This talk will challenge the alleged double-dissociation between optic ataxia and visual agnosia or the use of optic ataxia to argue for a specific vision-for-action occipito- posterior parietal stream. Clinical cases reveal that perceptual deficits have been long shown to accompany ataxia. Importantly the term ataxia appears to be misleading as patients exhibit a combination of visual and non-visual perceptual deficits, attentional deficits and visuo-motor guidance deficits, which are confirmed by experimental approaches. Three major features of optic ataxia are described. A spatial feature whereby the deficits exhibited by patients appear to be specific to peripheral vision, akin to the field-effect.
Visuo-motor field examination allows to quantify this deficit and reveals that it consists of a highly reliable retinocentric hypometria. A temporal feature whereby these deficits are exacerbated under temporal constraints, i.e. when attending to dynamic stimuli. These two aspects cumulate in a situation where patients have to quickly respond to a target presented in peripheral vision that is experimentally displaced upon movement onset. In addition to the field-effect, a motor feature, also called hand-effect can be described in conditions where the hand is not visible. Spatial and temporal aspects as well as field- and hand-effects may rely on several posterior parietal modules that remain to be precisely identified both anatomically and functionally. It is concluded that optic ataxia is not a visuo-motor deficit and there is no dissociation between perception and action capacities in optic ataxia, hence a fortiori no double dissociation between optic ataxia and visual agnosia.