Memory systems
How do the hippocampus and prefrontal cortex build imagined scenes?
Open access · cc by · source: Europe PMC
When people imagined new scenes, the ventromedial prefrontal cortex and hippocampus synchronised their slow rhythms, and the prefrontal region appeared to drive the hippocampus.
Study at a glance
- Design
- Human experiment — Within-subject MEG experiment: imagine novel scenes vs single objects vs counting baseline, cued by spoken words, with beamformer source power, theta coherence and dynamic causal modelling.
- N
- N=22 · 22 native English-speaking adults; one was dropped from the remembered-vs-forgotten coherence analysis for recognising every scene word.
- Population
- Healthy young adults (mean age 27) at University College London
- Outcome
- Theta power in the hippocampus, hippocampal-vmPFC theta coherence (scenes vs objects; remembered vs forgotten), and direction of effective connectivity
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Key findings
Both kinds of imagery reduced theta power in the left anterior hippocampus to a similar degree compared with counting, so scenes and objects did not differ in local power. Scenes, however, produced greater theta coherence between the anterior hippocampus and vmPFC (and with fusiform and parahippocampal cortex), and this coherence was higher for scenes later remembered than for those forgotten. The model with vmPFC driving the hippocampus won with a group probability of 97.66%, supported by 11 individuals versus 2 for the reverse, and vmPFC theta changes began earlier than hippocampal ones.
Methodology
Twenty-two adults lay in an MEG scanner with eyes closed and, after hearing a word, spent 3 seconds either imagining a novel 3D scene (e.g. 'jungle'), imagining a single object floating on a white background, or counting in threes as a baseline. They rated how detailed their imagery was, and afterwards took a surprise recognition test for the words. The authors localised theta-band (4-8 Hz) activity, measured coherence between the anterior hippocampus and the rest of the brain, and used dynamic causal modelling to compare a model where the hippocampus drives vmPFC with one where vmPFC drives the hippocampus.
Limitations
The key vmPFC coherence effect only appeared at a lenient uncorrected threshold justified by a prior hypothesis, so it is weaker evidence than a corrected whole-brain result. Half the participants showed no conclusive evidence for either direction of influence, and dynamic causal modelling only compared two pre-specified models. MEG localisation of deep structures like the hippocampus is imprecise, and imagery quality was self-rated. The link to episodic memory rests on an incidental recognition test, and the high-versus-low detail difference only approached significance.
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