Summary: Researchers spotlight the thalamus, typically regarded merely as a relay station, as a pivotal entity in adult brain plasticity.
Contrasting prior beliefs focusing largely on the cortex, the thalamus demonstrates a significant role in adapting sensory and motor information, as illuminated through mouse models in vision studies.
While visual input does journey from the retina to the visual cortex via the thalamus, this study accentuates that the thalamus isn’t merely a passive conduit but an active participant in adaptive processes.
Insights garnered could pave the way for innovative therapeutic approaches in treating conditions like lazy eye, shifting focus from the cortex to also encompass the thalamus.
- Critical Component: The removal of the GABA-alpha 1 subunit in the thalamus during mice’s critical vision development period significantly altered traditional visual cortex response, underscoring the thalamus’s role in neural plasticity.
- Adaptability Insights: Experiments demonstrated that plasticity occurred in the adult thalamus and was pivotal for alterations in the cortex, revealing its influential presence beyond mere sensory relay.
- Potential Therapeutic Applications: The findings suggest that issues traditionally attributed to the cortex, such as lazy eye, may also involve the thalamus, providing a new potential therapeutic focus.
It is generally believed that the adaptability of the adult brain mainly takes place in the cortex. However, a new study from the Netherlands Institute for Neuroscience shows that the thalamus, a relay station for incoming motor and sensory information, plays an unexpectedly important role in this process.
“This could be an interesting starting point for various therapies,” says Christiaan Levelt.
Learning new things requires a tremendous capacity of our brains. The adaptation of our brain as a result of new experiences is called plasticity. There are periods during our development when neural networks show a lot of plasticity, known as critical periods. But also the adult brain is capable of adapting. Where this plasticity takes place in adult brain is not well understood.
To gain more insight into this question, Yi Qin and his colleagues, under the supervision of Christiaan Levelt, examined the visual system of mice. This is a popular model for studying plasticity due to its ease of manipulation. Visual information reaches the thalamus through the retina. This brain nucleus then transmits processed information to the visual cortex and vice versa. An experiment in mice can clearly demonstrate how well the adult brain is able to adapt.
“When one eye of the mouse is occluded for several days, the visual cortex starts to respond less effectively to the closed eye and better to the open eye. How this is precisely regulated has been unclear for a long time. But these new results bring an important player to the forefront: the thalamus.
Christiaan Levelt: “Five years ago, we discovered that the thalamus plays a crucial role in the plasticity of the visual cortex during critical periods of development. This has changed our perspective on how this whole system works.
“We all thought that this process was regulated by the visual cortex, but it turned out not to be the whole story. We found out by removing a very specific component, the GABA-alpha 1 subunit, in the thalamus of mice during their critical period for vision. This component is responsible for inhibiting the thalamus, so its removal resulted in reduced inhibition.
“When we closed one eye in these mice, the shift in responses no longer occurred. Because the adult brain uses different plasticity mechanisms than the developing brain, an important question was whether adaptation in the adult visual system also involved the thalamus”.
Yi Qin: “In the current study, we performed the same experiment in adult mice and observed similar results. We observed that plasticity also took place in the adult thalamus, but disappeared when we removed the alpha-1 subunit. Consequently, there was no longer a shift in the cortex either.
“Since we know that the visual cortex also sends information back to the thalamus through a feedback mechanism, we were curious if the visual cortex also plays a role in plasticity of the thalamus.
“We investigated this by reversing the experiment and shutting down the visual cortex. What happens to the shift in responses in the thalamus then? In adult animals, we did not see any difference: the shift persisted.
“However, in animals during their critical period, we observed that when we shut down the visual cortex, the shift reverted back in the thalamus. So, at a young age, plasticity in the thalamus and cortex influence each other much more, while in the adult brain, the thalamus is particularly important for plasticity in the cortex but not the other way around.”
Involved in many processes
Levelt continues: “Plasticity is important in many processes. We are currently focusing on sensory plasticity (vision), but plasticity is also fundamental for memory and other functions. These new insights could be relevant, for example, in understanding learning disabilities.
“It is possible that the origin of these problems lies in the thalamus rather than the cortex. Therefore, a different approach is needed. Instead of solely looking at the cortex, we should also consider the thalamus when it comes to therapies and the pathogenesis of these issues. This is an important new interpretation.”
Qin: “Even in the case of lazy eye, it is assumed to be a problem of the cortex, but it could also involve the thalamus. In Europe, we test for the presence of lazy eye from a young age. This condition can be corrected during the critical period by temporarily patching the ‘good eye,’ which strengthens the connections to the weaker eye.
“In the US, for example, this is not routinely checked at a young age, resulting in more people carrying a lazy eye into adulthood. Since the critical period has already passed for them, it becomes more challenging to treat these individuals. Our study provides a hint that we need to look beyond the cortex, which can provide guidance for a new treatment strategy.”
About this neuroplasticity research news
Original Research: Open access.
“Thalamic regulation of ocular dominance plasticity in adult visual cortex” by Christiaan Levelt et al. eLife
Thalamic regulation of ocular dominance plasticity in adult visual cortex
Experience-dependent plasticity in the adult visual system is generally thought of as a cortical process.
However, several recent studies have shown that perceptual learning or monocular deprivation can also induce plasticity in the adult dorsolateral geniculate nucleus (dLGN) of the thalamus. How plasticity in the thalamus and cortex interact in the adult visual system is ill understood.
To assess the influence of thalamic plasticity on plasticity in primary visual cortex (V1), we made use of our previous finding that during the critical period, ocular dominance (OD) plasticity occurs in dLGN and requires thalamic synaptic inhibition.
Using multielectrode recordings we find that this is also true in adult mice, and that in the absence of thalamic inhibition and plasticity, OD plasticity in adult V1 is absent.
To study the influence of V1 on thalamic plasticity we silenced V1 and show that during the critical period, but not in adulthood, the OD shift in dLGN is partially caused by feedback from V1. We conclude that during adulthood, the thalamus plays an unexpectedly dominant role in experience-dependent plasticity in V1.
Our findings highlight the importance of considering the thalamus as a potential source of plasticity in learning events that are typically thought of as cortical processes.