3.1. Number of words produced
The groups produced similar total numbers of words across their responses (F(5, 51) = 1.64, p = 0.17) (, ). Non-fluent/agrammatic PPA patients produced somewhat fewer words than controls, and semantic PPA patients produced somewhat more, but these differences were not significant.
Responses to the elicited syntactic production task. (A) Total number of words produced. (B) Frequency with which the targeted structures were attempted. (C). Accuracy on the targeted structures, when they were attempted.
Responses to the elicited syntactic production task
3.2. Frequency of attempts at targeted structures
The groups differed significantly in the frequency with which they attempted the targeted structures (F(5, 52) = 3.05, p = 0.017) (, ). Follow-up comparisons revealed that all neurodegenerative groups except for the bvFTD group attempted targeted structures less frequently than healthy controls (non-fluent/agrammatic: t = 3.39, p = 0.0094; semantic: t = 2.73, p = 0.055; logopenic: t = 2.87, p = 0.039; mixed: t = 2.76; p = 0.051). The three PPA variants did not differ from one another (all t ≤ 0.035) in how often they attempted the targeted structures.
The 22 items differed in the frequency with which participants attempted the targeted structure (χ2(21) = 186.38, p < 0.001) (). While most targeted structures were obtained more than half the time, a few items were particularly unsuccessful: the two items designed to elicit ditransitives (5a and 5b), one of the items intended to elicit an embedded clause (9a), and one intended to elicit a comparative structure (11a).
Fig. 2 Item-by-item performance of individual participants. Each participant is represented by a column, and participants are grouped based on diagnosis. Each item is represented by a row, and items are grouped based on similarities obtained in performance, (more ...)
For each item, we used a χ2 test to determine whether the three PPA and two neurodegenerative control groups differed in the frequency with which they attempted the intended structure (we omitted controls because we have already shown that they attempt the targeted structures more frequently in general). We set an alpha criterion of p < 0.01 to informally correct for multiple comparisons. No items met this threshold. At an uncorrected threshold of p < 0.05, three items showed different distributions: 3a (p = 0.050), 9a (p = 0.030) and 11b (p = 0.020).
3.3. Accuracy on targeted structures
The groups differed significantly in the frequency with which they produced targeted structures correctly, when they did attempt them (Kruskal-Wallis χ2 = 29.41, df = 5, p < 0.001) (, ). Patients with non-fluent/agrammatic PPA were less accurate than controls (p < 0.001), logopenic PPA patients were marginally less accurate (p = 0.068), and mixed neurodegenerative patients were less accurate (p = 0.028). Non-fluent/agrammatic PPA patients were less accurate than semantic (p = 0.015) or logopenic (p = 0.0067) patients, but semantic and logopenic patients did not differ from one another (p = 0.79).
The 22 items differed in the frequency with which participants produced attempted targeted structures correctly (χ2(21) = 193.76, p < 0.001) (). The most challenging structures were the declarative transitive with 3sg agreement (3a), and the two embedded clauses (9a and 9b).
We used sets of chi square tests to determine which items patients with each PPA variant had most difficulty with, using an alpha criterion of p < 0.01 to informally correct for multiple comparisons. Non-fluent patients performed worse than controls on items 3a, 9a and 9b. Inspection of suggests that 3sg present tense agreement and embeddings posed the most problems for non-fluent patients, followed by wh-questions. There were no items on which semantic or logopenic PPA patients performed significantly worse than controls, though it should be noted that for both groups most errors occurred on item 9b, an embedded clause.
Examples of targeted structures that were attempted but resulted in syntactic errors are shown in .
Examples of syntactic errors produced by PPA patients
3.4. Voxel-based morphometry
Using voxel-based morphometry, we found that the only region where atrophy was significantly predictive of reduced accuracy on targeted structures was the left posterior inferior frontal gyrus, pars opercularis (center of mass: MNI coordinates –53, 12, 13; maximum t = 3.47; cluster extent = 3192 mm3; corrected p = 0.025; ).
Voxel-based morphometry. The posterior left inferior frontal gyrus was the only region where atrophy was predictive of decreased accuracy in production of targeted structures (p < 0.05, corrected for multiple comparisons).
The same region was found to predict reduced accuracy when covariates of executive function and/or working memory were included in the model, however its volume was large enough to survive correction for multiple comparisons only for the calculation covariate (extent = 3648 mm3); the cluster extent for syntactic accuracy was reduced to 456 mm3 when digit span backwards was included as a covariate, and 512 mm3 when modified trails was included as a covariate.
3.5. Syntactic and semantic errors
We counted syntactic errors irrespective of whether or not the targeted structure was attempted, and divided by the total number of words each participant produced. The groups differed significantly in syntactic errors per word (Kruskal-Wallis χ2 = 34.26, df = 5, p < 0.001) (). Non-fluent/agrammatic PPA patients produced the most errors.
We also counted semantic errors. The groups differed marginally in semantic errors per word (Kruskal-Wallis χ2 = 10.91, df = 5, p = 0.053) (). All patient groups produced some semantic errors, but controls did not produce any.