## 4 Discussion
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Tone** &
\begin{tabular}{c} **Tonal** \\ **Distance** \\ \end{tabular} & \(DCT1\) & \(DCT2\) & \(DCT3\) \\ \hline T1 & \(\chi^{2}(1)=2.907\), & \(\chi^{2}(1)=8.497\), & \(\chi^{2}(1)=0.009\), & \(\chi^{2}(1)=14.359\), \\  & \(p=0.088\) & \(p=0.004\) & \(p=0.924\) & \(p<0.001\) \\ T2 & \(\chi^{2}(1)=3.136\), & \(\chi^{2}(1)=0.386\), & \(\chi^{2}(1)=6.107\), & \(\chi^{2}(1)=2.559\), \\  & \(p=0.077\) & \(p=0.535\) & \(p=0.013\) & \(p=0.110\) \\ T3 & \(\chi^{2}(1)=2.490\), & \(\chi^{2}(1)=1.120\), & \(\chi^{2}(1)=2.086\), & \(\chi^{2}(1)=0.208\), \\  & \(p=0.115\) & \(p=0.290\) & \(p=0.149\) & \(p=0.648\) \\ T4 & \(\chi^{2}(1)=10.905\), & \(\chi^{2}(1)=15.626\), & \(\chi^{2}(1)=6.526\), & \(\chi^{2}(1)=13.474\), \\  & \(p<0.001\) & \(p<0.001\) & \(p=0.011\) & \(p<0.001\) \\ T5 & \(\chi^{2}(1)=3.048\), & \(\chi^{2}(1)=0.020\), & \(\chi^{2}(1)=5.998\), & \(\chi^{2}(1)=0.712\), \\  & \(p=0.080\) & \(p=0.889\) & \(p=0.014\) & \(p=0.399\) \\ T6 & \(\chi^{2}(1)=5.522\), & \(\chi^{2}(1)=7.790\), & \(\chi^{2}(1)=0.213\), & \(\chi^{2}(1)=1.788\), \\  & \(p=0.019\) & \(p=0.005\) & \(p=0.645\) & \(p=0.181\) \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Summary statistics of main effects on native listeners’ responses from the GLMMs (significant effects have been highlighted in bold). sandhi tone and the Cantonese T4. This hypothesis is supported by the perceptual assimilation results from [2], where the Cantonese T4 was considered similar to the Mandarin T3 as much as 78% of the time. The remaining three tones, namely T3, T5 and T6, cannot be linked to an existing tonal category in Mandarin and require extra effort to be acquired. Our findings thus provide further evidence for the SLM/SLM-r and the PAM-L2 based on the suprasegmental features, and suggest that familiar phonetic categories might be easier to acquire than are unfamiliar ones. Moreover, the immigrants appeared to have merged T3, T5 and T6 in their productions. This was evident based on our post hoc acoustic analysis (\(p\)s \(>\) 0.915) and on the native-speaking listeners' judgements. According to the five listeners, the most frequently mispronounced patterns involving these three tones were T3 \(\rightarrow\) T6, T5 \(\rightarrow\) T6, and T6 \(\rightarrow\) T3. The confusion of T3 and T6 was not unexpected because these two tones are reported to be in the process of being merged by some speakers [24]. When the input that the immigrants received was the merged T3-T6 category, we could not expect them to separate these two tones in their production. What was surprising was that almost one third of the immigrants' T5 tokens were classified as T6, which was not a pattern that was evident in the native speakers' data. This revealed the immigrants' difficulty in correctly establishing the T5 and T6 categories because these categories are missing in their L1 system. Moreover, the acoustic and perceptual similarity between these two tones (23 versus 22) might have exceeded their linguistic input and caused confusion in their L2 category formation. Another possible explanation for the merging of T5 and T6 is the narrower F0 space in Mandarin speakers compared to Cantonese speakers [44]. When the tonal space is more limited, it becomes challenging (if not impossible) for all the tonal categories to remain distinguishable. Additional support for this point comes from a recent study of Cantonese spoken by South Asian Cantonese speakers, which also suggested a narrower tonal space compared to native speakers of Cantonese [28]. There are some limitations in this study that could be addressed in future research. Firstly, although both acoustic analysis and perceptual evaluation were included to provide an appropriate assessment of the speech data, we only addressed Cantonese tone production by L2 learners in this study. Previous studies on L2 speech acquisition suggest that the development of productive abilities is not synchronous with that of perceptive abilities (e.g., [9]).