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Vision and the Visual System

Vision and the Visual System:

Dynamic displays in the book by Peter H. Schiller and Edward J. Tehovnik entitled Vision and the Visual System published by Oxford University Press.

The twelve displays are listed according to the chapter number in which they are described in the book. Clicking on each will activate the display. For each display a brief description is provided below:

Dynamic Display

The Dynamic Display 11-1 shows a movie of a subject with the 13 light spots walking. Although the information is limited to just 13 moving points, just about everyone can infer a walking human being. These findings highlight how incredibly effective our visual system is in deriving shape information from motion cues.


In Dynamic Display 11-2, if a viewer sees see-saw motion initially, prolonged viewing can lead to a shift resulting in the perception of zig-zag motion. By varying the composition of this display, one can gain further insights about the principles of apparent motion.


When viewed in Dynamic Display 11-3, with an array consisting of 16 quartets arranged in four rows and four columns, viewers will perceive the same direction of movement in all the quartets. After prolonged viewing a switch will occur in all of the units. Thus there appears to be some general feedback circuits in the visual system to create a unity in perception that applies even to apparent motion.


When viewed in Dynamic Display 11-4, a display in which the relative position and the size of the quartets is varied reveals that there is still a unity in the direction of motion perceived in all the quartets and that when there is a switch it occurs at the same time in all of the units.


In Dynamic Display 11-5, viewers will see that color cues fail to disrupt the unity of apparent motion in the display; thus it appears that color information is not utilized as a significant cue.


In Dynamic Display 11-6, when using shape cues in the display, viewers will notice that the unity of motion and the unity in switching persists. This once again suggests that the midget system, which can readily detect small differences in shape, does not play a significant role in the perception of apparent motion.


In Dynamic Display 11-7, by systematically varying size differences among the dots, curves can be generated showing that small size differences are ineffective in breaking down the unity of apparent motion among the quartets displayed but that large differences. This reveals that the unity of motion direction breaks down under these conditions.


Dynamic Display 11-8 tests the role of the proximity of stimuli presented successively and shows that the unity of motion under these conditions is broken down. In some rows, horizontal motion is perceived whereas in other rows vertical motion is perceived. This indicates that proximity plays significant roles in how we perceive apparent motion.


Dynamic Display 11-9 considers the significance of proximity in the perception of the direction of motion, as seen in the movies and on TV. As can be seen, the spokes of the wheels of the chariot rotate backwards when a car or a chariot is moving forward. The movie camera takes rapid intermittent stationary shots of the wheel movement, and when the movie is displayed, the amount of movement of the spokes in successive frames depends on the speed of the rotation.

Because of the principle of proximity, under slow motion the wheel will be seen in the movie or on TV as rotating clockwise in synchrony with the forward movement of the vehicle. Under fast rotation conditions, however, when the spokes appear closest to the next spoke, the rotation is seen as occurring counterclockwise.

If the link at left does not work, click here for an AVI version of the movie. Right-click and save .AVI file locally if it will not play in browser.


In Dynamic Display 12-1, as an example of an experiment to assess the extent to which depth cues are integrated in the brain, viewers are exposed to random dot stereograms that are rocked back and forth. This display shows the depth created by motion parallax, demonstrating how effective motion parallax is in creating a depth impression based on motion parallax.


Dynamic Display 14-1 examines adaptation and color vision. This figure has a circular array of red Gaussian disks, and each of the dots is turned off in successive steps going clockwise. If fixation is maintained on the central cross, after a short period of time (20-30 seconds) a rotating green spot is perceived. The rule is that after adaptation to a colored stimulus, the afterimage created has the color located at the opposite side of the color circle from the initial image. In accordance with these colored afterimage rules, when the dots are made yellow, the rotating dot will be seen as having a blue color.


Dynamic Display 14-2 shows the “waterfall illusion,” an illusory effect due to motion. When the motion in the display is stopped, the impression created is that the water flows upward for a brief time period. The effect appears to be due to the adaptation that takes place in single neurons in the cortex that are directionally selective: those cells that are activated say by downward motion, as in the case of the waterfall illusion, gradually become less responsive over time. Conversely, those cells that respond to upward motion will briefly increase their firing rate once the motion is stopped. This then explains the reason for the illusory effect created when motion is induced for a prolonged time period.

If the link at left does not work, click here for an animated GIF version, or here for the original source.