Matthijs A. A. van der Meer

The mammalian head direction (HD) system is thought to update its internal compass using angular head velocity (AHV) signals relayed from the brainstem. We tested this canonical interpretation by recording from the nucleus prepositus hypoglossi (NPH), a proposed AHV relay known to be important for accurate HD tracking, while simultaneously measuring eye and head movements in mice. Neurons classified as AHV-tuned instead predominantly encoded eye movements: most showed saccade-related bursts or pauses, and apparent AHV tuning was largely explained by eye rather than head motion. Optotagging showed that even NPH neurons projecting directly to the HD circuit predominantly encoded eye movements. Empirically constrained population models reconstructed eye position and gaze direction, but head-direction readouts failed when eye and head motion were dissociated. These findings overturn the canonical view of NPH as a dedicated AHV relay, revealing eye movements as a major, previously unmeasured component of the signals entering the mammalian HD circuit and thereby changing the computational problem that the HD system must solve to maintain a stable representation of heading.

Memory shapes how we explore the visual world, but the neural mechanisms linking mnemonic processes to eye movements during naturalistic viewing remain poorly understood. Theta-band oscillatory activity in the hippocampus is time-locked with eye movements in primates, suggesting a putative mechanism for coordinating mnemonic processing and oculomotor behavior. Yet, it remains unknown whether this coupling generalizes to episodic memory-guided viewing in humans, whether slow (3–6 Hz) and fast (6–10 Hz) theta bands play dissociable functions, and whether this coupling is sensitive to the direction of upcoming eye movements. Here, we used intracranial EEG and eye-tracking data from 11 neurosurgical patients of either sex (Keles et al., 2024) to address these questions. Using fixation-locked analyses, we found that hippocampal theta dynamics differentiate memory-guided from memory-independent fixations of episodically encoded naturalistic scenes (i.e., movies) through power and coherence mechanisms that are temporally and spectrally dissociable. First, immediately after fixations (∼0–250 ms), slow-theta power was more strongly suppressed during memory-guided trials (i.e., true positive, TP) than correct memory-independent trials (i.e., true negative, TN). Second, in the period around fixations (−80 to 60 ms), theta phase coherence increased independent of power during TP versus TN trials. This increase in coherence was most pronounced for contraversive relative to ipsiversive fixations, consistent with direction-sensitive hippocampal–oculomotor coordination during memory-guided viewing. Together, these findings suggest that hippocampal theta plays dissociable, time-locked roles in memory-guided fixations during naturalistic retrieval, supporting an ecologically relevant role of the hippocampus in coordinating memory and active visual behavior.

Memory shapes how we explore the visual world, but the neural mechanisms linking mnemonic processes to eye movements during naturalistic viewing are not well understood. Theta-band oscillatory activity in the hippocampus is time-locked with eye movements in primates, suggesting a putative mechanism for coordinating mnemonic processing and oculomotor behaviour. Yet, it remains unknown whether this coupling generalises to episodic memory-guided viewing in humans, whether slow (3–6 Hz) and fast (6–10 Hz) theta bands play dissociable functions, and whether this coupling is sensitive to the direction of upcoming eye movements. Here, we used intracranial EEG and eye-tracking data from 11 neurosurgical patients of either sex (Keles et al., 2024) to address these questions. Using fixation-locked analyses, we found that hippocampal theta dynamics differentiate memory-guided from memory-independent fixations of episodically encoded naturalistic scenes (i.e., movies) through power and coherence mechanisms that are both temporally and spectrally dissociable. First, immediately after fixations (∼0 to 250 ms), slow-theta power was more strongly suppressed during memory-guided trials (i.e., true positive, TP) than correct memory-independent trials (i.e., true negative, TN). Second, in the period around fixations (-80 to 60 ms), theta phase coherence increased independent of power during TP vs TN trials. This increase in coherence was most pronounced for contraversive relative to ipsiversive fixations, consistent with direction-sensitive hippocampal–oculomotor coordination during memory-guided viewing. Together, these findings suggest that hippocampal theta plays dissociable, time-locked roles in memory-guided fixations during naturalistic visual retrieval, supporting an ecologically relevant role for the hippocampus in coordinating memory and active visual behaviour. Significance statement: Memory influences how we sample the visual environment, but the brain signals that link memory systems to eye movements are poorly understood. Using intracranial recordings from the human hippocampus during visual exploration of naturalistically encoded scenes, we demonstrate that theta-band activity differentiates memory-guided from memory-independent fixations in two ways: increased theta synchrony around fixation onset and reduced theta power modulations after fixation onset. These results reveal novel insights into how hippocampal theta helps coordinate memory-guided visual exploration.

Summary: Learning from aversive experiences often generalizes beyond the context in which they occurred. In rodents, a strong aversive event can induce retrospective memory linking (RLI), whereby fear generalizes to a previously neutral context encountered days earlier. Although prior work has shown that RLI is associated with increased co-activity of hippocampal CA1 neurons across neutral and aversive contexts, it remains unclear how broader representational changes support generalization without affecting the ability to discriminate between contexts. Here, we reanalyzed calcium imaging data from dorsal CA1 during RLI to examine how hippocampal representational geometry changes during fear generalization. Using robust, non-parametric measures of population similarity, we show that in mice exhibiting RLI, the representation of the neutral context not only changes over time but becomes more similar to the aversive context during recall. Beyond this similarity increase, we provide evidence for a higher-dimensional geometric transformation consistent with a shared “fear” operation that can be applied across contexts while preserving their identity. Crucially, these two representational signatures dissociate by behavioral state: similarity to the aversive context emerges during freezing, whereas a shared transformation is expressed during active exploration. Together, these findings demonstrate that hippocampal representations support retrospective fear generalization through state-dependent geometric transformations, highlighting representational geometry as a key computational mechanism to resolve the apparent tension between generalization and discriminability.

Leading theories suggest that hippocampal replay drives offline learning through coupling with an internal teaching signal such as ventral striatal dopamine (DA); however, the relationship between hippocampal replay and dopamine is unknown. Simultaneous recording of putative hippocampal replay events (dorsal CA1 sharp-wave ripples, SWRs) and fiber photometry of ventral striatal DA in mice revealed a significant increase in DA following SWRs, peaking ∼0.3 s after SWRs. This result establishes for the first time a core theoretical requirement of offline learning in the mammalian brain.

The striatum is thought to switch flexibly between multiple converging inputs to support adaptive behavior. The “communication through coherence” (CTC) hypothesis is a potential mechanism to implement such a flexible switching. For CTC to work in the striatum, striatal excitability must show rhythmic fluctuations, such as those related to the phase of the striatal local field potential (LFP). To test this fundamental requirement, we delivered a constant input stimulus to ChR2-expressing striatal fast-spiking PV+ interneurons (FSIs) in head-fixed awake mice (PV-Cre:Ai-32,n = 18, 9 female) and determined whether the response to this stimulus varied with LFP phase. We found that approximately one-third (41.2%) of FSIs exhibited significant phase-dependent excitability in at least one LFP frequency band. Phase-dependent excitability was most prominent in the delta (2–5 Hz) frequency band, both in terms of prevalence (23.5% of FSIs sampled) and magnitude (phase modulation strength: 22% of average response). The most excitable phase tended to align with endogenous phase-locking, again most clearly in the delta band. These results bolster the functional relevance of the striatal field potential and spike-field relationships and provide proof-of-principle support for the possibility of CTC in the striatum.

Experience replay is a powerful mechanism to learn efficiently from limited experience. Despite several decades of compelling experimental results, the factors that determine which experiences are selected for replay remain unclear. A particular challenge for current theories is that on tasks that feature unbalanced experience, rats paradoxically replay the less-experienced trajectory. To understand why, we simulated a feedforward neural network with two regimes: rich learning (structured representations tailored to task demands) and lazy learning (unstructured, task-agnostic representations). Rich, but not lazy, representations degraded following unbalanced experience, an effect that could be reversed with paradoxical replay. To test if this computational principle can account for the experimental data, we examined the relationship between paradoxical replay and learned task representations in the rat hippocampus. Strikingly, we found a strong association between the richness of learned task representations and the paradoxicality of replay. Taken together, these results suggest that paradoxical replay specifically serves to protect rich representations from the destructive effects of unbalanced experience, and more generally demonstrate a novel interaction between the nature of task representations and the function of replay in artificial and biological systems. Highlights We provide an explicit normative explanation and simulations of the experimentally observed puzzle of “paradoxical replay”, which we show can serve to protect certain task representations from the destructive effects of unbalanced experience We confirm with new analyses the main prediction of the theory, that “rich” task representations, measured using representational distance in the rodent hippocampus, show more paradoxical replay compared to “lazy” task representations Our theory refines the notion of consolidation in complementary learning systems theory in showing that not all task representations benefit equally from interleaving, and provides an example of how the use of replay in artificial neural networks can be optimized

The striatum is thought to switch flexibly between multiple converging inputs to support adaptive behavior. The “communication through coherence” (CTC) hypothesis is a potential mechanism to implement such a flexible switching. For CTC to work in the striatum, striatal excitability must show rhythmic fluctuations, such as those related to the phase of the striatal local field potential (LFP). To test this fundamental requirement, we delivered a constant input stimulus to ChR2-expressing striatal fast-spiking PV+ interneurons (FSIs) in head-fixed awake mice (PV-Cre:Ai-32, n = 18, 9 female), and determined whether the response to this stimulus varied with LFP phase. We found that approximately one-third (41.2%) of FSIs exhibited significant phase-dependent excitability in at least one LFP frequency band. Phase-dependent excitability was most prominent in the delta (2–5 Hz) frequency band, both in terms of prevalence (23.5% of FSIs sampled) and magnitude (phase modulation strength: 22% of average response). The most excitable phase tended to align with endogenous phase-locking, again most clearly in the delta band. These results bolster the functional relevance of the striatal field potential and spike-field relationships, and provide proof-of-principle support for the possibility of CTC in the striatum. Significance Statement: The striatum is thought to switch dynamically between multiple converging inputs. A leading idea for how this is accomplished is through communication through coherence (CTC). A fundamental, but previously untested, requirement for CTC to work is that striatum must show changes in excitability that depend on local field potential phase. We find that about one-third of striatal neurons show phase-dependent excitability, providing proof of principle for CTC-like switching in striatum.