New research shows the connection between cognition and Aphasia

Published first in “Perspectives of the ASHA Special Interest Groups SIG 2, Vol. 2 (Part 1), 2017.

Authors: Sofia Vallila-Rohter Swathi Kiran

The nature of language impairment in aphasia is well studied and documented. Individuals with post-stroke aphasia have been reported to have trouble with almost all aspects of language processing that span phonology, semantics, morphology, grammar, pragmatics, and suprasegmental prosody. As these are the deficits that define aphasia, the majority of work in aphasia has focused on delineating the different taxonomic components in each of the broad domains outlined above, with the aim of improving diagnosis or therapy. Understanding whether a person with aphasia presents with semantic processing deficits, phonological deficits or both, for example, contributes to the characterization of that person’s impairment and subsequent treatment (Kiran & Sandberg, 2011; Meier, Lo, & Kiran, 2016).

Evidence is accumulating; however, that suggests that aspects of cognition such as attention, executive functions and cognitive control contribute to language processing in individuals with aphasia more than previously thought (Corbett, Jefferies, Ehsan, & Lambon Ralph, 2009; El Hachioui et al., 2014; Martin & Allen, 2008; Villard & Kiran, 2016). In addition, El Hachioui et al. (2014) found that 88% of the patients they tested on a battery of language and cognitive assessments presented with impairments in cognitive domains (including visuospatial processing, abstract reasoning, and executive functions) that tended to persist at even a year post-stroke. Importantly, studies that have compared measures of language and nonverbal cognitive ability reveal that language ability does not correlate with nonverbal cognitive ability (Gainotti, Silveri, Villa, & Caltagirone, 1983; Helm-Estabrooks, Bayles, Ramage, & Bryant, 1995; Kertesz & McCabe, 1977; Vallila-Rohter & Kiran, 2013). Furthermore, language ability accounts for only a small percentage of the variance observed on nonverbal cognitive task performance in aphasia (Fucetola, Connor, Strube, & Corbetta, 2009). Thus, cognitive deficits arise frequently and with variability aphasia.

Advances in brain imaging and network perspectives of the brain may provide an opportunity to explain the presence of these accompanying cognitive deficits in aphasia and also to shed light on mechanisms of recovery in aphasia. Two of the networks that have been identified in the brain are of particular interest in aphasia. The first—the language network—involves left hemisphere language regions and their right hemisphere homologues engaged in language processing. The second—the domain general multiple demand network—involves fronto-parietal cortices and is recruited under effortful conditions across diverse tasks such as arithmetic and visuospatial processing (Fedorenko, Duncan, & Kanwisher, 2012, 2013). What is fascinating is that these latter regions (domain general regions), thought to be recruited in effortful conditions across a variety of tasks, are involved in effortful language processing of complex linguistic tasks. Thus, regions of the brain traditionally involved in nonlinguistic cognitive processing may also be involved in complex language processing. This suggests that the brain networks for processing language and non-language cognitive information may overlap after all.

There are recent examples of non-language, domain general regions being recruited in stroke patients. For instance, Brownsett et al. (2014) demonstrated the engagement of the domain general network involved in attention and salience control when patients with aphasia were performing language processing tasks in the scanner that involved listening to and repeating simple sentences. Neural activations observed in individuals with aphasia were similar to the neural activations produced by control participants under more complex language processing conditions (speech in noise). These observations have profound implications for aphasia as they imply the engagement of domain general regions to support residual language processing in individuals with aphasia.

The following set of articles provides a more thorough insight into the presence and nature of cognitive deficits in aphasia (including attention, cognitive control and non-language learning) and how this relates to principles and considerations of therapy. First, Villard describes the attention deficits that have been observed in aphasia, drawing further attention to the notion that attention may not only impact language processing in a modular way, but that attention may actually underlie successful language processing in aphasia. In this review, Villard discusses the various constructs of attention and how they are challenged through the process of therapy. She discusses a relatively novel notion that examines attentional fluctuations within a subject over time. Subject-to-subject variability pervades throughout the literature in aphasia. Less typical is a discussion of variability that might arise within a single person, how this influences their language ability, engagement in therapy and eventual outcomes.

Then, Gray discusses some of the complexities of bilingualism and bilingual aphasia. This manuscript provides a roadmap for clinicians, highlighting factors to consider in the diagnosis and intervention of bilingual individuals presenting with aphasia. Nonverbal cognition is intrinsically linked the topic of bilingualism, as bilinguals must constantly exert control to activate lexicosemantics of their target language without interference from the non-target language. A bilingual cannot simply “turn-off” one language and utilize the other. Instead, cognitive control is required to manage language access and use in a flexible way. Bilingual aphasia provides a unique window into the potential interaction between language and cognitive control because this population presents with language deficits and presumably intact cognitive skills. Gray provides novel evidence that this may actually not be the case.

Finally, Vallila-Rohter considers the importance of learning in aphasia rehabilitation, briefly reviewing the literature that suggests that multiple learning systems exist (implicit and explicit). Research in individuals with amnesia, Parkinson’s disease, and Alzheimer’s have demonstrated that considerations of learning approach and mechanisms can be critical to success with learning. Tasks for amnestics, for example, must be adapted to minimize demands on the hippocampus. The field of aphasia has a limited understanding of how aphasia-inducing strokes impact networks of learning and how this might impact an individual’s interaction with therapy. Interestingly, Vallila-Rohter has demonstrated that individuals with aphasia vary in their ability to learn abstract categories, and that the ability to learn new information is not directly related to an individual’s language impairment.

All the studies that are part of this issue highlight two important points. First, language deficits in aphasia may not be modular, and attention, cognitive control, and learning may be just some of the factors that underlie successful language processing in aphasia. These papers underscore the emerging and compelling evidence for domain general mechanisms supporting language and cognitive processing. Second, these papers emphasize the implications of the interaction between language and cognitive processing (and impairment) on rehabilitation for patients with aphasia. Specifically, if indeed cognitive factors such as attention or cognitive control influence language processing in individuals with aphasia, they might provide a clearer explanation of the effects of language treatment in patients with aphasia. Lambon-Ralph et al., (2010) administered a series of language and cognitive assessments to patients receiving a naming language therapy and found that both language and cognitive skills predicted treatment outcomes. If cognitive and linguistic factors both contribute to outcomes, they should both be systematically considered in the evaluation and treatment of aphasia.

Additionally, understanding the interaction between language and cognitive processing may provide new avenues for developing behavioral treatments. Along these lines, a few novel treatments that focus on behavioral training of executive functions with corresponding improvements in language function have been recently reported (Des Roches, Balachandran, Ascenso, Tripodis, & Kiran, 2015; Zakarias, Keresztes, Marton, & Wartenburger, 2016). This type of cross-domain generalization can be capitalized in behavioral intervention studies and may validate the brain imaging studies that suggest interaction between language and cognitive processing in the brain. Clearly, more work needs to be done to establish the circumstances of when interaction between language and cognitive processing can be beneficial for the patient. However, it is imperative for clinicians and researchers to devote their efforts to identify and rigorously test novel treatments especially in the current health care climate, where therapy sessions are severely limited for patients with chronic aphasia (who may indeed struggle with both language and cognitive impairments)!

** Editor’s Note: This Perspectives also contains a carryover article form our December 2016, Part 4 publication that focused on contribution made by the Veteran’s Health Administration in the research and treatment of individuals with neurogenic language and cognitive-communication disorders. The final article, written by Linda Picon and Don MacLennan discusses the unique presentation of traumatic brain injury in military personal and shed a light on the complexities of this population.

References

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