Glucose utilization in human visual cortex is abnormally elevated in blindness of early onset but decreased in blindness of late onset.

Glucose utilization has been studied in the visual cortex of blind human subjects, by comparison with normal volunteers, using positron emission tomography. In 6 subjects who became blind early in life ('early blindness'), metabolism in visual cortex was elevated, comparable to that of normal subjects studied with the eyes open. By contrast, glucose utilization in visual areas of 6 human subjects who became blind after completion of visual development ('late blindness') was decreased, slightly lower than in normal volunteers studied with the eyes closed. This unexpected difference between early and late blind subjects might reflect the persistence, in early blindness, of supranumerary synapses which would escape the normal developmental decrease in synaptic density during infancy.