Monitoring frequency of occurrence without awareness: evidence from patients with Alzheimer's disease

People are surprisingly accurate at judging how often an event occurs. Patients with Alzheimer's disease (AD), however, perform poorly on such tasks, suggesting that this ability is compromised when episodic memory is impaired. The tasks used to assess this ability in previous studies, however, placed demands on retrieval that could obscure whether frequency of occurrence was adequately encoded. We developed an indirect test of frequency monitoring based on changes in reading time as a function of item repetition. Using this procedure, patients with AD showed normal frequency monitoring for novel information (Turkish words) even though they were unable to remember the words or judge how often individual words had been presented. These findings suggest the existence of a mechanism that automatically monitors frequency of occurrence and operates outside of conscious awareness.