How the carrot and the cane use different brain pathways to drive movement

What is it about?

When something exciting or threatening happens, it can change how we move. But how do emotional signals in the brain actually reach the parts that control movement? The well-known route for voluntary movement runs through a region called the dorsal putamen, but this circuit is walled off from the brain's emotional processing areas. Studies tracing neural pathways in monkeys have hinted at a second route through the ventral putamen, the underside of the structure. This region receives input from emotional brain areas like the amygdala, and critically, appears to reach the motor cortex through a separate relay. We carried out two neuroimaging experiments in healthy humans. First, we scanned people at rest using ultra-high-field (7T) MRI and mapped how brain regions communicate with each other. Consistent with a functioning second route, the ventral putamen maintained independent connections with both emotional regions (the amygdala) and motor areas (the cingulate motor area), even after statistically removing the influence of the dorsal putamen. In a second experiment, a different group of participants performed a reaching task while lying in the scanner. By hitting targets fast enough they won rewards. But every once in a while, they could win a large reward, or avoid a large loss. We found that when they chased the big reward, the brain quieted the routine motor circuit while keeping the emotional route active, and people moved faster but less carefully. When avoiding a loss, both circuits went quiet, and brain regions linked to hesitation and stopping took over instead. These findings suggest the brain flexibly switches between different control systems depending on how we feel about what is at stake. Relishing a win and dreading a loss are not simply experiences; they are different neural blueprints for action.

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Photo by Chandler Cruttenden on Unsplash

Photo by Chandler Cruttenden on Unsplash

Why is it important?

In Parkinson's disease, the dorsal putamen loses its dopamine supply, degrading the brain's primary motor circuit and making it difficult to initiate movement. Yet doctors have long observed that emotionally charged situations, such as emergencies or strong incentives, can temporarily restore normal movement in some patients, a phenomenon known as paradoxical kinesia. The ventral putamen is relatively spared by this dopamine loss, even in advanced disease. Our findings suggest a candidate explanation: when emotions run high, the second route through the ventral putamen may bypass the damaged circuit and reach the motor cortex through an alternative relay. Understanding how the brain flexibly recruits different circuits for movement could inform new therapeutic approaches, from context-based rehabilitation strategies to refined brain stimulation protocols.

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