Fig. 2 |. Schematic depictions of the properties of delay activity in different brain regions.

In each panel, simplified schematic diagrams are provided to illustrate the type of information represented by delay activity in different parts of the brain. Note that not all of the findings were shown in humans, but the regions have been depicted on a human brain for illustrative purposes. a | Delay activity in the lateral prefrontal cortex (lPFC) represents task rules. Population analyses involving pattern classification approaches have demonstrated that classification of the task rule (for example, which feature of a memorandum is relevant for response⁷⁵) is above chance during the memory delay, suggesting that lPFC delay activity represents aspects of task rules. b | lPFC delay activity represents working memory (WM) content. Neurons in the lPFC of non-human primates (NHPs) exhibit delay-period activity that varies in spike frequency with the properties of the memory stimulus, such as vibrotactile frequency¹¹⁴. c | Electroencephalogram (EEG) electrodes over PPC reveal delay activity that increases in magnitude and oscillatory power with WM load, and plateaus at an individual’s WM capacity²⁷. This is consistent with the notions that PPC delay activity encodes WM content and that PPC delay activity represents internal attention directed to items in WM. Challenges in localizing EEG activity make it unclear where in the brain these signals originate. d | Basal ganglia (BG) delay activity is associated with the upcoming behavioural response. BG delay activity is greater in magnitude when participants can anticipate the upcoming response (prospective representation) than when they cannot (retrospective representation)¹⁹¹. e | Neurons in motor association cortex show preference for specific response rules. Tasks that require NHPs to maintain response rules (for example, which memory item to indicate first with a behavioural response¹⁵⁸), suggest that this region encodes the anticipation of specific planned responses. f | Population activity in visual sensory regions is tuned to features of WM content. Functional MRI studies can identify meso-scale responses to features (such as orientation) of memory items, and have found that populations that represent features of WM content are preferentially active during the delay¹⁷⁴. g | Delay activity in the medial temporal lobe (MTL) may be involved in storing complex information in WM. The magnitude of MTL delay activity is larger for novel items than for familiar items¹⁵³, and MTL delay activity is often observed during WM for complex items¹⁵. h | The thalamus exhibits delay activity that seems to drive delay responses in PFC. Experimental disruption of thalamic delay activity in mice resulted in reduced or abolished delay activity in PFC^202,203.