Research suggests that the virus may gain access to the brain via the forebrain’s olfactory bulb, which is important for the processing of smell. Loss of smell is a symptom in many patients with Covid-19.
As part of the system responsible for your sense of smell, the olfactory bulb sends information about smell to be further processed in other brain regions – including the amygdala, orbitofrontal cortex and the hippocampus – which play a major role in emotion, learning and memory.
As well as having extensive connections to other brain regions, the olfactory bulb is rich in the chemical dopamine, which is important for pleasure, motivation and action. It may be that Covid-19 alters the levels of dopamine and other chemicals, such as serotonin and acetylcholine, in the brain, but we can’t say for sure yet. All these chemicals are known to be involved in attention, learning, memory and mood.