Can an eye that's disconnected from the brain see?

In regenerative medicine, in cases of transplantation or graft, innervation, or the supply of nerves to a body part, remains a major difficulty. This is particularly true for the sensory organs since they must be connected to the brain in order to communicate auditory, visual, and tactile information. But a recent study, carried out on tadpoles seems to indicate that it's possible to use the eye without it being directly connected to the brain.

In a previous study, D.J. Blackiston, K. Vien and M. Levin succeeded in demonstrating that eyes grafted to the outside of the tadpole’s head were sensitive to light. However, visual tests were disappointing, since innervation failed in most of the grafts. In order to better understand how to improve innervation, the researchers from the Allen Discovery Center at Tufts University took embryo eyes from Xenopus tadpoles and grafted them to the tails of blind tadpoles. They administered zolmitriptan to some of the tadpoles. The drug activates serotonin receptors (associated with neuron development) and is currently used to treat migraines. Next, various experiments were carried out to test the tadpoles’ ability to distinguish between different colors and follow optical patterns.

The first test consisted of placing the tadpoles in a sort of box (equipped with motion tracking cameras) in which they were able to swim. The box was divided into several spaces illuminated with either a blue or red light. By associating the red light with a slight electrical shock, the tadpoles learned to prefer the blue areas. The researchers found that 29% of the tadpoles that received the zolmitriptan managed to distinguish between blue and red, while only 11% of grafted, but untreated tadpoles succeeded at the task. It should be noted that 76% of the seeing tadpoles also succeeded at the test.

In another test, the scientists placed the tadpoles in a Petri dish (a transparent, cylindrical container) that was set on a computer screen picturing different groups of rotating triangles. The aim was to see whether the tadpoles were capable of forming an image using their vision (following the shapes). 80% of the tadpoles with normal vision succeeded. As in the previous test, among the blind tadpoles with an eye graft, those that did not receive the zolmitriptan performed more poorly (38%) when compared to the treated tadpoles (57%).

These results show that the grafted eye has the capacity to transmit sensory information without being directly connected to the brain. According to D. Blackiston, this suggests that: “the central nervous system contains a remarkable ability to adapt to changes both in function and connectivity.” This study brings new proof of the brain’s remarkable plasticity and its capacity for adaptation to changes in the body; in this case an eye attached to the spinal cord. In addition, it shows that zolmitriptan could stimulate the growth of neuronal connections (in tadpoles at least). New studies must be carried out to test its role in regenerative procedures involving organ transplants.

Finally, this research suggests that biomedical implants may not necessarily require a direct connection to the brain, limiting the risks of brain surgery.
Source: Blackiston, D., Vien, K., Levin M. “Serotonergic stimulation induces nerve growth and promotes visual learning via posterior eye grafts in a vertebrate model of induced sensory plasticity,” in npj Regenerative Medicine, March 2017.

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