The fickleness of forgetting: When, why, and how do patient groups differ (or not)?

This commentary will review recent findings regarding forgetting rates in patient groups, including observations from some older, less cited studies. It will draw attention to studies (and reviews) indicating faster forgetting of recalled or recollected memories, relative to recognition or familiarity-based memory. Secondly, it will focus upon the variability of findings in forgetting rate studies, including variability of performance between individuals within groups, inconsistency by individuals across test sessions and/or when tested many years apart, and discordance between equivalent or near-equivalent studies. Thirdly, it will consider the distinction between studies finding early forgetting or progressive/quantitative memory loss and those suggesting a later, 'qualitative' change in forgetting rate. The latter pattern, most commonly seen in epilepsy cases, may be relatively infrequent when appropriate account has been taken of variation in controls' performance, and effect sizes can be low. There is also a need for an adequate neurobiological account of this delayed (or 'later') forgetting. Fourthly, the major contributions of Sergio Della Sala, Alan Baddeley, and their colleagues will be reviewed, drawing our attention to important factors in experimental design, such as the presence or absence of repeated practice, recall of gist versus peripheral detail, and parallel forgetting curves from different levels of initial learning. The paper concludes with a summary of the major findings in (i) healthy participants (including studies of normal ageing), (ii) memory-disordered patients arising from focal lesions, (iii) Alzheimer and MCI patients, and (iv) epilepsy patients.