Biology subjects

Lifanov, J.

Publications and source records attributed to Lifanov, J..

3 recordsLinked to original sources

bioRxiv2020

Theta rhythmicity governs the timing of behavioral and hippocampal responses in humans specifically during memory-dependent tasks

1.Memory formation and reinstatement are thought to lock to the hippocampal theta rhythm, predicting that encoding and retrieval processes appear rhythmic themselves. Here, we show that rhythmicity can be observed in behavioral responses from memory tasks, where participants indicate, using button presses, the timing of encoding or retrieval of cue-object associative memories. We found no evidence for rhythmicity in button presses for visual tasks using the same stimuli, or for questions about already retrieved objects. The oscillations for correctly remembered trials center in the slow theta frequency range (1-5 Hz), while responses from later forgotten trials do not lock to the behavioral oscillation. Using intracranial EEG recordings, we show that the memory task induces temporally extended phase consistency in hippocampal local field potentials at slow theta frequencies, but only for correctly remembered trials, providing a mechanistic underpinning for the theta oscillations found in behavioral responses.

neuroscience↗

bioRxiv2020

Feature-specific reaction times reveal a semanticisation of memories over time and with repeated remembering

Memories are thought to undergo an episodic-to-semantic transformation in the course of their consolidation. We here tested if repeated recall induces a similar semanticization, and if the resulting qualitative changes in memories can be measured using simple feature-specific reaction time probes. Participants studied associations between verbs and object images, and then repeatedly recalled the objects when cued with the verb, immediately and after a two-day delay. Reaction times during immediate recall demonstrated that conceptual features were accessed faster than perceptual features. Consistent with a semanticization process, this perceptual-conceptual gap significantly increased across the delay. A significantly smaller perceptual-conceptual gap was found in the delayed recall data of a control group who repeatedly studied the verb-object pairings on the first day, instead of actively recalling them. Our findings suggest that wake recall and offline consolidation interact to transform memories over time, strengthening meaningful semantic information over perceptual detail.

neuroscience↗

bioRxiv2019

Hippocampal-striatal functional connectivity supports processing of temporal expectations from associative memory

The hippocampus and dorsal striatum are both associated with temporal processing, but they are thought to play distinct roles. The hippocampus has been reported to contribute to storing temporal structure of events in memory, whereas the striatum contributes to temporal motor preparation and reward anticipation. Here, we asked whether the striatum cooperates with the hippocampus in processing the temporal context of memorized visual associations. In our task, participants were trained to implicitly form temporal expectations for one of two possible time intervals associated to specific cue-target associations, and subsequently were scanned using 7T functional magnetic resonance imaging. During scanning, learned temporal expectations could be violated when the pairs were presented at either the learned or not-learned time intervals. When temporal expectations were not met during testing trials, activity in hippocampal subfields CA3/CA2 and CA1 decreased while right putamen activity increased, compared to when temporal expectations were met. Further, psycho-physiological interactions showed that functional connectivity between left CA1 and caudate, as well as between putamen and caudate, decreased when temporal expectations were not met. Our results indicate that the hippocampus and striatum cooperate to process implicit temporal expectation from mnemonic associations, with different but complementary contributions from caudate and putamen. Our findings provide further support for a hippocampal-striatal network in temporal associative processing.

neuroscience↗