The Task Structures the Response: Reference Frame Alignment in Toddlers’ Emerging Object Search Strategies Lynn K. Perry (
[email protected]) Department of Psychology, E11 Seashore Hall Iowa City, IA USA
Larissa Samuelson (
[email protected]) John Spencer (
[email protected]) Department of Psychology and Iowa Center for Developmental and Learning Sciences E11 Seashore Hall Iowa City, IA USA Abstract We investigate experimentally how children’s increasingly flexible use of reference frames enables accurate search for hidden objects. Children watch as an object is rolled down a ramp, behind a panel of doors, and stops at a barrier visible above the doors. Prior studies have found that 3-year-olds can accurately retrieve the object but that 2-year-olds fail to do so. We gave 2- and 2.5-year-olds a strong reference frame by increasing the relative salience and stability of the barrier. We found that 2.5-year-olds could successfully locate the hidden object. This work highlights the importance of the task structure in creating performance differences during transitional phases in cognitive development. Keywords: cognitive development; object representation.
Introduction Recent studies have provided surprising results indicating that older children sometimes fail to show competencies that infants have been thought to have. For example, Berthier, Deblois, Poirier, Novak, and Clifton (2000) presented 2and 3-year-old children with a ball that rolled down a ramp. The ball went behind an occluder panel of four solid doors, and stopped at a barrier. The barrier was visible above the occluder and could be placed directly to the right of any of the four doors so that opening the door directly to the left of the barrier would reveal the ball (see Figure 1). Berthier et al. (2000) found that 2-year-old children are not able to select the correct door when asked to find the ball in this “ramp task.” In fact, it was not until 3 years of age that children were able to choose the correct door at abovechance levels. Comparison of this result with those from research with infants leads to a conundrum: how can infants demonstrate understanding of the object concept (see Spelke, Breinlinger, Macomber, Jacobson, 1992)—that solid objects stop at solid barriers and continue to exist behind occluders—while toddlers cannot demonstrate such understanding? One possibility is that the discrepancy is due to differences in the methods used to study infants’ and toddlers’ understanding of object solidity. In particular, measures used with infants rely on looking behaviors, while those with toddlers require reaching responses. Thus, it is
possible that the apparent difference between infants’ understanding of the solidity of objects and the lack of understanding seen in 2-year-old children, is due to the added reaching demands of the ramp task compared to the violation of expectation paradigm. In support of this idea, toddlers have been shown to perform exactly as infants in a looking (i.e. violation-of-expectation) version of the ramp task (Hood, Cole Davies, & Dias, 2003). Thus, it appears that differences in the demands of looking versus reaching tasks can explain differences between infants’ and toddlers’ performance in tasks designed to measure the object concept. However, because a reaching measure has been used to test both 2- and 3-year-old children, differences in the type of required response cannot explain the apparent developmental differences in this age range. Thus, a question that remains is why can 3-year-olds solve the ramp task while 2-year-olds cannot? The argument that a reaching version of an object-concept task is harder for young children than a looking version depends on the assumption that reaching is more cognitively demanding than looking (Berthier et al., 2000). In particular, looking does not require the coordination of egocentric and allocentric cues while reaching does. In addition, reaching involves motor planning and coordination with visuo-spatial information (Berthier et al., 2000; Keen & Berthier, 2004). This is certainly true in the ramp task; to solve the task, you have to coordinate what you see as possible locations of the hidden object with possible reaching locations. We know that over development there are changes in the way in which children use reference frames. Evidence from reaching tasks indicates that young children initially encode locations egocentrically, using information about their own body position to remember space (Smith, Thelen, Titzer & McLin, 1999). By about 22 months, children are also able to use allocentric encoding, or information about objects’ positions (Newcomb, Huttenlocher, Drummey, &Wiley, 1998). Likewise, we know that there are decreases in the amount of perseverative reaching over development; for example, children under three years reach perseveratively on a majority of trials in the ramp task (Berthier et al., 2000). The ramp task, then, effectively pits the increase in allocentric encoding against the decrease in perseverative
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reaching. Furthermore, the particular structure of the ramp task may make successful use and coordination of allocentric encoding especially difficult for children between 2 and 3 who are transitioning from use of egocentric reference frames to using allocentric reference frames and simultaneously undergoing a reduction in perseverative reaching. In addition, the ramp task may be more difficult than other reaching tasks like the A-not-B task, because in the ramp task the location to which children must reach in order to correctly retrieve the object changes from on every. Thus, children must realign their allocentric and egocentric frames of reference on each trial. Doing so requires that the child find a good landmark, or frame of reference, to use for this alignment. Good reference frames are typically sturdy, large, and stable; making them both noticeable and reliable. In contrast, things that are flimsy, small, and move around are not typically good reference frames. In the ramp task, the thing that indicates where the hidden object and thus, that children should use as their frame of reference—the barrier—is flimsy, small and moves around from trial to trial. The challenge of finding the correct reference frame in the ramp task is compounded by the fact that the largest, sturdiest and most stable thing present—the ramp itself—is not what the children should use as their frame of reference. Clearly, then, the ramp task imparts additional difficulties not present in other reaching tasks and specifically related to the challenge of attending to and aligning reference frames. Furthermore, recent work on spatial cognition suggests children’s ability to use reference frames undergoes dramatic changes during the toddler years (Spencer, Simmering, Schutte, & Schoner, 2007). Thus, the age range tested in previous ramp studies represents a period of great change in spatial cognition. Spencer and colleagues have formalized the processes that underlie reference frame selection and calibration and developmental changes in these abilities during this age-range in a dynamic field model of spatial cognition (Spencer et al., 2007). Central to this account is an enhancement in the precision with which children align memories of particular target locations to specific reference cues in the task space. In summary then, three factors all suggest reference frame alignment may be the central issue underlying differences in 2- and 3-year-old children’s ability to solve the ramp task: 1) changes in children’s use of egocentric and allocentric encoding, 2) the use of a flimsy, movable barrier as the critical landmark, and 3) developmental changes in reference frame selection and calibration By this idea, then, 3-year-old children perform accurately in the ramp task because they are more skilled at coordinating and aligning the relevant reference frames. Two-year-olds, however, are not as skilled in this coordination, and thus are unable to perform accurately in the challenging ramp task. Children in the middle of this age range, 2.5-year-olds, should be on the verge of performing accurately. Thus, we may be able to boost performance of children in this middle range via small changes to the task that aid in reference frame alignment.
This experiment tests this possibility by altering the ramp task to make the barrier a sturdy, immobile landmark, and the ramp a moving (i.e. bad) landmark, thereby helping younger children attend to the correct reference frame and find the hidden object.
Method Participants Thirty-two 2.5-year-olds and 32 2-year-olds participated. Half the children from each age group were randomly assigned to either a condition designed to make the barrier more salient and ease reference-frame alignment (moving ramp condition), or a condition that made the barrier more salient but did not ease alignment (stationary ramp condition).
Apparatus A wooden ramp, a barrier, an occluder panel with four doors, a table, and a toy car were used. The ramp was 57.75cm wide, 20.3cm deep, and 28cm tall at the tallest point and was painted white (see Figure 1). The sloped surface of the ramp had four slots into which a bright yellow barrier (17.25cm tall by 28.25cm wide) could be placed to stop the car from rolling down the ramp. The slots were positioned such that when stopped the car would be visible through one of the four doors on the occluding panel that could be attached to the front of the ramp. The occluder was 27.94cm tall by 57.15cm wide. Each of the four black doors on the occluder were 10.75cm wide, 14cm tall. When the occluder was on the ramp, the car was visible for 15.24cm before disappearing from view. The ramp was placed upon a table that was painted the same bright yellow as the barrier. The table was 25cm tall, 49cm wide, and 32cm deep. The child sat on a chair placed approximately 60cm from the center of the table and aligned so that the center of the chair lined up with the table’s midline. A curtain was hung between the table and child. The curtain was brought down between trials to prevent the child from seeing the barrier or ramp being moved so as to aid the perception of the barrier as a stable frame of reference and also establish the illusion that the barrier was connected to the table. The curtain was pinned out of the way during each trial. While this apparatus is generally similar to that used in previous studies (see, Berthier et al., 2000; Thelen & Whitmyer, 2005), it incorporates three changes to the standard apparatus in order to support children’s use of the barrier as a stable frame of reference. First, the barrier was thicker, wider, and more brightly colored than those used in previous studies. Second, to support the perception that the barrier and table were connected and solid, they were painted the same color. Third, the child’s view of the movement of the ramp and barrier was blocked between trials by lowering the curtain. These changes were done in to draw attention to the barrier and make it appear to be a more stable reference frame for both conditions.
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Procedure Our general procedure closely followed that of Berthier et al. (2000). The session began with a familiarization period in which the child was introduced to the ramp and the car and shown that each door could open and close. The child was encouraged to approach the ramp and open and close each door. After the child returned to his or her seat, the experimenter removed the occluder and doors to reveal the bare ramp and barrier. The barrier was positioned in the slot corresponding to the fourth door from the left. The experimenter then rolled the car down the ramp two times, commenting each time that it stopped at the barrier. Next the experimenter replaced the occluder, opened all four doors, and rolled the car down the ramp two more times. On these trials the experimenter drew the child’s attention to the barrier by tapping it before releasing the car. The experimenter again commented that the car stops at the barrier (which was still at the fourth slot). Following this familiarization period, the child was given four open-door and four closed-door training trials. For the open-door training trials, the experimenter closed all but the fourth door from the left. The child was told that the car was going to practice hiding and it was the child’s job to find it. The experimenter then brought down the curtain and positioned the barrier according to one of four random orders. Only the correct door was left open. After reopening the curtain, the experimenter brought the child’s attention to the barrier and rolled the car down the ramp. The child was then allowed to retrieve the car. If the child retrieved the car correctly on his or her first reach, he or she was given a sticker and praised heavily. This was repeated for a total of four trials (so that the car stopped once at each door). For the closed-door training trials, the experimenter closed all doors, explaining that the car was going to really hide this time and that the child would have to open a door to find it. These trials then proceeded in the same manner as the open-door training trials (but with all four doors closed). As with the open-door trials, the car stopped once at each door. Finally, there were 12 closed-door testing trials. The procedure for these was identical to that of the closed-door training trials, and to the testing procedure of Berthier et al. (2000). The car stopped at each door once within a block of four trials so that it stopped at each door three times throughout testing in a pseudorandom order. This same procedure was followed for both moving mamp and stationary ramp conditions. The only difference between conditions was the placement of the ramp during the familiarization, four open-door, and four closed-door training trials. In the moving ramp condition, the ramp position changed from trial-to-trial such that the barrier, directly to the right of the correct door, was always aligned with the center of the table and the child’s midline (see top two pictures in Figure 1). In contrast, for the stationary ramp condition, the ramp remained centered on the table throughout training (see bottom tow pictures in Figure 1). Thus, in the stationary ramp condition, the barrier was only
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Moving ramp condition
Stationary Ramp Condition Figure 1. Placement of the ramp when the barrier is at the first and fourth door for the moving ramp condition (top two pictures) and the stationary ramp condition (bottom two pictures).
aligned with the center of the table, and therefore the child’s midline, only when it was placed by the third door from the left, or door 3, (1 time in the open-door and one time in the closed-door training trials, and 3 times total during testing). Figure 1 shows the position of the ramp and barrier during both a door 1 and door 4 trial for both conditions. Note that the stable ramp position of the Stationary condition matches the procedure used by Berthier (2000). The closed-door testing trials were identical in the two conditions; the ramp was positioned at the center of the table and did not move. Besides the movement of the ramp in the moving ramp condition, our procedure differs from that of Berthier et al. (2000) in two ways. First, we added the reinforcement of stickers for correct first reaches. This was done to ensure that children were engaged and interested in the task. Second, we allowed children to reach during the open-door training trials where only the correct door was open. All sessions were videotaped. The tapes were coded offline to ensure procedure accuracy and to record the first reach children made on each trial.
Results and Discussion Average accuracy on first reach during the test phase was calculated for each age group and condition. Three children (two 2-year-olds, one in each condition, and one 2.5-yearold in the moving ramp condition) failed to reach correctly on more than one trial; thus, they scored below .1. This is well-below chance levels and more than one standard deviation away from the mean of these children’s respective groups. Thus, these children’s data were not included in subsequent analyses. Figure 2A presents average accuracy for each age group in each condition during test trials. As can be seen in the figure, 2.5-year-olds in the moving ramp condition (dark gray bars) were more accurate than 2.5-year-olds in the stationary ramp condition (black bars) and 2-year-olds in both conditions (light gray and white bars). In particular, 2.5-year-old children in the moving ramp condition were the
**
only group that performed significantly better than chance levels (.25), M=.38, t(1)=3.36, p.31 respectively. We also examined changes in performance over the course of the test trials. The mean number of correct door selections in each of the three blocks of four test trials for children in each condition at each age is graphed in Figure 2B. As can be seen in the figure, the block data suggests variability across testing. The 2.5-year-old children in the moving ramp condition (dark gray bars) showed little change across test blocks. The 2-year-olds in the moving ramp condition (light gray bars) were also generally consistent across blocks. Both ages in the stationary ramp condition (black bars for 2.5-year-olds and white bars for 2year-olds), however, showed an increase in accuracy during Block 2 and then decrease again by Block 3. This suggests the possibility of fatigue effects for children in the stationary ramp condition, who have been seeing the same kinds of trials throughout the experiment because the ramp did not move in training in this condition. An examination of trial-by-trial reaching behavior suggested the possibility of a large amount of perseverative reaching affecting the data. Prior ramp studies have shown that children reach perseveratively on a majority of trials when they cannot solve the task (Berthier et al., 2000; Thelen & Whitmyer, 2005). We analyzed the type of reach each child made on each test trial—perseverative or not. We defined perseverative reaching as a child repeating a response from the previous trial. Thus, we calculated the average rate of perseveration in each group by dividing the number of times a child repeated a response from the previous trial by the total number of reaches minus one. Figure 3 presents the average rate of perseveration in each group. Perseveration was generally high across groups.
*
**
* *
*
Figure 2A (left) shows average accuracy of reaches for each age group in each condition during testing trials. Figure 2B (right) shows average accuracy of reaches during each 4 trial block of the test trials for each age group in each condition. * indicates performance significantly better than chance (.25).
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condition, and were able to maintain higher performance across testing blocks. Conversely, 2.5-year-olds in the stationary ramp condition and 2-year-olds in both conditions were not only less accurate over all, but were more variable in performance across test blocks.
General Discussion
Figure 3 shows the average rate of perseverative reaching for each age group in each condition. However, as is clear in the figure, rate of perseverative reaching differed across conditions. Children in the stationary ramp condition (white and black bars) perseverated more than those in the moving ramp condition (light and dark gray bars), and within each condition, 2year-olds (light bars) perseverated more than 2.5-year-olds (dark bars). Within the stationary condition, a few children at each age level had extremely high rates of perseverative reaching (i.e. more than 1 standard deviation away from the group average). Thus, to get a clearer picture of the results, we removed any children who perseverated on more than .70 of reaches (2 children in the 2.5-year-old stationary ramp group, and 3 children in the 2-year-old stationary ramp group) and conducted a mixed design ANCOVA with block as a within-subjects factor, age and condition as between subjects factors, and perseverative reaching as a covariate. This analysis indicated that perseverative reaching was a significant covariate of accuracy, F(1, 49) = 9.0, p
