Unlike most people who can enjoy music at their leisure, approximately 4% of the population (Kalmus and Fry, 1980) suffers from lifelong problems in perceiving and producing music in the absence of brain injury (Ayotte et al., 2002). Importantly, these problems cannot be explained by hearing loss, intellectual deficiencies, or lack of music exposure. This developmental disorder is known as congenital amusia (Peretz, 2001) (“amusics” hereafter), and the core deficit lies in musical pitch processing (Foxton et al., 2004; Hyde and Peretz, 2004).

Accumulating evidence suggests that amusics may also have behavioral difficulties with linguistic tone processing (Patel et al., 2008; Nan et al., 2010; Tillmann et al., 2011; Liu et al., 2012; Yang et al., 2013). This condition applies to both speech intonation and lexical tones and can affect the amusics either at the group level (Jiang et al., 2012; Liu et al., 2012) or only for a subgroup of individuals. For example, Patel et al. (2008) found that approximately 30% of amusics among non-tone language speakers had impairments in processing speech intonation, which is used to discriminate questions and statements (Patel et al., 2008). Similarly, our recent research shows that only a subgroup of amusics demonstrated deficits in lexical tone perception (Nan et al., 2010; Yang et al., 2013).

These data suggest that although the linguistic tone deficits in amusics were generally first observed at the group level, it is essential to be aware of the possibility that subgroup differences may exist. This is especially important for follow-up research on amusics' related speech tone difficulties. For instance, a recent study found that Mandarin-speaking amusics were impaired with regard to the categorical perception (CP) of lexical tones (Jiang et al., 2012). It should be noted that CP reflects some of the fundamental aspects of speech processing (Harnad, 1987). However, behavioral lexical tone difficulties in these amusics were not reported, nor were the possible subgroup differences in behavioral lexical tone perception. It is thus unclear whether the observed CP impairment applies to all Mandarin-speaking amusics or only to a subgroup of amusics who might have behavioral lexical tone difficulties.

The current study thus aimed to investigate the categorical nature of lexical tone perception in Mandarin-speaking amusics with and without behavioral lexical tone deficits. In addition to taking the subgroup differences into account, we also carefully chose an appropriate step size for the CP measurement. In the previous CP study with amusics (Jiang et al., 2012), the step size of the discrimination pairs used was a 6-Hz difference, which fell in the normal range of the just-noticeable differences (JNDs) for F0 contour discrimination among normal Mandarin listeners (i.e., 4–8 Hz) (Liu, 2013). This 6-Hz step size might not be large enough to reveal the categorical nature of tone perception in the amusics given their congenital pitch impairments. We thus used a relatively larger step size (20 Hz) to minimize the possible confounding effect of the step size due to amusics' pitch deficits. Moreover, stimulus duration was included as an independent variable (at two levels: 100 and 200 ms) to examine its effect upon the CP of lexical tones in amusics. It has been suggested that a lengthened syllable duration significantly enhances the pitch contour identification performance among native speakers of both tone and non-tone languages (Blicher et al., 1990).

Overall, it was expected that only amusics with difficulties identifying and discriminating lexical tones would exhibit impaired CP of lexical tones, while amusics without these problems would perform the two tasks as well as the normal controls. Research into the duration effect on CP could also provide implications for improving tone perception performance in amusics.

It is important to note that the lack of peaks in the discrimination functions for the tone agnosics, as observed above, might reflect their non-categorical perception of Mandarin tones. Alternatively, it might also be due to their reduced psychophysical capacity, if any, to discriminate tone contour changes across the tone boundary. That is, the 20-Hz difference in each stimulus pair of the discrimination task might not be large enough, e.g., smaller than the JNDs of the tone contour changes of tone agnosics. In a pilot study on the JNDs of tone contour changes for controls, pure amusics and tone agnosics, the tone contour discrimination thresholds of tone agnosics ranged from 20 to 30 Hz, significantly higher than the thresholds of the other two groups and also greater than the step size of 20 Hz used in the current discrimination task. Thus, to rule out the possible confounding effect of psychophysical capacity, an additional tone discrimination task was conducted for the tone agnosic group by changing the step size from 20 to 35 Hz, higher than their JNDs. Five of the six tone agnosics completed this additional discrimination task, the procedure of which was identical to the one described above.

The results showed that the larger step size between the stimulus pairs did not result in any discrimination peaks for tone agnosics (see Figure 5). A Two-Way repeated-measures ANOVA (within-subjects factors: stimulus duration and discrimination location) was run with the discrimination scores as the dependent variable for the tone agnosic group. None of the main and interaction effects were significant (all ps > 0.05), indicating no peak in the discrimination functions with the 35-Hz step size (e.g., no significant difference between P_bc and P_wc). These results suggested that, for tone agnosics, the absence of a peak in either discrimination function was more likely due to their lack of CP of lexical tones rather than their reduced psychophysical capacity to discriminate tone contour changes.

Congenital amusia is a developmental disorder that affects the processing of musical pitch (Ayotte et al., 2002; Peretz et al., 2002). Recent evidence suggests that this deficit may extend to speech, affecting the processing of speech prosody (Patel et al., 2008; Liu et al., 2010) and lexical tones (Nan et al., 2010; Tillmann et al., 2011). Some studies indicate that the linguistic tone difficulties might implicate only a subgroup of amusics while sparing the rest (Patel et al., 2008; Nan et al., 2010; Yang et al., 2013). For instance, our earlier studies suggest that there are two subgroups of Mandarin-speaking amusics: one has behavioral lexical tone processing impairments while the other does not (Nan et al., 2010; Yang et al., 2013). The current study aimed to examine the CP of lexical tones in Mandarin-speaking amusics with and without behavioral lexical tone deficits. Our results revealed that only amusics with tone agnosia showed significant impairment in the CP of lexical tones, while the pure amusics perceived lexical tones in a categorical manner similar to that of the controls.

Our results thus refine a recent finding that Mandarin-speaking amusics had CP impairments for lexical tones (Jiang et al., 2012), showing that not all Mandarin-speaking amusics were impaired but only those with behavioral lexical tone difficulties. These data imply that congenital amusia alone does not necessarily result in a deficit in the CP of lexical tones. The impaired CP of lexical tones only implicates those amusics with behavioral lexical tone difficulties, whereas amusics without lexical tone deficits are spared. These findings are consistent with the previous standpoint that the pitch processing deficits in music might not always coexist with those in speech (Nan et al., 2010; Tillmann et al., 2011; Yang et al., 2013). The dissociation between musical pitch and lexical tone deficits corroborates the hypothesis that despite some overlap across music and speech, there might be two different pitch-related systems subserving pitch processing in these two domains (Zatorre and Baum, 2012).

The current study also demonstrates the necessity of taking the behavioral speech tone performance into account and being aware of the possible subgroup differences for any further attempt to understand the linguistic tone deficits related to congenital amusia. As in the current study, if the subgroup differences in behavioral lexical tone difficulties were ignored and the amusics with or without lexical tone deficits were taken together as one whole group of amusics, the results would have found that the combined amusic group performed significantly worse than the controls in both discrimination and identification tasks. Specifically, for the discrimination task, the combined amusic group had significantly smaller category differences (between-category vs. within category performance) than the controls, F_{(1, 26)} = 7.314, p = 0.012. For the identification task, the combined amusic group demonstrated a significantly shallower identification slope than the controls, F_{(1, 26)} = 5.170, p = 0.031. It is thus clear that if the possible subgroup difference in behavioral lexical tone deficits were overlooked, the CP impairment would have been wrongly attributed to the whole amusic group instead of the subgroup of amusics who had behavioral lexical tone deficits.

Please note that besides the subgroup differences in behavioral lexical tone deficits, step size might be another factor contributing to the previous finding of the lack of CPs of lexical tones in the amusic group (Jiang et al., 2012). Rather than the 6-Hz step size used in Jiang et al.'s study (2012), which might be too small to reveal the CP of lexical tones in congenital amusics, we employed a larger step size (20-Hz). The results showed that the pure amusics exhibited significantly higher discrimination accuracy for between-category tone pairs than the within-category tone pairs, resulting in clear discrimination peaks across the identification boundary, which was one of the essential characteristics of categorical perception. In contrast, tone agnosics had no discrimination peaks, as predicted by the shallow slopes of the identification function. Importantly, the lack of peaks in tone agnosics was not likely due to a reduced sensitivity to tone pitch change, as the same pattern persisted in tone agnosics at the enlarged step size of 35 Hz.

The stimulus duration was included for the first time in studies of categorical perception. Interestingly, the results showed that the length of stimuli had little effect on the performance of the CP of lexical tones in pure amusics and tone agnosics because both groups exhibited consistency across different stimulus durations (i.e., 100 and 200 ms). Moreover, the pure amusics showed robust CP of lexical tones, which was similar to that of the controls, even when the stimulus duration was relatively short (i.e., 100 ms). In contrast, tone agnosics failed to perceive lexical tones categorically under both duration conditions. Notably, as shown in their psychometric curves, tone agnosics did not reach 0 and 100% in the identification task for either duration, confirming their behavioral lexical tone deficits as suggested by the pre-test.

The observation of the lack of CP for lexical tones in Mandarin-speaking amusics with behavioral tone deficits raises new research questions regarding the relationship between the CP of lexical tones and tone language experiences. Previous studies comparing tonal and non-tonal language speakers with the CP paradigm came to a general conclusion that only those who spoke tone languages showed classic CP for lexical tones, while individuals without tone language experience perceived tones on a psychophysical basis (Wang, 1976; Francis et al., 2003; Xu et al., 2006; Peng et al., 2010; Liu, 2013). Interestingly, despite being exposed to the tone language environment since birth, tone agnosics could not successfully produce a peak for the between-category discrimination and exhibited a shallow slope of tone identification functions, showing a lack of CP for lexical tones similar to non-tonal language speakers (Wang, 1976; Francis et al., 2003; Xu et al., 2006; Peng et al., 2010; Liu, 2013). Our results thus suggest that tone language experience does not necessarily guarantee the CP of lexical tones.

Despite the common deficits in musical pitch processing, the pure amusics and the tone agnosics are different in terms of lexical tone pitch processing, as shown in the current study as well as in previous research (Nan et al., 2010; Tillmann et al., 2011; Yang et al., 2013). Note that the non-categorical perception of lexical tones may reflect the impairment of fundamental aspects of speech tone processing, future studies should further investigate the mechanisms of lexical tone deficits in tone agnosics to help elucidate the relations and interactions between music and speech perception. In addition, it would be interesting to determine whether the pure amusics who seem to perform well in various types of behavioral tone tests (e.g., Nan et al., 2010) also have similar integrity from a neuroimaging perspective. Furthermore, it is vital to understand how the currently observed lexical tone deficits might affect the learning of a tone language. As shown in a recent mismatch negativity research, Mandarin speakers exhibited fast learning effects for novel tone-segment combinations (Yue et al., 2014). Future research is warranted to examine whether the amusic individuals with lexical tone deficits would be able to demonstrate a similar learning pattern.

Whether having difficulty in lexical tone perception separates pure amusics and tone agnosics. The current study investigated lexical tone perception in congenital amusics with the traditional CP paradigm. The CP of lexical tones was intact in pure amusics but impaired in tone agnosics. These results reconcile the previous debate on whether tone pitch processing is compromised in congenital amusics. That is, pitch processing deficits in music do stretch into speech, but only a small proportion of this special group is affected and suffers from tone perception deficits. Overall, the present findings improve our knowledge about the lexical tone processing abilities in congenital amusics. The different performance between congenital amusics with and without tone agnosia has provided a new perspective on the relationship between music and speech perception.