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Selection and memory of a lower limb motor-perceptual task in 3-month-old infants
Infant Behavior & Development 24 (2001) 239 –257
Selection and memory of a lower limb motor-perceptual task in 3-month-old infants Rosa M. Angulo-Kinzler*, Christen L. Horn Center for Human Motor Research, Division of Kinesiology, University of Michigan, Ann Arbor, MI 48109-2214, USA Received 2 May 2001; received in revised form 10 September 2001; accepted 21 September 2001
Abstract The purpose of this study was to test infants’ ability to learn and memorize a specific motorperceptual task using a constraining reinforcement mobile protocol. Fifteen three-month-old infants received a mobile reinforcement for every correct movement they performed. The task was to cross 85 degrees of flexion with the right knee. Infants were tested over a series of three sessions, spaced 24 and 72 hr apart. Results indicated that infants increased the frequency of mobile jiggles by selecting a new and efficient motor solution: smaller movements around the required threshold. These data suggest infants modulated their preferred movement patterns in an economical way. The data also showed that infants memorized the task by the third session, indicating they can learn and retain a specific motor-perceptual task. This implies that infants at this age are sensitive to environmental information and to the consequences of their own posture and movements. © 2002 Elsevier Science Inc. All rights reserved. Keywords: Infancy; Movement; Retention; Mobile
1. Introduction The acquisition of new motor skills is an integral part of an infant’s growth and development. Imagine going through life never having learned to coordinate leg movements into functional walking. Perhaps more importantly, if an infant does learn specific tasks, what
* Corresponding author. Tel.: ⫹1-734-647-9851; fax: ⫹1-734-936-1925. E-mail address: [email protected] (R. Angulo-Kinzler). 0163-6383/01/$ – see front matter © 2002 Elsevier Science Inc. All rights reserved. PII: S 0 1 6 3 - 6 3 8 3 ( 0 1 ) 0 0 0 8 3 - 2
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purpose does learning serve without a memory of the specific motor skill? Learning and subsequent forgetting would seriously hinder the development of an infant. Recently, tremendous gains have occurred in our understanding of the memory capacity of infants and several factors have been shown to affect their retention responses. First, infants’ age influences the duration of their retention. Hartshorn et al. (1998) have demonstrated that between 2 and 18 months of age infants’ retention increases monotonically. Second, the length of the familiarization period, the reactivation schedule and the delay between stimulus presentation and memory test are also relevant factors affecting retention in infancy. For instance, 1-day delay and 30 s of familiarization time are enough for 3-month-old infants to show long-term retention to a dynamic visual stimulus (Courage & Howe, 2001). Similarly, Gulya et al. (2001) demonstrated that 3-month-olds remember a serial list with a 24-hr delay but not with a 1-week delay independent of providing a reactivation schedule. Another important aspect in evaluating infants’ memory is the inherent or conditioned “value” of the stimulus. Infants’ preferences and therefore their memories are enhanced when the stimulus has a greater value. Several studies have provided evidence to suggest that infants’ preferences should not be evaluated solely on the amount of stimulus experience but consider simultaneously the changes in stimulus value (Najm-Briscoe, Thomas & Overton, 2000; Schwartz & Reznick, 1999). Finally, the modality of the infants’ response will also affect their retention performance. Hofstadter & Reznick (1996) claimed that infants showed poorer retention performance when they were asked to reach as compared to just direct their gaze to the stimulus. One of the most effective ways to study learning and memory in preverbal infants is the Conjugate Reinforcement Mobile task (Rovee & Rovee, 1969), which will be referred to as the Traditional Mobile Procedure (TM). In this procedure a 3/8” polyester ribbon binds the infant’s ankle to an overhead mobile forming a conjugate relationship (the mobile moves at the same rate and amplitude as the infant’s leg). This experimental procedure allows the manipulation of the aforementioned factors affecting memory. In 1976, Rovee-Collier and Fagen used the TM procedure to examine the effect length of training on 24-hr recall of the TM task. Rovee-Collier, Evancio & Earley (1995) studied how the length of time between sessions of the TM procedure affects long-term memory in 3 month-old infants when the same mobile is used in all sessions. They found that retention was best when infants had 48 hrs to consolidate their experience. Further studies have shown that the optimal time delay between training and long-term retention test is between 1 and 4 days (Hayne, 1990; Rovee-Collier, Sullivan, Enright, Lucas & Fagen, 1980; Sullivan, Rovee-Collier & Tynes, 1979). These results imply that there is a time window of 3 days during which information from the first session can be integrated with information from a second session. Years of research conducted by Rovee-Collier and her colleagues have provided a large body of information on learning and memory in 3-month-olds. Some of these contributions can be summarized as: (1) the TM procedure provides an environment for studying infant motor acquisition, (2) at 3 months, infants are capable of learning the contingency between rate of kicking and movement of a mobile, (3) the mobile movement serves as reinforcement for leg movement, (4) through comparisons of baseline and extinction rates of kicking, both short term and long term memory can be studied, (5) at 3 months, infants can remember a learned contingency, (6) the amount of time between sessions affects how much the infant
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retains, and (7) the context of the situation affects what is retained. It is clear that at 3 months, infants have the ability to learn and remember the contingency between leg movement and mobile movement. These contributions stream from manipulating constraints either in the mobile or the proximate environment, such as modifying the number or details of the mobile’s elements, or changing the distal environment such as the location of testing, color of crib bumpers, or background music. However, no study has manipulated the constraints of the motor component of this task. The question then is to what extent infants can acquire and retain a constrained motor solution. In order to study how infants acquire and retain a specific motor skill, it is necessary to place movement constraints on the training environment. Constraints limit the possible motor solutions by requiring an infant to produce a specific movement that is not the infant’s preferred movement pattern. The results of Thelen (1994) are an excellent example of skill acquisition through placing constraints on a training environment. Previously, Thelen (1985) determined that most 3-month-old infants prefer to display single or alternating kicks as compared to simultaneous kicks. Therefore, in-phase kicking (both legs together) is not a preferred pattern of coordination for 3-month-olds. To test if infants could change their kicking patterns, Thelen used the TM procedure but changed the constraints of the task by yoking infants’ legs together with a loosely tied elastic. The yoke made single and alternating kicks more difficult but still possible. However, the “yoking” favored in-phase kicks as a solution requiring less effort. As predicted, infants whose legs were yoked produced a higher frequency of in-phase kicks than those not yoked. Through exploration, the infants discovered that their original coordination pattern was no longer functional for them. Instead, a new movement pattern (in phase kicking) was much easier for the infants to produce, and infants greatly increased their frequency of in phase kicks. These data show that 3-month-old infants can learn patterns of interlimb coordination through exploration and selection of movements that are not within their original coordination tendencies. If infants are capable of learning and remembering the contingency between leg movement and mobile movement when there is a physical connection between the legs, are they also capable of learning and remembering a specific motor solution to a conditioning task without a physical connection between the legs or the mobile? Since the TM procedure requires a physical connection, and any general leg movement results in mobile movement, it was necessary to design a new procedure which makes task demands specific enough to test what motor aspects infants actually learn and remember. In 1997, Angulo-Kinzler implemented a Constraining Reinforcement Mobile Procedure (CM) which places a task requirement on infants such that they must explore their environment and identify the movement that is being reinforced. In comparison to the context of the TM procedure that has a broad proprioceptive and visual component, the CM procedure narrows the proprioceptive component by constraining the training environment. In addition, an auditory component was included. Therefore, the infant must finely map the multiple sensory modalities available in the context in order for learning and full retention to be evident (see Fig. 1). In essence, the CM procedure constrains infants’ repertoire of movements that will result in reinforcement to one very specific movement. In Angulo-Kinzler’s study (Angulo-Kinzler, 1997), right knee flexion at a threshold of 85 degrees was used as the position criteria necessary to receive a reinforcement from the mobile. Sensors called goniometers were
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Fig. 1. Representation of the Traditional and Constrained mobile procedures indicating their differences in involvement of various relevant sensory modalities, task requirements and motor solutions. Each sensory modality is represented by a circle filled with a different pattern. The region of motor solutions is represented by the black intersection among circles, and further expanded in the square presentation of task requirement versus movement amplitude. In the constrained method the task requirement is reduced to 85° flexion, while any degree of either flexion or extension might be valid in the traditional method.
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placed on both the infant’s knees to measure the degree of flexion in those joints. When the goniometers detected a crossing of the 85-degree threshold in the flexion direction, a computer triggered a controlled movement of the mobile. Hence, the amplitude of the knee movement has no effect on the mobile movement. The experimental session consisted of a 2-min baseline period, a 10-min acquisition period, and a 2-min extinction period. High flexion of the right knee joint triggered mobile movement only in the acquisition period. Angulo-Kinzler found that infants were in fact capable of learning the contingency between flexion of the right knee at 85 degrees and the reinforcement from the mobile. Upon further analysis, it became apparent that the constraints of the training environment made available two proprioceptively different solutions. Some infants preferred to use right leg kicks to cross the threshold and gain reinforcement (a high-amplitude movement-based solution). Other infants employed small movements around the threshold to receive almost continual reinforcement from the mobile (a low-amplitude posture-based solution). The movementbased and the posture-based solutions have significantly different involvement of movement amplitude to gain the reinforcement making posture-based solutions more economical. The differences seen in the way infants solved this task provided an opportunity to examine what infants remember about their movements. Based on past research, it was expected that infants would learn the constrained task during 2 days of consecutive training and subsequently show retention of the specific motor solution after a 3-day intermission period (retention 72 hrs later). The constraints placed on the training environment in the current study served to make the conditional motor response more specific than those in the TM procedure. We predicted that (1) infants would be able to increase the number of reinforcements gained to meet the learning criterion in all sessions. We also predicted that (2) infants would increase the number of threshold crossings produced in the baseline of session 3 as compared to the baseline of session 1 thereby showing retention of the constrained task. Finally, we expected that (3) there would be a change in the initially preferred leg movement patterns to one that required less movement and therefore was more economical.
2. Method 2.1. Participants Fifteen 3- to 31⁄2-month-old (corrected age, M ⫽ 97.2 days, SD ⫽ 8.1) infants, nine males and 6 females, participated in this study. Age was corrected for one 4-month-old infant who was born 1 month prematurely. Potential participants were identified through birth announcements in the local newspaper. A letter explaining the study was then sent out and followed up with a phone call. If the parents were interested in participating, they were asked to bring their infant into the laboratory for three sessions, the first two spaced 24 hrs apart and the third one 72 hrs later. All participants signed consent forms that were approved by the institutional review board. Five additional infants were tested but their data were not included in the analyses due to excessive crying (3) and technical problems (2). Also, the
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baseline data file for one infant was lost in session 2, and the video recording of one infant was not captured for sessions 1 and 2. 2.2. Apparatus A medium-sized mobile made from black, white, red, and blue designed plastic squares oriented both vertically and horizontally (The First Years Inc., Discovery mobile) served as the reinforcer. This mobile was a novel object for all infants as reported by the parents. Researchers confirmed with the parents that this mobile was novel for the infant. A chime, hung from the center of the mobile, provided acoustic stimulation in addition to the visual stimulation available from the mobile components. A motor was connected to the mobile by a transparent string, allowing the motor to provide a controlled pull on the mobile (0.6 sec duration) specified by a customized PC computer program. Sensors, called goniometers (Penny & Giles, XM-75), were used to measure the degree of angular displacement in the infants’ right and left knees. Each goniometer was approximately 7.5 cm long and consisted of two plastic rectangular pieces which measured about 3 cm ⫻ 0.75 cm and were connected by a thin wire spring (zero degrees ⫽ full extension). The goniometer signal was recorded by the computer and the customized program evaluated the movement conditions that were required to provide the reinforcement. If the conditions were fulfilled the reinforcement was provided (for more details, see Angulo-Kinzler, 1997 & 2001). The program saved the goniometer signals for the baseline and extinction but not while it was delivering reinforcements during the acquisition. A video camera on an elevated tripod in the corner of the room was used to record the entire session. A SMPTE time code was inserted on the video image to record time during the session. An LED connected to the computer flashed in front of the lens of the video camera each time the mobile was activated and was therefore recorded on tape. The only light in the testing area came from a halogen lamp placed near the infant’s head to illuminate the mobile. 2.3. Procedure Once the parent signed the consent form, the experimenter and a trained assistant undressed the infant from the waist down and loosened the diaper so the infant could kick freely. Two double sided adhesive electrode collars were attached to each end of the goniometer. One end of the sensor was placed on the anterior right thigh and the other end was placed on the anterior right shank so that the goniometer was in position to measure the angular displacement of the knee. A piece of hypoallergenic tape was placed over each end of the goniometer to ensure it was held in place. A pair of oversized footless cotton tights was put on the infant to keep the infant from grasping the sensors and cords. The goniometers were calibrated by holding the infant’s right leg fully extended (full extension ⫽ 0 deg), then bending the infant’s leg to 90 degrees flexion to make sure it reads approximately 90 degrees on the sensor display (see Fig. 2). The infant was then placed in a supine position on a padded surface underneath the mobile and held in place by a soft foam belt secured with Velcro strips. The VCR and the time code
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Fig. 2. Biofeedback system consisting of a PC computer, 4 goniometers and a motor-driven mobile. In this study, goniometers were only placed on the knees.
generator were activated to record the session. After a 2-min baseline period in which right leg flexion did not trigger the mobile to move, one “free” mobile reinforcement initiated the 8-min acquisition period. The task requirement for the infant was right knee flexion at 85 degrees in the flexion direction. During the acquisition period, each time the infant crossed the 85 degree threshold in the flexion direction, a controlled mobile reinforcement was received. The mobile moved just enough for the pieces of the mobile to shake and the chime to produce a small sound. Following the acquisition period, a 2-min extinction period in which knee flexion did not result in mobile reinforcement was provided. Throughout the session, the parent sat about 6 ft from the infant, far enough to be out of sight, allowing the infants to focus on the mobile task. The same procedure was used during all three sessions. Session 2 took place 24 hrs after session 1 and session 3 took place 72 hrs after session 2. The parent received $5.00 at the end of each session regardless of the infant’s performance. 2.4. Data reduction The data were coded in two different ways. During the baseline and extinction periods, the computer program searched for the 85 degree threshold crossing in the flexion direction and
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counted the number of times the threshold was crossed in each minute. During the acquisition period, the number of LED flashes that indicated a threshold crossing was counted in each minute. An infant was considered to have learned the contingency if the number of mobile reinforcements received during two out of three consecutive minutes of the acquisition period exceeded 1.5 times the average baseline value (the learning criterion). Long-term retention was measured by obtaining baseline and retention ratios of reinforcements (Rovee-Collier, Enright, Lucas, Fagen & Gekoski, 1981; Rovee-Collier & Hayne, 1987). These measures are the standard means of determining retention in TM procedures. A baseline ratio was calculated for each infant by dividing the number of 85°-threshold crossings during the initial 2-min nonreinforced period at the outset of session 3 (long-term retention test) by the number of 85°-threshold crossings emitted during the initial 2-min of baseline period in session 1. A retention ratio was also calculated for each infant by dividing the number of 85°-threshold crossings during the long-term retention test by the number of 85°-threshold crossings during the final 2-min nonreinforced period at the end of session 2. Because the task allowed movements of varying amplitudes to result in crossing the 85° threshold, all reinforcements were then coded for causation. Movements resulting in reinforcement were divided into 3 categories: large movements (⬎45 degrees of movement), small movements (7– 45 degrees of movement) and minimal movement (less than 7 degrees of movement). The computer program determined the types of movements that were used to cross the threshold in the baseline and extinction periods by measuring the minimum and maximum degrees of movement around the threshold. Acquisition periods were visually coded by minute to determine what motor solution caused each reinforcement. Coders were trained to visually determine the distinction between movement types until they reached 80% point-by-point agreement with the second author (Harris & Lahey, 1978). Coders were tested for reliability midway through the data coding and reached an 87% point-by-point agreement. To assess retention, baseline and retention ratios were computed for all three different types of motor solutions.
3. Results 3.1. Learning For analysis purposes, each session was divided into 6 2-min blocks: 1 in baseline, 4 in acquisition, and 1 in extinction. The absolute frequency of reinforcements was analyzed via a 3 (session) ⫻ 6 (block) analysis of variance (ANOVA) with repeated measures in both factors. This analysis revealed a significant block effect (F(5,232) ⫽ 4.37, p ⫽ .0008), indicating a significant increase in the number of reinforcements received for all sessions (see Fig. 3). No other significant effects were found. Posthoc comparisons using the Bonferroni method revealed a significant difference between baseline and acquisition blocks 2 and 3 (base vs. acq. 2, F(1,227) ⫽ 3.06, p ⬍ .04; base vs. acq. 3, F(1,227) ⫽ 3.42, p ⬍ .02). The majority of infants reached the learning criterion in each session. In session 1 and 3, 60% of infants met such criteria. In session 2, 67% of infants learned the task (see Table 1). The mean time it took infants to reach the learning criterion in each session is also shown
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Fig. 3. Group means and standard errors of the frequency of reinforcements produced during baseline, acquisition, and extinction conditions in each session.
in Table 1. Of the 9 infants who reached the learning criterion in the first session, the mean time to do so was 3.9 min (SD ⫽ 2.20). In session 2, the mean time for 10 infants to reach the learning criteria was 3.4 min (SD ⫽ 1.43). Finally, the 9 infants in session 3 who reached the learning criterion further reduced the mean time to reach the learning criterion to 2.9 min (SD ⫽ 1.17). 3.2. Retention Baseline and retention ratios were used to measure long-term retention. These two measures are routinely used in the mobile reinforcement task (see, Rovee-Collier & Fagen, 1981; Rovee-Collier & Hayne, 1987). Retention is primarily measured by the baseline ratio Table 1 Number of learners in each session and time elapsed to reach learning criterion
KM JH AD AB MM SB AT AH HH HW NW TK CM JN OW TOTAL
Session 1
Time (minutes)
Session 2
Time (minutes)
Session 3
Time (minutes)
Yes No No Yes Yes No Yes Yes Yes No No Yes No Yes Yes 9
5 — — 8 2 — 2 6 2 — — 2 — 5 3 Avg.3.9
Yes No Yes Yes Yes No Yes No Yes Yes No No Yes Yes Yes 10
3 — 3 3 2 — 2 — 2 3 — — 5 5 6 Avg.3.4
No No Yes Yes Yes No Yes Yes Yes No Yes No Yes No Yes 9
— — 5 4 4 — 2 2 2 — 2 — 3 — 2 Avg.2.9
Learning criterion was defined as 2 out of 3 consecutive minutes above 1.5 times baseline level. Time elapsed to satisfy criterion was defined as the second of the two consecutive minutes in which the infant first learned the task.
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Fig. 4. Group means and standard errors for the long-term baseline and retention ratios.
and is operationally defined as a baseline ratio greater than the theoretical population value of 1.00. A second measure, retention ratio, is used in conjunction with the baseline ratio to indicate whether the retention is complete or partial. A baseline ratio significantly greater than 1.00 together with a retention ratio significantly smaller than 1.00 indicates partial retention. Complete retention requires a baseline ratio significantly greater than 1.00 and a retention ratio not significantly smaller than 1.00. To determine whether infants retained the task, t tests were conducted to compare the mean baseline and retention ratios of each infant against the corresponding theoretical baseline and retention ratios of one. These analyses revealed a baseline ratio significantly greater than 1.00 (t ⫽ 2.14, p ⫽ .043) and a retention ratio not significantly smaller than 1.00 as shown in Fig. 4. These results suggest that infants achieved complete retention after 2 days of training. 3.3. Motor solutions Within the tight constraints of the task, infants could still use three motor solutions to make the mobile move: minimal movement, small movement, and large movement. To understand how each motor solution changed over the course of the sessions, three separate 3 (session) ⫻ 6 (block) analysis of variance with repeated measures in both factors were conducted.
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The minimal-movement strategy changed from being a nonpreferred pattern in baseline to becoming one of the preferred patterns during the acquisition blocks in all 3 sessions. A 3 ⫻ 6 ANOVA yielded a significant block effect (F(5,227) ⫽ 8.66, p ⫽ .0001) and a significant session effect (F(2,227) ⫽ 5.83, p ⫽ .003). The block by session interaction effect was not significant. Posthoc comparisons using the Bonferroni method revealed a significant difference between session 2 and the other 2 sessions (session 1 vs. 2, F(1,227) ⫽ 2.93, p ⬍ .02; session 2 vs. 3, F(1,227) ⫽ 2.99, p ⬍ .01). These results suggest that the frequency of minimal-movement motor solution during session 2 was significantly higher compared to sessions 1 and 3 (see Fig. 5a-c). Further posthoc comparisons using the Bonferroni method revealed a significant difference between baseline and the 3 first periods of the acquisition phase (base vs. acq.1, F(1,227) ⫽ 3.55, p ⫽ .007; base vs. acq.2, F(1,227) ⫽ 4.14, p ⫽ .0007; base vs. acq.3, F(1,227) ⫽ 3.71, p ⫽ .04), indicating an increase in the frequency of minimal-movement motor solutions from baseline to acquisition (see Fig. 5a-c). The small movement strategy changed in its frequency through time in all 3 sessions. The 3 ⫻ 6 ANOVA yielded a significant block effect (F(5,227) ⫽ 2.66, p ⬍ .03). No other effects were significant. (see Fig. 5a-c). Posthoc comparisons using the Bonferroni method revealed a significant difference between acquisition block 1 and the extinction phase in session 2 (acq.1 vs. ext., F(1,227) ⫽ 4.41, p ⬍ .003). These results suggest that the small movement strategy was significantly increased at the immediate retention phase (i.e. extinction) at the end of the training period. Finally, the analysis of the large movement strategy yielded no significant effects, indicating that the frequency of this motor solution did not change significantly across blocks and sessions. The analysis of minimal, small, and large movements suggested that minimal movement was the motor solution preferentially selected. To confirm this statement, we calculated for each infant the proportion of minimal movements in each session and an analysis of the proportion of minimal movements was conducted. A 3 (session) ⫻ 6 (block) ANOVA with repeated measures yielded a significant block effect (F(5,238) ⫽ 4.49, p ⫽ .001). No other main or interaction effects were significant. Analysis of simple main effects, analyzed separately for each session (Keppel, 1991), revealed a significant effect for session 2 (F(5,238) ⫽ 2.49, p ⫽ 0.03) and a trend for session 3 (F(5,238) ⫽ 2.19, p ⫽ 0.056). These results parallel the previous findings based on absolute frequency of minimal movements and suggest that the proportion of minimal-movement motor solution during session 2 was significantly higher compared to sessions 1 and 3. Individual preferences in motor solutions are presented in Table 2. On average, there were no changes in the frequency of small or large movements as motor solutions from average baseline value to the average value of all 4 acquisition periods in all 3 sessions. On the contrary, the minimal-movement strategy increased from average baseline value to average acquisition value in all three sessions. Ten out of 15 infants increased the minimal-movement solution from baseline to acquisition in session 1. The same change was observed in 9 out of 15 in sessions 2 and 3. In contrast, only 6 infants increased their frequency of large movement motor solutions from baseline to acquisition in sessions 1 and 2. In session 3, 8 out of 14 infants increased large movements as a motor solution to the task although these changes were minimal for most of them (see Table 2).
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Fig. 5. Group means of frequency of motor solutions during baseline, acquisition, and extinction: (a) session 1, (b) session 2, and (c) session 3.
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Figure 5. (Continued)
3.4. Motor solutions and retention To examine the partial contribution of the different motor solutions to retention, we calculated individual baseline and retention ratios for each motor solution in each infant. These ratios were calculated for long-term retention and are presented in Table 3. Looking at long-term retention, the baseline ratio for small movements was significantly larger than the theoretical population value of one (t ⫽ 2.77, p ⫽ .015), and the retention ratio for the same motor solution was not significantly smaller than 1. These results suggest that infants showed complete retention for an intermediate strategy (small movements) between the one preferred most initially (large movements) and the most economical one (posture-based solution).
4. Discussion In this paper three hypotheses were explored: (1) that 3-month-olds could learn a constrained motor task, (2) that 3-month-olds would remember the task after a 2-day training period and a 72-hr retention test, and (3) that they would change their preferred movement patterns to solve the task in a more efficient way. Our results supported all three hypotheses. We found that infants learned the contingent relationship between the mobile movement and
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Table 2 Frequency of motor solution per infant and session Subject
Session 1
Base
Acq. Avg
Base
Acq. Avg
Base
Acq. Avg
Base
Acq. Avg
Base
Acq. Avg
LM
Acq. Avg
SM
Base
MM
Acq. Avg
LM
Base
SM
Acq. Avg
MM
Base
LM
Acq. Avg
KM JH AD AB MM SB AT AH HH HW NW TK CM JN OW TOTAL
SM
Session 3
Base
MM
Session 2
1 1 1 4 6 1 4 1 2 2 1 1 1 9 3 3
4 7 12 11 35 1 8 21 – 5 0 1 3 7 4 9
3 5 5 3 1 4 13 2 6 1 1 1 1 4 1 3
4 7 7 6 2 3 11 3 – 5 0 2 1 4 1 4
1 1 44 10 4 4 39 1 5 1 1 1 12 16 0 10
34 30 9 5 0 17 8 2 – 25 0 2 3 4 3 10
1 1 3 4 8 1 2 0 6 1 1 12 7 6 8 4
2 8 30 21 41 0 22 32 – 16 0 1 6 7 4 13
3 4 20 3 2 2 12 – 7 7 1 1 3 2 2 5
3 8 8 9 2 0 8 1 – 4 0 0 5 8 3 4
21 13 10 1 1 19 3 0 2 30 1 1 1 9 4 8
29 18 5 5 1 10 3 4 – 25 0 2 6 6 12 9
1 1 1 8 3 1 3 7 1 1 1 1 9 4 1 3
1 8 6 29 26 2 3 20 3 10 3 1 4 3 9 9
8 21 8 2 6 7 4 10 4 3 1 1 3 3 1 5
1 5 3 3 3 14 10 1 1 4 1 0 2 9 2 4
44 15 11 1 1 44 10 4 4 39 1 1 1 1 1 12
20 21 13 2 0 21 13 7 3 24 1 1 6 11 14 11
Table 2 displays the changes in minimal-movement (MM), small-movement (SM), and large-movement (LM) motor solutions from average baseline to the average response during the 4 blocks in acquisition.
their leg actions. These results replicated the findings of our previous study (Angulo-Kinzler, 1997). Also, these results showed that a similar percentage of infants learned the task in all three sessions. Interestingly, the average time to reach learning criteria decreased from session to session. It seems that infants in session 1 took longer to reach the learning criterion and the increase in reinforcements was not as large compared to previous studies using the TM procedure. Possibly, the constraints imposed in the CM procedure were greater than in the TM procedure. The lack of haptic and visual information from the ribbon, the limited range of successful motor solutions, and the nonconjugate nature of the reinforcement could account for such differences. However, further research is needed to clarify the impact of these differences. Table 3 Long-term baseline and retention ratios per motor solution
Reinf. MM SM LM Mean ratio and SE.
Baseline (⫽ Base 3/Base 1)
Retention (⫽ Base 3/Ext 2)
1.79 (0.37) p ⫽ 0.04 1.80 (0.66) N. S. 2.17 (0.45) p ⫽ 0.015 1.25 (0.25) N. S.
1.84 (0.71) N. S. 1.08 (0.46) N. S. 0.81 (0.18) N. S. 3.65 (2.89) N. S.
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In accordance with the second hypothesis, we found infants were able to remember the task after a two-day training period and 72-hr retention test. We already know that changes in training and context, such as testing location, affect 3 month-old infants’ capacity to remember (Enright, Rovee-Collier, Fagen & Caniglia, 1983; Hayne, Rovee-Collier & Borza, 1991). We also know that changes in the mobile’s characteristics affect this recall capacity (Fagen & Rovee, 1976; Fagen, Rovee & Kaplan, 1976; Rovee-Collier, Adler & Borza, 1994). Our results also suggested that the availability and use of two motor solutions during the training period had an impact on how infants remembered the task. Infants remembered the appropriate task, crossing 85-degree flexion with the right knee, to make the mobile move. However, they mainly used a combination of two motor solutions to accomplish the task. The first solution was large movements resembling kicks that already formed a significant part of their initial preferred repertoire. The second was a posture-based solution and it increased in preference in their repertoire as a result of the task constraints. Our results suggested that infants at 3-months of age remembered the required task and modified their motor repertoires. Our interpretation is that the visual stimulation of a stationary mobile brought to infants the memory of the overall goal of the task—to make the mobile move. However, the specific details of the motor solutions might not be recognized until the contingent relationship started. Rovee-Collier & Sullivan (1980) found similar results in the visual domain when using mobiles with different components. When infants were tested after a greater long-term retention period, the authors suggested that memory may be inhibited due to the increased specificity of the task. Therefore, they proposed that access to general perceptual features persist longer than access to specific details. We hypothesize that a similar phenomenon may have occurred in the motor domain in the current study. More recently, Bhatt & Rovee-Collier (1994; 1997) have shown the effects of changes in individual mobile features, relations among features and number of elements in the mobile in infants’ memory capacity. They demonstrated that infants at 3-months of age are sensitive to both feature details and relations, but individual features are remembered for longer periods of time than relation among features. In addition, they showed that an increase in the number of mobile elements (memory load) constrained long-term memory for relational information. All these studies have concentrated on the characteristics of the reinforcer. However, we propose that changes in the required task may also have relevant effects on infants’ memory. Looking at the relative contribution of each motor solution, it is clear that the minimalmovement strategy was the predominant solution in session 2 while it emerged as a competing strategy together with large movements in sessions 1 and 3. That is, in these sessions the minimal-movement strategy increased more than other solutions compared to baseline value, though it was not the predominant solution. These results suggest that infants at this age may need longer periods of acquisition and/or longer periods of training to retain a highly specific component of the motor task—the minimal-movement strategy in our case. We propose that relevant task factors (such as mobile characteristics or type of motor solution) may have a larger impact in memory retention than context manipulations. Previous research has already shown that changes in mobile details impair retention even if novelty is introduced 24 hr later (Fagen et al., 1976; Rovee & Fagen, 1976). Likewise, the relative contribution of one motor strategy, minimal-movement, changed after 24 hr into the study. We suggest that minimal-movement not being a predominant strategy during both training
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sessions may have impaired the retention of that specific motor solution. On the other hand, when context is modified by changing training locations (Hayne et al., 1991) or different music in the environment (Fagen et al., 1997), the decreasing effect is only seen when retention is tested many more days later (2 weeks in Hayne’s study, 7 days in Fagen’s). These two studies suggest that task constraints have an immediate effect on memory retention. Our results also showed that task constraints may have an effect on retention of a highly specific motor solution. The preferred leg movement pattern of 3-month-old infants is a single leg kick (Thelen, 1981). A single leg kick has been defined in different ways depending on the research question. In the TM procedure, Rovee-Collier and collaborators characterized (and continue to characterize) a leg kick as a vertical or horizontal excursion of the tied foot which at least partially retraces its original path in a smooth and continuous motion (definition still in use since Rovee & Rovee, 1969). On the other hand, most research in the motor domain has defined a leg kick as a hip and knee flexion which brought the knee closer to the trunk (Heriza, 1991; Piek, 1996; Piek & Carman, 1994; Thelen, 1979; Thelen, 1981; Thelen, 1985). Assuming that the most relevant aspect of a leg kick in young infants in a supine position is the range of motion of the knee, our data supported the previous findings of leg-kick as a preferred motor pattern at 3 months of age because we found a prevalence of large movement strategy in all baseline periods. However, our data also indicated that infants are able to modify their preferred behavior to fit the task demands, confirming the hypothesis that infants would change their usual kicking pattern to a more economical one. Since the 1960s researchers have been exploring questions related to infants’ ability to learn contingent relationships between their actions and an external stimulus in the environment (Janos & Papousek, 1977; Milewski & Siqueland, 1975; Papousek, 1967). We already know that infants as young as a few days-old turn their heads in response to milk availability, or modify their sucking rate in order to obtain a visual reinforcer. Only in the past 20 years, researchers have been using instrumental conditioning procedures to study other aspects of infants’ behavior such as perception and memory. This study was a first attempt to address infants’ motor control and motor memory abilities by using a constraining mobile procedure. Our data showed that infants discovered a very economical strategy to increase the number of mobile movementsa flexion posture-based strategy which required less energy output. Infants not only discovered and selected this efficient strategy in session 1, they also rediscovered it and used it in sessions 2 and 3. Initially, infants produced more large movements than small or minimal movements to gain reinforcements. The competition between the two main strategies is evident throughout the acquisition phase in session 1. In the second session, infants clearly used the minimal-movement strategy predominantly, but in combination with the previously preferred large movement strategy. Finally, in session 3 infants immediately increased the contribution of the minimalmovement solution throughout the acquisition period maintaining an equally distributed combination between the two major strategies. Looking at what motor strategy was retained, our data suggest that infants remembered small-movements as the strategy to make the mobile move. This is a strategy that is in between large- and minimal-movement in terms of amplitude and therefore, generation of proprioceptive feedback. Remembering small movements, but increasing minimal movements during acquisition, might be the result of an on-going competition between the most practiced
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motor pattern and the most efficient solution. While the mobile movement (contingent reinforcement) is available, the context is salient enough to modify the strength of this competition toward the economical solution. But when the contingent relation is disrupted, the pattern that predominates is large movement. If the three motor solutions are considered as a continuum of attractors that differ in amplitude, it is not too difficult to imagine a shift from large-movement to minimal-movement during the contingent period and a “regress” to an intermediate solution once the contingency is removed (see Fig. 1, right center square). The argument above is based on the assumption that there is a significant proprioceptive difference between the large movements and the small and minimal movement strategies. Unfortunately, our current knowledge about proprioceptive development and learning of proprioceptively different tasks is very limited. Even more limited is our understanding about the impact of proprioceptively different tasks in the memory capabilities of 3-month-olds. Our results indicate that infants can remember a highly specific motor action, but the selection of a more economic solution was dependent on the availability of the contingent relationship. Furthermore, our findings suggest infants remembered an intermediate motor solution between their initially preferred one and the one required by the task. These results indicate a competition between organismic and task constraints originated by infants sensitivity to movement amplitude. Future research efforts should continue to address the selection and retention capabilities of infants for different motor components such as movement amplitude and expand to other attributes. Acknowledgments We appreciate the participation of parents and their infants without whom this study would have not come to fruition. We also thank several undergraduate students for their assistance in data collection and reduction. Finally, our appreciation goes to the anonymous reviewers who improved the quality of this manuscript. References Angulo-Kinzler, R. M. (1997). Exploration and control of leg movements in infants. Doctoral Dissertation. Indiana University. Angulo-Kinzler, R. M. (2001). Exploration and selection of intralimb coordination patterns in three-month-old infants. Journal of Motor Behavior 33 (4), 363–376. Bhatt, R. S., & Rovee-Collier, C. (1994). Perception and 24-hour retention of feature relations in infancy. Developmental Psychology, 30 (2), 142. Bhatt, R. S., & Rovee-Collier, C. (1997). Dissociation between features and feature relations in infant memory: effects of memory load. Journal of Experimental Child Psychology, 67 (1), 69 – 89. Courage, M. L., & Howe, M. L. (2001). Long-term retention in 3–5-month-olds: familiarization time and individual differences in attentional style. Journal of Experimental Child Psychology, 79 (3), 271–293. Enright, M. K., Rovee-Collier, C. K., Fagen, J. W., & Caniglia, K. (1983). The effects of distributed training on retention of operant conditioning in human infants. Journal of Experimental Child Psychology, 36 (2), 209 –225. Fagen, J., Prigot, J., Carroll, M., Pioli, L., Stein, A., & Franco, A. (1997). Auditory context and memory retrieval in young infants. Child Development, 68 (6), 1057–1066.
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Fagen, J. W., & Rovee, C. K. (1976). Effects of quantitative shifts in a visual reinforcer on the instrumental response of infants. Journal of Experimental Child Psychology, 21 (2), 349 –360. Fagen, J. W., Rovee, C. K., & Kaplan, M. G. (1976). Psychophysical scaling of stimulus similarity in 3-month-old infants and adults. Journal of Experimental Child Psychology, 22 (2), 272–281. Gulya, M., Galluccio, L., Wilk, A., & Rovee-Collier, C. (2001). Infants’ long-term memory for a serial list: recognition and reactivation. Developmental Psychobiology, 38 (3), 174 –185. Harris, F. C., & Lahey, B. B. (1978). A method for combining occurrence and nonoccurence interobserver agreement scores. Journal of Applied Behavior Analysis, 11, 523–527. Hartshorn, K., Rovee-Collier, C., Gerhardstein, P., Bhatt, R. S., Wondoloski, T. L., Klein, P., Gilch, J., Wurtzel, N., & Campos-de-Carvalho, M. (1998). The ontogeny of long-term memory over the first year-and-a-half of life. Developmental Psychobiology, 32 (2), 69 – 89. Hofstadter, M., & Reznick, J. S. (1996). Response modality affects human infant delayed-response performance. Child Development, 67 (2), 646 – 658. Hayne, H. (1990). The effect of multiple reminders on long-term retention in human infants. Developmental Psychobiology, 23 (6), 453– 477. Hayne, H., Rovee-Collier, C., & Borza, M. A. (1991). Infant memory for place information. Memory & Cognition, 19 (4), 378 –386. Heriza, C. B. (1991). Implications of a dynamical systems approach to understanding infant kicking behavior. Physical Therapy, 71 (3), 222–235. Janos, O., & Papousek, H. (1977). Acquisition of appetitional and palpebral conditioned reflexes by the same infants. Early Human Development, 1 (1), 91–97. Keppel, G. (1991). Design and analysis: a researcher’s handbook. New Jersey: Prentice Hall. Milewski, A. E., & Siqueland, E. R. (1975). Discrimination of color and pattern novelty in one-month human infants. Journal of Experimental Child Psychology, 19 (1), 122–136. Najm-Briscoe, R. G., Thomas, D. G., & Overton, S. (2000). The impact of stimulus ‘value’ in infant novelty preference. Developmental Psychobiology, 37 (3), 176 –185. Papousek, H. (1967). Experimental studies of appetitional behavior in human newborns and infants. In H. W. Stevenson, F. H. Hess & H. L. Rheingold (Eds.), Early behavior: comparative and developmental approaches. New York: Wiley. Piek, J. P. (1996). A quantitative analysis of spontaneous kicking in two-month-old infants. Human Movement Science, 15, 707–726. Piek, J. P., & Carman, R. (1994). Developmental profiles of spontaneous movements in infants. Early Human Development, 39 (2), 109 –126. Rovee, C. K., & Fagen, J. W. (1976). Extended conditioning and 24-hour retention in infants. Journal of Experimental Child Psychology, 21 (1), 1–11. Rovee, C. K., & Rovee, D. T. (1969). Conjugate reinforcement of infant exploratory behavior. Journal of Experimental Child Psychology, 8 (1), 33–39. Rovee-Collier, C., Adler, S. A., & Borza, M. A. (1994). Substituting new details for old? Effects of delaying postevent information on infant memory. Memory & Cognition, 22 (6), 644 – 656. Rovee-Collier, C., Evancio, S., & Earley, L. A. (1995). The time window hypothesis: spacing effects. Infant Behavior & Development, 18, 69 –78. Rovee-Collier, C., & Hayne, H. (1987). Reactivation of infant memory: implications for cognitive development. Advances in Child Development & Behavior, 20, 185–238. Rovee-Collier, C. K., Enright, M., Lucas, D., Fagen, J. W., & Gekoski, M. J. (1981). The forgetting of newly acquired and reactivated memories of 3-month-old infants. Infant Behavior & Development, 4, 317–331. Rovee-Collier, C. K., & Fagen, J. W. (1981). The retrieval of memory in early infancy. In L. P. Lipsitt (Ed.), Advances in infancy research (pp. 225–254). Norwood, NJ: Ablex. Rovee-Collier, C. K., & Sullivan, M. W. (1980). Organization of infant memory. Journal of Experimental Psychology: Human Learning & Memory, 6 (6), 798 – 807. Rovee-Collier, C. K., Sullivan, M. W., Enright, M., Lucas, D., & Fagen, J. W. (1980). Reactivation of infant memory. Science, 208 (4448), 1159 –1161.
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Schwartz, B. B., & Reznick, J. S. (1999). Measuring infant spatial working memory using a modified delayedresponse procedure. Memory, 7 (1), 1–17. Sullivan, M. W., Rovee-Collier, C. K., & Tynes, D. (1979). A conditioning analysis of long-term memory. Child Development, 50, 152–162. Thelen, E. (1979). Rhythmical stereotypies in normal human infants. Animal Behaviour, 27 (Pt 3), 699 –715. Thelen, E. (1981). Kicking, rocking, and waving: contextual analysis of rhythmical stereotypies in normal human infants. Animal Behaviour, 29 (1), 3–11. Thelen, E. (1985). Developmental origins of motor coordination: leg movements in human infants. Developmental Psychobiology, 18 (1), 1–22. Thelen, E. (1994). Three-month old infants can learn task-specific patterns of interlimb coordination. Psychological Science, 5 (5), 280 –285.
Selection and memory of a lower limb motor-perceptual task in 3-month-old infants Rosa M. Angulo-Kinzler*, Christen L. Horn Center for Human Motor Research, Division of Kinesiology, University of Michigan, Ann Arbor, MI 48109-2214, USA Received 2 May 2001; received in revised form 10 September 2001; accepted 21 September 2001
Abstract The purpose of this study was to test infants’ ability to learn and memorize a specific motorperceptual task using a constraining reinforcement mobile protocol. Fifteen three-month-old infants received a mobile reinforcement for every correct movement they performed. The task was to cross 85 degrees of flexion with the right knee. Infants were tested over a series of three sessions, spaced 24 and 72 hr apart. Results indicated that infants increased the frequency of mobile jiggles by selecting a new and efficient motor solution: smaller movements around the required threshold. These data suggest infants modulated their preferred movement patterns in an economical way. The data also showed that infants memorized the task by the third session, indicating they can learn and retain a specific motor-perceptual task. This implies that infants at this age are sensitive to environmental information and to the consequences of their own posture and movements. © 2002 Elsevier Science Inc. All rights reserved. Keywords: Infancy; Movement; Retention; Mobile
1. Introduction The acquisition of new motor skills is an integral part of an infant’s growth and development. Imagine going through life never having learned to coordinate leg movements into functional walking. Perhaps more importantly, if an infant does learn specific tasks, what
* Corresponding author. Tel.: ⫹1-734-647-9851; fax: ⫹1-734-936-1925. E-mail address: [email protected] (R. Angulo-Kinzler). 0163-6383/01/$ – see front matter © 2002 Elsevier Science Inc. All rights reserved. PII: S 0 1 6 3 - 6 3 8 3 ( 0 1 ) 0 0 0 8 3 - 2
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purpose does learning serve without a memory of the specific motor skill? Learning and subsequent forgetting would seriously hinder the development of an infant. Recently, tremendous gains have occurred in our understanding of the memory capacity of infants and several factors have been shown to affect their retention responses. First, infants’ age influences the duration of their retention. Hartshorn et al. (1998) have demonstrated that between 2 and 18 months of age infants’ retention increases monotonically. Second, the length of the familiarization period, the reactivation schedule and the delay between stimulus presentation and memory test are also relevant factors affecting retention in infancy. For instance, 1-day delay and 30 s of familiarization time are enough for 3-month-old infants to show long-term retention to a dynamic visual stimulus (Courage & Howe, 2001). Similarly, Gulya et al. (2001) demonstrated that 3-month-olds remember a serial list with a 24-hr delay but not with a 1-week delay independent of providing a reactivation schedule. Another important aspect in evaluating infants’ memory is the inherent or conditioned “value” of the stimulus. Infants’ preferences and therefore their memories are enhanced when the stimulus has a greater value. Several studies have provided evidence to suggest that infants’ preferences should not be evaluated solely on the amount of stimulus experience but consider simultaneously the changes in stimulus value (Najm-Briscoe, Thomas & Overton, 2000; Schwartz & Reznick, 1999). Finally, the modality of the infants’ response will also affect their retention performance. Hofstadter & Reznick (1996) claimed that infants showed poorer retention performance when they were asked to reach as compared to just direct their gaze to the stimulus. One of the most effective ways to study learning and memory in preverbal infants is the Conjugate Reinforcement Mobile task (Rovee & Rovee, 1969), which will be referred to as the Traditional Mobile Procedure (TM). In this procedure a 3/8” polyester ribbon binds the infant’s ankle to an overhead mobile forming a conjugate relationship (the mobile moves at the same rate and amplitude as the infant’s leg). This experimental procedure allows the manipulation of the aforementioned factors affecting memory. In 1976, Rovee-Collier and Fagen used the TM procedure to examine the effect length of training on 24-hr recall of the TM task. Rovee-Collier, Evancio & Earley (1995) studied how the length of time between sessions of the TM procedure affects long-term memory in 3 month-old infants when the same mobile is used in all sessions. They found that retention was best when infants had 48 hrs to consolidate their experience. Further studies have shown that the optimal time delay between training and long-term retention test is between 1 and 4 days (Hayne, 1990; Rovee-Collier, Sullivan, Enright, Lucas & Fagen, 1980; Sullivan, Rovee-Collier & Tynes, 1979). These results imply that there is a time window of 3 days during which information from the first session can be integrated with information from a second session. Years of research conducted by Rovee-Collier and her colleagues have provided a large body of information on learning and memory in 3-month-olds. Some of these contributions can be summarized as: (1) the TM procedure provides an environment for studying infant motor acquisition, (2) at 3 months, infants are capable of learning the contingency between rate of kicking and movement of a mobile, (3) the mobile movement serves as reinforcement for leg movement, (4) through comparisons of baseline and extinction rates of kicking, both short term and long term memory can be studied, (5) at 3 months, infants can remember a learned contingency, (6) the amount of time between sessions affects how much the infant
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retains, and (7) the context of the situation affects what is retained. It is clear that at 3 months, infants have the ability to learn and remember the contingency between leg movement and mobile movement. These contributions stream from manipulating constraints either in the mobile or the proximate environment, such as modifying the number or details of the mobile’s elements, or changing the distal environment such as the location of testing, color of crib bumpers, or background music. However, no study has manipulated the constraints of the motor component of this task. The question then is to what extent infants can acquire and retain a constrained motor solution. In order to study how infants acquire and retain a specific motor skill, it is necessary to place movement constraints on the training environment. Constraints limit the possible motor solutions by requiring an infant to produce a specific movement that is not the infant’s preferred movement pattern. The results of Thelen (1994) are an excellent example of skill acquisition through placing constraints on a training environment. Previously, Thelen (1985) determined that most 3-month-old infants prefer to display single or alternating kicks as compared to simultaneous kicks. Therefore, in-phase kicking (both legs together) is not a preferred pattern of coordination for 3-month-olds. To test if infants could change their kicking patterns, Thelen used the TM procedure but changed the constraints of the task by yoking infants’ legs together with a loosely tied elastic. The yoke made single and alternating kicks more difficult but still possible. However, the “yoking” favored in-phase kicks as a solution requiring less effort. As predicted, infants whose legs were yoked produced a higher frequency of in-phase kicks than those not yoked. Through exploration, the infants discovered that their original coordination pattern was no longer functional for them. Instead, a new movement pattern (in phase kicking) was much easier for the infants to produce, and infants greatly increased their frequency of in phase kicks. These data show that 3-month-old infants can learn patterns of interlimb coordination through exploration and selection of movements that are not within their original coordination tendencies. If infants are capable of learning and remembering the contingency between leg movement and mobile movement when there is a physical connection between the legs, are they also capable of learning and remembering a specific motor solution to a conditioning task without a physical connection between the legs or the mobile? Since the TM procedure requires a physical connection, and any general leg movement results in mobile movement, it was necessary to design a new procedure which makes task demands specific enough to test what motor aspects infants actually learn and remember. In 1997, Angulo-Kinzler implemented a Constraining Reinforcement Mobile Procedure (CM) which places a task requirement on infants such that they must explore their environment and identify the movement that is being reinforced. In comparison to the context of the TM procedure that has a broad proprioceptive and visual component, the CM procedure narrows the proprioceptive component by constraining the training environment. In addition, an auditory component was included. Therefore, the infant must finely map the multiple sensory modalities available in the context in order for learning and full retention to be evident (see Fig. 1). In essence, the CM procedure constrains infants’ repertoire of movements that will result in reinforcement to one very specific movement. In Angulo-Kinzler’s study (Angulo-Kinzler, 1997), right knee flexion at a threshold of 85 degrees was used as the position criteria necessary to receive a reinforcement from the mobile. Sensors called goniometers were
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Fig. 1. Representation of the Traditional and Constrained mobile procedures indicating their differences in involvement of various relevant sensory modalities, task requirements and motor solutions. Each sensory modality is represented by a circle filled with a different pattern. The region of motor solutions is represented by the black intersection among circles, and further expanded in the square presentation of task requirement versus movement amplitude. In the constrained method the task requirement is reduced to 85° flexion, while any degree of either flexion or extension might be valid in the traditional method.
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placed on both the infant’s knees to measure the degree of flexion in those joints. When the goniometers detected a crossing of the 85-degree threshold in the flexion direction, a computer triggered a controlled movement of the mobile. Hence, the amplitude of the knee movement has no effect on the mobile movement. The experimental session consisted of a 2-min baseline period, a 10-min acquisition period, and a 2-min extinction period. High flexion of the right knee joint triggered mobile movement only in the acquisition period. Angulo-Kinzler found that infants were in fact capable of learning the contingency between flexion of the right knee at 85 degrees and the reinforcement from the mobile. Upon further analysis, it became apparent that the constraints of the training environment made available two proprioceptively different solutions. Some infants preferred to use right leg kicks to cross the threshold and gain reinforcement (a high-amplitude movement-based solution). Other infants employed small movements around the threshold to receive almost continual reinforcement from the mobile (a low-amplitude posture-based solution). The movementbased and the posture-based solutions have significantly different involvement of movement amplitude to gain the reinforcement making posture-based solutions more economical. The differences seen in the way infants solved this task provided an opportunity to examine what infants remember about their movements. Based on past research, it was expected that infants would learn the constrained task during 2 days of consecutive training and subsequently show retention of the specific motor solution after a 3-day intermission period (retention 72 hrs later). The constraints placed on the training environment in the current study served to make the conditional motor response more specific than those in the TM procedure. We predicted that (1) infants would be able to increase the number of reinforcements gained to meet the learning criterion in all sessions. We also predicted that (2) infants would increase the number of threshold crossings produced in the baseline of session 3 as compared to the baseline of session 1 thereby showing retention of the constrained task. Finally, we expected that (3) there would be a change in the initially preferred leg movement patterns to one that required less movement and therefore was more economical.
2. Method 2.1. Participants Fifteen 3- to 31⁄2-month-old (corrected age, M ⫽ 97.2 days, SD ⫽ 8.1) infants, nine males and 6 females, participated in this study. Age was corrected for one 4-month-old infant who was born 1 month prematurely. Potential participants were identified through birth announcements in the local newspaper. A letter explaining the study was then sent out and followed up with a phone call. If the parents were interested in participating, they were asked to bring their infant into the laboratory for three sessions, the first two spaced 24 hrs apart and the third one 72 hrs later. All participants signed consent forms that were approved by the institutional review board. Five additional infants were tested but their data were not included in the analyses due to excessive crying (3) and technical problems (2). Also, the
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baseline data file for one infant was lost in session 2, and the video recording of one infant was not captured for sessions 1 and 2. 2.2. Apparatus A medium-sized mobile made from black, white, red, and blue designed plastic squares oriented both vertically and horizontally (The First Years Inc., Discovery mobile) served as the reinforcer. This mobile was a novel object for all infants as reported by the parents. Researchers confirmed with the parents that this mobile was novel for the infant. A chime, hung from the center of the mobile, provided acoustic stimulation in addition to the visual stimulation available from the mobile components. A motor was connected to the mobile by a transparent string, allowing the motor to provide a controlled pull on the mobile (0.6 sec duration) specified by a customized PC computer program. Sensors, called goniometers (Penny & Giles, XM-75), were used to measure the degree of angular displacement in the infants’ right and left knees. Each goniometer was approximately 7.5 cm long and consisted of two plastic rectangular pieces which measured about 3 cm ⫻ 0.75 cm and were connected by a thin wire spring (zero degrees ⫽ full extension). The goniometer signal was recorded by the computer and the customized program evaluated the movement conditions that were required to provide the reinforcement. If the conditions were fulfilled the reinforcement was provided (for more details, see Angulo-Kinzler, 1997 & 2001). The program saved the goniometer signals for the baseline and extinction but not while it was delivering reinforcements during the acquisition. A video camera on an elevated tripod in the corner of the room was used to record the entire session. A SMPTE time code was inserted on the video image to record time during the session. An LED connected to the computer flashed in front of the lens of the video camera each time the mobile was activated and was therefore recorded on tape. The only light in the testing area came from a halogen lamp placed near the infant’s head to illuminate the mobile. 2.3. Procedure Once the parent signed the consent form, the experimenter and a trained assistant undressed the infant from the waist down and loosened the diaper so the infant could kick freely. Two double sided adhesive electrode collars were attached to each end of the goniometer. One end of the sensor was placed on the anterior right thigh and the other end was placed on the anterior right shank so that the goniometer was in position to measure the angular displacement of the knee. A piece of hypoallergenic tape was placed over each end of the goniometer to ensure it was held in place. A pair of oversized footless cotton tights was put on the infant to keep the infant from grasping the sensors and cords. The goniometers were calibrated by holding the infant’s right leg fully extended (full extension ⫽ 0 deg), then bending the infant’s leg to 90 degrees flexion to make sure it reads approximately 90 degrees on the sensor display (see Fig. 2). The infant was then placed in a supine position on a padded surface underneath the mobile and held in place by a soft foam belt secured with Velcro strips. The VCR and the time code
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Fig. 2. Biofeedback system consisting of a PC computer, 4 goniometers and a motor-driven mobile. In this study, goniometers were only placed on the knees.
generator were activated to record the session. After a 2-min baseline period in which right leg flexion did not trigger the mobile to move, one “free” mobile reinforcement initiated the 8-min acquisition period. The task requirement for the infant was right knee flexion at 85 degrees in the flexion direction. During the acquisition period, each time the infant crossed the 85 degree threshold in the flexion direction, a controlled mobile reinforcement was received. The mobile moved just enough for the pieces of the mobile to shake and the chime to produce a small sound. Following the acquisition period, a 2-min extinction period in which knee flexion did not result in mobile reinforcement was provided. Throughout the session, the parent sat about 6 ft from the infant, far enough to be out of sight, allowing the infants to focus on the mobile task. The same procedure was used during all three sessions. Session 2 took place 24 hrs after session 1 and session 3 took place 72 hrs after session 2. The parent received $5.00 at the end of each session regardless of the infant’s performance. 2.4. Data reduction The data were coded in two different ways. During the baseline and extinction periods, the computer program searched for the 85 degree threshold crossing in the flexion direction and
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counted the number of times the threshold was crossed in each minute. During the acquisition period, the number of LED flashes that indicated a threshold crossing was counted in each minute. An infant was considered to have learned the contingency if the number of mobile reinforcements received during two out of three consecutive minutes of the acquisition period exceeded 1.5 times the average baseline value (the learning criterion). Long-term retention was measured by obtaining baseline and retention ratios of reinforcements (Rovee-Collier, Enright, Lucas, Fagen & Gekoski, 1981; Rovee-Collier & Hayne, 1987). These measures are the standard means of determining retention in TM procedures. A baseline ratio was calculated for each infant by dividing the number of 85°-threshold crossings during the initial 2-min nonreinforced period at the outset of session 3 (long-term retention test) by the number of 85°-threshold crossings emitted during the initial 2-min of baseline period in session 1. A retention ratio was also calculated for each infant by dividing the number of 85°-threshold crossings during the long-term retention test by the number of 85°-threshold crossings during the final 2-min nonreinforced period at the end of session 2. Because the task allowed movements of varying amplitudes to result in crossing the 85° threshold, all reinforcements were then coded for causation. Movements resulting in reinforcement were divided into 3 categories: large movements (⬎45 degrees of movement), small movements (7– 45 degrees of movement) and minimal movement (less than 7 degrees of movement). The computer program determined the types of movements that were used to cross the threshold in the baseline and extinction periods by measuring the minimum and maximum degrees of movement around the threshold. Acquisition periods were visually coded by minute to determine what motor solution caused each reinforcement. Coders were trained to visually determine the distinction between movement types until they reached 80% point-by-point agreement with the second author (Harris & Lahey, 1978). Coders were tested for reliability midway through the data coding and reached an 87% point-by-point agreement. To assess retention, baseline and retention ratios were computed for all three different types of motor solutions.
3. Results 3.1. Learning For analysis purposes, each session was divided into 6 2-min blocks: 1 in baseline, 4 in acquisition, and 1 in extinction. The absolute frequency of reinforcements was analyzed via a 3 (session) ⫻ 6 (block) analysis of variance (ANOVA) with repeated measures in both factors. This analysis revealed a significant block effect (F(5,232) ⫽ 4.37, p ⫽ .0008), indicating a significant increase in the number of reinforcements received for all sessions (see Fig. 3). No other significant effects were found. Posthoc comparisons using the Bonferroni method revealed a significant difference between baseline and acquisition blocks 2 and 3 (base vs. acq. 2, F(1,227) ⫽ 3.06, p ⬍ .04; base vs. acq. 3, F(1,227) ⫽ 3.42, p ⬍ .02). The majority of infants reached the learning criterion in each session. In session 1 and 3, 60% of infants met such criteria. In session 2, 67% of infants learned the task (see Table 1). The mean time it took infants to reach the learning criterion in each session is also shown
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Fig. 3. Group means and standard errors of the frequency of reinforcements produced during baseline, acquisition, and extinction conditions in each session.
in Table 1. Of the 9 infants who reached the learning criterion in the first session, the mean time to do so was 3.9 min (SD ⫽ 2.20). In session 2, the mean time for 10 infants to reach the learning criteria was 3.4 min (SD ⫽ 1.43). Finally, the 9 infants in session 3 who reached the learning criterion further reduced the mean time to reach the learning criterion to 2.9 min (SD ⫽ 1.17). 3.2. Retention Baseline and retention ratios were used to measure long-term retention. These two measures are routinely used in the mobile reinforcement task (see, Rovee-Collier & Fagen, 1981; Rovee-Collier & Hayne, 1987). Retention is primarily measured by the baseline ratio Table 1 Number of learners in each session and time elapsed to reach learning criterion
KM JH AD AB MM SB AT AH HH HW NW TK CM JN OW TOTAL
Session 1
Time (minutes)
Session 2
Time (minutes)
Session 3
Time (minutes)
Yes No No Yes Yes No Yes Yes Yes No No Yes No Yes Yes 9
5 — — 8 2 — 2 6 2 — — 2 — 5 3 Avg.3.9
Yes No Yes Yes Yes No Yes No Yes Yes No No Yes Yes Yes 10
3 — 3 3 2 — 2 — 2 3 — — 5 5 6 Avg.3.4
No No Yes Yes Yes No Yes Yes Yes No Yes No Yes No Yes 9
— — 5 4 4 — 2 2 2 — 2 — 3 — 2 Avg.2.9
Learning criterion was defined as 2 out of 3 consecutive minutes above 1.5 times baseline level. Time elapsed to satisfy criterion was defined as the second of the two consecutive minutes in which the infant first learned the task.
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Fig. 4. Group means and standard errors for the long-term baseline and retention ratios.
and is operationally defined as a baseline ratio greater than the theoretical population value of 1.00. A second measure, retention ratio, is used in conjunction with the baseline ratio to indicate whether the retention is complete or partial. A baseline ratio significantly greater than 1.00 together with a retention ratio significantly smaller than 1.00 indicates partial retention. Complete retention requires a baseline ratio significantly greater than 1.00 and a retention ratio not significantly smaller than 1.00. To determine whether infants retained the task, t tests were conducted to compare the mean baseline and retention ratios of each infant against the corresponding theoretical baseline and retention ratios of one. These analyses revealed a baseline ratio significantly greater than 1.00 (t ⫽ 2.14, p ⫽ .043) and a retention ratio not significantly smaller than 1.00 as shown in Fig. 4. These results suggest that infants achieved complete retention after 2 days of training. 3.3. Motor solutions Within the tight constraints of the task, infants could still use three motor solutions to make the mobile move: minimal movement, small movement, and large movement. To understand how each motor solution changed over the course of the sessions, three separate 3 (session) ⫻ 6 (block) analysis of variance with repeated measures in both factors were conducted.
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The minimal-movement strategy changed from being a nonpreferred pattern in baseline to becoming one of the preferred patterns during the acquisition blocks in all 3 sessions. A 3 ⫻ 6 ANOVA yielded a significant block effect (F(5,227) ⫽ 8.66, p ⫽ .0001) and a significant session effect (F(2,227) ⫽ 5.83, p ⫽ .003). The block by session interaction effect was not significant. Posthoc comparisons using the Bonferroni method revealed a significant difference between session 2 and the other 2 sessions (session 1 vs. 2, F(1,227) ⫽ 2.93, p ⬍ .02; session 2 vs. 3, F(1,227) ⫽ 2.99, p ⬍ .01). These results suggest that the frequency of minimal-movement motor solution during session 2 was significantly higher compared to sessions 1 and 3 (see Fig. 5a-c). Further posthoc comparisons using the Bonferroni method revealed a significant difference between baseline and the 3 first periods of the acquisition phase (base vs. acq.1, F(1,227) ⫽ 3.55, p ⫽ .007; base vs. acq.2, F(1,227) ⫽ 4.14, p ⫽ .0007; base vs. acq.3, F(1,227) ⫽ 3.71, p ⫽ .04), indicating an increase in the frequency of minimal-movement motor solutions from baseline to acquisition (see Fig. 5a-c). The small movement strategy changed in its frequency through time in all 3 sessions. The 3 ⫻ 6 ANOVA yielded a significant block effect (F(5,227) ⫽ 2.66, p ⬍ .03). No other effects were significant. (see Fig. 5a-c). Posthoc comparisons using the Bonferroni method revealed a significant difference between acquisition block 1 and the extinction phase in session 2 (acq.1 vs. ext., F(1,227) ⫽ 4.41, p ⬍ .003). These results suggest that the small movement strategy was significantly increased at the immediate retention phase (i.e. extinction) at the end of the training period. Finally, the analysis of the large movement strategy yielded no significant effects, indicating that the frequency of this motor solution did not change significantly across blocks and sessions. The analysis of minimal, small, and large movements suggested that minimal movement was the motor solution preferentially selected. To confirm this statement, we calculated for each infant the proportion of minimal movements in each session and an analysis of the proportion of minimal movements was conducted. A 3 (session) ⫻ 6 (block) ANOVA with repeated measures yielded a significant block effect (F(5,238) ⫽ 4.49, p ⫽ .001). No other main or interaction effects were significant. Analysis of simple main effects, analyzed separately for each session (Keppel, 1991), revealed a significant effect for session 2 (F(5,238) ⫽ 2.49, p ⫽ 0.03) and a trend for session 3 (F(5,238) ⫽ 2.19, p ⫽ 0.056). These results parallel the previous findings based on absolute frequency of minimal movements and suggest that the proportion of minimal-movement motor solution during session 2 was significantly higher compared to sessions 1 and 3. Individual preferences in motor solutions are presented in Table 2. On average, there were no changes in the frequency of small or large movements as motor solutions from average baseline value to the average value of all 4 acquisition periods in all 3 sessions. On the contrary, the minimal-movement strategy increased from average baseline value to average acquisition value in all three sessions. Ten out of 15 infants increased the minimal-movement solution from baseline to acquisition in session 1. The same change was observed in 9 out of 15 in sessions 2 and 3. In contrast, only 6 infants increased their frequency of large movement motor solutions from baseline to acquisition in sessions 1 and 2. In session 3, 8 out of 14 infants increased large movements as a motor solution to the task although these changes were minimal for most of them (see Table 2).
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Fig. 5. Group means of frequency of motor solutions during baseline, acquisition, and extinction: (a) session 1, (b) session 2, and (c) session 3.
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Figure 5. (Continued)
3.4. Motor solutions and retention To examine the partial contribution of the different motor solutions to retention, we calculated individual baseline and retention ratios for each motor solution in each infant. These ratios were calculated for long-term retention and are presented in Table 3. Looking at long-term retention, the baseline ratio for small movements was significantly larger than the theoretical population value of one (t ⫽ 2.77, p ⫽ .015), and the retention ratio for the same motor solution was not significantly smaller than 1. These results suggest that infants showed complete retention for an intermediate strategy (small movements) between the one preferred most initially (large movements) and the most economical one (posture-based solution).
4. Discussion In this paper three hypotheses were explored: (1) that 3-month-olds could learn a constrained motor task, (2) that 3-month-olds would remember the task after a 2-day training period and a 72-hr retention test, and (3) that they would change their preferred movement patterns to solve the task in a more efficient way. Our results supported all three hypotheses. We found that infants learned the contingent relationship between the mobile movement and
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Table 2 Frequency of motor solution per infant and session Subject
Session 1
Base
Acq. Avg
Base
Acq. Avg
Base
Acq. Avg
Base
Acq. Avg
Base
Acq. Avg
LM
Acq. Avg
SM
Base
MM
Acq. Avg
LM
Base
SM
Acq. Avg
MM
Base
LM
Acq. Avg
KM JH AD AB MM SB AT AH HH HW NW TK CM JN OW TOTAL
SM
Session 3
Base
MM
Session 2
1 1 1 4 6 1 4 1 2 2 1 1 1 9 3 3
4 7 12 11 35 1 8 21 – 5 0 1 3 7 4 9
3 5 5 3 1 4 13 2 6 1 1 1 1 4 1 3
4 7 7 6 2 3 11 3 – 5 0 2 1 4 1 4
1 1 44 10 4 4 39 1 5 1 1 1 12 16 0 10
34 30 9 5 0 17 8 2 – 25 0 2 3 4 3 10
1 1 3 4 8 1 2 0 6 1 1 12 7 6 8 4
2 8 30 21 41 0 22 32 – 16 0 1 6 7 4 13
3 4 20 3 2 2 12 – 7 7 1 1 3 2 2 5
3 8 8 9 2 0 8 1 – 4 0 0 5 8 3 4
21 13 10 1 1 19 3 0 2 30 1 1 1 9 4 8
29 18 5 5 1 10 3 4 – 25 0 2 6 6 12 9
1 1 1 8 3 1 3 7 1 1 1 1 9 4 1 3
1 8 6 29 26 2 3 20 3 10 3 1 4 3 9 9
8 21 8 2 6 7 4 10 4 3 1 1 3 3 1 5
1 5 3 3 3 14 10 1 1 4 1 0 2 9 2 4
44 15 11 1 1 44 10 4 4 39 1 1 1 1 1 12
20 21 13 2 0 21 13 7 3 24 1 1 6 11 14 11
Table 2 displays the changes in minimal-movement (MM), small-movement (SM), and large-movement (LM) motor solutions from average baseline to the average response during the 4 blocks in acquisition.
their leg actions. These results replicated the findings of our previous study (Angulo-Kinzler, 1997). Also, these results showed that a similar percentage of infants learned the task in all three sessions. Interestingly, the average time to reach learning criteria decreased from session to session. It seems that infants in session 1 took longer to reach the learning criterion and the increase in reinforcements was not as large compared to previous studies using the TM procedure. Possibly, the constraints imposed in the CM procedure were greater than in the TM procedure. The lack of haptic and visual information from the ribbon, the limited range of successful motor solutions, and the nonconjugate nature of the reinforcement could account for such differences. However, further research is needed to clarify the impact of these differences. Table 3 Long-term baseline and retention ratios per motor solution
Reinf. MM SM LM Mean ratio and SE.
Baseline (⫽ Base 3/Base 1)
Retention (⫽ Base 3/Ext 2)
1.79 (0.37) p ⫽ 0.04 1.80 (0.66) N. S. 2.17 (0.45) p ⫽ 0.015 1.25 (0.25) N. S.
1.84 (0.71) N. S. 1.08 (0.46) N. S. 0.81 (0.18) N. S. 3.65 (2.89) N. S.
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In accordance with the second hypothesis, we found infants were able to remember the task after a two-day training period and 72-hr retention test. We already know that changes in training and context, such as testing location, affect 3 month-old infants’ capacity to remember (Enright, Rovee-Collier, Fagen & Caniglia, 1983; Hayne, Rovee-Collier & Borza, 1991). We also know that changes in the mobile’s characteristics affect this recall capacity (Fagen & Rovee, 1976; Fagen, Rovee & Kaplan, 1976; Rovee-Collier, Adler & Borza, 1994). Our results also suggested that the availability and use of two motor solutions during the training period had an impact on how infants remembered the task. Infants remembered the appropriate task, crossing 85-degree flexion with the right knee, to make the mobile move. However, they mainly used a combination of two motor solutions to accomplish the task. The first solution was large movements resembling kicks that already formed a significant part of their initial preferred repertoire. The second was a posture-based solution and it increased in preference in their repertoire as a result of the task constraints. Our results suggested that infants at 3-months of age remembered the required task and modified their motor repertoires. Our interpretation is that the visual stimulation of a stationary mobile brought to infants the memory of the overall goal of the task—to make the mobile move. However, the specific details of the motor solutions might not be recognized until the contingent relationship started. Rovee-Collier & Sullivan (1980) found similar results in the visual domain when using mobiles with different components. When infants were tested after a greater long-term retention period, the authors suggested that memory may be inhibited due to the increased specificity of the task. Therefore, they proposed that access to general perceptual features persist longer than access to specific details. We hypothesize that a similar phenomenon may have occurred in the motor domain in the current study. More recently, Bhatt & Rovee-Collier (1994; 1997) have shown the effects of changes in individual mobile features, relations among features and number of elements in the mobile in infants’ memory capacity. They demonstrated that infants at 3-months of age are sensitive to both feature details and relations, but individual features are remembered for longer periods of time than relation among features. In addition, they showed that an increase in the number of mobile elements (memory load) constrained long-term memory for relational information. All these studies have concentrated on the characteristics of the reinforcer. However, we propose that changes in the required task may also have relevant effects on infants’ memory. Looking at the relative contribution of each motor solution, it is clear that the minimalmovement strategy was the predominant solution in session 2 while it emerged as a competing strategy together with large movements in sessions 1 and 3. That is, in these sessions the minimal-movement strategy increased more than other solutions compared to baseline value, though it was not the predominant solution. These results suggest that infants at this age may need longer periods of acquisition and/or longer periods of training to retain a highly specific component of the motor task—the minimal-movement strategy in our case. We propose that relevant task factors (such as mobile characteristics or type of motor solution) may have a larger impact in memory retention than context manipulations. Previous research has already shown that changes in mobile details impair retention even if novelty is introduced 24 hr later (Fagen et al., 1976; Rovee & Fagen, 1976). Likewise, the relative contribution of one motor strategy, minimal-movement, changed after 24 hr into the study. We suggest that minimal-movement not being a predominant strategy during both training
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sessions may have impaired the retention of that specific motor solution. On the other hand, when context is modified by changing training locations (Hayne et al., 1991) or different music in the environment (Fagen et al., 1997), the decreasing effect is only seen when retention is tested many more days later (2 weeks in Hayne’s study, 7 days in Fagen’s). These two studies suggest that task constraints have an immediate effect on memory retention. Our results also showed that task constraints may have an effect on retention of a highly specific motor solution. The preferred leg movement pattern of 3-month-old infants is a single leg kick (Thelen, 1981). A single leg kick has been defined in different ways depending on the research question. In the TM procedure, Rovee-Collier and collaborators characterized (and continue to characterize) a leg kick as a vertical or horizontal excursion of the tied foot which at least partially retraces its original path in a smooth and continuous motion (definition still in use since Rovee & Rovee, 1969). On the other hand, most research in the motor domain has defined a leg kick as a hip and knee flexion which brought the knee closer to the trunk (Heriza, 1991; Piek, 1996; Piek & Carman, 1994; Thelen, 1979; Thelen, 1981; Thelen, 1985). Assuming that the most relevant aspect of a leg kick in young infants in a supine position is the range of motion of the knee, our data supported the previous findings of leg-kick as a preferred motor pattern at 3 months of age because we found a prevalence of large movement strategy in all baseline periods. However, our data also indicated that infants are able to modify their preferred behavior to fit the task demands, confirming the hypothesis that infants would change their usual kicking pattern to a more economical one. Since the 1960s researchers have been exploring questions related to infants’ ability to learn contingent relationships between their actions and an external stimulus in the environment (Janos & Papousek, 1977; Milewski & Siqueland, 1975; Papousek, 1967). We already know that infants as young as a few days-old turn their heads in response to milk availability, or modify their sucking rate in order to obtain a visual reinforcer. Only in the past 20 years, researchers have been using instrumental conditioning procedures to study other aspects of infants’ behavior such as perception and memory. This study was a first attempt to address infants’ motor control and motor memory abilities by using a constraining mobile procedure. Our data showed that infants discovered a very economical strategy to increase the number of mobile movementsa flexion posture-based strategy which required less energy output. Infants not only discovered and selected this efficient strategy in session 1, they also rediscovered it and used it in sessions 2 and 3. Initially, infants produced more large movements than small or minimal movements to gain reinforcements. The competition between the two main strategies is evident throughout the acquisition phase in session 1. In the second session, infants clearly used the minimal-movement strategy predominantly, but in combination with the previously preferred large movement strategy. Finally, in session 3 infants immediately increased the contribution of the minimalmovement solution throughout the acquisition period maintaining an equally distributed combination between the two major strategies. Looking at what motor strategy was retained, our data suggest that infants remembered small-movements as the strategy to make the mobile move. This is a strategy that is in between large- and minimal-movement in terms of amplitude and therefore, generation of proprioceptive feedback. Remembering small movements, but increasing minimal movements during acquisition, might be the result of an on-going competition between the most practiced
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motor pattern and the most efficient solution. While the mobile movement (contingent reinforcement) is available, the context is salient enough to modify the strength of this competition toward the economical solution. But when the contingent relation is disrupted, the pattern that predominates is large movement. If the three motor solutions are considered as a continuum of attractors that differ in amplitude, it is not too difficult to imagine a shift from large-movement to minimal-movement during the contingent period and a “regress” to an intermediate solution once the contingency is removed (see Fig. 1, right center square). The argument above is based on the assumption that there is a significant proprioceptive difference between the large movements and the small and minimal movement strategies. Unfortunately, our current knowledge about proprioceptive development and learning of proprioceptively different tasks is very limited. Even more limited is our understanding about the impact of proprioceptively different tasks in the memory capabilities of 3-month-olds. Our results indicate that infants can remember a highly specific motor action, but the selection of a more economic solution was dependent on the availability of the contingent relationship. Furthermore, our findings suggest infants remembered an intermediate motor solution between their initially preferred one and the one required by the task. These results indicate a competition between organismic and task constraints originated by infants sensitivity to movement amplitude. Future research efforts should continue to address the selection and retention capabilities of infants for different motor components such as movement amplitude and expand to other attributes. Acknowledgments We appreciate the participation of parents and their infants without whom this study would have not come to fruition. We also thank several undergraduate students for their assistance in data collection and reduction. Finally, our appreciation goes to the anonymous reviewers who improved the quality of this manuscript. References Angulo-Kinzler, R. M. (1997). Exploration and control of leg movements in infants. Doctoral Dissertation. Indiana University. Angulo-Kinzler, R. M. (2001). Exploration and selection of intralimb coordination patterns in three-month-old infants. Journal of Motor Behavior 33 (4), 363–376. Bhatt, R. S., & Rovee-Collier, C. (1994). Perception and 24-hour retention of feature relations in infancy. Developmental Psychology, 30 (2), 142. Bhatt, R. S., & Rovee-Collier, C. (1997). Dissociation between features and feature relations in infant memory: effects of memory load. Journal of Experimental Child Psychology, 67 (1), 69 – 89. Courage, M. L., & Howe, M. L. (2001). Long-term retention in 3–5-month-olds: familiarization time and individual differences in attentional style. Journal of Experimental Child Psychology, 79 (3), 271–293. Enright, M. K., Rovee-Collier, C. K., Fagen, J. W., & Caniglia, K. (1983). The effects of distributed training on retention of operant conditioning in human infants. Journal of Experimental Child Psychology, 36 (2), 209 –225. Fagen, J., Prigot, J., Carroll, M., Pioli, L., Stein, A., & Franco, A. (1997). Auditory context and memory retrieval in young infants. Child Development, 68 (6), 1057–1066.
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