Evidence for cognition without grammar from causal reasoning and ‘theory of mind’ in an agrammatic aphasic patient

Understanding the inter-relationship between language and thought is fundamental to the study of human cognition [1] [2] [3]. Some investigators have proposed that propositions in natural language serve to scaffold thinking, by providing, for example, a sequential structure to a massively parallel process [4]. Others have maintained that certain thoughts, such as inferring the mental states of others, termed 'theory of mind' (ToM) reasoning, and identifying causal relationships, necessarily involve language propositions [5]. It has been proposed that ToM reasoning depends upon the possession of syntactic structures such as those that permit the embedding of false propositions within true statements ('Mary knows that John (falsely) thinks chocolates are in the cupboard') [6]. The performance on reasoning tasks of individuals with severe agrammatic aphasia (an impairment of language following a lesion of the perisylvian areas of the language-dominant hemisphere) offers novel insights into the relation between grammar and cognition. We report the unusual case of a patient with agrammatic aphasia of such severity that language propositions were not apparently available at an explicit processing level in any modality of language use. Despite this profound impairment in grammar, he displayed simple causal reasoning and ToM understanding. Thus, reasoning about causes and beliefs involve processes that are independent of propositional language.