Figure 2 shows the STE histogram for each subject group’s ISIs from 600 ms to 3600 ms. In normal subjects, tapping begins to lag behind the tones as the ISI length increases, and tapping time distribution relative to the tones formed a narrow single STE peak approximately 200 ms after the tones, which corresponds to the timing of the “delayed tapping”. This phenomenon was also observed in other SCA groups, such as SCA6, MJD, and MSA. However, in the SCA31 group, this delayed tapping was rarely observed, showing no peak of STE 200 ms after the tones.
Overall, the proportion of delayed tapping apparently increased along with the ISI length in SCA patients, which was also affected by disease types (Fig. 3A). A repeated measures two-way ANOVA showed that the effects of ISI (F [5, 235] = 36.772, p < 0.001, ε = 0.474) and disease type (F[4, 47] = 5.179, p = 0.00154) were both significant. The interaction between ISI and disease type was also significant (F[20, 235] = 2.09, p = 0.0333, ε = 0.474), suggesting that the effect of ISI on delayed tapping depended on the disease types. In SCA31, however, the delayed tapping stayed at a low level even when the ISI became longer, while in the other groups, the occurrence of delayed tapping increased as the ISI duration increased.
The mean STE tended to shift toward the negative direction as the ISI increased (Fig. 3B). Using a repeated measures two-way ANOVA, the effect of ISI was significant (F[5, 215] = 29.573, p < 0.001, ε = 0.466) while the effect of disease type was not significant (F[4, 43] = 2.144, p = 0.0917), and the interaction between these factors was significant (F[20, 215] = 2.180, p = 0.0279, ε = 0.466). These results demonstrate that the extent of STE mainly depended on the ISI, and this dependency varied between disease types. SCD patients, especially SCA6 and MSA patients, tended to show a strongly negative STE as the ISI duration increased.
The SD of STE increased linearly as the ISI increased in all subject groups (Fig. 3C), conforming to the scalar property. A repeated measures two-way ANOVA indicated that the effect of ISI (F [5, 215] = 126.668, p < 0.001, ε = 0.552) and disease type (F [4, 43] = 5.542, p = 0.00224) were both significant, while the interaction between these factors was not significant (F[20, 215] = 1.476, p = 0.149, ε = 0.552). These results show that the STE linearity between ISI and SD was maintained across all subject groups. SCD patients, especially those of SCA6 and MSA, showed a longer SD for STE than normal subjects and SCA31 patients, reflecting their inaccurate and irregular tapping relative to the tones.
As mentioned above, the relationship between the SD of STE and ISI was approximately linear. To visualize this linearity, we divided the SD of STE by ISI (Fig. 3D). The SD curves of STE and ISI were almost level in the normal and SCA31 groups. However, in the other SCD groups, the SD of STE was large at shorter ISIs such as 600 ms and 900 ms and decreased at longer ISIs. Using a repeated measures two-way ANOVA, the effects of ISI (F [5, 215] = 9.907, p < 0.001, ε = 0.695) and disease type (F [4, 43] = 5.542, p = 0.00109) were significant, while the interaction between these factors was not significant (F [20, 215] = 1.18, p = 0.299, ε = 0.695). These results showed that the linearity of SD for STE, corresponding to the scalar property, was maintained in normal and SCA31 patients through all ISIs, but not in SCA6, MJD, and MSA especially at shorter ISIs. This is because patients with SCA6, MJD, and MSA were unable to keep pace with fast tapping rhythms, while this was not a problem for normal and SCA31 patients.
The coefficient of variation (CV) for ITI was stable with an increasing ISI, except for MSA patients (Fig. 3E). A repeated measures two-way ANOVA showed that both ISI (F [5, 235] = 15.117, p < 0.001) and disease type (F [4, 47] = 3.414, p = 0.0157) significantly affected the CV of ITI, and the interaction between them was also significant (F [20, 235] = 1.986, p = 0.00869). Although the effect of ISI was significant, the CV of ITI curves tended to show negative peaks for ISIs of 900 ms or 1800 ms, and the overall curves did not show a monotonous increase or decrease. This tendency mostly fits the scalar property similar to the SD of STE (Fig. 3C). Among all disease subtypes, MSA patients showed an increased CV for ITI especially for short ISIs.
Figure 3F compares the limits of temporal integration among different subject groups. A one-way ANOVA showed that the effect of disease types was significant (p = 0.00197). Post-hoc Tukey multiple comparisons revealed that the differences between SCA31 and MJD (p = 0.00725), between SCA31 and MSA (p < 0.001), and between SCA31 and SCA6 (p = 0.0418) were significant, indicating that SCA31 patients have a longer limit of temporal integration than the other groups, while the other groups were comparable. The difference between the normal subjects and each of the other SCA groups was not significant (p > 0.05).
Figure 3G compares the SRTs in each group. A one-way ANOVA showed that there was a significant difference among these groups (F [4, 18.10] = 7.427, p = 0.0010). Post-hoc Tukey multiple comparisons revealed that SCA6 patients have significantly longer SRTs than normal subjects (p = 0.0147).
Supplementary Fig. 1 shows the relationships between ISI and the mean + 2SD of STE, which would approximate the upper limit of the STE distribution, and this includes data from Fig. 3B and 3C. Both the effects of ISI (F [5, 215] = 69.629, p < 0.001, ε = 0.746) and disease type (F [4, 43] = 5.526, p = 0.00111) were significant, indicating that the mean + 2SD, which is the upper limit of the STE distribution, increases as the ISI duration increases, and it is also affected by the disease type. The interaction between ISI and disease type was not significant (F [20,215] = 1.669, p = 0.0624, ε = 0.746), showing that the effect of ISI on the upper limit of STE distribution is maintained in all disease types.
To clarify which disease group has an abnormally large STE, one-to-one comparisons between normal subjects and each disease group were performed using repeated measures two-way ANOVA, with a within-subject factor of ISI and a between-subject factor of subject group (Normal vs. MJD, Normal vs. SCA6, Normal vs. SCA31, and Normal vs. MSA). The effect of subject group became significant only when the Normal vs. MSA group was selected (p = 0.0135, Bonferroni correction), while there was no significant difference in the other comparisons (p > 0.05). These results show that MSA patients have a larger STE than other groups.
Figure 4 shows the STE time-courses in each session plotted against the tap count, where the ISI was 900 ms, which were averaged for all subjects in each group. Initially, the STE decreased dramatically with the trial number up to approximately the tenth tap in all groups. After this initial decrease, the STE continued to decrease slowly. After the tenth tap, the correlation between the tap count and STE was significant in normal subjects (R = − 0.633, p < 0.001) and the SCA31 (R = − 0.388, p = 0.00536) group, but not in the SCA6 (R = − 0.202, p = 0.159), MJD (R = − 0.155, p = 0.283) and MSA (R = − 0.259, p = 0.0694) groups. These correlations were strongly negative only in the normal and SCA31 groups, which may be because the noise levels in the STE curves was relatively small in these subjects, reflecting accurate and regular tapping.
In Supplementary Fig. 2, the STE time-course where the ISI was 3600 ms was plotted in the same way as in Fig. 4. Because these sessions are divided into three parts rather than continuing with 60 taps in one session, the maximal tap count is 25 in this plot. There was no significant correlation between the tap count and STE (p > 0.05).
Table 2 shows the result of multiple linear regression analysis using the limit of temporal integration as the outcome variable, and age, disease duration, and ataxia scores (SARA) as predictor variables. None of these predictor values significantly correlated with the limit of temporal integration.
Table 2
Results of multiple linear regression analysis where the outcome variable is the limit of temporal integration (ms).
Explanatory variable
|
p value
|
β
|
Age (years)
|
0.207
|
−24.0
|
Disease duration (years)
|
0.525
|
15.9
|
SARA score
|
0.374
|
−26.8
|
SARA; Scale for the Assessment and Rating of Ataxia |