The average results obtained from both monkeys confirmed this observation. The average overlap is plotted in
Figure 6D. Black regions in this figure indicate the average overlap, ±1
SE, necessary to reach a level of
p < .05.
Figure 6E shows the individual data of both monkeys by plotting the number of shapes per orientation for which the overlap was significantly larger than expected by chance (
p < .05). In contrast to the human data, the overlap – the degree to which the same features were used to identify upright and rotated shape versions – depended on the stimulus orientation for the monkeys. A repeated measures ANOVA performed on the overlap yielded a significant influence of orientation,
F(3) = 6.6,
p = .007. The most pronounced difference was observed when comparing the overlap for 30 deg and 90 deg rotations. While the overlap was on average 43% for both 30 deg rotations, it only reached 14% for the 90 deg rotations, a difference that was highly significant, paired
t test,
t(9) = 4.3,
p = .002. A similar trend was seen for both monkeys individually. Overall, the overlap for only a few pairs of upright and rotated diagnostic regions reached the level of
p < .05. However, most of these cases were observed for the 30 deg rotations. Thus, the monkeys showed a strong tendency to use different shape features the further the shapes were rotated away from the original. This pattern of behavior is different from the one observed for human observers. A direct comparison between the levels of overlap reached at each stimulus rotation furthermore revealed that in general the monkeys were using less similar shape features than humans. With the exception of shapes rotated by −30 deg, the average overlap was significantly lower for the monkeys than for the humans,
t test, 30 deg:
t(12) = 2.4,
p = .03; 90 deg:
t(18) = 2.6,
p = .02; −30 deg:
t(12) = 1.9,
p = .08; −90 deg:
t(18) = 6.2,
p < .001. Interestingly, there was a significant correlation between the performance in the initial generalization test (see
Figures 5A and
5B) and the degree to which the monkeys used orientation-independent shape features after training with rotated shapes. The better the initial performance for a specific orientation, the more similar were the features used to identify the upright and the rotated shape (Pearson correlation coefficient between overlap and performance in the initial generalization test:
r = .60,
p = .005). This suggests that for rotation angles for which the monkeys could not immediately identify the shapes, they had a larger tendency to use novel shape features to solve the task.