## 4 Discussion
Possible explanations for these findings can be traced to the short duration of the disease or the sensitivity of SPECT. It may be the case that the brain atrophy was not advanced for this group due to the short disease duration. In particular, cerebral hypoperfusion has been shown to appear prior to brain atrophy (see [PERSON] et al. [38] and references therein). Given that the average duration of disease in this group is only 34.67 months, it can be the case that atrophy is not that advanced, so that it correlates with language performance. Our last research question addressed the relationship between performance in the production of complex sentences and clinical (disease duration) and demographic (age and years of education) variables. Concerning disease duration, the data do not indicate any relationship between the two. This is not categorically opposite to previous studies that pointed to an effect of disease duration [18; 20], as the disease duration in this group was relatively short. Recall that [PERSON] et al. [14] did not find any severe syntactic impairment in patients with 3.4 years of disease duration. The statistical analysis showed that performance in complex sentences is related positively to the years of education and age. Whereas the former is not surprising, the latter is unexpected. We speculate that this is related to the age of onset. In order to explore this, we computed the correlation between the number of correct responses in the language task and the age of onset of the symptoms (computed by subtracting the months of the disease duration from the age at the time of testing). Indeed, we found a statistically significant strong correlation between the two (\(r=0.8\), \(p=0.009\)). This finding needs further investigation and in the context of a longitudinal study. Summing up, this dataset indicates that syntactic deficits can be present in svPPA, at least for some patients, while for others, core syntactical abilities are largely preserved. Future longitudinal studies are needed to provide more robust data about the presence, the extent and the nature of a syntactic deficit in svPPA. However, the present data shed light on some aspects of the effect of syntactic complexity. First, they suggest that it is independent of general cognitive decline as measured by the ACE-R. Second, it seems that the syntactic deficit is related to general, non-modality-specific language abilities. Third, hypoperfusion in specific brain areas does not predict performance in a specialized syntactic test, which might be dependent on the disease duration. Further, we found an expected association between years of education and performance, which implies that premorbid educational status can affect the progress and the manifestation of the language symptoms. Finally, we did not find any association between the performance in the language test and the disease duration; nevertheless, the unexpected positive relation between performance and age is probably indicative of the role of age of onset in the progress of the disease: the later the onset, the slower the decline. All these findings are very promising for our understanding of the language deficit in PPA, but they suffer from certain limitations and could be the ground for future research. First, our findings are based on a relatively small group of patients with svPPA. Second, syntactic complexity comprises several types of sentences, so future research should address this diversity. Third, due to the progressive nature of PPA, longitudinal studies provide a more comprehensive insight into the nature of the language deficits. Furthermore,although the focus of the study was to investigate the association between hypoperfusion in cortical regions and syntactic deficit, it is, of course, beneficial to combine different neuroimaging methods in order to further investigate the contribution of white matter regions, as suggested by previous studies [27]. Conceptualization, P.I. and S.S.; methodology, S.S. and P.I. ; formal analysis, S.S. and V.K. ; investigation, D.M.