PATH CONSTRAINTS ON MOVEMENT OF OBJECTS

by

Swetha Chinta

A thesis submitted in conformity with the requirements for the degree of Master of Arts

Graduate Department of Applied Psychology and Human Development

University of Toronto

© Copyright by Swetha Chinta (2014)

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Path Constraints on Movement of Objects

Swetha Chinta

Degree of Master of Arts

Graduate Department of Applied Psychology and Human Development

University of Toronto

2014

Abstract

The present study examined 8 -10 month-old and 11 -13 month-old infants’ capabilities to infer

constraints imposed by an explicit visual pathway on object movement of object. In a preferential

looking paradigm, infants observed a ball rolling down a U– or V–shaped path. In the U-shaped

path, infants observed a ball rolling from beginning to the end of the path (possible), and a ball

rolling down from the beginning and stopping midway of the path (impossible). In the V-shaped

path, infants observed a ball rolling from beginning and stopping midway (possible), and a ball

rolling down the beginning to end of the path (impossible). Analyses of looking times showed a

marginal effect on path, with longer looking times towards the possible V-shaped path by both

age groups. Overall, infants had weak representations for constraints induced by an explicitly

presented path on movement of the object.

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Acknowledgments

I would like to thank my supervisor, Dr. Mark Schmuckler, who has supported my research

interests and guided me throughout the year. I am grateful for your valuable insights, time and

patience during the course of this project.

I would like to thank my mother and my husband, who have supported me in every step of the

way. I also want to extend my gratitude to Dr. Diane Mangalindan for all her endless help in this

project. Finally, I want to thank the research assistants – Pouneh Kharabi and Amy Lin for their

hard work in recruiting, assisting during the experiment and coding.

This work was supported through funding from NSERC Discovery grant awarded to my

supervisor, Dr. Mark A. Schmuckler.

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Table of Contents

Acknowledgements ........................................................................................................................ iii

Table of contents ............................................................................................................................ iv

List of figures ................................................................................................................................... v

Chapter 1 Introduction ..................................................................................................................... 1

1 Infants’ knowledge about object motion .................................................................................... 1

2 Extrinsic factors on object motion .............................................................................................. 2

3 Motion through occlusion ........................................................................................................... 4

Chapter 2 ....................................................................................................................................... 10

1 Research questions and hypotheses ............................................................................................ 10

2 Methods ....................................................................................................................................... 11

2.1 Participants ............................................................................................................................... 11

2.2 Materials and Stimuli ............................................................................................................... 11

2.3 Design and procedure .............................................................................................................. 13

2.4 Scoring and analyses ................................................................................................................ 14

3 Results ......................................................................................................................................... 15

Chapter 3 General Discussion ........................................................................................................ 17

1. Main findings ............................................................................................................................. 17

2 Limitations and future directions ................................................................................................ 17

References ...................................................................................................................................... 19

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List of Figures

1. Figure 1. Schematic representation of U shaped path and V shaped path.

2. Figure 2. Schematic representation of possible and impossible events for U shaped path.

3. Figure 3. Schematic representation of possible and impossible events for V shaped path.

4. Figure 4. Comparison of mean percentages looking times as a function of Age (Path).

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Chapter 1 Introduction

We live in a complex dynamic environment filled with object(s) in motion. We encounter

constant examples of this motion in our everyday lives when we see a child on a slide or a swing,

a tree leaf falling towards the ground, a billiard cue ball striking other balls setting them in

motion or a car travelling along the road moving on a path laid out by the road. By the time we

become adults we have accumulated a vast knowledge about the dynamic environment and are

able to act upon this information in events. Aspects of this knowledge involves learning about

the featural properties of objects, how these properties place limitations placed on the movement

of the objects and a sensitivity for the different outcomes based on the movement of objects and

expectations for the future movement of objects. As adults, we also hold concepts that allow us

to reason not only visual seen events but also allow us to predict how objects will behave in the

future. For instance, we can catch a ball thrown in the air by understanding and predicting its

trajectory. Tracing the development of such capabilities in infancy raises important questions of

how and when infants retrieve such concepts and thus develop the capacity to use these concepts

to infer object motion. How do infants integrate external and internal properties to form

expectations about the movement of objects? For instance, do infants understand that objects

travelling with or without an explicit path produce different motions as constrained by internal

and external factors?

What is known about infants understanding of the movement of objects? According to the wealth

of literature exists on this topic, one way of categorizing infants’ understanding of object

movement is by looking at infants knowledge of how intrinsic (size, weight) and extrinsic factors

(gravity, inertia, speed, collision) related to object movement influences infants perceptions. The

studies reviewed here will specifically look at infants’ sensitivity to extrinsic factors that

influences object movement while moving on a defined path.

1. Infants’ knowledge of object motion

How do infants form representations of objects and its motion in the first place? A number of

theoretical frameworks have proposed that infants are advanced concept-formers with

information about motion properties represented within the first year of life (e.g. Leslie, 1995;

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Mandler 1992, Gelman, 1990; Spelke, 1994). According to the “core knowledge view”, infants

are born with knowledge about object motion and that motion is constrained by innate principles

about the laws of physics. Even though this knowledge improves over experience, the core

concepts are available very early in life. According to Spelke (1994), for instance, key principles

relating to object motion are cohesion (objects move as connected and bounded wholes),

continuity (objects move on connected, unobstructed paths), inertia (objects do not change their

motion abruptly and spontaneously) and gravity (objects move downward in the absence of

support). Spelke argues that this core knowledge guides infants in perceiving object motion and

thus allows infants to become capable of using these properties to make predictions or

expectations about objects movements. Similarly, Baillargeon suggests that infants possess

primitive concepts that are enriched through experience (1995, 1998, 1999, 2001). Through the

violation of expectation (VOE) paradigm, infants, as early as six months have been found to look

longer at events that violate these properties relating to events that are consistent with these

factors. Developmentally, Baillargeon argues that when infants watch an event for the first time,

their representations of the event are not complete and thus would attend to only a few of these

features. With experience however, infants strengthen their concepts and therefore exhibit

greater sensitivity to events that violate their representations (Baillargeon and Hespos, 2001,

2004).

2. Extrinsic factors on object motion

When considering object movement in the physical world, two critically important extrinsic

factors of motion the constraints induced by are gravity and the perception of object speed.

Developmental research over the years, examined the impact of these factors. Kim and Spelke

(1992) for instance, examined infants sensitivity to gravity by looking at perceptions of

movement of object on an inclined plane. These authors habituated five and seven month old

infants to one of the two possible events, either a ball either accelerating down an incline or a

ball decelerating up an incline. After habituation (i.e., once infants stopped attending to the

previous event) the infants were shown two events in which the direction of the incline had been

changed. One of the two events was possible whereas the other event was impossible, based on

the application of gravity. Thus, infants who had been habituated to the acceleration scene

(rolling down the ramp) saw a ball moving up the incline, either accelerating (an impossible

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event) or decelerating (a possible event). Similarly, infants who had seen the decelerating ball

during habituation (rolling up the ramp) saw the ball moving down the incline, either

accelerating (a possible event) or decelerating (an impossible event). Finally, some trials

included a change in the direction of motion in which the ball accelerated moving up the incline.

The main result was that the five month olds looked longer at those test trials in which

acceleration or deceleration changed, combined with change in direction, when only one of the

factors changed. Interestingly, the possibility of the scenario per se did not influence infants

looking behavior; instead, they reacted on the basis of the amount of perceptual deviance from

the familiar habituation event. In contrast, the seven month olds looked longer at the impossible

events, suggesting that 7-month-old infants are sensitive to effects of gravity on the speed of

objection motion. Kim and Spelke (1992) concluded that the sensitivity to changes in speed in

relation to gravitational constraints develops early but gradually, with infants initially responsive

to individual features and only later responding to the combination of features that differentiate

possible from impossible events.

In subsequent work, Kim and Spelke (1999) employed children between seven months and six

years and examined the development of sensitivity to gravity by examining the path of motion of

an object that moves off a supporting surface. In these experiments participants saw a ball roll

down a truncated incline or a horizontal surface, rolling off the edge and continuing to roll in a

parabolic path (natural path), a straight path (contrary to gravity) or turned sharply and moved

downward (contrary to inertia). The authors found that in the incline condition seven month old

infants showed no evidence that they perceived the parabolic path as natural, whereas in the

horizontal surface condition they showed weak evidence that they perceived the downward

motion as natural. Two year old children, when familiarized with downward accelerating or free

fall motion events, and tested with parabolic and straight down events, responded to the straight

down motion as more unnatural than the parabolic event. For children between three to six years

live ramp events were shown and then children were asked to judge the landing point after the

cliff. Four year old children chose a point on the straight down path, whereas six year old

children chose a point on the correct parabolic path. The authors concluded that sensitivity to

gravity develops slowly beginning at two years of age.

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Kannass, Oakes, and Wiese (1999) examined perception of objects moving along inclines with

10 month and 16 month old infants to further explore whether infants could use a combination of

features and respond to possible and impossible events. Experiment 1 tested whether infants

were sensitive to gravity and whether they had any prior preference for events in which a ball

rolled (a) down an incline, either increasing in speed (down acceleration, a possible event) or

decreasing in speed (down-deceleration, an impossible event) and (b) up an incline, in which the

ball either increased in speed (up-acceleration, an impossible event) or decreased in speed (up-

deceleration, a possible event). Experiment 2 tested infants’ responses to changes in speed,

direction and possibility in down-incline events by randomly assigning them to two habituation

conditions – down-acceleration and down-deceleration and then tested on all four incline events.

Finally in experiment 3 infants were randomly assigned to either an up-acceleration or down-

deceleration habituation and also subsequently tested on all four incline events. the findings of

these experiments revealed results in accord with those of Kim and Spelke (1999). Sixteen

month-old infants were found to be more responsive towards the changes in possibility of motion

as well as the direction of whereas 10 month-old infants responded only to changes in direction

of motion for down events. Thus, like Kim and Spelke (1999), a similar developmental trend was

observed in which a 16 month old demonstrated a more refined understanding about rolling

objects on an incline than a 10 month old.

These studies conclude that from five months to two years of age, there exists a developmental

trend where by infants’ responses become more sensitive to the effects of gravity and speed. The

studies described above show that even in these young infants there appears to be some

rudimentary understanding of these concepts, with development involving increasing ability to

judge how gravity and speed will affect an object's motion. It is a possibility that for the age

groups being studied here, infants’ sensitivity to the influence of kinematic properties on objects

could be a precursor in determining the movement of objects on paths in accordance with the

shape of the path and kinematics.

3 Motion through occlusion

Object occlusion is an extrinsic factor that is outside of the object itself, but may or may not

influence the perception of movement of the object. Motion occlusion is a topic that has been of

perennially interest in understanding infants’ abilities to represent objects during motion over

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time and space. Early research studies have shown that when an object disappears behind a

stationary occluder, infants could anticipate the reappearances by shifting their gaze towards the

edge of the occluder (Bower, 1982; Moore, Borton, & Darby, 1978). Similar results were also

found in longitudinal studies by Meer, Weel, and Lee (1994) in which 11 month infants could

anticipate the reappearance of a toy moving behind an occluder by shifting their gaze to the end

of the trajectory (Meer et al., 1994; Spelke & Hofsten, 2001). These early studies suggest that (1)

young infants form prospective expectations about object reappearances using information from

initial trajectories of movement to specify where and when to look, (2) infants form such

expectations regardless of having to act upon the objects (Meltzoff & Moore, 1998), and (3)

infants’ anticipation of emergences of moving occluded objects depend upon a number of

different factors (Woods, Wilcox, Armstrong, & Alexander, 2010). Specifically, studying

occlusion events have revealed certain factors that could affect infants’ estimations or predictions

of locations where an object will reappear and when it will reappear based on periods of non-

visibility, occluder size, speed through which objects move and ability to extrapolate motion

through various trajectories.

One factor that has been found to influence infants’ predictive gaze for object motion behind

occluders involves the size or width of the occluder. In a qualitative study Sergienko (1992)

reported that infants between 12 and 18 weeks of age made predictive saccades over an occluder

twice the size of the occluded object. Gredeback, Hofsten, and Boudreau (2004) also

demonstrated that when 25% of the circular trajectory was covered by the occluder infants

waited longer before making a predictive gaze. As a result their success rate for predictive

tracking was less than when trajectory was covered by only 10%. Johnson, Amso, and Slemmer

(2003) investigated the size of an occluder on linear trajectory in one of their experiments. Three

occluder width conditions (12.1 cm, 14.8 cm or 17.7 cm) were used. In the 12.1 cm condition,

the ball was visible in its entirety on either side of the occluder for 1333 ms and occluded for 400

ms whereas in the 14.8 cm condition the ball was visible for 1200 ms and occluded for 533 ms.

Their results, consistent with other findings, showed that 4 month old infants tend to look longer

at narrower widths (12.1 cm) compared with wider widths (17.7 cm). These studies have also

concluded that narrow occluder widths are associated with infants’ perception of trajectories as

continuous while wider occluder widths are associated with perceptions of trajectories as

discontinuous.

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Additionally, research has also indicated that the duration of occlusion is an important factor

when considering predictive behavior at the occluder and thus it becomes important to ask

whether the object representations degrade over varying occlusion intervals. Nelson (1974)

showed a train that was moving around a rectangular track, with a side of track covered by a

tunnel. When the train was visible, six to eight-month old infants were successful in tracking it.

However, as soon as the train disappeared into the tunnel for several seconds, infants failed to

anticipate its reappearance. Mailer and Garth (1980) tested five month old and nine month old

infants in a similar scenario with the presence of stop trials in which an object remained hidden

for long periods of time before it re-emerged. Nine-month old infants were more successful than

five-month olds in anticipating the reappearances. Meer et al. (1994) found that five-month old

infants made successful predictions for an object which was occluded for 0.3 – 0.6 sec even

before it arrived on the other side of the trajectory. Using a preferential looking paradigm,

Wilcox, Nadel, and Rosser (1996) presented 2.5, 4.5 and 6.5 month old infants with an object

which was lowered behind one of two occluders. Three types of delay was imposed – 5, 10 and

30 sec, after which the object reappeared from behind the right (possible) or wrong (impossible)

occluder. Infants 2.5- month old looked longer at the impossible event after a 5 sec delay, and

4.5 month olds showed the same result with delays up to 10 sec. Six and half month olds also

responded similarly across the three types of delay. Jonsson and Hofsten (2003) occluded a

horizontally moving object for 0.4, 0.8 or 1.2 sec and found that six-month old infants were able

to anticipate the reappearance of an object which was occluded for 0.4 sec. Similar results were

also obtained by Munakata, Jonsson, Hofsten, and Spelke (1996). Rosander and Hofsten (2002)

found that 17 to 21 week-old infants anticipated the reappearance of an oscillating object that

was occluded for 0.3 sec over the central part of its trajectory. Further evidence comes from S.

Johnson et al. (2003) who investigated how infants perceive occlusion events under varied

periods of invisibility. Using habituation methodology, the infants were either shown an object

traveling through a horizontal path without being occluded or were shown the object

disappearing behind an occluder over three interval times of 400, 800 or 1,200 ms. The authors

found that longer periods of non-visibility decreased infant’s ability to track the object.

Furthermore, results also showed that infants looked at the occluder to facilitate prediction of

reappearance of the object. Gredeback et al. (2004) showed similar results in their experiment in

which nine-month old infants tracked a fully visible or partly occluded object moving in a

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circular trajectory while measuring their horizontal and vertical gaze. Their results showed that

nine-month olds showed adult like horizontal tracking but poor vertical tracking performance at

0.4 hz. As the duration of occlusion increased from 250 to 5,000 ms, there was an increase in the

number of unsuccessful predictions and the amount of time infants took to gaze at the other side

of the occluder.

Studies also have indicated that the speed of objects while going behind the occluder could

potentially affect an infants’ ability to use predictive gazing. In an attempt to study infants use of

speed information to extrapolate object motion, Rosander and Hofsten (2004) showed 7 – 21

week old infants an oscillating moving object over occlusions, that moved in two motions: (1)

sinusoidal motion, where speed changed continuously over the trajectory and (2) triangular

motion, where speed was constant but the direction of the object was changed abruptly at end

points. The object was either occluded at the center of the trajectory (central occluder) or at one

turning point (peripheral occluder). For the sinusoidal motion, occlusion times were kept

constant by using a wider occluder. The object accelerated at the disappearance behind the

central occluder for the sinusoidal motion, but not for the triangular motion. At the peripheral

occluder for the sinusoidal motion, the object decelerated specifying reappearance on the same

side, whereas for triangular motion, the object moved in constant velocity specifying

reappearance on the other side of the occluder. Over the age differences studied, they found that

infants 12 weeks of age do not predictively track objects that become temporarily occluded for

300 ms since the occluder edge competed with their attention thus impairing their ability to

switch gaze when it reappeared. Infants seven to nine weeks old continued their gaze at the edge

of the occluder for almost one sec after the object had reappeared on the other side or reversed its

direction, re-approaching the occluder and were incapable of regaining quick tracking. After the

age of 12 weeks, infants began to form better representations of the moving objects that persisted

over temporary occlusions. By five months, these representations incorporated the dynamics of

the represented motion. Bremmer et al. (2005) examined four-month old infants’ perceptions of

trajectory continuity by manipulating object speed and duration of occlusion. In their experiment,

a ball moved back and forth in a linear trajectory and with the middle of the trajectory obstructed

by an occluder. In the acceleration condition, the ball moved at a rate of 15 cm/s when it was in

view, but sped up while occluded by 36.1 cm/s when it was occluded, thereby reducing the

occlusion duration to 233 ms. In the deceleration condition the ball slowed down by 13.3 cm/s

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during occlusion, thus increasing occlusion duration by 700 ms. This experiment revealed that

when occlusion duration was reduced because of increased speed while occluded, infants

perceived the trajectory as continuous, whereas they perceived the trajectory as discontinuous

when duration of occlusion was increased and speed decreased. Hespos and Rochat (1997) have

also demonstrated that infants four to eight months of age were surprised if the final orientation

of the object did not match the extension of the movement seen and thus looking times increased

for such errors in the final orientation of a rotating object that moves behind an occluder.

Final aspect of study pertaining to infants’ perceptions of object motion involves the ability to

track moving objects across different trajectories. A study by Hofsten, Vishton, Spelke, Feng,

and Rosander (1998), replicated by Rosander and Hofsten (2002), presented linear and non-

linear trajectories that were fully visible with (Rosander and Hofsten, 2002) and without

occluders (Hofsten et al.,1998) and showed that in both experiments, infants were successful in

visually tracking the natural continuation of how the object moved. Moreover, given more

familiarization time with non-linear trajectories, infants were able to accurately predict the

reappearance of the object. Gredeback et al. (2004), in a longitudinal study, investigated infants

ability to track objects in a circular trajectory that was occluded. This research studied 20 infants

from six months to the end of the first year. In this study, infants saw a small yellow happy face

moving in a circular motion with an occluder covering 20 percent of the trajectory in four

different positions. The results of this experiment indicated from six months of age, infants can

extrapolate a circular motion and assume that if an object is moving in a circular motion it would

continue to do so. These findings are consistent with those from Hofsten, Feng, and Spelke

(2000), who showed that an object moving in a straight path will continue to do so, and are also

in accordance with the results from Hofsten (1980, 1983) showing that infants could successfully

reach for future positions of targets that move along a circular trajectory. Collectively, these

results indicate that infants are able to extrapolate an object’s trajectory during occlusion and are

sensitive to violations of expectation of seen rotations.

The research described above has examined multiple conditions from speed (rate at which the

object moves), occluder widths, varying period of non-visibility as well as non-linear

trajectories, which can impact infants’ capabilities to detect changes in the events and predict the

reappearance of the object as it passes behind the occluder. Results of these studies have shown a

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development trend, whereby infants as young as 21 weeks are not sensitive to spatial and

temporal constraints on object trajectories, although can track objects trajectory to a certain

degree, when presented with occluders of narrow width and decreased object speed. As infants

get older, their representations for the dynamic motion become stronger and thus capable of

detecting a continuous trajectory and make successful predictions for reappearances for the

object that moves over greater speeds and longer occlusion rates.

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Chapter 2

1 Present study and hypotheses

By enlarge, research just described above has focused on the influence of different

spatiotemporal factors such as an infant’s interpretation of occluded motions and the

disappearance and reappearance of an object (Franz, 2010; Wilcox et al., 1996; Xu & Carey,

1996), or the influence of speed, gravity and acceleration on infants interpretation of objects

moving in different trajectories. One aspect that remains uninvestigated, however, has to do with

the impact of an explicit visual pathway on that might conceivably constraint the movement of

an object. Along these lines, one might wonder whether infants (a) recognize that an objects is

travelling along a predefined path (b) can perceive that the path acts as a constraint on the motion

of the object (i.e. motion of an object is based on the shape of the path it traverses), and (c)

understand that different paths will produce object movements in accord with the paths’ shape

(i.e., that different paths will constrain movements of the object). The current project is aimed at

providing an initial investigation of these constraints related to infants’ capabilities to represent

moving objects an explicitly presented visual pathway.

Specifically the present study was designed to investigate questions related to infants’ capability

to generate expectations or predictions about an object’s movement on a given path.

Methodologically, the most straightforward means of investigating these questions involves the

use of a preferential looking paradigm in which various possible and impossible events are

presented for different types of explicit paths and to then assess whether infants show a

preference for these possible versus impossible events. This study employs two different explicit

paths – a U- shaped ramp and a V- shaped ramp, with the expected object in motion varying in

accordance with physical constraints imposed by these ramps. Thus, there is an expectation of

continuous motion on an object along a U- shaped ramp, from one end to the other, whereas

there would be an expectation for interrupted motion for a V – shaped ramp at the 90 degree

angle juncture joining the two halves of the ramp. Thus, these two ramps provide varying

constraints on the object, and thus varying expectations for object motion.

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2 Methods

2.1. Participants

Two ages groups of eight to ten month-olds and 11 to 13 month-olds were used for this study.

These age groups were selected based on previous research (Spelke, 1994; Kim and Spelke,

1992) that have suggested principle of continuity emerges by four months of age whereas

principles of gravity, cohesion and inertia begins to emerge by seven months of age and

gradually strengths. A total of 24 infants (10 boys, 14 girls) participated in the study, with twelve

infants in the 8 to 10 month-olds group (M = 9, SD = 0.74) and twelve in the 11 to 13 month-olds

group (M = 12, SD = 0.74). Four additional infants (two 8-10 month olds, and two 11-13 month

olds) were not used in the final data analysis due to of being too fussy or crying too during the

experiment. Infants were recruited from a database maintained at the Laboratory for Infant

Studies at University of Toronto at Scarborough and the parents were contacted by telephone

and/or email. Infants’ received a toy and a certificate for participating in the study.

2.2 Materials and Stimuli.

The apparatus used for the stimuli consisted of a three sided wooden rectangular frame holding

one of two different paths - a U – shaped and V – shaped path (see Figure 1). The frame

consisted of a base which was 40 inches in length and 3.5 inch in width. Two perpendicular

sides, 20 inches in length, were attached to the ends of the base. For the paths, a flat white pliable

plastic slat was used, which was 0.5 inches thick, 3.5 inches wide and 54 inches long. For the U

shaped path, the slat was simply slid into the frame to form a distinct U shaped curve. For the V

shaped path, an identical slat of the same length was cut in half and joined with white duct tape

on the underside of the cut, forming a hinge which would allow the slats to open up without

falling apart. The hinged slat was then inserted into the frame and positioned to form the V

shape. The frame was painted black and placed against a black background.A red racquet ball 2.5

inches in diameter and weighed approximately 1.4 oz., was used as the object that would travel

along the paths.

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Figure 1: Schematic representations of the U shape and V shape paths.

Using the apparatus, two different stimuli were created for both the U – shaped and V – shaped

paths – a possible (P) and impossible (IP) events. Figure 2 shows the P and IP events for the U –

shaped path. In the P event, the ball was shown initiating at point A (one end of the path), was

held for a moment and was then released. The ball then rolled from point A to point B (the other

end of the path). The IP event was created by modifying the possible event clip. Specifically, this

event was edited such that the ball was seen at the top of the path and rolled to the middle of the

path, where it stopped and remained motionless. Both P and IP had duration of 1.15 seconds.

Figure 2. Schematic representations of the possible event (top) and the impossible event

(bottom) for the U shape path.

Figure 3 shows the P and IP events for the V – shaped path. In the V shaped path, the P event

consisted of the ball again held at point A and subsequently being released. The ball then rolled

down and, because of the angle to the inclined plane on the second half of the ramp, it is

subsequently stopped in the middle of the path. Again, the IP event was produced by editing the

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P event such that the ball appeared to roll back the inclined plane, to point B, decelerating as it

climbed. To create this part of the event, a subsequent video was recorded in which the ball was

rolled down the ramp, starting at point B. The temporal order of the video was then reversed and

joined with the first half of the possible event video to create a continuous motion of the ball

going from point A to point B. Both P and IP events had duration of 1.15 seconds. Finally, the P

and IP events for both the U – shaped and V – shaped paths were looped seven times with a one

second black frame in between repetitions, to produce stimulus display that were 15 seconds in

length overall.

Figure 3. Schematic representations of the possible event (top) and the impossible event

(bottom) for the V shape path.

2.3. Design and procedure

All experimental sessions were covered in a dedicated testing room, partially covered through

the use of hanging curtains. After the purpose of the experiment was explained to parents and

informed consent was obtained, parents and infants were positioned in the experimental room.

Specifically, parents were seated on a chair, with infants on their lap, facing a pair of 15” LG

computer monitors, positioned approximately 60 inches away. A Sony Vixia digital video

camera (HF R40) was covertly placed between the two monitors to record the infants’ looking

behavior during the experiment. An experimenter viewed infants’ visual patterns on a television

monitor and recorded infant looking time by toggling keys on a computer keyboard. This

keyboard was connected to a computer situated outside the experimental room, which tabulated

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infants’ looking times towards left and right monitors, as well as controls the presentation of all

stimulus displays.

Infants were randomly assigned to one of two conditions. In the UV condition, infants’ viewed

the U shaped path first and then the V shaped path. In the VU condition, infants viewed V

shaped path first and then U shaped path.

The experiment employed a preferential-looking procedure which consisted of eight 15 second

trials in all. Each trial consisted of a simultaneous presentation of the P and IP events for that

path condition. The first eight trials consisted of the P and IP stimuli in the ramp condition

(either the U – shaped or V – shaped ramp), with the P and IP stimuli appearing on the left

versus right monitors twice each, in a random order. The second eight trials then consisted of the

P and IP stimuli for the remaining condition, again with left and right position randomly

counterbalanced.

2.4. Scoring and analysis

A primary coder scored all sessions, and a secondary coder recoded 20 sessions (83%), eleven

from the younger infants and nine from the older infants. Four videos were not recoded due to

error in initial recording. Inter-rater reliability was evaluated by comparing the two sets of

looking times. These looking times were strongly correlated, r (22) = .90, p <.05.

Infants looking patterns towards each monitor were scored online as the experiment was running

and employs a range of dependent measures. First and foremost was the total duration of looking

time towards the two monitors. In addition, two additional measures were calculated - the total

number of looks to the P or IP events, and first looks to P and IP events. For all of these

measures, proportional scores were then calculated. Thus for the total looking time duration, a

proportion was calculated by dividing the total looking towards the P display by the total looking

towards P and IP (i.e. P/ P+IP) Similarly a proportion score for the total number of looks was

calculated (P/P+IP). A first look proportion was calculated by dividing the # of first looks

towards P by 8. For all of these measures, significant preferential fixation would be indicated by

scores significantly different than .50.

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3 Results

For the main analysis, proportion of looking preferences were examined using a mixed ANOVA

with Path (UP versus VP) as within – subject factors and Age (2 groups: 8 – 10 m, 11 – 13 m) as

between – subject factor1. The analysis revealed a marginal effect for Path, F (1, 22) = 3.41, p =

.08. There was however no significant interaction with Path x Age, F (1, 22) = .41, ns. A paired t

test revealed that for path, both younger and older infants looked longer towards VP (M = 0.54

and 0.54, SE = .0.29 and 0.29) than UP (M = 0.47 and 0.50, SE = 0.02 and 0.02) (as shown in

Figure 4).

Figure 4. Comparison of mean percentages looking times as a function of Age (Path)

The proportion looking times of UP and VP was compared to chance (50%) using a one sample t

test. The tests revealed that looking times for UP was not significantly different from chance, t

(23) = -.83, ns, whereas looking times for VP was marginally significant above chance, t (23) =

1.77, p = .09.

Two paired samples t tests were used to examine fatigue effects in both conditions (U and V).

For the U condition, there were no significant differences of the first half (M = 0.36, SD = 2.26)

of the trials versus the second half (M = -0.18, SD = 2.81) of the session, t (23) = -.47, ns. For the

1 Analyzes for other dependent measures of number of looks and first look was also examined. Both results

indicated similar trends with infants looking longer at VP than UP.

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V condition, there were also no significant differences of the first half (M = 1.10, SD = 2.55) of

the trials versus the second half (M = .91, SD = 2.37) of the session, t (23) = -.90, ns.

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Chapter 3 General Discussion

1. Main findings

The purpose of the study was to provide an initial exploration of infants’ capacity to deduce the

motion of an object as constrained by an explicitly given visual path. The present research

examined 8 to 10 – month old and 11 to 13 – month old infants’ ability to distinguish between

two pathways (a U shaped versus a V shaped) and the likely movement of a ball as a function of

these paths. Accordingly, it appears that, atleast with infants first year or so of life, infants are

relatively insensitive to the constraints on object motion induced by an explicitly presented path.

Why might infants have shown a preference for the V shaped path? In considering this question,

two explanations come to mind. First, infants might simply have more familiarity with a simple,

smooth and continuous path, and thus the VP path was a novel entity rather than UP path. In this

regard, the findings could be seen to converge with the core knowledge principles, in that

infants’ looking could be driven by the principle of continuity (Spelke, 1994), with the display

violating the pattern of more interest. Second, it might be that the the V pattern was complex for

infants’ to form an adequate representation in the time provided, accordingly, infants spent more

time looking at the display simply in an attempt to process it. Interestingly this explanation is in

line with the previous one. Moreover, this explanation is also in accordance with a core

principles argument, in that infants might be having a hard time comprehending that the two

halves of the V shape are actually part of a single pathway. Therefore, the increased attention to

the display results from the operation of a lack of development of the principle of cohesion

(Spelke, 1994)

2. Limitations and future directions

Although it might be that infants at this age are simply insensitive to the constraints on object

motion induced by external path, other possible explanations for the lack of significant results

exist. For instance, and somewhat fundamentally, this study did not examine a large number of

infants at either age (N = 24). Compared with studies by Hofsten et al.,(1998), Bremmer et

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al.,(2005) which employed over forty infants, the number of infants could be a factor for

attaining low preferential looking patterns.

Second, the failure to see the effects could have been due to the speed of the motion. Because we

used actual video tape of real life events (and not animated displays), the movement of object

down the path was fast, potentially too fast for infants to fully process and extrapolate. Infact,

evidence that the speed of object motion plays an important role in infants’ motion extrapolation

can be seen in studies by Gredeback et al., (2004), Bremmer, et.al., (2005), and Rosander and

Hofsten (2004). These studies have suggested that, slower the speed of the object, greater the

opportunity for infants to encode the dynamic information about the object and smoothly pursue

its motion. Specifically, infants as young as four months were able to make successful

predictions for reappearance of object (on visible trajectories) for slower speeds.

Infants’ generally prefer to attend to more colorful displays and thus are more attentive. The

stimuli used in the study were on darker side with a black background. The only colorful object

was the red ball and was probably not enough to captivate infants’ interest. It is a possibility that

having brighter displays would retain infants attention long enough to generate representations

for the motion and thus show a preference for events based on possibility. In addition, it is

unclear whether infants’ preference for an event was due to new representations of the motion as

constrained by the path or just representing with the motion only. One way to study whether

infants can form path representations with motion would be in a habituation paradigm, in which

infants would be familiarized with a ball depicting a movement in absence of a path and then

habituated to motion of the ball in the presence of the appropriate path.

A final possible explanation is that preferential looking might simply be too insensitive. An

alternative procedure to test infants’ expectations for object continuation would be to take one’s

cue from work on object motion behind occluders and to see if infants make preferential

anticipatory eye movements for the reappearance of an object that moved behind and occluder,

as a function of an external path. Current work is proceeding along this view.

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