The visual system, like the other sensorial systems, is subjected to intrinsic, complex control, originating both in the retina (CHANG et al., 1959; ARDUINI and HIRAO, 1960; STERIADE, 1967) and in the visual cortex (BUSER et a/., 1963; JASSIK-GERSCHENFELD and ASCHER, 1963; MEULDERS, 1965), which regulates its input at various levels of the specific pathways. However, the visual system is also influenced by subcortical structures which, though not exerting on it a strictly selective control, determine notable modifications in the level of excitability of the cortical sensorial neurons. It is in fact we11 known that activation of the mesencephalic reticular formation, by increasing the level of cortical excitability, produces a facilitation of the potentials evoked in the visual cortex (BREMER and STOUPEL, 1958 and 1959; DUMONT and DELL, 1958 and 1960; BREMER et al., 1960; FUSTER, 1961; FUSTER and DOCTER, 1962). On the other hand, electrical or chemical stimulation of the ventral pontine reticular formation inhibits the potentials evoked in the specific areas of the cerebral cortex and chemical inactivation of the same pontine area causes their facilitation (COURVILLE et al., 1962; DEMETRESCU and DEMETRESCU, 1962b). Again referring to subcortical control of the sensorial afferences, there is however the more recent demonstration of an inhibitory action by the caudate nucleus on activity evoked in the areas of specific projection (DEMETRESCU and DEMETRESCU, 1962a; DEMETRESCU et al.,