4. Results and discussion4.1 Babbling stageThe results of the evaluation of the babbling stage show a decrease in the quality of the systemic mappings (i.e., an increase in sum-squared Euclidean error) with increased severity for both MPD and MPD+APD (Figure 5, left panel), but the effect is much larger for MPD+APD. The quality of the systemic mapping at different stages in the training process shows the effects to be stable throughout (Table 1). Compared to the effect of underlearning in the baseline condition (Figure 5, right panel; 500–1000 training items), the results show a very drastic and consistent decrease in mapping quality with increased severity for MPD+APD. In the case of a MPD only, the results are less consistent. The sum-squared Euclidean error is slightly smaller for noise levels of 5% and 10%, respectively, as compared to the baseline condition and error only starts to increase at noise levels of 15% and higher. At these higher noise levels, the error-increase appears to remain small, though it does correspond to substantial underlearning when compared to the baseline condition.