Behavioral/Systems/Cognitive Functional Identification of a Pulvinar Path from Superior Colliculus to Cortical Area MT

The idea of a second visual pathway, in which visual signals travel from brainstem to cortex via the pulvinar thalamus, has had consid-erable influence as an alternative to the primary geniculo-striate pathway. Existence of this second pathway in primates, however, is not well established.Amajorquestioncentersonwhether thepulvinar acts as a relay, particularly in thepath fromthe superior colliculus (SC) to themotionarea inmiddle temporal cortex (MT).Weusedphysiologicalmicrostimulation to identifypulvinarneuronsbelonging to the path from SC to MT in the macaque. We made three salient observations. First, we identified many neurons in the visual pulvinar that received input from SC or projected to MT, as well as a largely separate set of neurons that received input from MT. Second, and more importantly, we identified a subset of neurons as relay neurons that both received SC input and projected to MT. The identification of these relay neurons demonstrates a continuous functional path from SC toMT through the pulvinar in primates. Third, we histologically localizeda subset of SC–MTrelayneurons to the subdivisionof inferiorpulvinarknown toprojectdensely toMTbut also localizedSC–MT relay neurons to an adjacent subdivision. This pattern indicates that the pulvinar pathway is not limited to a single anatomically defined region. These findingsbringnewperspective to the functional organizationof thepulvinar and its role in conveying signals to the cerebral cortex