Intact first- and second-order false belief reasoning in a patient with severely impaired grammar.

The retention of first-order theory of mind (ToM) despite severe loss of grammar has been reported in two patients with left hemisphere brain damage (Varley & Siegal, 2000; Varley, Siegal, & Want, 2001). We report a third, and more detailed, case study. Patient PH shows significant general language impairment, and severe grammatical impairment similar to that reported in previous studies. In addition we were able to show that PH's impairment extends to grammatical constructions most closely related to ToM in studies of children (embedded complement clauses and relative clauses). Despite this, PH performed almost perfectly on first-order false belief tasks and on a novel nonverbal second-order false belief task. PH was also successful on a novel test of "ToM semantics" that required evaluation of the certainty implied by different mental state terms. The data strongly suggest that grammar is not a necessary source of structure for explicit ToM reasoning in adults, but do not rule out a critical role for "ToM semantics." In turn this suggests that the relationship observed between grammar and ToM in studies of children is the result of an exclusively developmental process.