II. Questions, Theories, and the Nature of Scientific Explanation

The demonstration showed what we should see given the retinal events that occur while we look around the world.  Specifically we saw:

1. Discontinuous "snapshots" (fixations) of the scene,
2. Fixations interspersed with saccades, i.e., retinal image smearing or blurring (though this was not shown in the demonstration), and
3. Retinal displacement with each saccade, i.e., as different objects moved in and out of your foveal region, the entire scene appeared to jump.

Why don't we perceive saccadic retinal events (events 2 and 3)?
Two issues need to be addressed:

1. during a saccade, why is retinal motion not perceived as motion in the world, and
2. from one fixation to the next, why is the displacement of the light pattern on the retina not perceived as displacement in the world?

With regard to the first issue, researchers have made great gains in understanding the mechanisms involved in the suppression of a perception of motion during a saccade.  Specifically,  when the image of the world sweeps across the retina during a saccade, velocities of up to 400 degrees/sec can be reached.   Because the retinae cannot successfully process the detailed features of the stimulus at this speed, any perception of such a shifting retinal image is physically impossible (Grusser, 1972). At the same time, selective suppression of the magnocellular visual pathway suppresses the resultant, fast, low spatial-frequency retinal image blur that accompanies the saccade  (Burr, Morrone, & Ross, 1994).

Unfortunately, with regard to the second issue, we do not have comparable understanding of the mechanisms involved in the suppression of the perception of displacement, and this issue will not be explored here. (For more about this second issue, see  Bridgeman, van der Heijden, & Velichovsky, 1994; McConkie & Currie, 1996).