Severity of explicit memory impairment due to Alzheimer's disease improves effectiveness of implicit learning

J Neurol. 2008 Apr;255(4):502-9. doi: 10.1007/s00415-008-0717-x. Epub 2008 Mar 20.


Background: Consistent evidence from human and experimental animals studies indicates that memory is organized into two relatively independent systems with different functions and brain mechanisms. The explicit memory system, dependent on the hippocampus and adjacent medial temporal lobe structures, refers to conscious knowledge acquisition and intentional recollection of previous experiences. The implicit memory system, dependent on the striatum, refers to learning of complex information without awareness or intention. The functioning of implicit memory can be observed in progressive, gradual improvement across many trials in performance on implicit learning tasks. The influence of explicit memory on implicit memory has not been precisely identified yet. According to data from some studies, explicit memory seems to exhibit no influence on implicit memory,whereas the other studies indicate that explicit memory may inhibit or facilitate implicit memory.

Objectives: The analysis of performance on implicit learning tasks in patients with different severity of explicit memory impairment due to Alzheimer's disease allows one to identify the potential influence of the explicit memory system on the implicit memory system.

Patients and methods: 51 patients with explicit memory impairment due to Alzheimer's disease (AD) and 36 healthy controls were tested. Explicit memory was examined by means of a battery of neuropsychological tests. Implicit habit learning was examined on probabilistic classification task (weather prediction task).

Results: Patients with moderate explicit memory impairment performed the implicit task significantly better than those with mild AD and controls.

Conclusion: Results of our study support the hypothesis of competition between the implicit and explicit memory systems in humans.

MeSH terms

  • Aged
  • Aged, 80 and over
  • Alzheimer Disease / physiopathology*
  • Alzheimer Disease / psychology*
  • Brain / physiopathology*
  • Disability Evaluation
  • Disease Progression
  • Female
  • Humans
  • Learning* / physiology
  • Male
  • Memory / physiology
  • Memory Disorders / diagnosis*
  • Memory Disorders / etiology*
  • Middle Aged
  • Models, Neurological
  • Neuropsychological Tests
  • Predictive Value of Tests
  • Severity of Illness Index