. The entire heavy tool was 14 cm
long.
The two training tasks in this experiment required two sets of materials. The first task
used a clear hollow plastic tube. One tube had each end wrapped in red electrical tape, was 9.5
cm long, 2.5 cm in diameter and was used for the round handle training. The other tube with the
ends covered in yellow electrical tape was 11.5 cm long, 4.5 cm in diameter, and was used in the
Weight Distribution on Tool Use Training 10
straight handle training task. In addition, 3 cm craft pom-poms were inserted in the small tube
and 5 cm craft pom-poms were inserted in the big tubes. The other training task employed a bell
box, which was a large cow bell surrounded by a purple box made of craft board covered in
plastic lamination. Five sides of the box were covered, but on the sixth side of the box a hole
was cut around the mouth of the bell. This allowed the inside of the bell to be exposed while the
bottom of the bell was parallel to the box. During observational training, a large Plexiglas
window measuring 70 cm by 70 cm, supported by a 30.5 cm base was placed on the table to keep
the infant from reaching for the training materials.
The test task used a light box measuring 14.0 cm in length, 10.0 cm in height and 12.5 cm
in width. This box was painted green and the front was made of a clear Plexiglas window
through which one could see the inside of the box. On the right side of the box was a hole
measuring, 2 cm in diameter. Inserting an object in the hole would break an infrared beam of
light and activate a LED display of red and green colored lights inside. The box was attached
with Velcro to the right end of white wooden base, which was 41 cm in length, 2 cm in height
and 14 cm in width. Figure 1 shows the light box and heavy tool stimuli.
Procedure
The training procedure varied for each condition, but the hand preference task and test
task were always the same. A baseline trial was excluded in this study because this would
eliminate the unexpected weight difference intended for the observation condition. Each infant
was individually given the hand preference task while seated on the couch in the lab waiting
room. The experimenter kneeled in front of the infant who was sitting on his or her mother’s lap.
The experimenter held out the monkey to the infant, who could then reach for toy. The
Weight Distribution on Tool Use Training 11
experimenter recorded which hand the infant used to grab the toy and retrieved the monkey.
This procedure was repeated with the toy pig. A third trial was again repeated with the monkey.
Round handle observational training.
Infants in the round handle observational training sat on their mother’s lap at a table
where a Plexiglas screen had already been placed between the infant and the experimenter. The
experimenter retrieved the skinny plastic tube and the heavy tool. The infant watched the
experimenter pick up the skinny plastic tube with her right hand, while also picking up the round
end of the tool with her left hand. The experimenter would grasp the round end of the tool and
use the straight end of the tool to poke the pom-poms out of the tube. The tool was placed on the
table and the pom-poms were inserted back into the tube. This procedure was repeated two more
times. Next the experimenter would put the tube away and get out the bell box. The
experimenter would lay the bell box on its side on the table so that the opening was facing the
experimenter’s left hand. She would then grasp the tool by the round handle and use the straight
handle to ring the cow bell. The tool was placed next to the box then the procedure was repeated
two more times. This entire procedure from the beginning was repeated a second time and then
all the training tools were put away and the Plexiglas sheet was taken down.
Straight handle observational training.
Infants in this condition received the exact same training as in the round handle
observational training, except the infants were trained to grasp the straight handle of the heavy
tool. In this condition, the fat tube with the large pom-poms was used. The experimenter
grasped the straight end of the tool and used the round end to poke the pom-poms out of the tube.
The experimenter also grasped the straight handle of the heavy tool and used the round end to
ring the bell box.
Weight Distribution on Tool Use Training 12
Round handle hands-on training.
In round handle hands-on training, infants had active training with the tool. The
experimenter took out a skinny tube with the small pom-poms and placed the heavy tool on the
table. The experimenter then placed the round end of the tool into the infants preferred hand,
based on the hand preference task. Then the experimenter grasped the outside of the infant’s
hand, while holding the skinny tube in his/her other hand, and assisted the infant in using the tool
to push the pom-poms out of the tube. Then experimenter replaced the balls in the tube. Then
the infant was allowed to use the tool by themselves to poke the pom-poms out of the tube,
which the experimenter held. If the infant was unable to do it on their own, the experimenter
continued to assist the infant. This was repeated once more for a total of three trials. Next the
tube and pom-poms were put away and the bell box was placed on the table. The infant grasped
the round end of the tool and the experimenter took the infant’s hand and helped them use the
straight end to ring the bell box. Then the infant was allowed to explore the bell box for twenty
seconds on his or her own. Three separate trials were too difficult to determine so a twenty
second limit was set to match the average time of the observation trials. After this, the
experimenter put away the bell box. This entire training was repeated once more.
Straight handle hands-on training.
In the straight handle hand-on training condition, infants received the same training as
infants in round handle hands-on condition; however, the infants were trained to regard the
straight end of the tool as the handle. Infants did everything above except during the pom-pom
tasks they grasped the straight end of the tool and used the round end to poke the balls out of the
fat tube. In addition, during the bell box task the infants inserted the round end inside to ring the
bell.
Weight Distribution on Tool Use Training 13
Test trials.
After training all infants participated in the test task. This began with the experimenter
placing the light box on the table. The test task was composed of an easy and hard trial. During
the easy trial, the experimenter grasped the tool by its round end and placed it next to the light
box so the straight end was facing towards the box. Then the experimenter picked the tool up by
the round handle and inserted the straight end into the hole in the side of the box, activating the
light display. The tool was placed back on the table and then the procedure was repeated a
second time. With the tool sitting in this orientation, the light box was moved in front of the
infant and the experimenter said, “Now it’s your turn.” All the infant had to do was grasp the
tool by its round handle and insert the straight handle into the light box. If the infants got
distracted during the experimenter, the experimenter would turn the box toward the infant and
point at the hole saying, “Can you put it in here?” After 30 seconds, regardless of success at
solving the task, the light box was retrieved for the hard trial. This time the round end of the tool
was placed closest to the light box with the straight end pointed away from the box. The
experimenter then demonstrated two times how to solve the light box task, by grasping the round
handle and turning the tool to insert the straight end into the box. The tool was put down in the
hard orientation and the light box was pushed toward the infant with the same procedure as in the
easy trial. After 30 seconds all materials were collected and put away.
Measures
Each session was videotaped in order to be coded. Success in solving the light box task
was coded in order to examine the effect of both handle training and observational or active
training. Success was defined as inserting the straight end of the tool into the hole in the light
box and activating the light display. This was marked as yes or no.
Weight Distribution on Tool Use Training 14
To make sure measures were reliable, a research assistant who was unaware of the goals
of the study coded a random sample of the videos (N=8). Reliability for quantitative data was
calculated by a means of Pearson’s product correlation (r=1). Reliability for qualitative data was
determined using Cohen’s Kappa formula and yielded an r=1.0.
Results
Generalized Estimating Equations (GEE) analyses were used to examine the relationship
between the two between subject variables: handle preference (round versus straight) and type of
training (active versus observational). In addition, a within-subject variable of trial (easy and
hard) was analyzed by GEE. These independent variables were all analyzed to predict the one
dependent variable, solve. Age and Gender were also included as parameters to see if there were
any other predictors of solve. Finally, an interaction between handle preference and type of
training was conducted. SPSS was used to run the GEE analysis. A summary of the statistical
tests is shown in Table 1.
The GEE analysis did confirm that, although the tool had an uneven weight distribution,
handle preference training did have an effect on likelihood of solving the light box task. Infants
in the round handle training condition were 2.14 times more likely to solve the light box task
than infants in the straight handle training condition (p< .001). This result supports our
hypothesis that handle preference training will be effective even with the heavy tool. It is also
consistent with previous studies in which infants’ actions during test are strongly effected by
training (Barrett, Davis, & Needham, 2007).
Age also came out significant, but the effect was so small (1.03 times more likely to solve
the task with age) that it most likely did not have a large influence on the results.
Weight Distribution on Tool Use Training 15
The GEE once again did not find a significant result of type of training, hands-on or
observational, on likelihood to succeed (p> .1). However, there was a significant interaction
effect between type of training and handle preference on likelihood to solve. This effect had a
large Beta value of 2.483 (p< .05). This provides support that the hands-on condition produced a
greater effect of training. Infants in the hands-on round condition were most likely to solve the
light box task, whereas infants in the hands-on straight condition were least likely to solve the
light box task. The observation conditions fall in the middle, with round handle being more
likely than straight handle to solve the task. This result is illustrated in Figure 2.
Four 2x2 Chi-square follow-up tests of independence further support these results. The
first 2x2 Chi-square test analyzed the type of training, active or observational, as a between
subjects variable on overall success at solving the task. The X2= .308 (p>.1) shows that the type
of training alone was not related to success. Infants in the hands-on condition solved the task
52.9% of the time compared to infants in the observational condition who solved the task 62.5%
of the time, a non-significant difference. The second Chi-square test was for the effect of handle
preference, straight versus round, on success. This effect was significant with X2 = 5.125 (p<.
05) showing that round handle training was related to success in solving the light box task.
Infants in the round handle training solved the task 76.5% of the time whereas infants in the
straight handle training solved the task only 37.4% of the time, a significant difference.
The third and fourth Chi-square tests were on the interaction of the two between subject
variables of handle preference and type of training on solving the light box task. For the handson condition, the X2 value was 7.244 (p<. 05), revealing that the infants in the round handle
hands-on condition solved the task significantly more often (87.5%) than the infants in the
straight handle hands-on training condition (22.2%). In contrast, for the observation condition,
Weight Distribution on Tool Use Training 16
the X2 value was .152 (p>.05) showing that infants in the round handle observational condition
did not solve the light box task significantly more often (66.7%) than infants in the straight
handle observational condition (57.1%). Therefore the interaction effect is supported by the
third and fourth Chi-square values. These Chi-square tests can only be regarded as follow-up
descriptive statistics on the GEE because the data in this study had repeated measures, easy and
hard trials, so the result of overall success is not independent.
One of the most interesting results was that the overall success rate of all the infants in
this version of the study was much higher than in Davis’s previous study. The Chi-square value
was 3.921 (p<. 05) showing that Davis’s total success rate, which was 32.2% across all
conditions, is significantly different from overall success in this study of 58.4%. This is 1.8
times the success rate seen in the previous version of this study. A comparison of the results of
the two studies is shown in Figure 3. We hypothesized that success rates would be lower overall
due to the uneven weight distribution; however, we found the exact opposite. This result is
surprising because the one would think the uneven weight distribution would decrease an
infant’s ability to succeed during the test task.
Discussion
This study looked at the effects of two factors, active versus observational learning and
weight distribution, on the development of tool use in infancy. This study was conducted as a
follow up on Davis’ (2011) research that did not find a significant difference between
observational and active training on infant’s ability to use a tool to succeed during test. The
previous study was replicated with one change: this time the novel tool had an uneven weight
distribution that could not be determined through visual observation alone. The current study
produced results consistent with previous research that handle preference training, round or
Weight Distribution on Tool Use Training 17
straight, affects an infant’s ability to succeed during test (Barrett et al., 2007; Davis, 2011). Even
with unexpected weight distribution of the tool, there was still a significant effect of handle
preference training on infant’s behavior during test. Infants in the round handle training
condition were more likely to succeed during test than infants in the straight handle training
condition because they were trained to grasp the round handle.
This study, also consistent with the previous study, did not find a significant effect of
type of training, hands-on or observational, on overall success rate. However in this
experimenter, there was a significant interaction effect between the type of training, active or
observational, and handle preference training, round or straight, on overall success at solving the
light box task. This could provide evidence to support the second hypothesis that there would be
a stronger effect of handle preference training, round versus straight, for infants in the hands-on
condition. This interaction is due to infants in the hands-on round condition having a
significantly higher success rate than infants in the hands-on straight condition. In contrast,
infants in the observation round condition had a success rate that was not significantly higher
than the success rate of infants in the observation straight condition (Figure 2).
One possible interpretation of these results is that active training shapes an infant’s tool
use behavior more than observational learning, in cases where the all of the characteristics of the
tool are not available visually. The prior experiences of infants in the hands-on training
condition may have made them less flexible in the way they grasped the tool. They continued to
grasp the tool by the handle that they learned to use during training. This could explain why the
participants in the hands-on straight condition were unable to succeed during test; they had
effectively learned to grasp the tool by its straight handle and could not produce the behavior
necessary for success. In comparison, the infants in the observational condition may not have
Weight Distribution on Tool Use Training 18
effectively learned where to grasp the tool. This could explain why infants in the observational
straight condition were equally as likely as infants in the observational round condition to
succeed. Even though they were trained to regard the straight end or the round end as the handle,
if the training was not as effective they would be equally likely to solve the light box task. This
interaction result could therefore be explained by the observational handle preference training
not being as effective as hands-on handle preference training in producing grasp specific
knowledge, when the tool has an unexpected physical characteristic.
An alternate explanation for these findings is that the interaction is a result of infants in
the observational categories being more likely to adjust their grasp position on the tool after
feeling the surprising weight distribution. There is evidence that infants between 11 and 13
months use information from their initial grasp in order to adjust to a more stable grasp (Barrett
& Needham, 2008). This could have resulted in infants in the observational conditions adjusting
their grasps after noticing the uneven weight distribution. If this is the case, infants in the
observational straight condition might have tried to compensate for the weight distribution by
adjusting their grasp to the round handle, thereby increasing their success rate. This would also
explain why infants in the observational round condition did not have as high of a success rate as
infants in the hands-on round condition. If they were surprised by the weight they may have
adjusted their grasp to the wrong position decreasing their likelihood of succeeding. In contrast,
infants in the hands-on training have already had a chance to learn about the weight imbalance
and would be more likely to stick with their trained grasp. This could explain the large
difference between the hands-on round handle and the hands-on straight handle success rates.
Measures such as change in grasp placement during test trials as well as latency between
attempts could be compared between conditions to try to examine which explanation is more
Weight Distribution on Tool Use Training 19
likely. Further research in this area is important in understanding the development of tool use in
infancy, as well as, understanding the strength and weaknesses of hands-on and observational
learning in different contexts.
One of the most interesting and relevant findings was that the success rate over all of the
conditions in the heavy tool version of the study was almost twice as high as the success rate in
the previous, light tool, study by Davis (2011). As shown in Figure 3. This result went against
our original hypothesis that infants in the heavy tool study would have a lower overall success
rate than in the original study. This result is counter-intuitive because one would think the
uneven weight the tool would decrease the infant’s ability to correctly line the tool up with the
hole to be able to succeed during test. This supports and extends previous research which shows
that infants between age 11 and 13 months can use information about an object’s weight to adjust
their grasp (Barrett & Needham, 2008). The higher success rate in the heavy tool experiment
could be a result of the uneven weight helping infants focus on how they are holding the tool.
By increasing their focus on how they are holding the tool, they may be more likely to try to
adjust their grasp, which could eventually result in the correct grasp and a higher rate of overall
success. Again, other measures, such as number of changes in grasp position, should be
compared between the two studies to better understand the reasons behind these results.
One limitation in this study is the effect of age. Although age was a very weak predictor
of success, it could still play a role. The average age of the hands-on straight condition was
about 20 days younger than the average age of the hands-on round condition. An age effect was
not expected because it has not been found in these studies before. It might be beneficial to run
more participants to even out the average ages in all conditions and see if the results stay the
consistent.
Weight Distribution on Tool Use Training 20
The results from this study provide new insights into how unobservable physical
characteristics and previous experience combine to influence the development of tool use.
Interestingly, this study found that the unobservable characteristic of the tool actually increased
success rates compared to a tool without any unexpected features. This increase in success rate
was found regardless of whether or not infants had been exposed to the uneven weight
distribution before the test task. This provides support for Gibson’s view that the act of using a
tool generates feedback, which can help one learn how to use the tool (Baber, 2003; Greeno,
1994). Lifting the tool used in this study provides different feedback, due to its weight
imbalance, than interacting with the tool used in the regular study. When infants pick up this
tool and feel the uneven weight distribution, they may be more willing to adjust their grasp to try
to account for it (Barrett & Needham, 2008). This willingness to change grasps could allow
them to be more flexible about where to grasp the tool and therefore be more likely to succeed.
This study also provides insights about the differences between observational learning
and hands-on learning. It is possible that the lack of a difference between active and
observational learning in the previous study could be due to all the characteristics of the tool
being visually observable. Characteristics of many of the tools we use are not apparent just from
looking at them. For example because a hammer’s head is made of metal and the handle is made
from wood. There is uneven weight distribution which is more drastic than one would expect
when just observing it visually. This might be a case where active interactions with the tool
would be a more useful type of training.
Since so many tools we use in everyday life have unexpected characteristics, future
research should examine how infants learn to deal with the issues these types of tools create. It
would be interesting to look at the different effects of using a tool with an obvious weight
Weight Distribution on Tool Use Training 21
imbalance based on shape versus one with an unexpected weight imbalance. Would infants plan
their grasps differently for the tools? How would they adjust after the feedback they receive
from interacting with the tool? It would also be interesting to examine how these different types
of tools would affect the infant’s ability to succeed during test. In conclusion, the development
of tool use in infancy is influenced by an overwhelming number of factors that researchers
should continue to examine.
Weight Distribution on Tool Use Training 22
Table 1
Summary of GEE Estimation Analysis for Variables Predicting Infants’ Success in Reproducing
the Action
Variable
B
SE B
eB
CI
Handle Preference
.763**
.2370
2.14
.298 – 1.227
Type of Training
-1.472
1.2040
.229
-3.832 - .888
Interaction
2.483*
1.0706
11.98
.384 – 4.581
Age
.025**
.0072
1.03
.011 - .040
Gender
-.208
.4328
.812
-1.056 - .641
*p< .05
** p< .001
Note: CI = Confidence Interval. Handle preference indicated round handle training or straight
handle training, Type of Training refers to active or observational experience
Weight Distribution on Tool Use Training 23
Figure Captions
Figure 1. Stimuli for test trials. The test consisted of grasping the novel tool by the round handle
and inserting the straight handle into a hole in the side of the light box. This activated an LED
display inside the box. The straight handle was made of metal (weighing 1.5 oz) and the round
handle was made of wood (weighing .5 oz)
Figure 2. Mean success rate during test task as a function of condition. There were four possible
conditions: round handle hands-on (active) training, round handle observational training, straight
handle hands-on (active) training, and straight handle observational training.
Figure 3. Comparison of mean success rates by condition between heavy and light tool studies.
Conditions the same as Figure 2.
Weight Distribution on Tool Use Training 24
Figure 1
Weight Distribution on Tool Use Training 25
Figure 2
100
90
80
Success rate
70
60
50
40
30
20
10
0
hands-on round
observation round
hands-on straight
Condition
observation straight
Weight Distribution on Tool Use Training 26
Figure 3
100
90
Heavy Tool
80
Light Tool
Success rate
70
60
50
40
30
20
10
0
hands-on round
observation round
hands-on straight
Condition
observation straight
Weight Distribution on Tool Use Training 27
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