[Hemianopsias in cortical and subcortical lesions of the visual system: what is the difference?]

The neuronal correlate of perception in the visual area remains unimpaired in lesions of the optic tracts or the radiatio optica. Hemianopsias due to lesions of these pathways are perceived immediately. On the contrary, hemianopsias due to a lesion of the visual cortex are not perceived or are only indirectly realized. It has not been established if a lesion of the primary visual area alone is sufficient to make the patient unaware of a hemianopsia or if additional impairments of the peristriate visual areas are required. Data processing within the cortical visual projection chain makes it likely, however, that a lesion of the striate cortex is enough. Under physiological conditions an uniformly illuminated field is represented in the neuronal activity by the borders of the field. Neurons within the field are not activated. The higher visual cortical areas can so not discriminate if an input missing from V1 is a result of the physiological processing or of a lesion. Scotomas are therefore not perceived. By analogy this argument should also hold for a hemianopic "scotoma".