The percentage of correct judgments again served as the dependent measure in a mixed ANOVA. The between-subject factor was group (Weber-equal versus Michelson-equal), and the within-subject factors were the moving bar polarity (positive versus negative) and the spatial distribution variable (ipsilateral versus contralateral) depending on which hypothesis was being evaluated.
The major hypothesis predicted an interaction between the spatial distribution variable and the grouping variable. The spatial distribution variable should exert an effect in the Weber-equal group, but there should be no effect of this variable in the Michelson-equal group. The means from all conditions for both groups are shown in
Figure 4. The spatial distribution X group interaction was significant,
F(1, 46) = 6.10,
P = .017. The spatial distribution factor exerted a significant effect on the percentages of correct judgments in the Weber-equal group (
Figure 4 left panel) but not in the Michelson-equal group (
Figure 4 right panel). The effect in the Weber-equal group was in the predicted direction. The percentage of correct judgments was lower when most of the positive polarity static bars were on the same side as the moving target (ipsilateral in
Figure 4,
M = 62.3%) than when they were on the side opposite the moving target (contralateral in
Figure 4,
M =74.8%). Switching the location of most of the negative/positive polarity bars with respect to the moving bar modulated the percentage of correct judgments by an average of 12%. The negative polarity static bars in the Weber-equal condition had substantially more Michelson contrast (100%) than the positive polarity bars in this condition (33%). In the Michelson-equal group, the mean percentages of correct judgments did not depend on the spatial distribution of the two static bar types (
M = 64.8% and 66.0% for ipsilateral and contralateral, respectively). The major hypothesis was supported.
The data from individual infants also showed this spatial distribution effect. In the Weber-equal group, 17 infants showed less attention to the moving bar when most of the negative polarity bars were contralateral to the moving target than when they were ipsilateral; four infants showed the opposite pattern; and three infants showed no difference. A sign test showed this to be a nonrandom result in the predicted direction (P = .007). In contrast, in the Michelson-equal group, 12 infants showed better detection when most of the negative polarity static bars were ipsilateral to the moving bar than when these bars were contralateral to the moving bar; nine showed the opposite pattern; and three showed no difference (P = .664).
The mean judgment time in the Weber-equal group (M = 1.720 sec) was slightly less than the mean judgment time in the Michelson-equal group (M = 1.767 sec). This difference of 47 milliseconds was not significant, t(46) = 0.75, P = .459.
The auxiliary hypothesis discussed above led to the prediction of a target polarity X group interaction. The polarity of the moving target should exert an effect on the overall percentage of correct judgments, but only in the Weber-equal group because the Michelson contrasts of the two bar types are substantially different (
Table 1) in that group. The negative polarity moving bars should have produced higher percentages of correct judgments than the positive polarity targets. The pattern in the data supported the auxiliary hypothesis. There was an interaction between the polarity of the moving target and the grouping variable,
F(1,46) = 4.06,
P = .05). In the Weber-equal group, the mean percentage of correct judgments (
M = 71.3%) with a moving decrement was higher than the mean with a moving increment (
M = 65.8%). This difference was not observed in the Michelson-equal group. In the Michelson-equal group, infants oriented toward the moving decrements slightly but not significantly less often than they did toward the moving increments (
M = 64.4% versus 66.5%, respectively). Infants more readily oriented to the moving bar when it was dark than when it was light but only when their Michelson contrasts were unequal.
The pattern of results across the two groups in this experiment followed exactly from the hypothesis that the effective contrast of both the static and the moving bars more closely reflects the Michelson contrast metric than the Weber contrast metric. When the Michelson contrasts of the two polarities were equal at 33%, the detection rates for both polarity moving bars were equal, and the spatial distribution of the two polarities had no effect on the percentage of correct judgments despite large differences in the Weber contrasts of the light and dark bars in this condition. When the Michelson contrasts of the two polarities were unequal (100% for negative versus 33% for positive), the negative polarity static bars were more effective than the positive polarity bars in competing with the moving bar for the infant’s attention despite the fact that the Weber contrasts of the light and dark bars were equal in this condition.
Finally, we also note that prior to this experiment we ran the Weber-equal condition with lower contrasts for the light and dark bars (Weber contrasts 52%; Michelson contrasts 21% and 35% for the increments and decrements, respectively). The pattern of results was in the predicted direction, but the differences between the ipsilateral and contralateral conditions were close to null. Thus, the greater salience of the dark bars that we observed in the Weber-equal group in
Experiment 2 may only hold with very high contrasts.