Electrophysiology of prosodic and lexical-semantic processing during sentence comprehension in aphasia

Abstract

Event-related potentials (ERPs) were used to examine how individuals with aphasia and a group of age-matched controls use prosody and themattic fit information in sentences containing temporary syntactic ambiguities. Two groups of individuals with aphasia were investigated; those demonstrating relatively good sentence comprehension whose primary language difficulty is anomia (Individuals with Anomic Aphasia (IWAA)), and those who demonstrate impaired sentence comprehension whose primary diagnosis is Broca's aphasia (Individuals with Broca's Aphasia (IWBA)). The stimuli had early closure syntactic structure and contained a temporary early closure (correct)/late closure (incorrect) syntactic ambiguity. The prosody was manipulated to either be congruent or incongruent, and the temporarily ambiguous NP was also manipulated to either be a plausible or an implausible continuation for the subordinate verb (e.g., "While the band played the song/the beer pleased all the customers."). It was hypothesized that an implausible NP in sentences with incongruent prosody may provide the parser with a plausibility cue that could be used to predict syntactic structure. The results revealed that incongruent prosody paired with a plausibility cue resulted in an N400-P600 complex at the implausible NP (the beer) in both the controls and the IWAAs, yet incongruent prosody without a plausibility cue resulted in an N400-P600 at the critical verb (pleased) only in healthy controls. IWBAs did not show evidence of N400 or P600 effects at the ambiguous NP or critical verb, although they did show evidence of a delayed N400 effect at the sentence-final word in sentences with incongruent prosody. These results suggest that IWAAs have difficulty integrating prosodic cues with underlying syntactic structure when lexical-semantic information is not available to aid their parse. IWBAs have difficulty integrating both prosodic and lexical-semantic cues with syntactic structure, likely due to a processing delay.

Keywords: Aphasia; Event related potentials; N400; P600; Prosody; Sentence processing.

Figure 1.

Schematic of one trial. Participants were presented with the word “Ready” in the center of the screen to signal the beginning of a new trial. Next, a red cross was presented in the center of the screen, which corresponded with the sentence playing. The red cross remained on the screen throughout the sentence duration up to 1000ms after the sentence ended. A blue question mark was presented to signal that the participant could make their acceptability

Figure 2.

Electrode montage. The circled electrodes indicate the 15 electrodes used in data analysis.

Figure 3.

Grand average ERPs and voltage maps of the N400 epoch (300–600ms) and P600 epoch (600–1200ms) at the onset of the ambiguous NP (song/beer) in conditions with incongruent prosody (Pr−TF+, Pr−TF−) in A) age-matched controls, B) all of the individuals with aphasia, C) IWAA, and D) IWBA. Voltage maps depict the scalp distribution of the difference waves (incongruent – congruent prosody) in each epoch. The plausibility cue present in the implausible NP (beer) in Pr−TF− vs. Pr−TF+ sentences elicited a significant N400-P600 complex in age-matched controls. A small N400 effect, but no P600 effect, was found in the overall group of individuals with aphasia. Although, analyses of the subgroups of individuals with aphasia revealed significant N400 and P600 effects in the IWAA group, but no evidence of either N400 or P600 effects in the IWBA group.

Figure 4.

Grand average ERPs and voltage maps of the N400 epoch (300–600ms) and P600 epoch (600–1200ms) at the onset of the critical verb (pleased) in A) age-matched controls, and B) individuals with aphasia. Voltage maps demonstrate the scalp distribution of the difference waves (incongruent – congruent prosody) in conditions with a plausible NP (song in Pr−TF+ vs. Pr+TF+) and an implausible NP (beer in Pr−TF− vs. Pr+TF−) in each epoch. In the age-matched controls and individuals with aphasia, both sentences with incongruent prosody (Pr−TF+ and Pr−TF−) elicited an N400 effect, while a P600 effect was only revealed in Pr−TF+ sentences.

Figure 5.

Grand average ERPs and voltage maps of the N400 epoch (300–600ms) and P600 epoch (600–1200ms) at the onset of the critical verb (pleased) in the A) IWAA group, and the B) IWBA group. Voltage maps demonstrate the scalp distribution of the difference waves (incongruent – congruent prosody) in conditions with a plausible NP (song in Pr−TF+ vs. Pr+TF+) and an implausible NP (beer in Pr−TF− vs. Pr+TF−) in each epoch. In IWAAs, the implausible NP beer elicited a right-lateralized N400 effect in both the 300–600 and 600–1200ms time windows, whereas incongruent prosody (Pr−TF+ and Pr−TF−) evoked a negativity in the 600–1200ms window. Both types of sentences with incongruent prosody (Pr−TF+, Pr−TF−) elicited a sustained positivity across both time windows in the IWBAs.

Figure 6.

Grand average ERPs and voltage maps time locked to the onset of the final word across all four conditions. Two epochs are shown (300–600ms and 600–900ms) in A) age-matched controls, B) all of the individuals with aphasia, C) IWAAs, and D) IWBAs. The voltage maps depict the scalp distribution of the difference waves (incongruent – congruent prosody) in conditions with a plausible NP [song in (Pr−TF+) – (Pr+TF+)] and an implausible NP [beer in (Pr−TF−) – (Pr+TF−)] in each epoch. Sentences with incongruent prosody elicited a sustained negativity effect at the final word in both the 300–600ms and 600–900ms epochs in the age-matched controls, while delayed negativities were revealed in these sentences in the individuals with aphasia in the 600–900ms epoch. Only sentences with incongruent prosody and an implausible NP (Pr−TF−) elicited a significant sustained negativity effect in the 300–600ms epoch in IWAAs. Sentences with incongruent prosody (Pr−TF+, Pr−TF−) elicited a delayed negativity in the 600–900ms epoch in the IWBAs.