This day we’ll be delving into the neuroscience of eye movements. There are basically two different “types” of voluntary eye movement, so I’ll start with the definitions:
Saccades: rapid, intermittent movements as the eye fixes on different points in the visual field (watch video)
Smooth Pursuits: tracking of a moving object at a steady, coordinated velocity (watch video)
So this is where things get interesting. It turns out that – at least in most people – the ability to initiate and maintain smooth pursuits requires that there actually be an object to pursue. In other words, you can’t just “will yourself” to smoothly pursue an imaginary object across the room. Instead of smooth motion, your eyes will produce a series of rapid saccades.
(I’ll wait while you try it.)
You can scroll down to my “sources” for a more complete explanation, but here’s the short version:
Pursuit movements are often portrayed as voluntary but their basis lies in processes that sense retinal motion and can induce eye movements without active participation. The factor distinguishing pursuit from such reflexive movements is the ability to select and track a single object when presented with multiple stimuli. (1)
But wait! There’s more! It turns out that there are a few exceptions to this rule.
For example, humans are capable of prematurely initiating smooth pursuit if they anticipate that an object is about to move (i.e. watching a diver prepare to jump into a pool). They can also briefly maintain pursuit if the object momentarily vanishes (i.e. watching an airplane that moves behind a cloud).
Even more astonishing is the fact that a human can maintain smooth pursuit by tracking the motion of his own finger…even if he’s in a pitch-black room, unable to actually see his hand. (2) Just think about that for a minute.
I can think of a few other instances where higher-level cognitive functions interact with more primitive reflexes to produce bizarre results. But this has to take the cake.
(1) Barnes GR. Cognitive processes involved in smooth pursuit eye movements. Brain and Cognition, 2008, 68(3):309-326.
(2) Gauthier GM and Hofferer JM. Eye tracking of self-moved targets in the absence of vision. Experimental Brain Research, 1976, 26(2):121-139.