Smooth pursuit-like eye movements during mental extrapolation of motion: the facilitatory effect of drowsiness.

We studied eye movements during mental extrapolation of motion. Subjects tracked with the eyes a target oscillating sinusoidally by +/- 5 degrees on the horizontal plane at frequencies between 0.15 and 0.5 Hz. After 4 cycles the target disappeared and subjects had to mentally extrapolate its motion in imagery for 5 further cycles. Subjects were invited to relax. We recorded eye movements with infrared oculography and the lid aperture with a webcam. Typically, in the imagery phase, sequences of saccades "reproduced" the macroscopic characteristics of the to-be-imagined motion. However, slow eye movements substituting the sequence of saccades emerged repeatedly in the course of the experiment, to the extent that sometimes it was almost impossible to distinguish the pattern of eye movements during tracking from that during imagery. These smooth pursuit-like eye movements lasted up to 17 s, although they were more commonly intermingled with saccades. It turned out that this phenomenon occurred mostly, although not exclusively, when the lids were partially lowered suggesting an association with drowsiness. The average gain of smooth pursuit-like eye movements in imagery passed from 0.09 when the lids were open, to 0.27 when they were partially lowered. A control experiment excluded that the lowering of the lids per se had played any important role. This finding has implications for both the physiology of the oculomotor system and for the debate on the relation between imagery and perception.