Short-term retrospective versus prospective memory processing as emergent properties of the mind and brain: human fMRI evidence.

The functional-neuroanatomical substrates for short-term retrospective versus prospective memory processing were examined in a delay task, in which associative choices were made conditionally based on the presenting discriminative/cue stimulus. Delay-period prospection could be of the intended choice and/or the expected response outcome, whereas delay-period retrospection would be of the just-presented cue stimulus. Previous results have shown that the spontaneous process of unique outcome prospection did not implicate the lateral prefrontal cortex (PFC) but instead implicated the lateral posterior parietal cortex (LPPC) in a modality-independent fashion (Mok et al., 2009). Spontaneous retrospection was more dependent on the medial temporal lobe (MTL). Nevertheless, it was anticipated that the more explicit process of prospecting an intended choice would implicate the lateral PFC. To verify this, Mok et al.'s data were further analyzed, with new control data. Healthy, young adults performed delayed discriminative choices under procedures that biased them toward different degrees of delay-period prospection: higher-using cue-unique, differential outcomes (DO); versus lower-using a non-unique, common outcome (CO), or unpredictable, non-differential outcomes (NDO). Experimental participants performed the DO versus CO procedures concurrently, while undergoing event-related functional magnetic resonance imaging (fMRI). Separately, control participants provided data for: the NDO condition; related comparison tasks, which biased them toward different degrees of delay-period retrospection; and null-event trials. Expectedly, the more explicit process of prospecting an intended associative choice implicated the lateral PFC, as part of and together with other components of the multiple-demand network. Comparisons against null-event trials indicated that the sustained delay activity observed in MTL and LPPC, respectively, was part of default brain activity. These results demonstrated that short-term retrospection and prospection may emerge without necessarily relying on working memory-specific brain networks. Furthermore, attention may not necessarily be recruited to realize working memory. When cognitive processes are spontaneously experienced, they may be facilitated by the default brain network.