Prospective memory in thalamic amnesia

The contribution of the thalamus to the functioning of prospective memory (PM) is currently unknown. Here we report an experimental investigation of the performance of two patients with bilateral infarcts in the anterior-mesial regions of the thalami on an event-based PM paradigm. One patient, G.P., had a pervasive declarative memory impairment but no significant executive deficit. The other patient, R.F., had a memory deficit limited to verbal material with associated behavioral abnormalities (inertia and apathy); she performed poorly on tests of executive functions. Although both patients performed poorly on the PM task, a qualitative analysis of performance revealed different mechanisms at the base of their impaired PM. G.P. had reduced declarative memory for target words compared with normal controls; but, unforgotten words were normally able to elicit his recall of the prospective intention. Conversely, R.F.'s declarative memory for target words was as accurate as that of normal controls, but she presented a dramatically reduced ratio between the number of target words she recalled and the number of times she activated the prospective intention on the PM task, suggesting that her deficit consisted of difficulty in activating the intention despite normal declarative memory for the target events. In conclusion, results of the present study demonstrate that thalamic structures have an important role in PM processes. They also document that damage to the anterior-mesial regions of the thalami affects PM abilities by two different mechanisms, respectively based on the relative disruption of declarative memory or executive processes functioning, which, in turn, is related to the specific intrathalamic structures involved by the lesions. Indeed, while G.P.'s pervasive declarative memory deficit was underlain by bilateral involvement of the mammillo-thalamic tract, R.F.'s executive and behavioral abnormalities were likely related to bilateral damage of the midline, intralaminar, and medio-dorsal nuclei.