Processing Time Reference in Healthy Brain: an ERP study
People suffering from agrammatic aphasia are known to have problems with different kinds of verbal inflection. Tense inflection is mostly more impaired than subject-verb agreementinflection. Problems with tense could either be morphosyntactic or morphosemantic in nature. Indications for the latter come from studies in which the past was found to be more impaired than present tense, even when the verbs where in their base position and both for uninflected as for inflected verbs. In the study presented here this issue is further explored, first in a grammaticality judgment task administered to agrammatic patients, then in a reaction time (RT) experiment with healthy participants.
Cross-linguistic data suggest that the grammatical categories of tense and aspect are not generally impaired in individuals with aphasia (see Bastiaanse et al., 2011 for a review). Rather, and more specifically, verb forms expressing reference to the past or conveying perfective semantics are more impaired than verb forms expressing reference to the non-past (present or future) or conveying imperfective semantics, both in comprehension and production. The present study used some structural properties of Russian to systematically test the interaction of time reference and aspect in non-fluent and fluent aphasia.
The Test for Assessment of Reference of Time (TART; Bastiaanse, Jonkers, & Thompson, 2008; Russian version Dragoy & Bastiaanse, 2010) was used to elicit production of four verb forms in sentence context: past perfective, non-past perfective, past imperfective and non-past imperfective. The results showed that non-past time reference had a general advantage over past time reference, all aspectual forms being collapsed. However, an interaction between time reference and aspect was found: imperfective verbs were better produced in the non-past, whereas production of perfective verbs was better preserved in the past time frame. Non-fluent and fluent aphasic speakers showed largely overlapping performance.
These results demonstrated that the advantage of a particular time reference depends on aspectual characteristics of the verb. The performance of both non-fluent and fluent aphasic speakers can be explained in terms of prototypical and non-prototypical matches of time reference and aspectual semantics: perfectives primarily refer to completed, past events while imperfectives prototypically describe ongoing, non-past events.
Reference to a time frame in which an event takes place can be done by verb inflection. If the time frame (past, present, future) is set by a temporal adverb, the verb inflection should correspond (yesterday he walk. ed; today he walk. s). Temporal violations by simple verbs (single, lexical verbs inflected with tense) in the present tense and with present time reference elicit a P600 effect (Baggio, 2008; Dragoy, Stowe, Bos, & Bastiaanse, 2012). However tense does not always coincide with time reference; in languages such as Dutch and English, reference to the past can be established by using the present tense in the present perfect (e.g. 'he has eaten the cake'). The current study investigates whether the P600 effects described by Dragoy et al. and Baggio are caused by tense or time reference violations of the verb. In the context of a past adverb, ERP responses to auxiliaries in present tense with either congruent past time reference or incongruent non-past time reference were compared. The findings show that the P600 effect for violations of the temporal context is caused by the time reference of the complete verb form, rather than by the tense.
Aphasia, a language impairment following stroke or brain trauma, is normally manifested behaviorally. Patients with the so-called non-fluent aphasia experience major difficulties at the level of morphosyntax (that is, producing and comprehending complex morphology and syntax). In contrast, patients with fluent aphasia predominantly show problems at the lexical-semantic level (that is, accessing word forms and lexical semantics). However, a few studies have proven that the language deficit in patients with aphasia is expressed not only in their linguistic performance, but also in specific electrophysiological responses.
In healthy individuals, incongruencies at different linguistic levels cause distinct event-related brain potentials (ERPs) registrated at the scalp. Syntactic incongruency elicits the ELAN and the P600 potentials, lexical-semantic incongruency elicits the N400 potential. Critically, in standard for those potentials linguistic contexts the lack of N400 was reported for fluent patients, while the lack of the ELAN and a reduced and delayed P600 – for non-fluent patients (Friederici et al. 1998, Wassenaar and Hagoort 2005). These findings support the idea of electrophysiological brain mapping on specific linguistic problems observed in different aphasia types.
The study was aimed at further investigating electrophysiological evidence for the suggestion that spoken sentence comprehension problems in individuals with fluent and non-fluent aphasia are caused, at least partly, by breakdowns at different levels of language processing – lexical-semantic and morphosyntactic respectively. To test this, we performed a study in healthy and aphasic Russian individuals using the method of event-related potentials (ERPs) that has become a powerful tool in addressing temporal aspects of language processing.
The goal of the present study was to investigate event-related potential (ERP) responses to Dutch negative and positive polarity adverbs of degree presented in licensed and unlicensed contexts with negative and affirmative particles directly preceding the polarity item. To control for effects of the processing of negation as such, neutral adverbs were also presented in negative and affirmative contexts. The results did not show any significant effect of negation for the non-polar adverbs, allowing context effects for polarity items to be interpreted as being due to the appropriateness of the context. Negative polarity violations elicited an N400 response that might reflect the lack of semantic congruity of the negative polarity item in an affirmative context. In contrast, processing positive polarity items in context of negation resulted in a positive effect resembling the P600, which may be considered as a marker of a different sort of integration difficulty caused by violation of licensing conditions and/or a search for a licensor in the wider discourse context. The study presented here is the first to show an unambiguous dissociation between responses to negative and positive polarity violations. This dissociation argues for different mechanisms underlying the processing of these two types of polarity; we propose that positive polarity items are sensitive to wider discourse context, while negative polarity items are more sensitive to local lexical context.
Recent studies have shown that reference to the past is more problematic in aphasic individuals than reference to the present (Anjarningsih et al. 2009; Bastiaanse, 2008). A fundamental issue concerns the extent to which past time reference difficulties are specific to aphasia. The present experiment was aimed at testing universal differences in processing present and past reference. We hypothesized that if past time reference problems are specific to aphasia, the same ERP effects should be observed while healthy individuals process verbs referring to the present and verbs referring to the past. However, if differences between present and past are more universal, non-similar ERP effects should be found.
Processing present and past time reference expressed through verbs is different in healthy individuals. Violation of the previously set time frame with a present tensed verb elicits a clear P600 response time locked to the critical verb. It means that present time reference is started being decoded and integrated into the preceding time context as soon as such it is presented. In contrast, there is no response time locked to the target verb when time frame is violated with a past tensed verb. This could be due to the fact that the proper decoding of time frame from verbal morphology does not happen right after a past tensed verb is presented. Instead, processing past time reference is delayed. These findings are in line with aphasiological data and suggest that problems with past in aphasia are based on more universal differences between processing present and past time reference.