Importance of the visual cortex for postural stabilization: inferences from pigeon and frog data.

Abstract

Optokinetically induced self-motion with its consequences for postural balance is based upon visual-vestibular convergence. It is a matter for speculation which visual pathways--subcortical accessory optic tract and/or cortical striate projection--convey optokinetic information to the central vestibular system. The functional significance of the visual cortex was tested by a behavioral approach in two animals, selected for their different evolutionary stage: frog (midbrain visual center); pigeon (primitive neocortical center). Lateral postural sway during optokinetic stimulation in roll served as a measurement for induced rollvection and apparent body tilt. Roll motion elicits a tonic 'compensatory' postural adjustment towards the direction of pattern motion in pigeon as in man but not in frog. From the lack of this reaction we infer that the frog does not perceive rollvection because it has no visual cortex. This agrees with the absence of visual-vestibular convergence in the frogs vestibular nuclei neurons as well as the absence of a nystagmus velocity storage in the brainstem. The animal experiments fit human data in hemianopic patients who also only experience rollvection when stimulated in the unaffected hemifield.