online sentence comprehension in ppa: verb-based integration and prediction

Introduction. Impaired language comprehension is frequently observed in primary progressive aphasia (PPA). Word comprehension deficits are characteristic of the semantic variant (PPA-S) whereas sentence comprehension deficits are more prevalent in the agrammatic (PPA-G) and logopenic (PPA-L) variants (Amici et al., 2007; Gorno-Tempini et al., 2011; Thompson et al., 2013). Word and sentence comprehension deficits have also been shown to have distinct neural substrates in PPA (Mesulam, Thompson, Weintraub, & Rogalski, in press). However, little is known about the relationship between word and sentence comprehension processes in PPA, specifically how words are accessed, combined, and used to predict upcoming elements within a sentence. A previous study demonstrated that listeners with stroke-induced agrammatic aphasia rapidly access verb meanings and use them to semantically integrate verb-arguments; however, they show deficits in using verb meanings predictively (Mack, Ji, & Thompson, 2013). The present study tested whether listeners with PPA are able to access verb meanings and to use this information to integrate and predict verb-arguments. Methods. Fifteen adults with PPA (8 with PPA-G, 3 with PPA-L, and 4 with PPA-S) and ten age-matched controls participated in two eyetracking experiments. In both experiments, participants heard sentences with restrictive verbs that were semantically compatible with only one object in a four-picture visual array (e.g., eat when the array included a cake and three non-edible objects) and unrestrictive verbs (e.g., move) that were compatible with all four objects. The verb-based integration experiment tested access to verb meaning and its effects on integration of the direct object (e.g., Susan will eat/move the cake); the verb-based prediction experiment examined prediction of the direct object (e.g., Susan will eat/move the …). The dependent variable was the rate of fixations on the target picture (e.g., the cake) in the first 500 ms after the offset of the verb. Logistic regression was used to compare the rate of target fixations between PPA and controls and to test for differences between PPA subtypes. Results. In the verb-based integration experiment (Fig. 1a), PPA listeners as well as controls showed rapid access to verb meaning, making more target fixations in the restrictive than unrestrictive conditions in the first 500 ms after verb offset. No significant differences were found between participant groups. In the verb-based prediction experiment (Fig. 1b), control listeners exhibited a greater difference between the restrictive and unrestrictive conditions compared to PPA listeners. In both experiments, no significant differences were found between PPA subtypes. Conclusion. The results of this study suggest that access to verb meaning is relatively preserved in PPA and can facilitate integration of verb-arguments. However, prediction of verb-arguments is impaired. These findings are in line with stroke-induced agrammatic aphasia, in which prediction is markedly impaired (Mack et al., 2013). The similar pattern of results across PPA subtypes should be interpreted cautiously due to small sample sizes. However, these findings suggest that – despite well-established differences in word and sentence comprehension impairments – there may also be shared deficits across PPA subtypes that affect the ability to use lexical information predictively during sentence comprehension.