Periodic Reporting for period 3 - NEUROMEM (A Neurocomputational Model of Episodic Memory)
Période du rapport: 2019-10-01 au 2021-03-31
We have provided a detailed model of spatial memory and imagery, incorporating representations of objects into egocentric parietal and allocentric medial temporal representations to combine the content and context of an experience within flexible representations (Bicanski and Burgess, 2018). This new model offers an account on how the brain stores complex representations in memory, via associating the content of an experience with the surrounding context, and can flexibly use these representations to generate imagery to guide future behaviour. This model also incorporates the provision for imagined movement via connections between grid cells in entorhinal cortex and hippocampal place cells. The model is important in consolidating our understanding and making predictions about how memories are formed, retrieved and updated within a complex system of brain regions.
To investigate and model the interactions between place cells and grid cells to support dynamic imagery and memory representations, we have recently proposed a model of recognition memory in which grid cells encode translation vectors between features of an attended stimulus and thus guide eye movements between expected features to accumulate evidence to identify experienced stimuli (Bicanski and Burgess, 2019). The model is timely in providing a neural account of recent empirical data demonstrating the grid cells show responses coupled with eye movements in non-human primates and humans.
To examine the associative structure of long-term memory and its reactivation at retrieval via hippocampal pattern completion, we have provided an account of how negative experiences can affect memory and contribute to memory disturbances often seen in posttraumatic stress disorder (PTSD; Bisby and Burgess, 2017). This review consolidates the current empirical evidence and makes novel predictions about how negative experiences can affect amygdala- and hippocampal-dependent memory in opposing ways, strengthening memory for the negative content and weakening the associative and contextual structure of the memory representation. The result of these opposing interactions are thought to support distressing intrusive imagery in PTSD. We have expanded this work further, carrying out behavioural experiments to assess the associative structure of negative events in memory and the way in which they are retrieved. The results of this work support our initial proposals, that negative events weaken the associative structure of memory representations to disrupt pattern completion processes leading to memories being retrieved in a less holistic manner (Bisby et al., 2018). This work also utilises computation modelling to account for the neural mechanisms how negative events can alter hippocampal function to reduce memory coherence via a Hopfield model of associative learning in the hippocampus.
In collaboration with research projects with other funding streams, we have also assessed the way in which sequence information is incorporated within memory representations. To do this, we have combined a sequential motor learning task with magnetoencephalography (MEG) to investigate the temporal dynamics of brain areas required to retrieve sequence information associated with action responses (Kornysheva et al., 2019). We show that the brain learns and controls multiple sequences of complex actions by flexibly combining distinct representations of the actions required, the interval timing and the sequence position. In addition, we have examining oscillatory changes within the hippocampus, utilising desktop virtual reality to assess spatial memory and examining intracranial recordings from electrodes targeting the hippocampus in epilepsy patients admitted for pre-surgical monitoring (Bush et al., 2017). This work demonstrates how power increases in the theta range are observed prior to and during navigation to remembered objects, shedding light on the dynamics that occur during memory retrieval and navigation. We have also used a similar virtual reality approach combined with fMRI to examine the brain areas involved in learning about locations within an environment that might predict a mild aversive shock to the wrist (Suarez-Jimenez et al., 2017). Results highlight distinct networks that work in concert to support learning about the spatial environment and whether an individual will likely encounter a threat within the environment. These findings are important in understanding how anxiety influences spatial memory and how learning might be altered in anxiety disorders.
For the remaining period of the project, we will continue to use novel approaches in behavioural tests, neuroimaging and computational modelling and advanced analysis methods to understand the brain systems involved in human memory. We envisage that future studies within the project will generate results that have further our understanding of the role of the hippocampus in memory storage and retrieval and how sub-regions differently contribute to these processes. One particular focus will be on finding out how the spatial system (comprising place cells, grid cells etc) supports sequential memory - a seemingly non-spatial function which is critical to episodic memory. We also expect to further our previous work on how negative events can influence the brain systems involved in memory and how alterations might lead to intrusive memories, a prominent feature in posttraumatic stress disorder.