At the outset of STREAM, we developed a novel experimental paradigm to decompose visual memories into their fundamental features, to then track the re-emergence of the relevant memory components in neural time and space. Over the runtime of STREAM, we found robust empirical evidence for STREAM's hypothesis that memory reconstruction follows a reverse gradient compared to perception. A perception-memory flip in the representational feature hierarchy was found using EEG (Linde-Domingo et al., 2019), feature-specific reaction times (Linde-Domingo et al., 2019; Lifanov et al., 2021; Postzich et al., in progress), simultaneous EEG-fMRI recordings (Lifanov et al., 2022, bioRxiv), and parallel MEG and fMRI recordings (Postzich et al., in progress). The EEG and behavioural work revealed when in time, after presenting a memory cue, different features of a visual memory are reconstructed. This work consistently showed that high-level semantic features are reactivated significantly earlier than low-level perceptual features during recall, in reverse order of their initial perceptual processing. The simultaneous EEG-fMRI work additionally allowed us to map this timeline onto the brain areas processing different types of features during perception and recall.
Consistent with the original proposal, we also find that this memory reactivation is clocked by neural oscillations in the theta frequency range, and that these rhythmic fluctuations can be observed in the hippocampus (ter Wal et al., 2021; Kerren et al., 2018, 2022), and even in button press behaviours (Ter Wal et al., 2021). This includes work in epileptic patients using intracranial EEG (iEEG) to record signals directly from the human hippocampus (ter Wal et al., 2021), speaking directly as to the sources of these memory signatures. The methodological developments related to this objective also led to the publication of the “Brain Time Toolbox” (van Bree et al., 2022), a tool allowing researchers to link brain oscillations to the content that is decodable from EEG/MEG/iEEG recordings. We summarised our first milestone findings from STREAM in a highly-cited review paper about the timeline (“neural chronometry”) of memory recall (Staresina & Wimber, 2019). The data from several other experiments are still in the analysis or write-up stage, with first results having been presented at international conferences.