Contralateral tectal projections from single areas of the visual cortex in the cat.

1. The projections from cat cortical visuals areas to the contralateral superior colliculus (SC) were studied by the autoradiographical tracing method. Microinjections of L- [5-3H] proline were carried out in cortical visual areas 17, 18, 19 and the lateral suprasylvian visual area (LS) in different cats. Only one cortical area was injected in each animal. Survival times of 30 hours or 8 days were allowed. 2. Areas 17, 18, 19 and LS send projections to laminae, I, II and III of the frontal pole of the contralateral SC. Areas 19 and LS project also the contralateral pretectal nuclei, mainly to the posterior pretectal nucleus. 3. Cortical fibres reaching the contralateral SC pass through the brachium of the ipsilateral SC. They run along the caudal part of this structure, cross the midline and then run along the caudal part of the contralateral SC; finally, they turn anterolaterally and reach the rostal part of SC. This pathway is the tectal semidecussation (5). Cortical fibres from LS (in particular from PLLS; 15) reach the contralateral SC also via the commissure of SC. 4. Our data support the suggestion (3) that the frontal poles of SC, in the cat, may subserve straight-ahead attention and orientation to visual stimuli.