Transcranial magnetic stimulation (TMS) for treatment of major depression involves placing the coil to target a portion of the brain located just above and behind the left temple, called the dorsolateral prefrontal cortex (DLPFC). This region of the brain is associated with abstract reasoning, attentional control, and working memory. The prefrontal cortex is uniquely developed in humans. It is what allows for meta-cognition, executive function and sophisticated decision-making following conceptual reasoning. The prefrontal cortex is essential in making us who we are: it gives us the ability to evaluate our emotional responses and adapt them according to circumstances.
The left dorsolateral prefrontal cortex (DLPFC) is strongly involved in higher-order perceptual reasoning which can helps regulate mood. Those suffering from major depressive disorder have under-active left DLPFC. Besides mood regulation and executive functioning, however, the DLPFC is also involved in a lesser-known function: working memory. Working memory is described as that temporary storage area that holds sensory memory as it comes in for a brief time, before it is further processed and consolidated into long-term memory. Though it is short-lasting and often limited in “storage space,” working memory is essential in attention. It is the ability to keep in mind one’s to-do list and manipulate it effectively in one’s mind.
For patients with major depression, poorer working memory often accompanies symptoms of low mood, low energy, and blunted affect. This particular manifestation of depression often leads to reduced attention span, difficulty focusing on tasks, and generally what many patients describe as “brain fog.”
There is increasing evidence that TMS has neurocognitive effects, including enhancing working memory and improving speed of perceptual, motor and executive processing. TMS, which uses a pulsing magnet to electrically stimulate neurons in the dorsolateral prefrontal cortex, induces long-term potentiation, or “learning” by the neurons such that with stimulation over time, they eventually “learn” to become more active. In a 2013 study, adolescents with major depressive disorder, who received 30 sessions of daily rTMS, showed not only decrease in severity of depression symptoms, but also showed significant improvement in working memory and delayed verbal recall. Another study found that attention and processing speed were improved following repetitive TMS therapy. In another sham-controlled trial, 20 sessions of TMS in patients with treatment-resistant bipolar depression resulted in significant improvement in language, immediate and long-term verbal memory, attention and processing speed, and working memory at 4-8 weeks after starting TMS.
Further studies on the effects of TMS on working memory are underway. However, from the current literature, there is strong evidence that TMS may have positive neurocognitive effects that accompany the mood regulation in its treatment of major depression and other psychiatric illnesses.