Brain meeting: Yunzhe Liu

Brain meeting

Humans exhibit remarkably flexible behaviour. We can choose how to act based on experiences that are only loosely related and imagine the consequences of entirely novel choices. Such flexibility is thought possible because the brain builds internal models of the world (i.e., cognitive map) that accounts for the relationships between isolated experiences that enable a generalization of knowledge to new situations.

 How the brain builds and updates a world model remains a central question in cognitive neuroscience. Replay and/or Preplay is one proposed mechanism. In replay, patterns of cellular firing during rest spontaneously play out past spatial trajectories in both forward and reverse directions, and reverse replays are increased for rewarded trajectories. In preplay, potential future trajectories are played out, perhaps constrained by structural knowledge of relationships in the world. This computation is also implied by Dyna-type learning algorithms. However, despite the theoretical importance of neural replay/preplay, its study has so far been largely restricted to spatial navigation tasks in rodents. It is unknown whether knowledge of task structure can indeed impact on the sequences that are (pre- or re-) played. 

 In this talk, I will show two studies conducted in humans using magnetoencephalography (MEG) to measure fast spontaneous sequences of representations. By building pattern classifiers of MEG sensor activity for each stimulus, we detected sequential reactivation of their trajectories during rest. These sequences recapitulated known features of neural replay and reflected correctly re-assembled orderings, rather than experienced trajectories, this supports the notion that task structural knowledge imposed a constraint on replay. We provide further evidence that neural pre-play is a manifestation of abstract structure knowledge, and the representation in the replay is factorised so that a sensory code of object representations was preceded 50 ms by a code factorized into sequence position and sequence identity. The forward replay of a correctly re-assembled sequence transitioned to that of reverse replay when a sequence was rewarded. Together they help build, maintain and update the world model.

 I will also show some preliminary data of replay in aversive learning and episodic memory retrieval, and its implications in psychiatric disorder, like Schizophrenia.

 There will be coffee, tea and cake in the conservatory directly after the talk.