Effects of verb meaning on lexical integration in agrammatic aphasia: Evidence from eyetracking
Introduction
Agrammatic (Broca's) aphasia involves not only deficits in morphosyntactic processing, but also impaired lexical processing. At the level of single-word processing, priming effects have sometimes failed to emerge, or have emerged at a slowed time course, in adults with agrammatic aphasia relative to unimpaired adults (Milberg, Blumstein, & Dworetzky, 1988; Prather, Zurif, Love, & Brownell, 1997; Prather, Zurif, Stern, & Rosen, 1992; Utman, Blumstein, & Sullivan, 2001). Eyetracking studies have also revealed deficits in single-word lexical processing, including generally slowed lexical access (Choy, 2011) and abnormal lexical competitor effects (Mirman, Yee, Blumstein, & Magnuson, 2011; Yee, Blumstein, & Sedivy, 2008). These findings have motivated several accounts of single-word processing deficits in agrammatic aphasia, including decreased levels of lexical activation (Janse, 2006; Milberg et al., 1988; Utman et al., 2001; Yee et al., 2008), slowed lexical activation (Prather et al., 1992, 1997), and impaired ability to select between competing lexical representations (Mirman et al., 2011).
Several studies have also investigated the role that lexical processing deficits play in sentence comprehension in agrammatic aphasia. Cross-modal priming studies of filler-gap structures, which involve displacement of a sentence constituent from its canonical position (e.g., Whoi did the boy kiss GAPi at school?), found that participants with agrammatic aphasia showed delayed or absent reactivation of the filler at the gap site when sentences were presented at a normal speech rate (Burkhardt, Piñango, & Wong, 2003; Love, Swinney, Walenski, & Zurif, 2008; Swinney, Zurif, Prather, & Love, 1996; Zurif, Swinney, Prather, Solomon, & Bushel, 1993). On this basis, Love et al. (2008) argued that slowed lexical activation may underlie impaired comprehension of noncanonical sentences. Eyetracking studies have also lent support to the hypothesis that lexical processing deficits contribute to sentence comprehension impairments. Visual-world eyetracking experiments on gap-filling in agrammatic aphasia have reported intact and timely online processing, as reflected by increased looks to a picture of the filler at the gap site (Choy, 2011; Dickey, Choy, & Thompson, 2007; Dickey & Thompson, 2009). However, the participants in these studies also exhibited impaired comprehension of noncanonical sentences. A similar contrast between intact online processing and impaired offline processing was observed in an experiment investigating pronoun resolution (Thompson & Choy, 2009). One interpretation of these findings, proposed by Thompson and Choy (2009; cf. Choy, 2011) is that in agrammatic aphasia the syntactic processes supporting gap-filling and pronoun resolution are intact, resulting in timely reactivation of the antecedent, but lexical integration, i.e. the incorporation of the critical word into the unfolding representation of the sentence, is impaired. Further support for the lexical integration hypothesis comes from studies of lexical ambiguity resolution, which have revealed a deficit in timely selection of the appropriate meaning of ambiguous words in agrammatic aphasia (Hagoort, 1993; Swaab, Brown, & Hagoort, 1998).
Timely access to the lexical representations of verbs, including subcategorization (syntactic combinatory potential), argument structure (semantic combinatory potential), and selectional restrictions (the semantic restrictions that verbs place on their arguments), is essential to lexical and grammatical integration processes in unimpaired adults (e.g., Altmann & Kamide, 1999, 2007; Boland, 2005; Shapiro, Zurif, & Grimshaw, 1987, 1989; Trueswell, Tanenhaus, & Kello, 1993). It is well-documented that people with agrammatic aphasia often exhibit deficits in verb production, with greater impairment for verbs with complex lexical representations (e.g., Kim & Thompson, 2000; Thompson, 2003; Thompson & Lee, 2009; Zingeser & Berndt, 1990). However, relatively few studies have investigated verb access during on-line sentence comprehension in agrammatic aphasia. Studies investigating access to subcategorization and argument structure information have yielded mixed results. Cross-modal lexical decision studies (Shapiro & Levine, 1990; Shapiro, Gordon, Hack, & Killackey, 1993) provided evidence that people with agrammatic aphasia, like unimpaired controls, activate argument structure information immediately upon hearing the verb. However, a recent ERP study (Kielar, Meltzer-Asscher, & Thompson, 2012) reported abnormal electrophysiological responses to argument structure violations in individuals with agrammatic aphasia relative to control participants. Myers and Blumstein (2005) investigated the effects of a verb's selectional restrictions on the processing of a subsequent argument noun phrase in an auditory lexical decision paradigm using both single-word and sentence contexts. Participants made a lexical decision to a noun (e.g., letter) preceded by a (a) semantically associated and compatible verb (i.e., compatible with the verb's selectional restrictions) (e.g., mail), (b) semantically compatible but not associated verb (e.g., find), and (c) semantically incompatible verb (e.g., persuade). In the single-word experiment, the verb/noun pairs were presented in isolation, whereas in the sentence experiment, the pairs were embedded within grammatical (e.g., The men mail/find/persuade the letter) or ungrammatical environments (e.g., *The men mail/find/persuade over letter). Across all contexts, unimpaired adults showed priming effects for nouns following semantically compatible/associated and semantically compatible/unassociated verbs relative to semantically incompatible verbs. Adults with agrammatic aphasia showed priming for semantically compatible/associated verbs across all contexts, but priming for semantically compatible/unassociated verbs emerged only in single-word and grammatical sentence contexts. Myers and Blumstein (2005) argue that these results suggest that access to selectional restriction information is vulnerable to disruption in agrammatic aphasia. Thus, some evidence suggests that impaired access to verb representations may affect lexical and grammatical integration processes in agrammatic aphasia.
The present study investigates effects of access to verb meaning on subsequent lexical integration processes in aphasic and age-matched control listeners using a visual-world eyetracking paradigm, following Altmann and Kamide (1999). In that study, young adult participants made anticipatory fixations to a target object (e.g., a cake, within a scene also containing several non-edible objects) while processing a restrictive verb that was semantically compatible only with the cake (e.g., The boy will eat the cake) but not while processing an unrestrictive verb compatible with all objects in the scene (e.g., The boy will move the cake). In addition, the effects of verb type persisted into the noun itself, reflecting facilitated lexical integration in the restrictive condition. These findings indicate that healthy listeners use verb meaning to predict upcoming target nouns. Several other studies with young adult participants have replicated and extended these findings (e.g., Altmann & Kamide, 2007; Boland, 2005; Kamide, Scheepers, & Altmann, 2003), and such effects have also been found in 10- and 11-year old children (Nation, Marshall, & Altmann, 2003).
No previous research has investigated the eye movement patterns of healthy older adults or participants with aphasia using this paradigm. Two experiments were conducted. In Experiment 1, full sentences with restrictive and unrestrictive verbs (e.g., Tomorrow Susan will open/break the window) were presented in order to test the effects of verb meaning on the processing of a subsequent overt argument. As in previous experiments (e.g., Altmann & Kamide, 1999), this experiment investigates both prediction (anticipatory eye movements before the critical noun) and lexical integration (eye movements during the critical noun). In Experiment 2, the noun was omitted (e.g., Tomorrow Susan will open/break the…). This allowed us to investigate effects of verb meaning on the prediction of a subsequent argument, while providing a more generous time-window for prediction that is absent when the critical noun phrase immediately follows the verb. An additional motivation for including Experiment 2 is that prediction and integration of non-overt arguments are essential for comprehension of noncanonical sentences and have been argued to be impaired in agrammatic aphasia (e.g., Choy, 2011; Dickey, Choy, & Thompson, 2007; Dickey & Thompson, 2009).
Although we anticipate that older adults will perform similarly to younger listeners, event-related potential research suggests that predictive processes in sentence comprehension may be less robust in older compared to young adults (e.g., Delong, Groppe, Urbach, & Kutas, 2012; Federmeier & Kutas, 2005; Wlotko, Federmeier, & Kutas, 2012). If this is the case, verb-driven anticipatory eye movements (i.e., effects of verb type that emerge before noun onset) may be absent in the age-matched control listeners in Experiment 1. However, we expect to see effects of verb type on lexical integration following the verb (i.e., during processing of the noun in Experiment 1), as well as on prediction of a subsequent argument in Experiment 2, in which time constraints on prediction are eliminated. If the ability to access and use verb meaning to generate predictions and facilitate lexical integration is intact in agrammatic aphasia, then effects of verb type should emerge on the same time course as that observed for age-matched controls. However, if agrammatic aphasia involves impaired access to verb meaning or subsequent prediction and integration processes, then effects of verb meaning should be reduced, delayed, or absent relative to the control listeners.
Section snippets
Participants
Nine adults with agrammatic aphasia (five male) and ten unimpaired adults (three male) participated in the study. All aphasic participants had sustained a left-hemisphere CVA at least two years previously. Detailed lesion information was available for six aphasic participants (all but A5, A6, and A9); see Table 1. For all six of these participants, the lesion included left inferior frontal cortex. The participant groups were matched for age (control M (SD) = 56.0 (10.8); aphasic M (SD) = 56.7
Participants
The participants were the same as in Experiment 1.
Stimuli
There were 40 critical sentence pairs, each with a corresponding visual array of four objects. The form of the sentences was the same as in Experiment 1, except that the sentences were lacking a target noun (e.g. Tomorrow NP will V the…). Each sentence pair contained one restrictive and one unrestrictive verb. We used the same verb pairs as in Experiment 1 (see Table 2 for a summary of their properties), but with different noun phrases and
General discussion
The present study used the visual-world eyetracking paradigm to investigate the time course of access to verb meaning and its effects on lexical integration in agrammatic aphasic and age-matched control listeners. The study included two experiments that investigated the effects of verb meaning on processing of subsequent overt (Experiment 1) and non-overt (Experiment 2) arguments. The eye movement data were analyzed using mixed-effects empirical logit regression (e.g., Barr, 2008), in addition
Conclusion
The results of the present study provide new insights into the nature of lexical processing deficits in agrammatic aphasia and their effects on sentence comprehension. Participants with agrammatic aphasia demonstrated intact access to and use of verb meaning to integrate the verb with subsequent overt arguments. However, the ability to use verb meaning to predict upcoming arguments was impaired. Additional research is necessary to better understand how impaired lexical-semantic prediction
Acknowledgments
The authors would like to thank Mari Feuer and Abigail Hogan Brown for help with stimulus construction, Melissa Randazzo Wagner for assistance with data analysis, and Dr. Aya Meltzer-Asscher for assistance with verb classification and helpful discussions. This research was supported by NIH 4 R01 DC001948 to C. K. Thompson and NIH 5 T32 DC009399 to Northwestern University.
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