Looking with the Head and Eyes (Gibson, 1979, Chapter 12)

In this chapter, Gibson places head and eye movements on an ecological footing, and makes them the activity of a distributed perceptual system (c.f. Gibson, 1966) who's job it is to look around. 

We have to look around because our heads get in the way of our eyes. Even animals with eyes on the side have this problem, but for animals with front-facing eyes over half the optic array that is ambient to a point of observation is occluded by the head (which of course also specifies the presence of the self). Looking around is about reversing that occlusion and revealing things as required. 

Gibson first proposes that we do not see the world with just the eyes; but with the visual perceptual system, which contains eyes in a head on a body on a ground. He then notes that there is the visual field (the current view) of the visual world (the ambient experience). The visual perceptual system supports the perception (not memory) of the entire environment that surrounds us by providing access to information about occlusion and by enabling the behaviours that move us through that environment. 

Eye movements come in a few well-defined flavours:

• Fixations. These are typically described as when the eye is still and acquiring an image to be joined together with previous images. Gibson reformulates them as a posture the eye can take, in which the eye is never completely still and the amount of motion is about supporting access to information for the task as hand. 
• Saccades. These are high-speed movements from one fixation to another. Gibson notes that saccades can vary in size and speed and blend into fixations, and so they are another posture the eyes can adopt. He also claims that while these postures relate to visual attention, they are not identical to it. 
• Pursuit. These are eye movements than entail fixating an object or location that is moving relative to the observer; the eye moves to maintain the fixation. Gibson calls this an adjustment to optic flow. 
• Convergence/divergence. The eyes can move separately, in and out with distance to the target. Gibson denies that there is any fusion of the two views happening anywhere; the binocular system overall adjusts to search for matching and mismatching perspective structure. Using two eyes at the same time provides access to some invariants that require the two views: "The human binocular system extracts the similarities of structure between two arrays, I suggest, just as each eye extracts the invariants of structure in its own array" (pg. 204). 
• Compensatory. Eyes also counter-rotate when the head rotates to maintain a view of relevant surface layout. 

Then he notes things such as blinking, lens focusing (accommodation) and pupil adjustments, as well as the dark adaptation of the retina. These are about maintaining a clear view of the array across a wide variety of conditions. 

The overall function of the visual system is to search for information in the optic array. Importantly, the function is not to convert a sequence of images into perception of a scene. The comparison of the eye to the camera is flawed, and to understand the anatomy of the eye we need to understand the information it is trying to access.