Human eyeballs constantly are on the move. They shift, in tandem, in regular saccadic rhythms at a rate of 3-4 redirections per second. Saccades can be voluntarily, but “for the most part operate well below the level of conscious awareness.” Most everyday saccades are only a few degrees in size. Gilchrist and Findlay (2003: 5) state that:

Orienting movements larger an about 20 degrees are normally achieved by a combination of head and eye movements and for very large re-orientation, trunk and whole body movements also take place.

Saccadic redirections are referred to as “ballistic.” They are aimed in advance of their execution. For Gilchrist and Findlay (2001: 88):

No useful vision occurs during these movements and information intake is restricted to the successive fixations of the eye, pauses of a fraction of a second… The important questions become: What information is taken in at each fixation? How is this information integrated with that from preceding and subsequent fixations? How are the scanning movements planned and orchestrated?

In a landmark study of ophthalmoplegia (an extremely rare condition), Gilchrist, Brown and Findlay (1997) confirmed that their subject, who reported “no major visual problem associated with her deficit,” used movements of the head to compensate for a complete lack of eyeball mobility. By rapid fine adjustments to the neck she scanned texts in an entirely “saccadic manner.” Gilchrist, Brown and Findlay describe normal saccadic eye movements while reading as:

...alternating between short fast movements and fixations, where the eye is stable... Steady fixation is maintained between saccades, typically lasting between 200 and 250 ms. It is during fixation that information is gathered from the text.

 

Gilchrist, I. D., Brown, V. & Findlay, J.M. (1997) Saccades without Eye Movements. Nature 390, 130-131.

Findlay , J. M. and Gilchrist, I. D. (2001). Visual Attention: the Active Vision Perspective. In M. Jenkins and L. Harris (Eds). Vision and Attention. Springer Verlag.

John M. Findlay and Iain D. Gilchrist (2003) Active Vision: The Psychology of Looking and Seeing. Oxford University Press. Oxford, UK.