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Kari Hoffman @karihoffman.bsky.social · 30/07/2026

To more directly test this idea, we measured the local cohesiveness of assembly members. Cohesiveness increased with memory age: new < recent < old, suggesting that experience progressively embeds assemblies within more interconnected and potentially more stable networks. 9/n

graph connectedness of assembly members increased from new to recent to old

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Kari Hoffman @karihoffman.bsky.social · 30/07/2026

“Drifting assemblies” = gradual change in activation strength during the task, was greater for New assemblies. Was this drift due to member cells "competing" across fellow assemblies? No! The more exclusive an assembly's members, the greater the drift, i.e. integration->stabilization w/ time. 8/n

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Kari Hoffman @karihoffman.bsky.social · 30/07/2026

The old metassemblies don't just co-activate more, they coordinate for LONGER ⏱️ Markov modeling and cross-correlograms show mutual reinforcement: half-widths can extended 500ms+, beyond the SWR window we’d assumed. --> The temporal reach of the metassembly groups grows with experience. 6/n

schematic from the paper's HMM showing transitions from baseline and inactive states to the active state. The activation from one assembly to another of the "metassemblies" was stronger for old then new metassemblies. Below, the crosscorrelograms shows that strong 'instantaneous' pairs of old metassemblies are more sustained in time than new metassemblies.

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Kari Hoffman @karihoffman.bsky.social · 30/07/2026

Old assemblies don't just reactivate more, they are more organized. When pairs of old assemblies reactivate together (or in opposition) during the task, the pattern is preserved in sleep; New assembly pairs also have such 'Metassemblies' in the task, but they aren't preserved in sleep. 5/n

Metassemblies - pairwise activation of assemblies - shown sorted for task and again in sleep. Old metassemblies were preserved between task and sleep.

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Kari Hoffman @karihoffman.bsky.social · 30/07/2026

Distinct patterns of population activity (cell assemblies) represented the different sequence types (New/Old), and were better decoded than single-unit activity. --> The populations carried memory-specific information during the task. But what about sleep? Sharp-wave ripples? Reactivation? 3/n

from the population activity covariance matrix, we find cell assemblies - population cofiring motifs - some of which occur preferentially during new-sequences or during old-sequences.

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Kari Hoffman @karihoffman.bsky.social · 30/07/2026

The setup: macaques learned multi-touchscreen sequences in an immersive enclosure: a "new" or "recent" (1-day old) sequence on one side and an "old" sequence (learned ~6 weeks' prior) on the other, while we recorded from the hippocampus and connected regions during the task and then in sleep. 2/n

enclosure with touchscreens on the left, single unit ensembles activated during the task on the right

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Kari Hoffman @karihoffman.bsky.social · 11/12/2025

#1 The Discussions! Best explained by founding member Jim McGaugh. OK, what did I miss? parkcitywinterconf.org/register/ 6/6

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Kari Hoffman @karihoffman.bsky.social · 11/12/2025

#2 Latest, sometimes controversial, findings. Hear unpublished results, hot takes that speakers haven’t yet disclosed to the big crowds. Every year, I hear it here first. During the meeting or after, at the Hotel Bar. 👇 5/6

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Kari Hoffman @karihoffman.bsky.social · 11/12/2025

#5 Activities – and not just skiing. Snowshoeing, hiking, fat tire biking, bougie spas, excursions to the main drag, High West Distillery. Touch gra- well, snow. 👇 2/6

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Kari Hoffman @karihoffman.bsky.social · 11/12/2025

📣Register now for the 50th Anniversary Park City Winter Conference on the Neurobiology of Learning and Memory parkcitywinterconf.org/register/ Deadline tomorrow, Friday, the 12th! Spread the word. 🧠🔬👩‍🔬 ➡️Top 5 reasons to attend: 1/6

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Kari Hoffman @karihoffman.bsky.social · 17/11/2025

Happening now ss3 ss4 dentate spikes, beta, category learning in freely moving 🐒

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Kari Hoffman @karihoffman.bsky.social · 14/11/2025

Here are our contributions for #SFN2025 #SFN25, Sunday and Monday morning. Hope to see you there!

List of SFN presentations from the Hoffman lab

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Kari Hoffman @karihoffman.bsky.social · 21/05/2025

We agree. KenjiLee your data. @sabbaspoor.bsky.social's version uncovered surprisingly specific spike-field motifs and short latency spike timing interactions in macaque CA1: doi.org/10.1016/j.ce...

Functional cell groups identified from linear arrays through macaque hippocampal CA1.

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Kari Hoffman @karihoffman.bsky.social · 25/04/2025

Another one in the Krakauer et al., 2017 Neuron perspective: www.cell.com/neuron/fullt...

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Kari Hoffman @karihoffman.bsky.social · 15/04/2025

Finally, assemblies can flexibly team up (co-activate) during the task and in later sleep, but this propensity depends on pairing with/among OLD (and RECENT) assemblies. This property could allow for selective generalization while preserving constituent representations.

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Kari Hoffman @karihoffman.bsky.social · 15/04/2025

After careful curation of single units from CA1 and connected structures, we identified cell assemblies - repeated patterns of synchronous firing (typically N~100, some approaching 300). Some assemblies were more active in NEW trials, others OLD. But did either or both groups reactivate in sleep?

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Kari Hoffman @karihoffman.bsky.social · 15/04/2025

We recorded wirelessly from high-density probes in freely-moving macaques as they learned item-context sequences. In the same session, they had to remember an OLD sequence, learned weeks earlier, and a NEW sequence learned that day. We came back the next day, too, calling the NEW sequence RECENT).

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Kari Hoffman @karihoffman.bsky.social · 24/11/2024

🎉Ach, calretinin+, and more (-) expression in Rh🐒means we can start to connect functional cell groups like here doi.org/10.1016/j.ce... to “omics” cell types, and selectively manipulate circuits! It’s a big deal for understanding population dynamics. Thanks to authors and funders: Brain Initiative!

Figure from Abbaspoor and Hoffman 2024 showing different functional cell types from macaque hippocampal CA1

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