Behavioral/Systems/Cognitive Signals Conveyed in the Pulvinar Pathway from Superior Colliculus to Cortical Area MT

Wepreviously established a functional pathway extending from the superficial layers of the superior colliculus (SC) through the inferior pulvinar (PI) to cortical areaMT in the primate (Macacamulatta). Here, we characterized the signals that this pathway conveys to cortex by recording frompulvinar neurons thatwe identified bymicrostimulation as receiving input fromSCand/or projecting toMT.Thebasic properties of these ascending-path PI neurons resembled those of SC visual neurons. Namely, they had brisk responses to spots of light, inhibitory surrounds, and relatively large receptive fields that increasedwith eccentricity, aswell asminimal presaccadic activity. Beyond these basic properties, there were two salient results regarding themodulatory andmotion signals conveyed by this ascending pathway. First, the PI neurons appeared to convey only a subset of the modulations found in the SC: they exhibited saccadic suppression, the inhibition of activity at the time of the saccade, but did not clearly show the attentional enhancement of the visual response seen in SC. Second, directional selectivity was minimal in PI neurons belonging to the ascending path but was significantly more prominent in PI neurons receiving input fromMT. This finding casts doubt on earlier assumptions that PI provides directionally selective signals to MT and instead suggests that PI derives its selectivity fromMT. The identification of this pathway and its transmitted activity establishes the first functional pathway from brainstem to cortex through pulvinar and makes it possible to examine its contribution to cortical visual processing, perception, and action