The Neuronal Substrate Of Prospective Memory

Objective Prospective memory (PM) refers to the function that enables a person to carry out an intended act after a delay. Despite of the ubiquity of this behaviour, little is known about the supporting brain structures and the roles that they play. A substantial body of literature, primarily based upon the findings of behavioral studies, indicates that there are robust declines in the efficiency of prospective memory in healthy elders. However, the cognitive and neural mechanisms mediating age-related deficits in prospective memory are not well understood.The present study aimed is to test the hypothesis that prefrontal cortex is involved in the PM network ,and to find out the difference between event-based prospective memory (EBPM) and time-based prospective memory (TBPM) impairment in healthy elders and in patients with MCI.Methods Subjects, including 40 healthy adults, 40 healthy elders, 30 patients with lesions in prefrontal cortex, and 30 patients with MCI, were administered with a neuropsychological battery including EBPM and TBPM tasks. In EBPM task, subjects were asked to perform an action whenever the animal Chinese words were presented. In TBPM task, subjects were asked to perform an action at 5', 10', 15'minute timepoints.Results Compared with healthy adults (EBPM, 6.25±1.60; TBPM, 5.38±0.87), healthy elders were impaired in EBPM (2.50±0.85) as well as in TBPM (4.93±1.07) (P<0.001, P<0.05), furthermore were impaired in EBPM more significantly (P<0.001).Double dissociation between the performance of EBPM and TBPM was also found in healthy elders;Compared with healthy elders (EBPM,2.33±0.66; TBPM,4.90±1.03), patients with MCI were impaired in EBPM (0.90±1.09) as well as in TBPM (4.23±1.14)(P<0.01,P<0.05), furthermore were impaired in EBPM more significantly (P<0.01);Compared with the healthy adults (EBPM, 6.00±1.91; TBPM, 5.30±0.92), the patients with lesion in prefrontal cortex were significantly impaired in EBPM (1.47±1.07, P<0.001) , but not significantly impaired in TBPM (4.77±1.17, P>0.05).There was no difference in performance of EBPM (1.40±1.12, 1.64±1.01) and TBPM (4.60±1.06,4.86±1.29) between the left PFC and the right PFC.Conclusions These results confirmed that prospective memory is obviously impaired, and EBPM is impaired more significantly than TBPM in healthy elders. The evidence of double dissociation between EBPM and TBPM in healthy elders group supports that EBPM and TBPM may have different neural substrate;Prospective and retrospective memory disorders were all found in patients with MCI, but compared with RM and TBPM, EBPM is impaired more significantly in patients with MCI. This finding indicats that the impairment of EBPM may be as an early indicator of MCI;The patients with lesion in PFC were impaired in EBPM, but not in TBPM. This finding also confirmed that EBPM and TBPM may have different neural substrate. It is possible that prefrontal lobe is more selectively involved in EBPM, but less involved in TBPM.