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I. Retinal Events, Perceptual Consequences, and the Illusion of a Stable and Continuous World


Eye movements cause two types of retinal events:

1. The image of an object in the world changes its retinal location from one fixation to the next. For example, an object may lie somewhere off the fovea in your peripheral vision, but after a saccade, it will suddenly lie on the fovea, and what was on fovea has now moved off into your periphery of your retina.   Usually, only objects imaged on the fovea are "seen" in enough detail to allow you to recognize what they are.

2. During the saccade, you are, essentially, blind.  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 on the retina at this speed, the resultant retinal event is a blurring or smearing of the image (Grusser, 1972). In short, saccades represent a short "blank" period where no meaningful visual information processing occurs.

Perceptual Consequences:

Despite a retinal image that is shifting around with each saccade, despite the visual system taking in discrete "snapshots" of world, each separated by a small "blank" period of time when we are essentially blind, despite all this, our visual experience is one of smooth, uninterrupted visual continuity.  We feel we are seeing the entire scene all of the time.  The constant, stable environmental structure that we perceive must somehow be derived from, but can only be partially representative of, a sensory input stream that is noisy and discontinuous.  Perception, it seems, is a process designed to represent the "permanent possibilities of sensation(Mill, 1865).



The current challenge in cognitive-visual psychology is explain how our visual system derives from a series of seemingly disconnected and chaotic retinal events a perception of the world that is constant, stable and continuous.