“Wash your hands frequently and don’t touch your face.” This is generally the most practical advice we’ve been given throughout the COVID-19 pandemic — and it makes sense. The eyes, nose, and mouth are notorious portals for contagions to enter the body, particularly for something like coronaviruses that are transmitted through respiratory droplets.
Despite the constant warnings, actually applying this advice is a whole other story. On average, we tend touch our own faces 23 times an hour. It seems that being warned against touching your face only makes you compelled to do it more, even if we take the consequences of disease very seriously. So why do we do it? The brain’s basal ganglia may be to blame. Repeating a particular behavior over and over long enough, our brain learns the behavior subconsciously and we end up doing it with only minimal input.
Located at the base of the forebrain, this bundle of nuclei is closely connected with both the thalamus (regulation of consciousness and sleep) and the cerebral cortex (regulation of memory and perception). It is composed primarily of the striatum — both the dorsal striatum (known as the caudate nucleus and putamen) and the ventral striatum (including the nucleus accumbens and the olfactory tubercle). Striatum make up the basic channels of the brain’s motor and reward system — spiny neurons that receive input throughout regions of the brain, and chemical signals to produce dopamine for positive responses.
Input collected from the rest of the brain is swiftly transmitted to the basal ganglia, which serves as a large switchboard operator with its four structures: the globus pallidus (which regulates voluntary movement), the ventral pallidum (known as the brain’s pleasure center), the substantia negra (which processes motor planning and learning and receives dopamine input), and the subthalamic nucleus (known for increasing impulsive behavior in those conditioned by the brain’s reward center.)
Your face might itch — perhaps even more so after you’ve been strongly warned against touching it – and this information registers in the brain with a reaction similar to the kind produced by pain. You scratch your face in a particular place to relieve the itch, and can briefly get rid of the irritation – the act of when and where to scratch, and what the effect produces, is soon committed to and divided up among each of these structures.
More importantly, the basal ganglia is also the reason that you’re able to do this one task in full synchronicity with everything else you happen to be doing at the moment, such as walking up steps, or reaching for a tissue. It can also be our key to learning new behaviors that could keep us healthier — like washing your hands more frequently, and more thoroughly without giving it much thought.
Putting the Pieces Together
Researchers are still perfecting their model of the basal ganglia. The exact way in which it directs our movements is not quite clear, but there is one hypothesis that has been picking up momentum.
According to the direct/indirect model, there are pathways throughout the nuclei that either promote or inhibit movement. The model is based on connections between the basal ganglia, in particular, the links between the globus pallidus and substantia nigra that form with neurons in the thalamus. These neurons are connected to the motor cortex, which is involved with the planning and control of voluntary movement. The basal ganglia is able to influence the thalamic neurons in such a way that they fail to interact with the motor cortex, preventing the individual from acting.
When you find yourself desiring to touch your face, or even just to get up and go for a walk after a long day of working in isolation, the motor cortex relays a signal to the basal ganglia. This typically hits the striatum (divided between the putamen and the caudate.) It then travels along a circuit known as the direct pathway — which quiets neurons in the globus pallidus and substantia nigra and allows you to either scratch that itch or get up from your chair and walk towards the door.