Infants’ looking times to the incomplete and complete test displays were summed, and the proportion of looking toward the incomplete display relative to total looking was calculated for each infant. As seen in , the average proportion of looking was 52.7% and ranged from 77.8% (a strong preference for the incomplete display) to 29.8% (a strong preference for the complete display). Further analyses were geared toward assessing the effects of motor skill and exploration on looking preferences.
Figure 3 Individual infants’ contributions to each predictor variable. Mean values are represented by horizontal lines on each plot. A) Post-habituation looking preferences toward the incomplete object display. B) Infants’ ages at testing in days. (more ...)
Influences of Motor Skill and Visual-Manual Exploration on Infants’ Looking Preferences
We examined several potential predictors of infants’ post-habituation looking preferences: age at testing, tripod and self-sitting experience, holding skill, measures of manual exploration without looking, and measures of coordinated visual-manual exploration. We assumed that greater sitting skill would be associated with more days of tripod and self-sitting experience, and higher levels of manual skill would be associated with fewer drops and more frequent bouts of rotating, fingering, and transferring, especially while looking at the objects.
Infants displayed a wide range of scores for post-habituation looking preference () and for each of the potential predictors (). Notably, 2 infants never dropped toys and 9 infants never transferred objects between hands (with or without looking). Every infant rotated and fingered objects at least once, either while looking or not.
shows the zero-order correlations among predictor variables and with the proportion of looking to the incomplete display. Looking preferences were reliably predicted by sitting experience (for both tripod and self-sitting) and coordinated visual-manual exploration (including bouts of rotations, fingerings, and transfers), rs(27) ≥ .40, ps < .05. Infants’ age, holding skill, and—most importantly—measures of manual exploration without looking were not significantly related to post-habituation looking preferences. Independent sitting experience was related to several predictor variables: Infants with more self-sitting experience tended to be older, had more tripod sitting experience, and produced more rotations and transfers while looking, rs(27) ≥ .40, ps < .05. Measures of coordinated visual-manual exploration were highly intercorrelated: Infants who produced more rotations while looking tended to produce higher frequencies of fingerings and transfers while looking, rs(27) ≥ .50, ps < .01. Rotations and transfers while looking were correlated with the same manual actions without looking, rs(27) ≥ .40, ps < .05.
Correlations between infants’ post-habituation looking preferences, age at test, tripod and self-sitting experience, holding skill (drops), manual exploration without looking, and coordinated visual-manual exploration.
Because age, sitting experience, and manual exploration skills were intercorrelated, we used a hierarchical linear regression analysis to examine the unique contributions of these variables for explaining post-habituation looking preferences. The general strategy for this analysis was to enter each variable or block of variables into the regression equation one at a time to observe their individual effects on R2, a measure of between-infant variation in proportion of looking to the incomplete test display. As each block was entered into the regression, the contributions of previously entered predictors were partialled out, allowing the unique contribution of each subsequent set of variables to be observed. The blocks were ordered so as to assess the effects of sitting experience and manual exploration skills controlling for chronological age, and to observe the effects of manual exploration skills controlling for sitting experience. Within the general categories of sitting experience and manual exploration, we entered nuisance variables in earlier blocks to control for their influence in later blocks. So chronological age was entered first. Next, the sitting predictors were entered—with the nuisance variable, tripod sitting experience, entered before self-sitting experience. Holding skill and manual exploration without looking were also considered to be nuisance variables; we entered these factors before entering coordinated visual-manual exploration in the final block.
shows the change in R2 as each block of predictors was entered. In the first block, age at testing explained only 3.5% of the variance and was not a significant predictor. Likewise, in the second block, tripod-sitting experience did not explain a significant amount of unique, additional variance (13.2%). In the third block, after controlling for effects of age and tripod sitting experience, independent sitting experience explained an additional 13.6% of variance in infants’ looking preferences—a significant contribution. Holding skill was entered next but explained only 0.2% of unique variance. Manual exploration without looking only explained an additional 4.8% of variance in looking preferences. In the final block, after partialling out the effects of all previously entered predictors, coordinated visual-manual exploration accounted for an additional 29.3% of the variance. With all of the variables entered into the model, these predictors were able to account for 64.6% of the variability in infants’ proportion of looking to the post-habituation display, F(10, 27) = 3.10, p < .05.
Table 2 Hierarchical regression using age, tripod and self-sitting experience, holding skill (drops), manual exploration (bouts of rotations, fingerings, and transfers) 'without looking, and coordinated visual-manual exploration (bouts of rotations fingerings, (more ...)
The regression analysis showed that both self-sitting experience and coordinated visual-manual exploration were significant predictors of infants’ looking preferences. To assess the independent effects of self-sitting after controlling for visual-manual exploration, we tested a second regression model in which self-sitting experience was entered in the final block rather than the third block. Age at testing, tripod sitting experience, holding skill, and manual exploration without looking were entered in the first four blocks; as before, none of these factors could account for significant variance in looking preferences (all changes in R2 < 13.3%, ps > .05). Coordinated visual-manual exploration—entered as the penultimate, fifth block—still explained unique, additional variance, controlling for the previously entered variables (change in R2 = 34.2%, p < .01). Entered as the last block, self-sitting experience explained only 0.6% of unique variance, and the change in R2 was not significant. In sum, independent sitting experience does not appear to have unique predictive value, as it was only able to predict infants’ novelty preference scores when visual-manual exploration was not previously controlled. Coordinated visual-manual exploration, on the other hand, is a robust and powerful predictor of post-habituation looking preferences.
Comparison of Visual-Manual Actions by Sitting Ability
The regression analyses suggested that one route toward 3D object completion might stem from the development of self-sitting and subsequent increases in visual-manual exploration as infants’ hands are freed from supporting functions. We used an analysis of covariance (ANCOVA) to assess the effects of self-sitting on the frequency of manual exploration with and without accompanying visual inspection. The analysis compared the number of rotations, fingerings, and transfers with and without looking in non-sitters (including tripod sitters and infants with no sitting experience) and self-sitters. We considered tripod sitting to be a nuisance variable because it was only correlated with self-sitting, not with manual exploration (see ). Thus, to assess the unique effect of self-sitting, we treated tripod sitting as a covariate— equating all subjects on the mean days of tripod sitting experience for the sample.
As shown in , infants performed more rotations and fingerings than transfers (note differences in scale on graphs). In addition, self-sitters produced more coordinated visual-manual exploration than manual exploration without looking, but non-sitters produced equal amounts of manual exploration with and without looking. A 3 (manual exploration: rotations, fingerings, transfers) × 2 (visual inspection: with looking, without looking) × 2 (sitting skill: non-sitter, self-sitter) mixed-design ANCOVA with tripod sitting experience as a covariate confirmed a main effect of manual exploration (F(1.2, 29.87) = 6.60, p < .05, partial η2 = .21). (Because sphericity could not be assumed for manual exploration and its interactions, all F tests with this variable have been Greenhouse-Geisser corrected.) Post-hoc, Sidak-corrected pairwise comparisons showed that infants performed more rotations (M = 15.11, SD = 10.27) and fingerings (M = 24.42, SD = 22.20) than transfers (M = 3.50, SD = 5.52), ps < .01, but similar numbers of rotations and fingerings, p > .05.
Figure 4 Mean number of bouts (with and without looking at the toys) of A) rotations B) fingering and C) transfers during the structured play session grouped by infants’ independent self-sitting ability. Error bars represent standard errors of the mean. (more ...)
The analysis also confirmed a significant interaction between visual inspection and sitting skill (F(1, 25) = 7.90, p < .01, partial η2 = .24). Follow-up, Sidak-corrected pairwise comparisons within each sitting group revealed equal amounts of manual exploration with and without looking in non-sitters, p > .05, but more manual exploration with looking than without in independent sitters, p < .01. This finding is reflected in by the relative heights of the bars for each sitting group. We found no effects for the covariate, tripod sitting experience.