Depression is at its heart a disorder of the left prefrontal cortex in most cases. Research consistently demonstrates that these areas of the left brain demonstrate lower metabolic activity than other aspects of the cerebral cortex. But why?
A fundamental concept in neurology is that if neurological pathway “A” projects to pool of neurons “B,” and something decreases the frequency of firing of pathway A, pool B will show decreased activity, decreased regional metabolism, and decreased oxygen utilization. This mechanism, known as diaschisis, shows how the nervous system is interconnected, and that damage or dysfunction in one region will inevitably lead to problems in other areas.
The vestibular system is comprised of the inner ear apparatus and associated pathways. It allows us to understand where our head is in relation to gravity, how our head is moving in relation to the environment, and thus where our body is located in space. The input to the brain from both sides of the vestibular system needs to be symmetrical in order to function effectively; the brain needs the same input from both inner ears to be able to properly localize the body. If these systems become significantly impaired, this can lead to dizziness, balance problems, and vertigo.
The input from the vestibular system ascends to stimulate the cerebral cortex. Input from the right inner ear goes to both sides of the brain, but has a preferential activation of the left prefrontal cortex. Asymmetries in vestibular activation, even small ones, can create decreases in activation of the brain that need not manifest as dizziness or vertigo. Recently published research demonstrates that major depressive patients show an abnormal pattern of vestibular activity with lower function at the right side compared to left. This raises the possibility of vestibular asymmetries as a biomarker of major depression. It also reveals another potential cause of major depressive disorder for some patients. Vestibular function is another critical aspect of your framework that needs to be evaluated and treated when necessary.
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