The hippocampus as the switchboard between perception and memory

Abstract

Adaptive memory recall requires a rapid and flexible switch from external perceptual reminders to internal mnemonic representations. However, owing to the limited temporal or spatial resolution of brain imaging modalities used in isolation, the hippocampal-cortical dynamics supporting this process remain unknown. We thus employed an object/scene cued recall paradigm across three studies, spanning single neuron recordings, intracranial as well as high-density EEG and fMRI. First, a hippocampal gamma power/spike rate increase distinguishing successful vs. unsuccessful recall emerged 500 ms after cue onset. Intriguingly, hippocampal gamma peaks marked the moments at which extrahippocampal activation patterns shifted from perceptual cue towards mnemonic target representations. In parallel, source-localised EEG (corroborated by fMRI) revealed a recall signal progressing from hippocampus to posterior parietal cortex and then to medial prefrontal cortex. These results identify the hippocampus as the switchboard between perception and memory and elucidate the ensuing hippocampal-cortical dynamics supporting the recall process.

The role of hippocampal theta oscillations in working memory impairment in multiple sclerosis

“…MS patients performed significantly worse in accuracy and reaction time on all conditions, except accuracy in the 0-back condition. Only one of the previous n-back fMRI studies (Forn et al, 2007;Penner et al, 2003;Sweet et al, 2006Sweet et al, , 2004 was able to find significant differences in performance between the HC and MS sample (Wishart et al, 2004). A previous EEG study (Covey et al, 2017) and two neuropsychological nback studies (Covey, Zivadinov, Shucard, & Shucard, 2011;Parmenter, Shucard, Benedict, & Shucard, 2006) did also report prolonged reaction times during the n-back task in MS. We found no group differences in the ERFs in the precuneus, right inferior temporal and left inferior temporal.…”

“…In the frontal ROIs this observation seems to be the opposite of our hypothesis of a stronger theta response in MS patients based on previous observations of increased activations in the prefrontal cortex reported by most fMRI studies (Forn et al, 2007;Penner et al, 2003;Sweet et al, 2006Sweet et al, , 2004, and causal evidence for the excitatory role of frontal theta oscillations (Riddle et al, 2020). One fMRI study did report less activation in the prefrontal and specifically also in the middle frontal regions in MS patients (Wishart et al, 2004). More neurophysiological M/EEG studies are needed to elucidate the changes in frontal theta oscillations during WM in MS.…”

“…About 43 to 70% of the 2.3 million multiple sclerosis (MS) patients worldwide (Multiple Sclerosis International Federation, 2013) are estimated to suffer from cognitive impairment (Peyser, Rao, LaRocca, & Kaplan, 1990;Rao, Leo, Bernardin, & Unverzagt, 1991) together with the strong physical problems that are caused by the inflammatory demyelination and neurodegeneration that characterise this disorder. Particularly information processing speed (Deluca, Chelune, Tulsky, Lengenfelder, & Chiaravalloti, 2004;Van Schependom et al, 2015) and working memory (WM) (Chiaravalloti & DeLuca, 2008;D'Esposito et al, 1996) but also attention and visuospatial abilities have been shown to be affected in PwMS, even in persons with only mild physical problems (Ruchkin et al, 1994).…”

“…By increasing n, the WM load or the number of items to be retained in WM is increased. Those studies have reported relatively heterogenous findings of possibly compensatory mechanisms in different regions (Forn et al, 2007;Penner, Rausch, Kappos, Opwis, & Radü, 2003;Sweet, Rao, Primeau, Durgerian, & Cohen, 2006;Sweet, Rao, Primeau, Mayer, & Cohen, 2004;Wishart et al, 2004). The most common finding for these studies was increased activation in the prefrontal cortex in MS patients (Forn et al, 2007;Sweet et al, 2004).…”

Evidence of shifts towards neural states of stability during the retrieval of real-life episodic memories

“…Though pattern completion has been central to influential theoretical models of memory retrieval (Marr, 1971;McClelland et al, 1995;Rolls, 2016), it is only recently that we have received evidence for its existence in humans (Staresina et al, 2016;Horner et al, 2014 andTreder et al, 2020). This research has been successful in identifying neural signatures underlying pattern completion by using lab-based stimulus material thought to emulate episodic memory functioning in real-life.…”