Attention is known to exert at least three effects on MT neurons (Treue & Maunsell, J Neurosci 1999). When two competing stimuli, one preferred and one non-preferred, are simultaneously presented within the receptive field (RF), attention to the preferred stimulus increases the response (effect 1), and attention to the nonpreferred stimulus decreases the response (effect 2A). In addition, attention to a remote stimulus outside the RF also decreases the response (effect 2B). Recent evidence suggests these attentional effects may be mediated via feedback projections to visual cortex from oculomotor structures, e.g. the SC (Gattass & Desimone, SFN 1992) and the frontal eye fields (Moore & Fallah, PNAS 2001; Moore & Armstrong, Nature 2003). To test this idea further, we examined the effects of subthreshold microstimulation of the SC on the visual responses of MT neurons. We stimulated the SC (50 msec trains, biphasic pulses, 333 Hz, current < eye-movement threshold) while recording MT activity from an awake, f...