The Deep Wiring of Speech in the Human Brain
Post by Anastasia Sares
The takeaway
Compared to other animals, humans have a highly developed capacity for speech. This study showed that human speech areas have high-fidelity and fast connections to deep brain nuclei—meaning that we may indeed be hard-wired to learn language.
What's the science?
The basal ganglia are a set of structures deep in the brain that help to regulate almost all other activity: they form circuits with other brain areas, regulating them and deciding whether to perform an action, or when to stop. One of these circuits is called the hyperdirect pathway; this pathway puts the brakes on an action or a process. In rodents, it was discovered by injecting special viral proteins and dyes that can climb along neural pathways. This was not ethically possible to do in humans, so it was unclear whether our brain architecture was similar. Scientists came one step closer in the 2010s, when the hyperdirect pathway was found in primates. In 2018, evidence of this pathway in humans was observed by careful electrical recordings during surgery. This confirmed that the hyperdirect pathway exists in humans, and it links to many areas of the cortex. This week in Cell Reports, Jorge and colleagues used a similar electrical stimulation and recording technique to look at speech-related areas of the brain. They concluded from the timing of the electrical responses that there is a hyperdirect pathway that connects areas associated with speech to the deep parts of the brain, perhaps explaining humans’ unique ability with language.
How did they do it?
The study recruited patients with Parkinson’s disease who were already being implanted with electrodes in a procedure known as “deep brain stimulation.” In this procedure, a long, thin electrode is inserted into the interior of the brain and is connected to an exterior pacemaker-like device that can deliver pulses of electricity directly to stimulate that brain region.
For Parkinson’s, the target of these stimulations is the subthalamic nucleus: one of the brain’s deep nuclei that works to inhibit actions that are not necessary. Stimulating the subthalamic nucleus helps to suppress the resting tremors that are associated with Parkinson’s disease. The subthalamic nucleus also happens to be the first stop of the hyperdirect pathway, and when stimulated, electrical activity can actually travel backwards to the cortex (at least, this is true in animal models). So, the researchers were able to take advantage of some electrodes placed on the surface of the brain during the surgery to measure and map out these backward-traveling signals. They would stimulate the subthalamic nucleus, then time how long it took for the signal to reach cortex. If the timing was short (< 10 milliseconds), then it was likely that the two regions were directly connected.
What did they find?
First, the authors confirmed the 2018 finding: they were able to measure electrical signals traveling backward from the subthalamic nucleus to the cortex. These signals arrived quickly, under 10 milliseconds, which means the connections were likely only a single neuron in length.
They then looked at how the placement of the stimulating electrode affected the activity in the cortex. Stimulating areas closer to the midline of the brain generated stronger signals in the parts of cortex that control movement while stimulating further from the midline generated signals in parts of the cortex that deal with sensory perception and forming associations. Many areas known for processing speech were affected by subthalamic stimulation, including the inferior frontal gyrus (classically known as Broca’s area), the auditory cortex, and association areas in the temporal cortex (roughly equivalent to the classical Wernicke’s area).
What's the impact?
This study demonstrates that speech areas of our brain are just a single neuron away from the deep inner nuclei of the brain. This super speedy pathway may contribute to our extraordinary capacity for speech, and help us understand what makes humans unique. Many current models of speech skip or gloss over the role of these deep brain loops in speech, and therefore these models may need to be updated to reflect the importance of these pathways.