• Choose a category
• All
• Classic-Fiction

By Root 1063 0

fraction of a second, we can briefly see it afterward in the mind’s eye. In 1967 Neisser used the term “icon” for this very brief visual memory, measured its duration as about half a second (later research reported as little as a quarter of a second) to two seconds, and found that it is erased if a new pattern is presented before it has faded.45 Other vision researchers then suggested that since the eye sweeps across the field of vision or follows moving objects in a series of jumps known as “saccades,” it sees nothing while moving but at every momentary stop sends an iconic snapshot to the brain. The snapshots are assembled there into a perception of motion, somewhat as if one were watching a movie.46

This hypothesis was widely accepted in the 1970s and the early 1980s. But some leading investigators began to doubt that the icon, observed only under unnatural laboratory conditions, exists in normal perception; if it does not, the saccadic-iconic hypothesis of movement perception collapses. As Ralph Haber saw it:

Such presentations have no counterpart in nature, unless you are trying to read during a lightning storm. There are no natural occasions in which the retina is statically stimulated for less than about a quarter of a second, preceded and followed by blankness… There is never a fixed snapshot-like retinal image, frozen in time, but rather a continuously changing one… The icon was born in the laboratory, and it has life only there and nowhere else.47

The screen of the eye is not a photographic emulsion, and moving images on it are not captured, unblurred, in the form of stills. Rather, the retina is a tissue made up of millions of receptors, each of which, when stimulated, fires many times per second. As an image moves across the retina, a continuous flow of impulses from a series of receptors proceeds to the visual cortex. There is no blur, because the system generates not a series of stills but an unbroken stream of changing information.

And indeed, a dramatic discovery about movement perception, made four decades ago, was that some neurons in the retina and in the visual cortex fire in response to movement but that many others do not; the detection of movement begins at the single-cell level. This ancient evolutionary development helps prey avoid being eaten, and also helps predators locate and seize their prey. A frog will efficiently snap at any small moving object but starve to death if presented only with dead flies or worms, which it does not perceive as food;48 many other simple predators show similar behavior. The frog’s retina and brain apparently have neurons that respond to movement (and size), a capacity that has more survival value than other aspects of vision.

In the 1960s and 1970s, Hubel and Wiesel, among others, demonstrated the existence of movement detectors. They showed, when recording the activity of single cells in cats and monkeys by the microelectrode technique, that in both the retina and the visual cortex certain cells, and only these, respond strongly to movement. Some, in fact, fire in response only to movement in a particular direction, others only to movement in the opposite direction.49

Other investigators confirmed this by entirely different methods. In 1963 Robert Sekuler and a colleague projected an image of a grate moving upward, established the threshold (minimum speed) at which each human subject could see it moving, and then had each one look steadily at the moving image. After several minutes, subjects could no longer see the movement when the grate was crawling at the original threshold speed, although they could still do so when the speed was doubled, and could see downward motion at the slower speed. The results indicated that there were upward-motion detectors, which had become fatigued, and downward-motion detectors, which had not. Comparable results were obtained in reverse when the subjects watched a downward-moving grating for several minutes.50

Most of us have experienced movement-detector fatigue without knowing its neural basis. If we look steadily for some