We recorded the responses of 189 medial superior temporal area (MST) n
eurons by using optic flow, real translational movement, and combined
stimuli in which matching directions of optic flow and real translatio
nal movement were presented together. One-half of the neurons (48%) sh
owed strong responses to optic flow simulating self-movement in the ho
rizontal plane, and 24% showed strong responses to translational movem
ent. Combining optic flow stimuli with matching directions of translat
ional movement caused substantial changes in both the amplitude of the
best responses (44% of neurons) and the strength of direction selecti
vity (71% of neurons), with little effect on which stimulus direction
was preferred. However, combining optic flow and translational movemen
t such that opposite directions were presented together changed the pr
eferred direction in 45% of the neurons with substantial changes in th
e strength of direction selectivity. These studies suggest that MST ne
urons combine visual and vestibular signals to enhance self-movement d
etection and disambiguate optic flow that results from either self-mov
ement or the movement of large objects near the observer.