Noticing the new: novelty encoding and ageing in visual immediate memory

Emerging evidence suggests that age-related declines in memory may reflect a failure in pattern separation, a process that is believed to occur in the hippocampal dentate gyrus/ region CA3 which reduces the overlap between two similar stimulus representations during memory encoding. Furthermore, this process of pattern separation may be indexed by a visual continuous recognition task (various names are popular in the literature, but BPS for behavioural pattern separation has perhaps been the most popular label) in which items are presented to observers in sequence and observers report for each whether it is novel, previously viewed (old), or whether it shares features with a previously viewed item (similar). Thus, worldwide a number of laboratories currently employ the task to infer not only aspects of human memory encoding and retrieval, but also make published claims as to the mapping of patterns of performance in the task (for example, accuracy in identifying novel items, or errors in reporting similar items as ones previously encountered) onto neural circuits and regions within hippocampus. Indeed, clinical claims have been made – notably that the behavioural task captures aspects of both normal neurocognitive ageing but also pathologies affecting human memory such as Alzheimer’s disease. This thesis reports a series of six experiments which in summary support certain constraints that may be recommended to future investigators; the work was inspired and guided by contemporary theories of human immediate memory. First, a critical variable – the temporal spacing of items within the task – was experimentally identified. The theoretical justification here was that post-item time permitted the memory trace of the items to be consolidated into memory (in one of the reported experiments post-item time was directly manipulated using the introduction of a secondary attentionally-demanding task). Ageing and performance on the task was also investigated, and errors in noticing novelty with age was an important aspect observed in performance. Further, one new finding reported here questions whether these memory deficits (failure in recognising change in ongoing stimulation) in older adults result from the interference caused by the encoding of intervening items, or from the degrading of a memory representation over an extended time delay. Finally, a recommendation is made to use stimuli that do not permit the participants to engage in verbal labelling and maintenance through verbal rehearsal – here a single ‘class’ or category of stimulus achieved this aim. In summary, this thesis reports important advances in understanding an influential experimental task claimed to tap hippocampal function – a function that may be termed ‘noticing the new’.