Spatial neglect and paradoxical lesion effects in the cat - A model based on midbrain connectivity

Spatial visual attention in humans and mammals is severely disrupted after deactivation of specific cortical or subcortical regions. Such dysfunction has frequently been associated with impaired unilateral cortical mechanisms. Alternatively, I propose that essential mechanisms of spatial attention are based on the bilateral competition between subcortical midbrain structures. Consequently, visuospatial hemineglect and hemi-extinction arise from unbalanced bilateral competition. I present a simple mathematical model, based on known connectivity of the cat midbrain, that exemplifies the principle of competition through mutual inter-hemispheric inhibition. The model represents a linear, topographic integration of several cortical and subcortical inputs in the two halves of the feline superior colliculus. Such a system reproduced a variety of neglect symptoms and also explained the paradoxical effects observed in some lesion experiments where the consequences of a primary lesion could be reversed through a secondary lesion in structures of the contralateral hemisphere. The model generated predictions for future experiments.