Expectations of Task Demands Dissociate Working Memory and Long-Term Memory Systems

Abstract

Many aspects of the complex relationship between working memory (WM) and long-term memory (LTM) remain unclear. Here, we manipulated task demands on a brief delayed-recognition paradigm to reveal behavioral and neural dissociations between these systems. Variations from a Baseline task included 3 challenges: increased delay duration, distraction during maintenance, and more closely matched memory probes, which were presented in behavioral experiments and during functional magnetic resonance imaging. Each of the challenges resulted in a significant decline in WM accuracy, and interestingly, a concurrent improvement in incidental LTM. Neural data revealed that, in task blocks, when participants anticipated, and then experienced, increased demands, they engaged medial temporal lobe (MTL) regions more during both the encoding and delay periods. Overall, these results indicate that distinct memory systems are recruited based on anticipated demands of a memory task, and MTL involvement underlies the observed dissociation between WM and LTM performance.

Keywords: delayed recognition; fMRI; medial temporal lobe; prefrontal cortex; task difficulty.

Figure 1.

fMRI experimental paradigm. Each experiment consisted of 2 blocks for each of the 5 tasks. The Passive View task (not displayed) was similar to the Baseline task, but displayed an arrow as the probe stimulus. Participants were instructed before each block which task they were going to perform. Note: an additional task (Short Delay, not displayed) was included in the behavioral study.

Figure 2.

Behavioral results. Study 1—Behavioral experiments: (A) Baseline versus Distraction, (B) Baseline versus Short and Long Delays. Study 2—fMRI experiment: (C) Baseline versus Distraction, Long Delay and Tight Pairing. Gray asterisks represent LTM comparisons with P < 0.05; black asterisks represent WM accuracy comparisons with P < 0.05.

Figure 3.

Univariate BOLD activity. (A) During the encode period, MTL activity was increased in the Challenge tasks compared with the Baseline task (yellow box). (B) During the delay period, Challenge and Baseline tasks utilize differential PFC regions (Table 1).

Figure 4.

Functional connectivity data. Functional connectivity between the FFA and MTL increased in the Challenge tasks compared with the Baseline task during (A) the encoding period as well as during (B) the delay period (yellow boxes). Challenge tasks and Baseline task exhibit differential functional connectivity between FFA and PFC, although much more widespread during Challenge tasks (Table 2).