Burst firing represents learned composite stimuli in primary sensory cortices

The primary cortical areas of each sensory modality occupy a significant portion of the mammalian neocortex. Beyond mapping basic sensory features, such as visual object orientation or sound frequency, these regions may play a broader role in sensory processing. Here, we review recent advances in our understanding of sensory representations through a unique neuronal firing mode called bursting, with a particular focus on layer 2/3 (L2/3) pyramidal neurons. While maps of single-feature inputs are preserved in primary sensory cortices, individual L2/3 pyramidal neurons receive heterogeneous inputs from multiple basic features. The co-activation of these inputs can induce bursting, forming sparse yet persistent representations of composite sensory stimuli. Unlike basic sensory feature maps, which drift over time, experience-driven bursting patterns in L2/3 remain stable over long periods. Notably, these bursting representations are holistic, as single-featured component stimuli rarely elicit such activity. We propose that these holistic bursting neurons (HB neurons) in L2/3 play a crucial role in integrating sensory experiences, generating durable, sparse, and reliable representations that may serve as building blocks of long-term memory in the complexity of the real-world.