Mental Rotation
The very existence of mental images has been controversial.
Shepard and Metzler[Shepard, R. N., and Metzler, J. (1971). Mental rotation of three-dimensional objects. Science, 171, 701-703.] showed that response time in matching rotated 3-D images is proportional to angle of rotation.
Georgopoulos et al.[Georgopoulos, A. P., et al., ``Mental Rotation of the Neuronal Population Vector'', Science, vol. 243, no. 4888 (13 Jan. 1989), pp. 234-236.] demonstrated a mental rotation in a monkey.