Why can't our eyes smoothly transition from side to side without focusing on a moving object?
Short answer
Smooth pursuit is driven by retinal slip, which is determined by external input registered peripherally in the retina.
Background
The smooth pursuit system is a system designed to minimize retinal slip, i.e., the movement of an image across the retina. Saccades are rapid eye movements meant to fixate quickly on a new target (Purves et al., 2001).
Both movements are readily apparent when sitting in a train and staring outside; the eyes focus on a distant object and pursue that object. Once it exits the field of view, the eyes target a new object via a saccade and smooth pursuit is again initiated (the train window phenomenon).
Saccades can be triggered both involuntary (due to a sudden, unexpected stimulus) and voluntarily (Walker et al., 2000).
The smooth pursuit system cannot be voluntarily controlled, barred in some highly trained observers (Purves et al., 2001). Instead, it is guided by eye movements dependent on the movement (slip) of the fixated target on the fovea (Thier &amp; Uwe, 2005). Hence, this system is controlled by external stimuli, not by internal ones. Retinal slip can of course be voluntarily manipulated by moving the eyes away from the target, for example by moving the head. The smooth pursuit system detects this by registering retinal slip, and keeps the eyes fixated on the target, through smooth pursuit which is involuntary.
References
- Purves et al., Neuroscience, 2nd ed. Sunderland (MA): Sinauer Associates (2001)
- Thier &amp; Uwe, Curr Opinion Neurobiol (2005); 15: 645–52
- Walker et al., Exp Brain Res (2000); 130: 540–4