Compression as a neural mechanism of mnemonic chunking in prefrontal cortex.

Chunking is the mental process of grouping information into meaningful units and plays a central role in many cognitive processes such as working memory, yet the neural mechanisms that form chunks remain unknown. Here, we investigated how chunking impacts the representation of mnemonic information in the dorsolateral prefrontal cortex (dlPFC). Macaque monkeys performed a self-ordered visuospatial working memory task, in which spontaneous chunking could be behaviorally characterized. We found that chunking improved memory performance overall and produced lower accuracies and longer reaction times at chunk boundaries. In the dlPFC, chunking systematically biased neural representations of spatial targets toward common chunk centers. At the same time, chunking reduced the dimensionality of population activity, indicating a compression of mnemonic information. Memory-guided behavior was also biased in the same direction as the neural distortions, which suggests that the compression mechanisms are lossy rather than lossless. Together, these results offer a mechanistic account of chunking as a form of lossy compression in neural coding that sacrifices detail within a chunk but improves overall memory retention.