Brain Activity in Amblyopia

Amblyopia is a disorder of visual processing in the brain. We employed a nuclear medicine technique, positron emission tomography (PET), to study visual processing activity in the brains of three deeply amblyopic adults (corrected acuity ≤ 20/200) and two normally sighted control subjects during monocular viewing of a dramatic motion picture. Multiple PET scans were performed while varying the viewing conditions in each subject. Using the intravenous tracer 18F-2-deoxyglucose to observe relative cerebral glucose metabolism as an indicator of local brain activity, we found that activity in primary visual cortex of a control subject was similar during viewing with either normal eye. In two amblyopic patients, activity in primary visual cortex was 5–6% less during stimulation of the amblyopic than the sound eye. Optical blur (−20/200) reduced activity in primary visual cortex by about 8% in two control subjects. In one amblyopic patient, a combination of amblyopia and optical blur reduced activity in primary visual cortex by 23% relative to the sound eye. Subjects and patients often exhibited left-right asymmetries in the processing activities of the cerebral hemispheres during visual stimulation of sound or amblyopic eyes, with the contralateral hemisphere generally more active than the ipsilateral. Either amblyopia, or optical blur of a normal eye, altered the degree of asymmetry.These results indicate that PET is a useful new tool for the study of amblyopia in humans, and suggest that both amblyopia and optical blur reduce the amount of visual information processed in primary visual cortex. Although the major effect of deep amblyopia is on primary visual cortex, other cortical areas are probably also affected.