Visual Scanning Processes

Visual Scanning Processes

Visual Scanning Processes

Visual Scanning Processes

Excerpt

Vision for human beings, like audition for bats, is surely the principal channel through which information is acquired about the outside world, The physical and functional anatomy of the human eye, however, makes it impossible for information to be gathered equally efficiently from all parts of the surrounding world. Not only is the entrance angle of the eye limited to 180 degrees more or less, but there is also a reduction in resolution for objects or stimuli more than a few degrees from the line of sight. This reduction stems from three sources: 1) the retina has its greatest packing density of receptor cells around the center of the visual field; 2) these central receptor cells are connected on a one-to-one basis to corresponding cells in the cortex. Further away from the center of the visual field, the cells tend to become interconnected at ganglion layers immediately adjacent to the receptive surfaces; 3) there are refractive errors which degrade visual resolution away from the center of the visual field. As a consequence one does not see very well more than two or three degrees away from the line of sight. A human being must move the eyes either by moving the head, or by moving the eyes within the head, in such a way as to bring into the central part of the visual field whatever is to be examined in detail.

The human eye is thus constantly in motion under normal conditions. Part of this motion is the result of voluntary effort on the part of the observer who deliberately moves the point of regard from one place in the visual field to another. Most of the movements, however, are involuntary, and the observer is quite unaware of a good part of them. There is an in-between category where in the observer is aware that the eyes are moving, but mistakenly assumes they are making one kind of movement when in fact another is going on.

Since the eye is the most powerful channel that the brain has for taking up information, it is important to know where the eye looks and why, how long it looks there and why, what is there to be seen and where it looks next and why, if one is to understand something of what is happening in the brain.

Eye movements can be sorted into the large and small, and into the fast and slow. The large fast movements are called saccades. Slow small movements are called drifts. Large slow movements are called pursuit movements. Finally, fast small movements are called flicks or microsaccades. Even when the eye is fixated voluntarily there is residual movement. A typical eye movement record of a "fixated" eye will show a succession of random appearing drifts and flicks with the general position of the eye held fairly steady. On the average, the . . .

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