The cortex has long been considered the primary site of adaptability in the adult brain, but a recent study by the Netherlands Institute for Neuroscience challenges this belief. Their research reveals an unexpected role played by the thalamus, a relay station for sensory and motor information. According to Christiaan Levelt, this discovery could open new avenues for therapeutic approaches.
Our brains possess an immense capacity for learning, and the process of adapting to new experiences is known as plasticity. While critical periods of heightened plasticity occur during development, even the adult brain can adapt. However, the exact location of this adult plasticity has remained elusive.
To delve deeper into this question, Yi Qin and colleagues, under Christiaan Levelt’s guidance, focused on the visual system of mice—a convenient model for studying plasticity. Visual information flows from the retina to the thalamus, which then relays processed data to the visual cortex and vice versa.
In a mouse experiment, the adult brain’s adaptability becomes evident. After occluding one eye for several days, the visual cortex becomes less responsive to the closed eye and more so to the open one. The precise regulation of this process has long baffled researchers, but these new findings shine a spotlight on a crucial player: the thalamus.
Christiaan Levelt’s groundbreaking work revealed a significant shift in our understanding of brain plasticity. He states, “Five years ago, we uncovered the critical role of the thalamus in shaping plasticity within the visual cortex during developmental critical periods. This discovery reshaped our perspective on this intricate system. Contrary to earlier beliefs that the visual cortex held sole control, we discovered it was just part of the puzzle. This revelation came about when we selectively removed the GABA-alpha 1 subunit from the thalamus during the critical period for vision in mice.”
“The GABA-alpha 1 subunit is responsible for inhibiting thalamic activity, and its removal reduced this inhibition. Consequently, when we closed one eye in these mice, the expected shift in responses didn’t occur. Since plasticity mechanisms differ between the adult and developing brain, a pressing question arose: Does the thalamus also play a role in adult visual system adaptation?”
Yi Qin further elaborates, “In our latest study, we replicated the same experiment in adult mice and obtained similar results. Plasticity indeed occurred within the adult thalamus, but upon removal of the alpha-1 subunit, this plasticity vanished, resulting in no shift in the cortex. Given our knowledge that the visual cortex communicates back to the thalamus through a feedback loop, we ventured to explore the visual cortex’s role in thalamic plasticity.”
“We approached this by reversing the experiment—temporarily silencing the visual cortex. The question was, what happens to the thalamic responses in this scenario? Remarkably, in adult animals, we observed no significant change; the shift persisted. However, in animals during their critical period, when we silenced the visual cortex, the shift in the thalamus reverted.”
“In essence, during youth, thalamic and cortical plasticity strongly influence each other, whereas in adulthood, the thalamus emerges as a key player in cortical plasticity, with less influence in the opposite direction.”