Two monkeys are able to "see" and recognise letter shapes generated by arrays of electrodes implanted in their visual cortex rather than relying on light hitting their retina. It is the highest resolution achieved with implants in the brain, rather than the retina. "That's really good news," says Pieter Roelfsema at the Netherlands Institute for Neuroscience, whose team aims to restore some vision to people who have lost their sight. Many research groups around the world are working on restoring some sight in people who are blind by sending signals from a head-mounted camera to arrays of electrodes that stimulate the appropriate nerve cells. There have been numerous trials in people already, and one 60-electrode device, called the Argus II, was approved for use in the US in 2013.

For nearly 100 years, we have understood the idea that it might be possible to restore sight to those who have become blind through a device that delivers electrical stimulation to the brain [Mirochnik, Pezaris, 2019]. Visual prostheses, as they are called, form part of a constellation of approaches that seek to deliver input to the brain to replace a lost or missing sense, including cochlear implants for the deaf, and cortical implants for the insensate, such as amputees with robotic arms. The challenges faced by each approach are similar: biological compatibility, long-term functional stability, and interpretability of the evoked sensations. Biological compatibility has thus far been addressed by careful selection of materials and implant techniques, but much remains to be done to create devices that the body will tolerate for decades with a low risk of infection or rejection. The first major challenge is long-term functional stability; ensuring that the effectiveness of the devices do not degrade over time.