The mean
looking times during the pre-tests and the post-habituation tests are shown in
Table 2. The looking times on the two pre-test trials were combined and the looking times on the two post-test trials were also combined. Before analyzing, we excluded the infants whose combined looking times were less than 6 s in the two pre-test trials or in the two post-test trials.
As an index to test whether infants looked longer at the novel shape,
a novelty preference score was calculated for each infant in the pre-tests and the post-habituation tests. Hereafter “novel shape” in pre-tests indicates the shape that was not presented in the following habituation phase. This was done by dividing the infant's looking time to the novel shape by the total looking time.
Figure 5 shows the mean preference scores. Histograms for each cell were examined and no obvious deviations from normality were observed.
To examine whether infants showed a novelty preference and whether there were differences of the novelty preference between age groups, cue, and shape types, a 2 × 2 × 2 × 2 ANOVA was performed: (i) the pre- and post-test differences (the pre-test and the post-test) as a within-participants factor, (ii) the age of infants (4 to 5 months and 6 to 7 months) as a between-participants factor, (iii) the cue presented in the habituated displays (shading and line junction) as a between-participants factor, and (iv) the shape of the habituated displays (the cake and rocket) as a between-participants factor. The ANOVA revealed a significant main effect of the pre- and post-test differences [ F(1, 40) = 11.8, p < 0.01] and an interaction between the pre- and post-test differences and age [ F(1, 40) = 8.62, p < 0.01]. Tests of simple main effects showed a significant difference in the pre- and post-tests for 6- to 7-month-old infants [ F(1, 40) = 20.3, p < 0.01] but not 4- to 5-month-old infants. In other words, there was a significant difference in novelty preference between the pre- and post-tests for the 6- to 7-month-old infants but not the 4- to 5-month-old infants. Other main effects and interactions were not significant.
To directly explore the novelty preference in each condition and in each age, we conducted planned comparisons using multiple t-tests that were independent of each other. The t-tests compared the novelty preference scores in the pre- and post-tests for each condition and each age. Under the hypothesis that the preference scores should increase after habituation, the t-tests were one-tailed. This analysis showed that the older infants significantly increased in their novelty preferences in three out of four conditions and demonstrated a marginally significant change in the other condition [shading and cake, t(40) = 2.62, p < 0.01; shading and rocket, t(40) = 2.68, p < 0.01; line junction and cake, t(40) = 1.52, p < 0.07; line junction and rocket, t(40) = 2.13, p < 0.02]. In younger infants, no significant difference was found [shading and cake, t(40) = 0.13, ns; shading and rocket, t(40) = 0.13, ns; line junction and cake, t(40) = 0.23, ns; line junction and rocket, t(40) = 0.49, ns].