The complex relationship between cognition and language: illustrations from acquired aphasia

The relationship between language and thought has been hotly debated for decades (e.g., Evans, 2014; Fodor, 1975; Langland‐Hassan et al., 2021). Some authors, like the linguist Ferdinand de Saussure quoted above, believe that thought is barely possible without language. However, there are many arguments suggesting that this simply cannot be the case, including the quote above from Tom Lubbock who documented his language loss as a result of a brain tumor. Other examples include that babies show clear intent by reaching for things before they have acquired language, and that non‐human animals have been demonstrated to show complex cognition and problem‐solving behavior in the absence of language. It has also been demonstrated that many cognitive processes (e.g., arithmetic, solving complex problems, listening to music, thinking about other people's mental states, navigating in the world) engage distinct brain regions from, and do not depend on, language (Fedorenko & Varley, 2016). The relationship between language and cognition is clearly critical when thinking of individuals with language impairment – does a language impairment cause impairment in non‐linguistic cognition, and/or vice versa? We would contend that there is clear evidence that the answer is “no”, as over the years, cases have been reported where these abilities have dissociated (for review see Fedorenko & Varley, 2016). For example, Varley (2002) reported a man with severe impairments in both comprehension and production of language following brain damage. Yet, this man could drive, play chess against both human and machine opponents, and took responsibility for financial planning for his family. He also scored in the top 10% for his age and education on the Wisconsin Card Sorting Test (Grant & Berg, 1948) a complex task that is argued to tap problem solving abilities to figure out the changing rules of the game, requiring executive functions as well as working memory and attention.

Traditionally, this separation between language and cognition was clear in the definition of some language disorders. For example, “intellect” was generally considered to be unimpaired in aphasia (e.g., Broca, 1861) and children with specific language impairment were diagnosed on the basis that there were no other cognitive impairments. (Note: the broader term Developmental Language Disorder that does not include this criterion is now preferred, Bishop et al., 2017).

Nevertheless, while there is no necessary causal relationship between language and cognitive impairment, cognition and language likely function together (Perlovsky, 2011). Moreover, the language areas of the brain lie in close proximity to those associated with cognitive functions (Varoquaux et al., 2018), and consequently impairments to cognition and language are likely to co‐occur following brain damage. In order for language intervention to be most appropriately targeted, there is a need to determine the extent to which non‐linguistic cognition (i.e., attention, executive functions, memory, visuospatial abilities) is preserved in individuals with language impairment. In addition, the extent to which an individual may benefit from language intervention may also be related to their broader cognitive profile (Gilmore et al., 2019; Simic et al., 2020). For example, Gilmore et al. (2019) showed that enhanced executive functions predicted better gains following treatment in aphasia and suggested that the commonly seen heterogeneity of therapy outcomes among people with aphasia might be related to the varied profiles of non‐linguistic cognition before treatment. Crucially, once again, this does not necessarily imply a causal relationship between linguistic and executive factors, but instead that cognitive training is likely to provide people with aphasia with strategies that might enhance their performance (Brownsett et al., 2014; Kohnert, 2004). For example, the ability to monitor for errors, apply a newly learned grammatical rule in the correct circumstances, or use a strategy to improve word retrieval, all require attention, memory and executive function skills. However, other interventions may be successful despite cognitive impairments, for example, when word retrieval treatment results in priming of existing lexical representations thereby making them more accessible.

In this chapter, we address several issues relating to cognition and language, focusing first on the difficulties with assessment. We then look in more detail at two aspects of cognition – attention and executive functions, and conclude the chapter with a discussion of cognition in bilingual individuals. While, in each section, we focus on examples from people with acquired language disorder (aphasia) to illustrate the points raised, they also apply to populations of individuals with other language disorders.