By night, within two hours of sleeping, you may find yourself wandering the warm hued clouds of a dream world — vacationing in a paradise, making contact with relatives long gone, or uncovering the seemingly obvious secret to life — moments of grandeur where no obstacles stand in your way. For others, the night may be more terrifying — a chance to relive your fears that seem at once inevitable and insurmountable.
Even when morning finally comes, there’s still that uncertainty that echoes in your mind, a lingering fear of the irrational — or perhaps there’s that frustration of not being able to fully describe the vivid paradise that once surrounded you. In either case, you experienced the sensation of rapid eye movement — a sensation in which brain waves move at a more rapid frequency than normal — at a desynchronized, low amplitude, as electrical signals rush from the brainstem towards the visual cortex, causing the muscles to contract.
The structure responsible for this cycle of brain activity is the basal ganglia. Located at the base of the forebrain, this bundle of nuclei is closely connected with the thalamus (which regulates consciousness and sleep) and the cerebral cortex (which regulates memory and perception). It is composed primarily of the striatum: both dorsal striatum (known as the caudate nucleus and putamen) and the ventral striatum (both the nucleus accumbens and olfactory tubercle). The striatum makes up 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.
That feeling of satisfaction you get when someone likes the new photos on your Instagram is processed by the basal ganglia and is probably a big reason you spend too much time on it — your brain is constantly craving the next little reward — reinforced each time you upload a new picture and get feedback. The input received from the rest of the brain is then transmitted throughout only the basal ganglia: 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 brain’s reward center).
The subthalamic nucleus processes information sent by the striatum and the cerebral cortex, and then relays the messages onto the globus pallidus. As the basal ganglia plays a role in voluntary motor movements and leads to the development of habits — both sleep habits and even patterns like cognition and emotion — it is thought to play some role in “action selection”: deciding which movements to execute. The basal ganglia is also tasked with synchronizing all of these movements to come together; why you’re able to walk and chew gum at the same time. This could be the reason that emotions and learning are so closely linked within the structure. It’s why when you’re walking home at night, you learn to avoid parts of the asphalt that glisten in the dark. Your basal ganglia can quickly process the areas as covered with ice and, along with that, bring up painful memories of falling on ice. You may be in a hurry to get home at the end of a dark winter day — but the sudden rush of cold air when you step outdoors — the sight of ice and salt crystals on the walkway — are some bits of sensory information that tell the basal ganglia that running to your car in the darkness is probably not the safest plan of action.
One of the challenges associated with studying the basal ganglia in depth lies in building an effective model. While Mahlon DeLong constructed the first one in the 1990s, there is still a great deal of contention among the exact pathways of its sensory neurons — as researchers are still unsure about the exact nature of overlap among the direct and indirect pathways to the brain. One hyperdirect pathway around the thalamus acts as a safeguard, preventing the basal ganglia from deciding too quickly on limited information the senses bring in. Across just this small structure above the brainstem, nearly 20,000 such circuits are known to exist — making for quite an intricate highway. To make things more complex, some researchers subscribe to a “push-pull” theory suggesting that many of these circuits are regularly at odds with each other. There is also an alternative “center-surround” theory, suggesting that inhibitory signals from the other circuits can protect a focused message into the cortex.
Among DeLong’s research interests is Parkinson’s disease — a disorder that attacks the body’s central nervous system and makes it difficult to control movement voluntarily. In recent years, as researchers work on stem cell treatments for people suffering from Parkinson’s, the basal ganglia has come into focus. For patients with the disease, neurotransmitters among the substantia negra deteriorate, causing a domino effect throughout the nuclei — as it loses the dopamine it needs to communicate with the motor cortex.
Brach Poston, a professor of kinesiology at the University of Las Vegas, is currently involved in a research effort to treat neurodegenerative diseases like Parkinson’s with low levels of electrical stimulation — a therapy known as transcranial direct current stimulation (tDCS). Sponges soaked in saline are attached to rubber electrodes over the patient’s scalp, and a mild electric current is passed across them. At present, the treatment is largely used for outer areas of the brain, but Poston sees the possibility of using tDCS to treat the brain’s more complex structures in the near future. Last summer, he began the practice of applying this stimulation to the cerebellum, which has been known to compensate for the damage done to the basal ganglia. “During a single treatment, we and other research groups have typically seen a 10 to 15 percent performance improvement, with the effects lasting up to 90 minutes,” Poston said.
Poston added, “Daily application could produce a cumulative effect, and we hope to be able to elicit performance improvements of approximately 30 percent, which were seen in studies among young adults, when we apply stimulation over a two-week period.” Poston’s patients showed improvement in simple motor tasks involving their hands and arms — tasks like picking up coins from the floor or pinching and gripping — all of which require cortical neurons to signal the brain. Practicing these actions over time with tDCS from transportable wearable devices — leads to further stimulation of the neurons and can improve the patient’s accuracy.
Further research of the basal ganglia may be crucial to understanding the nature and treatment of autism — a range of conditions that medical science is just beginning to understand. Both the cerebellum and the cerebral cortex have been affected in cases of autism spectrum disorder, but only recently have researchers begun to look at the structure they are both interconnected by. Patients afflicted with carbon monoxide poisoning have sustained damage to the basal ganglia, and often exhibit the repetitive behaviors seen in people with autism — a pattern that caused postdoctoral researcher Krishna Subramanian of the Hussman Institute for Autism to investigate further.
Using the postmortem brain tissue of 21 patients, as well as 23 additional control samples, Subramanian’s team isolated the chemical messenger gamma-aminobutyric acid, or GABA, in the striatum, known for dampening brain activity. Those with autism show increased levels of the receptor GABAA in the striatum. They also had decreased levels of GABAA in the subthalamic nucleus — where they are cut off by excitatory neurons. This pattern could be weakening signals to the cortex, something that could be stopped by medications that boost the signaling. Researchers are still investigating further, announcing their findings in May 2017 at the International Meeting for Autism Research in San Francisco, California. While there is much yet to be learned about the basal ganglia, its centrality to the rest of the brain is already coming clearer.
This article was originally published in the Fall 2017 issue of Brain World Magazine.
