Long-Term Memory Can Replace Active Storage in Visual Working Memory
Humans have remarkable episodic long-term memory abilities, capable of storing thousands of objects with significant detail. However, it remains unknown how such episodic memory is utilized during the short-term maintenance of information. Specifically, if people have an long-term memory for an item, how does this affect subsequent working memory for that same item? Here, we demonstrate that under these conditions people can quickly and accurately make use of long-term memory and therefore maintain less information in working memory. We assessed how much information is maintained in working memory by measuring neural activity during the delay period of a working memory task using electroencephalography. We find that despite maintaining less information in working memory when long-term memory representations are available, there is no decrement in memory performance. This suggests people can dynamically disengage working memory and instead use long-term memory when it is available. However, this does not mean that participants always utilize long-term memory if it is available. In a follow-up experiment, we introduced additional perceptual interference into working memory and found participants actively stored items in working memory even when they had existing long-term memories of those items. These results clarify the conditions under which long-term and working memory operate. Specifically, working memory is engaged when new information is encountered or perceptual interference is high. Long-term memory is otherwise rapidly accessed and utilized in lieu of working memory. These data demonstrate the interactions between working memory and long-term memory are more dynamic and fluid than previously thought.
Figure 1. (A) General design of sequential working memory task. To allow measurement of the contralateral delay activity, a neural marker of working memory, participants were cued to remember only the objects on either the left or right side of fixation. For half the trials, both study images presented had never been previously encountered (LTM-unavailable condition). For the other half of trials, one of the images had been seen previously in during an episodic long-term memory encoding session (LTM-available condition). Objects were presented sequentially for 500ms each, with a 500ms ISI. After a 900ms delay, a perceptual discrimination 2-AFC test assessed detailed object memory. (B) Behavioral results for Experiment 1. We found no difference in performance based on whether observers had a previous long-term memory representation available or not. Error bars represent within-subject standard error.
Figure 2. Results of Experiment 1. (A) Contralateral-minus-ipsilateral waveforms for the LTM Unavailable (black) and LTM Available (red) conditions. The CDA is measured from 300 ms after offset during the delay period (black rectangle, labeled CDA). We observed significantly reduced CDA amplitudes for when participants had a LTM representation available, compared to when they did not. (B) CDA amplitudes across all three conditions. We observed significantly reduced CDA activity for when participants encountered an LTM first, but not second, consistent with the gating hypothesis of working memory. Error bars reflect within-subject standard error.