The whole cerebellum works together as a unit ― Discovery by RIKEN which upends conventional theory

The theory of modularity conceptualizes that various areas of the brain play different functions, while distributed processing conceptualizes the idea that the whole cooperates to play multiple functions. Even though the cerebellum comprises only about 10% of the total weight of the brain, it contains more nerve cells than the cerebrum and is essential for movement and perception. Whether the cerebellum follows the theory of functional localization or the theory of holism has been hotly debated by scientists.

A collaborative research group composed of Researcher Takayuki Michikawa and Team Leader Atsushi Miyawaki of the RIKEN Center for Advanced Photonics found that the sensory input that changes from moment to moment is expressed in real time throughout the cerebellum. This is the first time that this has been observed in the world, and upend the theory of localization in the cerebellum. The research group combined data from a genetically modified mouse model that expresses the fluorescent calcium sensor protein yellow chameleon in all cerebellar Purkinje cells and a proprietary ultra-wide-field macroscope system and image analysis technology. Consequently, they constructed an experimental system that can measure the entire dorsal side of the cerebellar cortex at the same time.

Researcher Michikawa said, "I first came up with this idea 20 years before the yellow chameleon was created, and it has finally succeeded." Using this system, the firing of complex spikes of 20,000 or more Purkinje cells observable from the dorsal surface of the mouse cerebellum were simultaneously measured. Results showed that individual Purkinje cells were not active independently, but that nearby Purkinje cells fired synchronously. The collaborative research group named this synchronously firing Purkinje cell cluster a "segment" and analyzed its role in sensory information expression.

In order to investigate the relationship between localized body parts and the cerebellar cortex, weak electrical stimulation was applied to the muscles of the extremities of the mouse under anesthesia and in wakefulness conditions; a complex spike response in the cerebellar cortex was observed. As a result, unlike what is suggested in the conventional modularity theory, it was found that the response was noted in almost the entire observed cerebellar cortex. Bayesian inference was used to analyze which muscles were stimulated from the response of the complex spikes observed in the cerebellar cortex. Subsequently, from the responses of multiple segments, it was found that the timing of electrical stimulation and the stimulated muscles could be accurately determined in each trial.

This result shows that the cerebellum expresses the information of the outside world and the body changes from moment to moment by performing distributed group coding as a whole. Team Leader Miyawaki said, "We have been developing the yellow chameleon and transparent technologies, and by utilizing them, brain research can be further advanced. We would like to continue working on cranial nerve diseases and other issues such as Brain Machine Interfaces."