ADAPTED PHYSICAL ACTIVITY QUARTERLY, 1990,7,325337

Assessing the Perceptual-Motor Interaction in Developmentally Disabled

and Nonhandicapped Children

Allen W. Burton University of Minnesota

The purpose of this experiment was to design a gross-motor task that would quantify the perceptual sensitivity of developmentally disabled (DD) and nonhandicapped (NHJ children to the relationship between their personal constraints and the constraints in the environment in a movement context. Three groups of subjects participated in this experiment: 17 DD preschoolers, 25 NH kindergarteners, and 27 NH fourth-graders. The subjects moved through a sequence of four high-jump barriers six times as quickly as possible, negotiating the barriers any way they wanted. They also went through the course without the barriers as quickly as possible to establish a baseline move- ment time. Relative to their own baseline, the fourth-graders moved through the obstacle course significantly faster than the kindergarteners, while the kindergarteners went through the course significantly faster than the DD pre- schoolers. In addition, significant differences were found between the NH kindergarteners and DD preschoolers for two sets of perceptual variables: percent error and the slopes of two identified transitions from one mode of locomotion to another. These results and further analyses showed that at least some of the movement problems experienced by DD children can be attributed to perceptual difficulties, and established the potential of the present rnethod- ology in examining perceptual sensitivity in a movement context in DD and NH children.

Thelen (1986), in her systems account of the development of locomotion, identified eight components or factors contributing to locomotor skill and proposed that some of these components are "rate-limiters" that constrain the emergence of locomotor behaviors. That is, even if all other components are ready to support bipedal locomotion in a given child, one or two ratelimiters or components that are not ready-such as postural control and extensor strength-would keep that child from beginning to walk. In this developmental context, Thelen (1986) emphasized the importance of identifying the components that are rate limiting in explaining the emergence of locomotor milestones. Similarly, when assessing children with

Request reprints from Men W. Burton, School of Physical Education and Recreation, 1900 University Ave. S.E., University of Minnesota, Minneapolis, MN 55455.

326 Burton

movement problems, it is important to determine which specific components or factors are limiting or constraining their movement behavior.

A list of important components or factors shaping and influencing the motor behavior of handicapped and nonhandicapped children should include body size1 morphology, muscular strength, cardiovascular endurance, flexibility, perceptual accuracy, postural control, the degree of coordination between body components, and the degree of control over the coordinated units. The potential of some of these factors to act as rate-limiters in the movement behavior of handicapped children has been demonstrated (e.g., body size-Dobbins, Garron, & Rarick, 1981 ; motor control-Davis 1986), but there is little research that would prompt a practitioner to consider perception as a possible rate-limiter of the motor behavior of a child with movement problems.

The idea that poor perception may negatively influence movement behavior was presented over 50 years ago, but strong empirical support for this idea never has been generated. Schilder, in his classic book fie Image and Appearance of the Human Body (1950, first published in 1935), stated that if one's body image or perception is faulty, then all related action also will be faulty. Later, during the 1950s and 1960s, one of the major tenets of the perceptual-motor movement was that perception and movement were intimately related and that participation in various movement activities could improve perceptual skill (Cratty, 1975). Kephart, a leader in this movement, obsemed in one of his early books that "much of the awkwardness of the brain-damaged child appears to be due to a lack of integration of perceptual and motor systems, as well as the failure of the visual perceptual processes to provide substantial and clearly structured patterns for the motor actions to follow" (Strauss & Kephart, 1955, p. 172).

The research and writing that emanated from the perceptual-motor move- ment emphasized the interaction between perception and movement; however, this was overshadowed by the questionable premise that "programs of enrichment containing specific movement experiences will probably improve perceptual attributes, including those underlying reading, writing, and thought, and thus, the academic competencies of all children exposed to them" (Cratty, 1975, p. 37). Most of the perceptual-motor research during this period focused on the effect of movement on academics through changes in perception, while the effect of percep- tion on movement remained relatively unexplored. Although the basic idea of perception and movement being closely linked was sound, the negative connota- tions of the term "perceptual-motor" stemming from the widely criticized connec- tion to academic performance (Cratty, 1975; Kavale & Mattson, 1983; Pitcher- Baker, 1976) discouraged work from being carried out in this area. Just recently, the Board of Trustees of the Council for Learning Disabilities (1986) recommended "a moratorium on assessment and training of perceptual and perceptual-motor functions in educational programs" @. 247). However, after the long dormant period that followed the perceptual-motor movement, new interest has been shown in the relationship between movement and perception in persons with movement disorders (e.g., Burton, 1987; Howard & Henderson, 1989; Lord & Hulme, 1988), with an emphasis on a new set of approaches to analyzing the perceptual- motor interaction.

The instruments most commonly used to assess perceptual functions primarily - -

nt context, such as the Motor-Free Visual , 1972), the Frostig Developmental Test - -.

Perceptual-Motor Interaction 327

of Visual Perception (Frostig, Maslow, Lefever, & Whittlesey, 1966), and the Test of Visual-Perceptual Skills (Non-Motor) (Gardner, 1982). The ecological approach to perception, forwarded by Gibson (1977, 1979), offers a conceptual basis for an alternate approach for assessing perception, keeping perception within the context of movement. In Gibson's ecological approach, movement and percep- tion are viewed as being inseparable because the environment is proposed to be functionally perceived in terms of a given individual's capacity to act on it, and thus movement is guided by the perception of these "affordances" for action. In perceiving hislher environment, a person must take into account hislher body sizelmorphology, muscular strength, and other key factors such as those listed earlier (including perceptual accuracy itself). Since perception is based on a person's awareness of hislher own movement limitations, at least in this model, it should be considered as a prime candidate in the search for potential rate-limiters in children with movement problems.

The broad purpose of this study was to explore how to assess the perceptual- motor interaction in handicapped and nonhandicapped children, using an ecological approach to perception, with the goal being to identify children whose motor behavior is limited by perceptual problems. The more specific objective was to design a gross-motor task that would allow for the measurement of the perceptual sensitivity of developmentally disabled and nonhandicapped children to the rela- tionship between their personal constraints (e.g., body size, movement skill) and the constraints in the environment in a movement context. This ecologically orient- ed problem has been investigated in nonhandicapped adults using gross-motor tasks such as stair-climbing (Warren, 1984), sitting (Mark & Vogele, 1987), and moving through doorways or "apertures" (Howard & Henderson, 1989; Warren & Whang, 1987). The effectiveness of the task in meeting the above-stated objec- tive was evaluated in terms of its ability to discriminate between the perceptual- motor behavior of different-age subjects, developmentally disabled and non- handicapped subjects, and normal- and low-skilled subjects in simple and complex situations.

Methods Subjects

Three groups of subjects participated in this experiment: 17 preschoolers with developmental disabilities (DD), 25 nonhandicapped (NH) kindergarteners, and 27 NH fourth-graders (see Table 1 for a detailed description of the subject groups). These groups were chosen to allow for a comparison between children of different ages (kindergarten vs. fourth-grade) and between DD and NH children of the same ages (DD preschool vs. NH kindergarten). The DD preschool children, who ranged in age from 4 years 11 months to 6 years 10 months, qualified for early childhood special education by demonstrating either (a) a substantial delay or disorder in development, or (b) an identifiable sensory, physical, mental, or sociallemotional condition or impairment known to hinder n o d development and to require special instruction and services. Two of these children had Down's syndrome, one was tentatively diagnosed as having Noonan's syndrome, one wore ankle-foot orthotics on both feet, and two wore eyeglasses (as did two of the fourth-graders). All were able to locomote independently without crutches or wheelchairs. As expected, the fourth-graders were significantly taller than the

328 Burton

kindergarteners (p<.0001), while there was no significant height difference between the two groups of younger children. However, the leg lengthlheight ratio was significantly greater for the fourth-graders than for the kindergarteners w.05) and significantly greater for the kindergarteners than for the DD preschoolers W.001) (see Table 1). All 69 children were students in the Osseo School District in the Minneapolis metropolitan area.

Table 1

Number of Subjects by Sex, Mean Age, and Mean Physical Measures for Each Group

Group

Variable Preschool Kindergarten Fourth grade

(DD) (NH) (NH)

No. of subjects Boys 12 Girls 5

Age (in months) 71.9 (5.6)a

Standing height (in cm) 114.7 (9.8)

Leg lengthlheight ratio 0.472 (0.014)

'Standard deviations in parentheses.

Apparatus

The experimental apparatus comprised four sets of high-jump barriers, two digital timers (Lafayette 54417-A), and two infrared photorelay sensors (Radio Shack 49-55 1) mounted on tripods. Each barrier consisted of two metal vertical supports or standards 1.55 m high, marked off in increments of 2.5 cm, with two adjustable brackets on which a 1.22-m wooden stick (1.3-cm square) rested. The actual distance between the standards was 1.17 m. The barriers were placed 2.44 m apart, with the crossbars perpendicular to an imaginary line passing between the stan- dards. A 1.22-m colored-tape line was placed 1.22 m before the frrst barrier and 1.22 m after the last barrier to create an obstacle course 9.76 m long. The photo- relay sensors and their accompanying reflectors were positioned such that an infra- red beam crossed 50.0 cm above each tape line. The digital timers were connected to the photorelay sensors, with one timer recording the time taken to move from one line to the other and the other timer recording movement time in the opposite direction. All subjects were tested individually in a large room with at least 5 m of space beyond each end of the obstacle course.

Perceptual-Motor Interaction 329

Procedures

When the subjects first entered the testing area, their standing height and sitting height were measured and recorded. Then they were instructed to stand behind the tape line nearest them and at the signal "Go!" to run past the other tape line as quickly as possible. After a 3- to 5-second rest, they were instructed to run as quickly as possible in the opposite direction at the appropriate signal. At this point, no crossbars were set across the barrier standards. The movement times for each pass through were recorded by hand from the digital timers.

Next, the crossbars were placed on the barriers at heights that were specified proportions of the subjects' standing height (20,32,44,56, 68, or 80%). Each of the six relative heights were encountered four times for a total of 24 barrier cross- ings. This required six passes through the four-barrier obstacle course, three times in each direction. Three relative-height orders, reversed on the return trips, were chosen so that (a) all heights would occur at each of the four sequence positions (first to fourth), and (b) there would be at least a 24% difference in consecutive heights, alternating heights above and below 50%. These three orders were counterbalanced across subjects within each age group. For each pass through the obstacle course, the experimenter recorded (a) movement time from the digital timer, (b) the movement pattern used to negotiate the barrier in terms of the number of body parts in contact with the floor as they went through the barrier (see Table 2), and (c) the number of times the crossbars or standards were touched. No subjects were filmed or videotaped and only one rater evaluated the subjects' movement pattern (the same one for all subjects). The proportion of agreement between raters for the body-parts score (BPS) was established to be .97 in a previous study also dealing with preschool children (Burton, Pick, Heimichs, & Greer, 1988).

Table 2

Body Part Scores (BPS) Reflecting No. of Body Parts in Contact With Floor as Barrier is Crossed

and the Most Common Movement Behaviors for Each Score

BPS Most common movement behavior

Jump, leap, or hop over Step over Walk under Walk under plus one hand on floor Under with two feet and hands on floor Under with two feet and hands plus one knee on floor Under with two feet, hands, and knees on floor (creep) Never observed Under with two feet, hands, knees, and elbows on floor Under with two feet, hands, knees, elbows, and trunk on floor (crawl)

330 Burton

Finally, the crossbars were removed and the subjects performed two more passes through the course (down and then back) as was done at the beginning of the experiment. This protocol took from 10 to 15 minutes to complete. No feedback about the subjects' performance was given until after the protocol was finished.

Data Analysis

Eight dependent variables were used in this study: number of errors (unintentional touches of the crossbar); movement time (MT) as measured by the digital timers; movement velocity (MV) in terms of leg length per second (r9.76 mlleg length]/MT) without the crossbars; a movement efficiency ratio (MER), defined as the ratio between the mean MT with the crossbars and the mean MT without the crossbars; and four variables related to transitions in the subjects' locomotor patterns as a function of increasing relative height of the barriers.

Touches of the barrier crossbars or standards were divided into intentional and unintentional touches. The intentional touches or grasps were those that a subject used for support in crossing the barrier, whereas unintentional touches simply were considered to be errors. MT was the mean of the three lowest times (out of four) required to move through the 9.76-m course without the crossbars (simple MT). MV, expressed in terms of leg lengths per second, was computed since it controlled for differences in stride lengths, which may account for a signifi- cant portion of the variance in running speed between subjects of different ages or developmental status (Roberton, 1982). The MER was calculated by dividing the mean MT across the six trials with the barriers (some of the DD preschoolers stopped when they knocked a crossbar off a barrier; in those few cases the MTs were not included in the calculation of the mean) by the mean simple MT (i.e., how many times longer did it take them to go through the course with the barriers than without them?). This variable was considered to be a measure of the subjects' loco- motor skill in a complex situation in which they were required to interpret or perceive constraints in the environment relative to their own personal constraints, controlling for difference in simple MT.

Two locomotor transitions were identified as the barrier heights varied from low to high: (a) from going over to not going over (i.e., going under) the cross- bar, and @) from not going under with just two feet (i.e., going over or going under using a pattern involving body parts in addition to the two feet) to walking under. These transitions can be graphically represented by plotting percent occur- rence of a particular mode (e.g., going over the barrier) against relative barrier height (barrier heightlleg length). In Figure 1, the subject always went over at a relative height of 0.62 or below and always went under at a relative height of 1.05 or above.

Two dependent variables were chosen to quantify the two identified loco- motor transitions. Both were determined by a regression line calculated across a restricted range of relative heights (for the over-to-under transition, from the relative height preceding the frrst deviation from 100% occurrence to the relative height following the last deviation from 0%). First, the critical transition height was defined as the relative height at which the locomotor pattern reached 50% occur- rence, as calculated from the regression equation. This point marked the disappear- ance of going over (Transition 1) or the appearance of walking under (Transition 2)

was assumed to reflect how subjects perceived their own movement skill and

Perceptual-Motor Interaction

Q From over + To walk under

110 ,

- l o ! . 1 . I . I . , . , . , . I 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6

Barrier height l leg length

Figure 1 - Percent occurrence of two locomotor behaviors-going over the crossbar and walking under the bar-as a function of barrier height relative to leg length for a developmentally disabled girl, 74 months old and 110 cm tall.

body size in relation to the constraints imposed by the barrier. (In Figure 1, the two transitions occurred at relative heights of 0.84 and 1.18, respectively.) Second, the abruptness or rate of the transition was defined as the slope of the linear regression and was assumed to reflect the subjects' perceptual sensitivity or acuity.

Results

This study focused on examining possible performance differences between the younger and older NH groups and between the age-matched DD and NH groups; thus the statistical analyses utilized two planned comparisons consonant with these two pairings. A MANOVA was considered to include all of the dependent variables in one analysis but was rejected for two reasons. First, several of the DD pre- schoolers did not show one or both of the locomotor transitions (i.e., they never went over or never walked under), causing values for the four transition variable values to be missing for these subjects. Second, only 6 of the 28 intercorrelations between the eight dependent variables were above .60, indicating a relatively low degree of dependence between these variables. Hence, the two planned compari- sons were evaluated separately for each dependent variable, with the alpha value set at .05. No adjustment was made for the increased probability of making a Type I error caused by the repetition of eight sets of comparisons since this was an

332 Burton

exploratory experiment in which an increased probability of Type II errors, and thus greater power, was tolerable.

Temporal/Movement Skill Variables

As could be predicted from previous research (e.g., Branta, Haubenstricker, & Seefeldt, 1984), the fourth-grade NH children took significantly less time than the kindergarten NH children to move through the course without crossbars, F(1,66) =5.88,6.05 (see Table 3). However, when differences in leg length were controlled for through the calculation of MV in terms of leg lengths per second, these significant differences in simple MT disappeared. In the more complex situation where the barriers needed to be negotiated, the younger children had significantly larger MERs than the older children, F(1,66) =5.19, p<.05, indicating that they required more time in comparison to their own simple MT to perceive and act on the constraints in the environment relative to their own personal constraints. The low MER standard deviation for the fourth-graders suggested that they might have been approaching a lower limit for this efficiency ratio (see Table 3).

The performance of the DD preschoolers was significantly poorer than the NH kindergarteners on all three temporal variables: MT, MV, and MER, F(1,66)=25.83, 42.95, and 87.88, respectively, 6 .0001 (see Table 3). Thus, even when differences in physical size were accounted for, the DD children moved slower in the simple situation, and even when differences in simple MT were accounted for, they took twice as much time as the same-age NH children to negotiate the obstacles.

Table 3

Mean Values of Performance Variables Across Groups

Group

Variable

Preschool Kindergarten Fourth grade (DD) (NH) (NH)

M SD M SD M SD

Movement time (in sec) 4.26 1.69 2.87 0.38 2.29 0.16 Movement vel. (in leg

lengthsls) 4.59 1.00 5.95 0.58 6.03 0.42 Mvmt. efficiency ratio 4.33 1.40 2.16 0.41 1.70 0.11 Errors (010) 24.3 17.6 8.3 4.5 5.9 4.5 Transition 1 relative height

(rel. to leg length) 0.679 0.254 0.878 0.130 0.962 0.090 Transition 1 slope (% A/

1 Oh rel. height) 1.94 1.13 3.45 1.31 3.71 1.21 Transition 2 relative height

(rel. to leg length) 1.42 0.34 1.11 0.15 1.04 0.12

Perceptual-Motor Interaction 333

SpatialIPerceptual Variables

The most basic spatialfperceptual variable was percent error, which reflected the degree to which subjects overestimated what they could do (i-e., an inaccurate perception of the environment in relation to their own movement skill and body size). There was no significant difference in percent error between the two groups of NH subjects, but the DD preschoolers made significantly more errors than the NH kindergarteners, F(1,66) =28.30, 6 .0001 (see Table 3).

There were no significant differences in the relative height at which the younger NH and older NH children switched from going over to under (Transi- tion 1) and fiom going under with body parts besides the feet in contact with the ground to just walking under (Transition 2). As might be predicted from a biomechanical model, Transition 1 occurred at about 5-15% below their leg length while Transition 2 occurred at about 5-15 % above their leg length (see Table 3). There was much greater variability in the relative heights of these transitions for the DD children in comparison to the younger NH children, with the over-to- under transition occurring an average of 32 % below their leg length and the transi- tion to walking under occurring an average of 42% above their leg length (see Table 3). These relative transition heights were significantly different from those exhibited by the NH kindergarteners, F(1,63) = 15.3 1, p<.001; F(1,62) = 22.18, 6.0001, indicating lower levels of perceived skill in the NH children. As noted earlier, three of the DD children never went over the crossbar even when it was set at 20% of their standing height, and four never walked under the crossbar even when it was set at 80% (two children did not show either transition).

There were no significant differences in the abruptness of the locomotor transitions between the two groups of NH children; however, the slopes of both transitions were significantly larger (over 70% more steep) for the NH kinder- garteners than for the DD preschoolers, F(1,63) = 13.36, p<.001; F(1,62) =6.25, 6 . 0 5 (see Table 3). Thus it appeared that the DD children were less sensitive to the environmental cues than their same-age NH peers.

A closer look at the distribution of specific locomotor patterns at particular relative heights should provide a more complete perspective of differences between the three groups in their locomotor patterns given the same environmental con- straints. At .44 of standing height (or about .91 of leg length), the fourth-graders almost always (94%) jumped or leaped over the bar. Most of the kindergarteners (61 %) did the same, with 18% going under on hands and feet and 8% creeping under. In contrast, most of the DD preschoolers (62 %) creeped under, with 15 % stepping over (none jumped or leaped over) and 12 % going under on hands and feet. At the next relative height up (.56 of standing height or about 1.15 of leg length), most of the fourth-graders switched to walking under on two feet (65 %), with 24% putting one or two hands on the ground as they passed under the bar. The kindergarteners demonstrated a similar transition, with 52% now walking under, 24% going under on hands and feet, and 14% creeping under. The primary locomotor pattern for the DD preschoolers continued to be creeping under (57 %), with 13 % walking under.

Discussion

The usefulness of the present gross-motor task in measuring the perceptual sensi- tivity of developmentally disabled and nonhandicapped children to the relation-

334 Burton

ship between their personal constraints and the constraints in the environment was evaluated in terms of its ability to discriminate between the perceptual-motor behavior of different-age subjects, handicapped and nonhandicapped subjects, and normal- and low-skilled subjects in simple and complex situations. The first two comparisons will be discussed in separate subsections whereas the discussion regarding the relationship between level of movement skill and perceptual skill will be included in the second subsection.

Comparison Between Age Groups

The only statistically significant differences between the NH kindergarteners and fourth-graders were found on movement skill variables, MT and MER. However, it was shown that the difference in MT could be accounted for by differences in the leg lengths between subjects in the two groups, by calculating movement velocity in terms of leg lengths per second (i.e., MV) and finding no group differences for this variable. This result reinforces the importance of taking into consideration body-size constraints as a possible rate-limiter of the movement skill of handicapped or nonhandicapped individuals (see also Dobbins et al., 1981).

The age-related differences in the more complex movement situation, as measured by MER, could have been caused by differences in actual movement skill or in the perceptualldecision-making processes. The lack of significant differences on percent error or the transition slopes suggests that it probably was not due to differences in perceptual sensitivity. A stepwise multiple regression analysis using MER as the predicted dependent variable and the other seven dependent variables treated as independent variables was carried out to determine which variables contributed most to the variance in MER for each group. 'For the kindergarteners, the only two variables with sufficient F values to be included in the equation were, in order, the critical transition values for Transitions 1 and 2 (multiple R= .59 at Step 1 and .68 at Step 2). Thus it appeared that the limiting factor for the kindergarteners was related to perceived skill, which should be a good indicator of actual skill, rather than perceptual skill or sensitivity (even though there was no significant difference between the two groups in the critical transition point values). This conclusion is consistent with the ecological model, which implies that the perceptual accuracy or skill of nonhandicapped persons of any skill level should be equal since they all perceive their world relative to their own personal capacities and limitations.

Comparison Between Developmentally Disabled and Nonhandicapped Groups

There were significant differences between the DD preschoolers and the NH kindergarteners on all eight dependent variables, suggesting that the DD children were poor in both movement skill and perceptual skill. As indicated from the comparison between the two NH groups, MV is a better relative measure of simple movement skill than MT. For MV, the value determined to maximally discriminate between these two groups was 5.09 leg lengthsls, with 71 % of the preschoolers and 8% of the kindergarteners scoring below this value. This maximum discrimi- nant value (MDV) can be considered a criterion of normal versus low levels of simple movement skill. Similarly, the MDV for MER (a ratio greater than 2.38)

- %- ws -- - -=am be-considered a criterion of mmal versus low levels-of e 0 m p l e x ' m o ~ n t - -

PerceptuaCMotor Interaction 335

skill. Using this criterion, 94% of the DD children and 12% of the NH children demonstrated low movement skill in the complex situation.

In an attempt to isolate the motor and perceptual aspects of performance limitations, the percentage of preschool and kindergarten children in each of three categories was determined. Using the above criteria: (a) 65 % of the preschoolers and none of the kindergarteners scored low on both MV and MER variables; (b) 29 % of the preschoolers and 12 % of the kindergarteners had low scores on MER but normal scores on MV; (c) 8 % of the kindergarteners and none of the preschoolers scored low on MV but normal on MER; and (d) 6% of the pre- schoolers and 80% of the kindergarteners had normal scores on both MV and MER variables. These results suggest that most DD children this age (65%) are likely to demonstrate problems in both simple and complex movement situations, with the possibility of both motor and perceptual skill acting as rate-limiters in different situations. However, it is those children who perform normally in simple situations but poorly in complex situations who are the most interesting and relevant to the present research problem.

Low MER scores for children in this category could have been caused by differences in actual movement skill beyond those required in the simple situation or by differences in perceptuaYdecision-making processes. It might be expected that a movement skill explanation would be supported by low perceived skill (as measured by critical transition points) and a perceptual explanation would be supported by low perceptual sensitivity (as measured by percent error and transition slopes). Of the three NH kindergarteners who fit this category, none scored below the MDV for percent error (below 17%) and only one scored below the MDV for each of the two slopes (below 233 and 209, respectively), arguing against a perceptual explanation. In line with the movement skill explanation presented in the previous section for kindergarteners, all three had critical transition values outside the normal range for Transition 1 (MDV < 0.862 or > 1.116) and one outside the normal range for Transition 2 (MDV > 1.190).

In contrast, possible perceptual difficulties in the DD preschoolers were evidenced by four of the five children fitting this category scoring below the MDV for percent error and four scoring below the MDV for both transition slopes (one failed to show either transition at all). The movement skill explanation was given less support, with low critical transition values being shown by only two subjects for Transition 1 and by only one subject for Transition 2. Further, the stepwise multiple regression analysis for MEV for the DD preschoolers revealed only one variable that had a sufficient F value to be loaded into the equation: percent error, a perceptual variable (multiple R = .65). Thus the data provided evidence that at least a portion of the movement problems experienced by DD children can be attributed to perceptual or, more precisely, perceptual-motor difficulties.

Future Directions

The measures used to quantify the children's performance on this locomotor task were able to successfully discriminate between NH kindergarteners and fourth- graders in relation to complex movement skill and between NH kindergarteners and DD preschoolers in relation to simple and complex movement skill and perceptual sensitivity or skill. However, the ultimate goal of this line of research is to identify children whose motor behavior is limited by perceptual problems, not just to be

336 Burton

aware that some movement problems exhibited by DD children in general are perceptual in origin.

In the present experiment, the transition slopes and percent enor were the dependent variables measuring perceptual sensitivity in a movement context, but there were a few problems with these variables. First, percent error only reflected inaccurate perception in one direction-overestimation. Perhaps the clearance distance of the subject's foot over the bar or head under the bar would provide a measure of the degree of perceptual underestimation; this would need to be obtained from videotape or film. Second, the transition slopes, conceptually the best measure of perceptual sensitivity from the ecological perspective, were too variable within subject groups. This variability perhaps could be reduced by increasing the number of trials per barrier height, by decreasing the increments between heights (from 12% of standing height), or by calculating the regression line differently. Further, test-retest reliability or the variability of the dependent variables within subjects needs to be established. And finally, the external validity or generalizability of the entire assessment protocol should be expanded by includ- ing an object-control task to go along with the present locomotor task. Such an object-control task is currently being developed.

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