Nature Neuroscience

How much does sight depend on visual experience? A stem cell transplant allowed a blind man to regain his sight after 40 years—and gave researchers an unusual opportunity to address this question. In the September issue of Nature Neuroscience, Ione Fine and colleagues report on MM, a man who became blind in an accident at three and a half years of age, then had his sight restored by surgery at age 43.

Two years after the surgery, MM's simple form, color and motion processing were essentially normal, but his three-dimensional perception and object and face recognition were severely impaired. For example, he had no difficulty recognizing simple shapes such as a square or circle, but he could only identify 25% of common objects that were presented to him, and he was able to tell whether an unfamiliar face was male or female only 70% of the time. His motion perception was perhaps the most well-preserved visual faculty, and functional imaging showed that a motion processing area of his brain was activated normally.

Like other sight-recovery cases, MM was not fully comfortable with his new sight after the operation. He was an expert skier as a blind person, but immediately after surgery he would close his eyes while skiing, as the visual information frightened him. Since his operation, he has learned to interpret visual images more effectively, but says that he is "still guessing" at what he sees in many cases.

Nature Neuroscience

Perhaps you really can't teach an old neuron new tricks. The plasticity of the nervous system is known to decrease with age. What is the underlying basis for this loss of flexibility? In the September issue of Nature Neuroscience, Jeff Lichtman and colleagues suggest an answer.

They studied naturally occurring rearrangements of axon terminals in the nervous system of young and old mice engineered to express a fluorescent protein. This allowed them to image individual nerve terminals over weeks and in some cases, years. They find that in adolescent mice, axon branches undergo rapid growth and retraction within minutes. However, synapses became progressively more stable as animals age, with a significant decrease in synaptic terminal remodeling by middle age. This finding suggests that the connections between neurons may become less flexible as animals age.