The parieto-collicular pathway: anatomical location and contribution to saccade generation

The monkey lateral intraparietal area (LIP), involved in reflexive shifts of visual attention, has two main oculomotor outputs: towards frontal oculomotor areas and towards the superior colliculus. Recent studies suggest that these two outputs do not carry similar information. Direct LIP-collicular neurons would convey visual signals providing the oculomotor system with on-line visuo-spatial information. Parietal visuo-spatial information regarding internal stimuli would access the brainstem oculomotor circuitry through a parieto-frontal network. Consequently, an interruption of parieto-tectal neurons should affect reflexive saccades towards unpredictable targets and have little or no effect on saccades towards predictable or memorised stimuli. In order to test this hypothesis in humans, we have determined in rhesus monkeys the location of LIP-tectal fibres in the region of the internal capsule, and found that these neurons travel in the most posterior region of the posterior limb of the internal capsule. We have then tested, in seven patients with a small lesion involving this region, several oculomotor paradigms designed to determine the influence of spatial predictability on saccade accuracy and the ability to withhold reflexive saccades. In all patients, saccade accuracy was affected in unpredictable conditions but was normal when target location could be predicted or memorised. Reflexive saccade inhibition was affected only in the three patients in whom the capsular lesion had the most anterior extent. These results therefore support in humans the hypothesis that parieto-tectal neurons (i) transmit an on-line signal that is used by the oculomotor system for reflexive saccade triggering, (ii) are not crucial for the computation of internally guided saccades and (iii) are not crucial for reflexive saccade inhibition.