Cognitive impairments in chronic temporal lobe epilepsy patients : an event-related potential study of prospective memory

Abstract

Cognitive impairments are common in patients with temporal lobe epilepsy (TLE), and impairment of prospective memory (PM) can seriously affect their daily independence. The focus of this thesis is on PM impairment in TLE patients. Working memory and inhibition were aslo studied as cognitive components of PM. Most importantly, we measured event-related potential (ERP) to explore the neural correlates of PM. The investigation of PM and its underlying neural correlates can help clinicians to better assess cognitive impairment in TLE patients, and this would pave the way for more efficient clinical interventions in the future. A total of 38 TLE patients were recruited from Queen Mary Hospital. They were divided into the well-controlled group (n = 18) and the refractory group (n = 20) according to the control of their TLE. For comparison, 26 sex- and age-matched healthy subjects were recruited as controls. Regarding the neuropsychological tests, healthy controls scored better than the TLE patients in the Digit Span Forward, Verbal Fluency Test and Symbol Digit Modalities Test Oral Edition; there was no significant difference in the Digit Span Backward and Digit Modalities Test Written Edition. Well-controlled TLE patients scored better than refractory TLE patients in the Verbal Fluency Test. In the ERP study, a PM paradigm was used; an Oddball task and a Go/Nogo task were also included to delineate which cognitive component is more impaired in TLE patients. Behavioural data showed no significant difference in reaction time in the three cognitive tasks, but reduced accuracy in the Go/Nogo Task and PM Task in the refractory TLE patients. The ERP data showed that TLE affected the performance of Oddball Task, Go/Nogo Task, and PM Task. Reduction of the P300 component in the Oddball Task revealed impaired working memory in the central-parietal sites, especially in the well-controlled TLE patients. This may indicate dysfunction in the working memory of TLE patients, and the deficit may be different between the well-controlled and refractory TLE patients. Impaired inhibition, as shown by decreased P3 amplitudes over the frontal-central sites, was seen during the Go/Nogo Task in the refractory TLE patients. During the PM Task, reductions in amplitude of the prospective positivity (PP) component were seen over the frontal, central and parietal sites in refractory TLE patients. The PM function impairment may be caused by a complicated mechanism involving multiple cortical sites including frontal and central sites. The present results provide both behavioral and ERP evidence of PM impairments in TLE patients. The adverse effects of TLE on PM may be attributed to the impairment of inhibitory function more than working memory function. The present study may reveal a need for neuropsychological rehabilitation in TLE patients with PM impairment.