Abstract
The relationship between recursive sentence embedding and theory-of-mind (ToM) inference is investigated in three persons with Broca's aphasia, two persons with Wernicke's aphasia, and six persons with mild and moderate Alzheimer's disease (AD). We asked questions of four types about photographs of various real-life situations. Type 4 questions asked participants about intentions, thoughts, or utterances of the characters in the pictures (“What may X be thinking/asking Y to do?”). The expected answers typically involved subordinate clauses introduced by conjunctions or direct quotations of the characters' utterances. Broca's aphasics did not produce answers with recursive sentence embedding. Rather, they projected themselves into the characters' mental states and gave direct answers in the first person singular, with relevant ToM content. We call such replies “situative statements.” Where the question concerned the mental state of the character but did not require an answer with sentence embedding (“What does X hate?”), aphasics gave descriptive answers rather than situative statements. Most replies given by persons with AD to Type 4 questions were grammatical instances of recursive sentence embedding. They also gave a few situative statements but the ToM content of these was irrelevant. In more than one third of their well-formed sentence embeddings, too, they conveyed irrelevant ToM contents. Persons with moderate AD were unable to pass secondary false belief tests. The results reveal double dissociation: Broca's aphasics are unable to access recursive sentence embedding but they can make appropriate ToM inferences; moderate AD persons make the wrong ToM inferences but they are able to access recursive sentence embedding. The double dissociation may be relevant for the nature of the relationship between the two recursive capacities. Broca's aphasics compensated for the lack of recursive sentence embedding by recursive ToM reasoning represented in very simple syntactic forms: they used one recursive subsystem to stand in for another recursive subsystem.
Highlights
Medical/clinical tests as well as cognitive tests, including Mini Mental State Examination (MMSE), were followed within 1 month by our own recursive sentence embedding tests administered in three sessions, with at least 10 days elapsing between subsequent occasions
There was no significant difference between the mild Alzheimer’s disease (AD) group and the control group in the proportions of replies involving recursive sentence embedding vs. situative responses
In the case of moderate AD group we found a significant difference in the proportion of responses involving recursive sentence embedding and simple situative statements: the ratio of situational statements presupposing recursive ToM reasoning was significantly lower than in the control group
Summary
Most linguists use a kind of inductive definition of recursion (Tomalin, 2007; Hulst, 2010b): they define it as the embedding of a constituent in a constituent of the same type in linguistic expressions. Recursion builds complex structures by increasing embedding depth whereas simple iteration yields output structures which do not increase depth (cf Karlsson, 2010). Watumull et al (2014) criticize the concept of recursion as articulated in linguistic analysis; they point out that “syntactic embedding is a sufficient, though not necessary, diagnostic of recursion” In the interpretation of our data we will extend the concept of recursion beyond linguistic syntax to the recursive logic of theory-of-mind (ToM) reasoning
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