Holistic Bursting Cells Integral to Cognition

Scientists have shared the discovery of "holistic bursting" cells—a novel functional class of cortical neurons that represent learned complex objects as wholes rather than parts. The discovery, from a collaborative research team from the Suzhou Institute of Biomedical Engineering and Technology of the Chinese Academy of Sciences, was published recently Nature Communications. 

The team used a combination of two-photon Ca²⁺ imaging of neuronal populations and loose-patch recording of single neurons in behaving mice. They found that in mice that had received an auditory association training, there existed a special subset of neurons in layer 2/3 of the auditory cortex each of which reliably exhibited a unique mode of high-rate, prolonged burst firing response (instantaneous firing rate ~ 100 Hz, firing duration 100–250 ms) to the trained sound in each trial.

In contrast, neurons with such strong burst firing responses were almost absent in the auditory cortices of untrained animals, where the typical neuronal responses were unreliable singlet firings. Further chronic imaging experiments revealed that the bursting response property emerged due to the associative training but occurred only in a sparse subset (~5%) of neurons in the auditory cortex.

Researchers also observed that mice could be trained with different chords, each consisting of multiple pure-tones. The behavioral response showed a "holistic" character—meaning that the mice exhibited a reliable behavioral response exclusively to the trained chords but not to any of the constituent pure tones, even though all sounds were played at nearly the same volume. The experimental precision was sufficient to show that the response strength to the preferred chord was significantly larger than the sum of individual response strengths to the four tones that constituted the chord.

"The groundbreaking finding of this paper is the emergence of few neurons which respond, very powerfully, to a stimulus as a whole, rather than to its components, in contradistinction to most neurons surrounding them,” says contributing author Israel Nelken. “These very few holistic bursting neurons could not be detected before the deployment of the combined technology of two-photon imaging and single-cell electrophysiology."