The cerebellum is an intriguing part of our brain. Its name is the diminutive form of cerebrum, so literally means ‘little brain’. It is true that, in humans, it occupies just 10% of the brain volume, yet recent research shows it accounts for approximately 80% of the nerve cells; a complex network of approximately 69 billion neurons! Why does the ‘little brain’ contain such a disproportionate number of neurons? How do we begin to understand the operation and function of such a complex network? What does the cerebellum do? And given that the cerebellum has the clear majority of our neurons, why is it so completely overshadowed by the cerebral cortex in psychology texts? An understanding of the cerebellum and what we might call the cerebellar ‘sense of self’ addresses these, and other, cerebellar questions.
An evolutionary perspective highlights the origins of the cerebellum in early vertebrates. Early lineages, like lamprey, have cerebellum-like structures in their hindbrain, but no cerebellum. Sharks and rays have both cerebellum-like structures and a true cerebellum. So cerebellum evolved in concert with jaws and paired fins. In some shark species, cerebellar structures occupy over 50% of the brain volume.
It also seems probable that the cerebellum-like structures in the shark brain represent the ancestral structure from which cerebellum evolved. So understanding the function of cerebellum-like structures in the shark is not only intriguing in its own right, but is a good first clue as to what cerebellum does.
Why does the ‘little brain’ contain such a disproportionate number of neurons?
The function of shark cerebellum-like structures is to discriminate ‘self’ from ‘other’ in sensory inputs. Sharks have an amazing electrosensory system, used to detect the weak electric fields produced by their prey. Recordings from electorosensory neurons clearly show that the inputs from this system are strongly driven by the sharks’ own electric fields. The contribution of the cerebellum-like structure in the hindbrain is to cancel this self-generated noise. In effect, the cerebellum-like structures operate like noise cancelling headphones. They use an adaptive filter to cancel input associated with the sharks own activity, and pass on the prey signal to other areas of the brain. The idea of discrimination of ‘self’ and ‘prey (=other)’, and the function of the cerebellar circuitry as an adaptive filter provide key insights into the function and operation of the true cerebellum.
With the evolution of the true cerebellum it seems the adaptive filter functionality was adopted for motor control and paved the way for the athleticism and movement finesse that we see in swimming, running, climbing, and flying vertebrates. The traditional view of the cerebellum has long been that the cerebellum is strongly linked to motor control, and some of our best understanding of cerebellar function comes from model systems such as reflexive and voluntary eye movements. The understanding derived from model systems can be used to claim that a core element of movement finesse is the anticipation of reactions resulting from the actions we take. Adaptive filters are proficient at generating what engineers would call a forward-model. Such a model is just what is required to anticipate and counteract the reaction forces that propagate through the body when we exert a force on the outside world. Efficient handling of these reactive forces is a somewhat subtle, but important, contribution to athleticism.
Of particular interest, is that forward models not only provide a means to tailor reactive forces, but also predict the sensory consequences of our own movements. It seems that the extent to which these expectations are matched by the actual sensory consequences of the movement is what gives us our ‘sense of agency’ in our interactions with the world.
So the idea of ‘cerebellar sense of self’, captures the key elements of distinguishing self and other in our sensory interactions with the world. Both in the way that the shark distinguishes ‘prey’ from ‘self’ in its electrosensory system, but also in the way we distinguish the sensory consequences of what we do, from sensory consequences of what is done to us. This view of cerebellar self is also consistent with the understanding of a number of traditional cerebellar model systems. More recent interest in the possible role of cerebellum in cognitive function is represented in sensory processing and in our sense of agency. The idea of cerebellar-self spans a wide range; from shark prey detection to human psychoses.
The evolutionary perspective of this approach also provides answers as to why the cerebellum scarcely features in our conscious experience of the world, why experimental, or injury induced, deficits in cerebellum are often subtle in effect, and how we can literally live without a cerebellum. The adaptive filter view of cerebellar circuit function makes sense of the anatomy of cerebellar networks, the diverse contribution that can be made to a wide range of brain processing tasks, and to why the cerebellum has so many neurons.
Cerebellar research is an exciting body of work of interest to anyone curious as to how brains work, not only to neuroscientists, neurologists, and psychologists, but also to computer scientists and engineers concerned with machine/human interactions and robotics.
Featured image credit: Shark by Jake Gaviola. CC0 public domain via Unsplash.