Deactivation of Anterior Thalamic Nuclei Underlies Associative Memory Encoding

Post by Shireen Parimoo

What's the science?

Associative memory refers to memory for multiple pieces of information (e.g., a face and a name) and is a critical component of episodic memory. The involvement of regions in the medial temporal lobe (MTL) cortex – such as the hippocampus – in episodic memory has been well-established. The thalamus is a subcortical brain region made up of several nuclei that has structural connections to the MTL and is also implicated in the formation and retrieval of memories. However, the precise role of thalamic nuclei, particularly the anterior and mediodorsal nuclei, in associative memory is not clear. This week in Neuropsychologia, Geier and colleagues used functional magnetic resonance imaging (fMRI) to investigate the contribution of thalamic nuclei and their connectivity to the hippocampus and MTL regions in associative memory encoding and retrieval.

How did they do it?

Twenty-seven young adults completed an associative memory task while undergoing fMRI scanning. In the encoding phase, they were shown a scene overlaid with a face, and participants made plausibility judgments about the face-scene pairs. The retrieval task consisted of three phases: (i) a cue phase in which a scene image was presented, followed by (ii) a delay phase, during which participants were instructed to mentally recall the face that was previously paired with the scene, and (iii) a probe phase in which three faces were presented with the scene and participants performed a memory judgment for the previously paired face.

The authors examined activity in the thalamus and MTL during memory encoding and the three memory retrieval phases. They also investigated how the time course of activation in each task phase was related to memory performance. That is, how did activity in the anterior and mediodorsal thalamic nuclei, as well as the anterior and posterior hippocampus, differ for correctly and incorrectly remembered faces? Finally, they assessed functional connectivity between thalamic nuclei and the MTL in associative memory and additionally compared the connectivity profiles of the anterior and mediodorsal thalamic nuclei to the MTL.

What did they find?

Thalamic activity significantly decreased during the encoding phase, whereas MTL regions like the hippocampus exhibited a nominal increase in activation. In particular, activation of the anterior hippocampus during memory encoding was associated with better face-scene memory performance. Conversely, reduced activation of the anterior thalamus at encoding was related to memory accuracy, while the mediodorsal thalamus showed little change across correct and incorrect trials. Interestingly, the opposite pattern of activation was observed at retrieval, with greater activation of the thalamus during the delay and probe phases but reduced delay-phase activation in the MTL. However, retrieval-phase activity in the hippocampus or thalamus was not significantly related to memory performance. These results indicate that the successful formation of associative memories is related to the increased involvement of the anterior hippocampus but deactivation of the anterior thalamus. Finally, the mediodorsal thalamus showed greater connectivity with the MTL compared to the anterior thalamus. However, differences in functional connectivity between the thalamic nuclei and the MTL were not related to memory performance.

What's the impact?

This study characterized the involvement of thalamic nuclei in memory and used neuroimaging to demonstrate the complementary role of the anterior thalamic nuclei in associative memory formation. Although patient and neuroimaging studies have shown that the thalamus is important for memory, we now know how specific nuclei in the thalamus are involved in the formation of associative memories, specifically. These results pave the way for future work that can further reveal how thalamic nuclei communicate with other regions of the brain to support memory encoding and retrieval.

Geier et al. The role of anterodorsal thalamus in associative memory encoding and retrieval. Neuropsychologia (2020). Access the original scientific publication here.