It has been shown that visual awareness in the blind hemifield of hemianopic cats that have undergone unilateral ablations of visual cortex can be restored by sectioning the commissure of the superior colliculus or by destroying a portion of the substantia nigra contralateral to the cortical lesion (the Sprague effect). We propose that the visual awareness that is recovered is due to synchronized oscillatory activities in the superior colliculus ipsilateral to the cortical lesion. These oscillatory activities are normally partially suppressed by the inhibitory, GABAergic contralateral nigrotectal projection, and the destruction of the substantia nigra, or the sectioning of the collicular commissure, disinhibits the collicular neurons, causing an increase in the extent of oscillatory activity and/or synchronization between activities at different sites. This increase in the oscillatory and synchronized character is sufficient for the activities to give rise to visual awareness. We argue that in rodents and lower vertebrates, normal visual awareness is partly due to synchronized oscillatory activities in the optic tectum and partly due to similar activities in visual cortex. It is only in carnivores and primates that visual awareness is wholly due to cortical activities. Based on von Baerian recapitulation theory, we propose that, even in humans, there is a period in early infancy when visual awareness is partially due to activities in the superior colliculus, but that this awareness gradually disappears as the nigrotectal projection matures.
Copyright 2000 Academic Press.