Chance in agrammatic sentence comprehension: What does it really mean? Evidence from eye movements of German agrammatic aphasic patients

Abstract

Background: In addition to the canonical subject-verb-object (SVO) word order, German also allows for non-canonical order (OVS), and the case-marking system supports thematic role interpretation. Previous eye-tracking studies (Kamide et al., 2003; Knoeferle, 2007) have shown that unambiguous case information in non-canonical sentences is processed incrementally. For individuals with agrammatic aphasia, comprehension of non-canonical sentences is at chance level (Burchert et al., 2003). The trace deletion hypothesis (Grodzinsky 1995, 2000) claims that this is due to structural impairments in syntactic representations, which force the individual with aphasia (IWA) to apply a guessing strategy. However, recent studies investigating online sentence processing in aphasia (Caplan et al., 2007; Dickey et al., 2007) found that divergences exist in IWAs' sentence-processing routines depending on whether they comprehended non-canonical sentences correctly or not, pointing rather to a processing deficit explanation.

Aims: The aim of the current study was to investigate agrammatic IWAs' online and offline sentence comprehension simultaneously in order to reveal what online sentence-processing strategies they rely on and how these differ from controls' processing routines. We further asked whether IWAs' offline chance performance for non-canonical sentences does indeed result from guessing.

Methods & Procedures: We used the visual-world paradigm and measured eye movements (as an index of online sentence processing) of controls (N = 8) and individuals with aphasia (N = 7) during a sentence–picture matching task. Additional offline measures were accuracy and reaction times.

Outcomes & Results: While the offline accuracy results corresponded to the pattern predicted by the TDH, IWAs' eye movements revealed systematic differences depending on the response accuracy.

Conclusions: These findings constitute evidence against attributing IWAs' chance performance for non-canonical structures to mere guessing. Instead, our results support processing deficit explanations and characterise the agrammatic parser as deterministic and inefficient: it is slowed down, affected by intermittent deficiencies in performing syntactic operations, and fails to compute reanalysis even when one is detected.

Keywords:

• Eye movements
• Non-canonical sentences
• Agrammatic aphasia
• Broca's aphasia
• Chance performance
• Online and offline processing
• Sentence comprehension disorders
• German syntax

Acknowledgments

This research was partially supported by the National Science Foundation under ADVANCE Grant #0137851 to the second author. Any opinions, findings, conclusions, or recommendations expressed in this article are those of the authors and do not necessarily reflect the views of the National Science Foundation. We wish to thank Titus von der Malsburg for substantial help in data preparation and with the data analysis. Special thanks go to all individuals who enthusiastically participated in the study. We also thank the anonymous reviewers for their comments.

Notes

¹By qualitative deviations we refer to differences in eye movements between controls and IWA that do not merely reflect a delay in an otherwise similar fixation pattern (which would be a quantitative difference) but which constitute a structurally different pattern of fixations to the objects or depicted events presented (cf. also Dickey et al., 2007).

²Chance performance was determined by carrying out a non-parametrical analysis of performance levels. The comprehension screening for canonical and non-canonical sentences (a sentence–picture matching task with two pictures) encompassed 20 items per condition. Hence, the chance value was at 50%, i.e., 10 correct responses. We calculated the chance range using the Fisher's exact test (one-sided) by comparing possible distributions of correct and incorrect responses against the 50% chance distribution (for example, comparing 10 correct and 10 incorrect responses against 12 correct and 8 incorrect answers). This procedure revealed that only a score that amounted to fewer than 4 incorrect answers (20%) or more than 15 correct responses (75%) was significantly different from chance. Therefore, scores that fell between 20% and 75% correct responses were considered as chance performance. This was true for each of the individuals with aphasia.

³It must be noted that no screening of motor responsiveness was done with the individuals with aphasia and of course motor ability plays an important role in the design we used. However, the ability to give motor responses to the task was observed and judged by the two experimenters during the practice phase of the experiment. In addition, we had determined a maximum response time of 15000 ms and answers exceeding this time span were considered time-outs. Hence, if any participant had striking deficits in motor responsiveness, we would have expected many time-outs. However, this was not the case for neither of the participants.

⁴For patient A1, who had a lesion in the right hemisphere and was pre-morbidly left-handed, the left hand was hemiparetic. Therefore two mirror buttons on the right side of the keyboard were used, and he was instructed to use two fingers of his right hand.

⁵We use the term ROI to refer to a specific part of the auditory sentence presented together with the two pictures. In using this term that way (and not for a specific area of the visual display), we oriented ourselves towards visual-world studies by for example Knoeferle and colleagues (Knoeferle, 2007; Knoeferle, Crocker, Scheepers, & Pickering, 2005) and Altmann and colleagues (Altmann & Kamide, 1999, 2004; Kamide et al., 2003).