Neural bases of subsequent forgetting in young and older adults

Objectives
Using functional MRI, we looked into the age-related difference in the neural underpinnings of
subsequent forgetting - cerebral activation at encoding for items that are later forgotten.
Methods
In an MRI scanner, during an incidental encoding phase, participants (20 young and 19 older
adults) were presented with black-and-white drawings of objects. They were instructed to
perform a size judgement on the depicted objects. Then, still in the scanner, the volunteers'
memory for the objects was tested by showing them pictures shown previously along with new
ones and asking them to make a Remember/Know/New judgement.
Results
Behaviourally, older participants showed decreased recollection, but intact familiarity at
recognition. In an event-related design (SPM8), we compared cerebral areas activated at
encoding for items subsequently forgotten compared to those leading to recollection (p<.001
uncorrected). In both groups, a pattern of activation consistent with the default-mode network
(DMN) was found. Furthermore, results pointed out to additional activations in the frontoparietal
control network in older adults. Also, contrasting activations for items subsequently
forgotten with those leading to familiarity revealed activations in DMN areas. In young adults,
these activations were limited to the posterior cingulate cortex.
Conclusions
The forgetting of information appears to be associated with a higher recruitment of the DMN,
which might reflect disengagement from encoding-supportive processes, both in young and
older participants. Moreover, the additional fronto-parietal activity found in the older group
could indicate that their failure to recollect the pictures was related to inefficient encoding
mechanisms, in addition to disengagement from the task.