A blind man can see again after being given a stem cell transplant.
Cells were transplanted into the eye
Mike May, of California, had been blind for 40 years since an accident at the age of three where he lost one eye and was blinded in the other.
During that time he had some ability to perceive light, but could not make out form or contrast.
He said he had no visual memories from his early childhood.
The operation transplanted corneal and limbal stem cells into his right eye.
The cornea is the clear part of the outer layer of the eye that covers the iris and the pupil. The limbus is the thin area that connects the cornea and the sclera, the white part of the eye.
Researchers followed his recovery in order to study how Mr May learned to see again.
They measured areas such as how he perceived shapes, his spatial awareness and how he saw 3D images.
When he was tested five months after surgery, the patient was able to perceive slight movements of a bar and was able to recognise simple shapes.
Two years after the surgery, Mr May was able to see form, colour and motion almost normally.
However, his 3D perception and face and object recognition was still severely impaired.
Mr May could only identify around a quarter of common objects shown to him.
And he was also only able to tell if an unfamiliar face was male or female 70% of the time.
His perception of motion was found to be the most well-preserved visual faculty.
But Mr May was not fully comfortable with his newly gained sight.
Before the operation he had been a keen skier, using verbal directions as a guide.
But after he recovered his sight, he was frightened he would crash into something.
Over two years, he has learnt to use shading patterns on the snow to estimate the shape of the slope.
Mr May is also nervous of crossing the road, where he was confident of doing so while blind.
He said: "The difference between today and over two years ago is that I can better guess what I am seeing. What is the same is that I am still guessing."
The details of Mr May's case are published in the journal Nature Neuroscience.