Discussion

This study examined language performance in both possible and probable bvFTD using a unified battery of single word processing, with two primary aims: first, to determine if language changes observed in bvFTD resemble the pattern found in SD; and second, to examine the relative contributions of executive and semantic measures to single-word processing in bvFTD. Overall, results identified reductions in language performance in both bvFTD groups. Naming deficits were present in approximately half of individuals with probable bvFTD, with semantic association deficits also frequently observed in this group. In possible bvFTD, naming, comprehension and semantic association deficits were found in roughly a quarter of participants. Importantly, the pattern of relative impairment across subtests differed between bvFTD and SD. In the SD group, naming was significantly more impaired than the other subtests, whereas this pattern was not observed in either bvFTD group. Although semantic functioning was associated with single-word language performance in both bvFTD groups, some executive functioning measures also made an independent contribution.

Consistent with previous studies of probable bvFTD [5, 6, 38], group-level impairments were found across naming, single word comprehension, and semantic association. In contrast to earlier work, which only found significant reductions in naming, as compared with healthy controls, for cases with abnormal brain imaging (i.e., probable bvFTD) [11], comparable impairments were evident in the possible bvFTD group for the majority of language subtests. This earlier study, however, only included nine cases of “MRI-normal” bvFTD participants and only examined naming performance, rather than testing a broader range of single-word processing abilities. As noted within their study, intragroup variability was high, and while MRI-abnormal cases were clearly below healthy control performance, no significant differences were found when comparing directly between MRI abnormal and MRI normal patients [11]. While Kipps et al. [11] did not report individual frequencies of impairment, the current study found over a quarter of possible bvFTD patients showed naming and comprehension deficits. This suggests that language impairments may be present in both possible and probable bvFTD, although more commonly found in probable bvFTD. Given that the present analyses focused on baseline assessments (i.e., performance at a patient’s first assessment), these findings support the view that deteriorations in language can emerge early in the disease course rather than only at more advanced stages [21, 22].

An unexpected finding from the current study was the significant reduction in word repetition when comparing bvFTD groups with healthy controls. Although this finding differs from previous studies in bvFTD [15], reduced performance occurred in the context of very few repetition errors made by healthy controls on this task. Despite this, it is interesting to note that performances were substantially reduced for three participants with probable bvFTD and three participants with SD, but no participants with possible bvFTD. On review of audio files of these individuals, clear language difficulties were observed in the bvFTD cases (e.g., phonemic errors). This finding aligns with large cohort studies and genetic bvFTD series that have also reported a small but non-trivial subset of patients demonstrating impaired word and/or sentence repetition [39–41].

In line with previous research [5, 6], the probable and possible bvFTD groups showed significantly less impairment than the SD group on all subtests other than repetition, where the patient groups did not differ significantly. Contrary to prediction, however, the pattern of performance across the subtests typically seen within an individual with SD was not replicated in the two bvFTD groups. Only patients with SD showed a pattern wherein naming was significantly more impaired relative to the other subtests. Although the probable bvFTD group did show a similar preservation of repetition when compared to the other language tasks, patients with possible bvFTD demonstrated a flat profile of performance across tasks. This suggests that, while some overlap in language deficits may exist between SD and bvFTD, the nature of the language impairments may differ. Regarding the cognitive factors associated with the language declines, our results were consistent with past suggestions that both semantic and executive functioning deficits play a role in language performance [6]. In both bvFTD groups, single-word language performance was strongly associated with our measure of semantic abilities from the ACE-III and, to a lesser extent, with executive functioning measures. However, the models were more strongly predictive in possible bvFTD, explaining up to 65% of the variance, than in probable bvFTD, where they explained 23–31%. This could suggest that in the presence of significant frontotemporal brain changes (as required in a probable vs possible diagnosis), language performance may be influenced by a broader range of factors not captured by the current models. Supporting this interpretation, visuospatial performance was reduced in probable but not possible bvFTD on the ACE-III. If this result reflects true visuospatial decline rather than executive inefficiency, it may provide an additional explanatory factor to performance on subtests which require visual discriminations and picture matching (as in the comprehension and semantic association tasks of the SYDBAT). Predictors of word repetition differed between probable and possible bvFTD subgroups. While none of the variables considered were associated with performance in the probable bvFTD group, three contributions to the score were found in the possible bvFTD group. The strongest predictor, explaining the most unique variance, was semantic knowledge (from the ACE-III), followed by working memory capacity. This finding is consistent with the reliance of repetition on intact lexical–semantic representations and short-term maintenance. In contrast, a negative contribution was found from the Hayling, which taken together with the negligible direct association with repetition performance and limited variance within this sample, suggests that individual differences in response initiative and suppression may modulate the contribution of other cognitive processes.

The relatively high frequency of semantic association impairment in probable bvFTD, together with evidence of significant associations with both semantic and executive measures is interesting to note. One possibility is that a subset of individuals with bvFTD has genuine disruption of semantic knowledge, potentially reflecting involvement of anterior temporal systems. Alternatively, poor performance may arise from executive difficulties affecting the controlled retrieval, selection, and monitoring of semantic information, rather than loss of the underlying representations themselves, or a combination of these two factors. Although the regression findings indicate that both semantic and executive measures were associated with this aspect of word-processing performance, it is important to note that the semantic and executive measures included do not constitute process-pure measures, instead sharing to some extent verbal, retrieval, selection, attentional, and monitoring demands. Verbatim responses and qualitative error data were not retained, preventing analysis of perseverative, off-task, semantic, phonological, or other error types. Given the overlap in task demands, it is therefore not possible to definitively distinguish degradation of semantic knowledge from impaired executive control of semantic processing. Future studies, combining qualitative analysis of error types together with measures designed to contrast automatic and controlled semantic processing, may aid in resolving this question.

Clinical Implications and Future Directions

The occurrence of single-word language impairments in both the probable and possible bvFTD groups suggests that current diagnostic criteria for bvFTD may benefit from revision. Incorporating assessments of naming and broader semantic knowledge into diagnostic evaluations may improve diagnostic confidence and provide insights into the likely clinical course.

Appreciating that language deficits may arise from a combination of executive and linguistic aspects may inform more effective intervention strategies. Declines in naming and semantic abilities can lead to everyday communication problems, such as word-finding difficulties and semantic paraphasias [42]. Interventions targeting language in bvFTD may need to consider not only linguistic elements but also executive demands (e.g., providing bvFTD patients with structured, cued response options) and support initiation and organisation of information to help address potential difficulties with coherency and goal-directed expression.

Future research will benefit from adopting fine-grained approaches, such as examining error types, to further distinguish semantic and executive contributions (e.g., perseverative responding from various types of semantic errors). In the only study to do this to-date, patients with probable bvFTD were found to produce high rates of associative errors and omissions, similar to SD patients [6]. Expanding this to examine the type of errors made by those with possible bvFTD may provide further opportunities to distinguish between bvFTD types and identify which individuals are likely to progress from possible to probable bvFTD. Longitudinal studies are needed to characterise trajectories of language decline and identify early predictors of rapid progression. Finally, expanding assessment beyond single-word processing to include connected speech and narrative language will strengthen links to everyday communication function.