The time-course of lexical activation during sentence comprehension in aphasia

We present a cross-modal priming study using simple active sentence constructions to map out the time-course of lexical activation throughout a sentence. Preliminary findings suggest that in contrast to our unimpaired listeners who demonstrated immediate initial activation of a lexical item and a linear, relatively gradual decay, our participants with agrammatic aphasia demonstrated a pattern of late initial activation, and then immediate decay.  We surmise that this delayed lexical access coupled with the accelerated time-course of decay contributes to what appears to be a disrupted integration process

Structure-function correspondences in Broca’s aphasia: Evidence from MRI and comprehension of verb phrase ellipsis constructions

We describe an effort to map lesion to behavior by studying the comprehension of complex VP-Ellipsis constructions (e.g., The policeman defended the child, and the dedicated fireman did___ too…) in participants with Broca‟s aphasia. We quantified the lesions of our individual participants using cytoarchitectonic probability maps of the human brain. We found that our Broca participants evinced delayed priming of the object in the ellipsis clause, while off-line comprehension was largely spared. Structure-function analyses revealed that lesions in both temporal and frontal areas participated in the behavioral outcomes, though each region seems to have played a distinct role

More than Words : Lexical Processing During Sentence Comprehension in Broca's Aphasia

Though Broca's aphasia is traditionally defined as an expressive language impairment, listeners with Broca's aphasia (LWBA) typically evince sentence comprehension deficits as well. These comprehension deficits are characterized by difficulty understanding certain types of sentences that contain complex syntax. While some research proposes that the source of the comprehension disorder can be attributed to a syntactic processing delay, other research argues that syntactic processing impairments are secondary to primary lexical processing impairments. The Delayed Lexical Activation Hypothesis (DLA) suggests that a slowed lexical activation system results in lexical information "feeding" syntactic processing too slowly, leading to a mismatch between processing rate and what is required for fast-acting processing routines. A series of three studies with LWBA are presented to explore (a) if the DLA holds in the face of simple, canonically ordered sentences (Chapter 3), and (b) if lexical access delays can be mitigated through manipulations of speech input rate (Chapter 4) and/or cue based prediction (Chapter 5). Chapter 2 reviews current research with LWBA and details prior empirical evidence supporting the presence of a processing delay in both lexical and syntactic processing. Chapter 3 presents evidence of real-time lexical access during processing of syntactically simple sentences. Results showed LWBA demonstrated a pattern of protracted lexical access as compared to unimpaired controls. Chapter 4 explores if slowed input rate would combine with the purported slowed lexical activation to yield 'on-time' lexical access patterns. While a LWBA group effect of rate was not found, interesting patterns emerged when considering individual patterns of brain damage. It was found that the proportion of damage to a brain region of interest implicated in lexical, but not syntactic processing, significantly predicted the effect of rate of speech on the time-course of lexical access. Finally, Chapter 5 investigates if LWBA demonstrate predictive processing by showing 'on-time' access with contextual cues that unimpaired listeners have been shown to use - biased adjectives. LWBA were able to use semantic, not structural, cues to mitigate a lexical access delay. These results taken together support the DLA hypothesis; that a lexical access delay underlies the comprehension disorder in LWB

More than Words : Lexical Processing During Sentence Comprehension in Broca's Aphasia

Though Broca's aphasia is traditionally defined as an expressive language impairment, listeners with Broca's aphasia (LWBA) typically evince sentence comprehension deficits as well. These comprehension deficits are characterized by difficulty understanding certain types of sentences that contain complex syntax. While some research proposes that the source of the comprehension disorder can be attributed to a syntactic processing delay, other research argues that syntactic processing impairments are secondary to primary lexical processing impairments. The Delayed Lexical Activation Hypothesis (DLA) suggests that a slowed lexical activation system results in lexical information "feeding" syntactic processing too slowly, leading to a mismatch between processing rate and what is required for fast-acting processing routines. A series of three studies with LWBA are presented to explore (a) if the DLA holds in the face of simple, canonically ordered sentences (Chapter 3), and (b) if lexical access delays can be mitigated through manipulations of speech input rate (Chapter 4) and/or cue based prediction (Chapter 5). Chapter 2 reviews current research with LWBA and details prior empirical evidence supporting the presence of a processing delay in both lexical and syntactic processing. Chapter 3 presents evidence of real-time lexical access during processing of syntactically simple sentences. Results showed LWBA demonstrated a pattern of protracted lexical access as compared to unimpaired controls. Chapter 4 explores if slowed input rate would combine with the purported slowed lexical activation to yield 'on-time' lexical access patterns. While a LWBA group effect of rate was not found, interesting patterns emerged when considering individual patterns of brain damage. It was found that the proportion of damage to a brain region of interest implicated in lexical, but not syntactic processing, significantly predicted the effect of rate of speech on the time-course of lexical access. Finally, Chapter 5 investigates if LWBA demonstrate predictive processing by showing 'on-time' access with contextual cues that unimpaired listeners have been shown to use - biased adjectives. LWBA were able to use semantic, not structural, cues to mitigate a lexical access delay. These results taken together support the DLA hypothesis; that a lexical access delay underlies the comprehension disorder in LWB

Neural networks supporting audiovisual integration for speech: A large-scale lesion study

Auditory and visual speech information are often strongly integrated resulting in perceptual enhancements for audiovisual (AV) speech over audio alone and sometimes yielding compelling illusory fusion percepts when AV cues are mismatched, the McGurk-MacDonald effect. Previous research has identified three candidate regions thought to be critical for AV speech integration: the posterior superior temporal sulcus (STS), early auditory cortex, and the posterior inferior frontal gyrus. We assess the causal involvement of these regions (and others) in the first large-scale (N = 100) lesion-based study of AV speech integration. Two primary findings emerged. First, behavioral performance and lesion maps for AV enhancement and illusory fusion measures indicate that classic metrics of AV speech integration are not necessarily measuring the same process. Second, lesions involving superior temporal auditory, lateral occipital visual, and multisensory zones in the STS are the most disruptive to AV speech integration. Further, when AV speech integration fails, the nature of the failure-auditory vs visual capture-can be predicted from the location of the lesions. These findings show that AV speech processing is supported by unimodal auditory and visual cortices as well as multimodal regions such as the STS at their boundary. Motor related frontal regions do not appear to play a role in AV speech integration