How the Visual Cortex Conducts the Thalamus ✎ express
The brain is like an orchestra, with the cortex as the conductor and the thalamus as the first violin. The thalamus relays signals from the eyes to the visual cortex. But the cortex can also send commands back, tuning the thalamus for the task at hand. This was once a hypothesis, but now it's been tested in humans.
Participants watched videos of talking faces without sound. In one task, they had to understand speech by reading lips; in another, they had to name the color of the lips. Activity in the thalamus (specifically, in its visual part, the lateral geniculate nucleus) changed depending on the task. Then the researchers temporarily 'switched off' the cortical area V5/MT, which processes motion, using magnetic stimulation. After that, the difference in thalamic activity between the tasks almost disappeared.
So the cortex really does control the thalamus, and without it, the thalamus can't switch from one task to another. This finding helps us understand how the brain flexibly processes visual information — for example, when we look at a face and read lips at the same time.
The lateral geniculate nucleus, part of the thalamus, receives signals from the eyes and sends them to the visual cortex. But there are ten times more feedback connections from the cortex to it than there are direct connections from the eyes.