The visual span has a width of 9 letters across the diameter of the central clear region. There are 4-letter-wide zones of low-resolution peripheral vision on each side of the visual span. The line of text at the top shows what Mr. Chips is trying to read. In the examples on this page, Mr. Chips is reading the text across the diameter of the circle, as indicated by the solid black line across the page. Mr. Chips could just as easily read anywhere within the 2-D retinal area. As he identifies words, they appear in italics below his moving retina. Stars (*) indicate the presence of unrecognized letters. You are seeing repetitions of a sequence of 6 fixations.
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| Animation by J. Stephen Mansfield & Timothy S. Klitz |
Notice that the saccades in this sequence have a regular pattern, jumping approximately 9-10 characters on each saccade. On the sixth fixation, Mr. Chips has partial information about the word; he knows its length and two of the three letters "ol.". From his lexicon, he can determine that only the word "old" can fit this pattern.
This example uses a retina with a visual span of 9 letters across the diameter of the central clear region with a three-letter-wide scotoma at the center. Notice that the central scotoma also extends into the clear region above and below the line of text that Mr. Chips is currently reading.
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| Animation by J. Stephen Mansfield & Timothy S. Klitz |
Notice the presence of a regression in this sequence, that is a short leftward saccade. On the sixth fixation, Mr. Chips has partial information about the word; he knows its length and three of the four letters ".hat". From his lexicon, this could be "what" or "that." To resolve this uncertainty, he has to make a regression to uncover the first letter of the word.
Regressions are often considered a mark of poor reading, but this example shows that an Ideal strategy results in regressions. In fact, an ideal observer with a central scotoma makes substantially more regressions than an ideal observer with intact central vision. Recent research by Mark Bullimore & Ian Bailey (University of California at Berkeley) has shown that human readers with central scotomas also make an unusually large number of regressions.
This example uses a retina in which islands of vision alternate with scotomas across the diameter of the visual field. In addition, there are also randomly scattered islands of vision within the field of view.
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| Animation by J. Stephen Mansfield & Timothy S. Klitz |
In the repeating sequence, notice that the saccades have a distinctive long-short, long-short pattern. The long saccade shows half the letters of a new part of the text, and then the short saccade brings the other half into view.
This unusual long, short, long, short, pattern of saccades is quite abnormal. Nevertheless, it's optimal for a retina with this pattern of scotomas.
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| Animation by J. Stephen Mansfield & Timothy S. Klitz |
In the three previous examples, there was no saccade noise. In this fourth example, there is Gaussian error associated with the landing positions. Although Mr. Chips takes this noise into account in planning saccades, he sometimes runs into severe difficulty. In this example, when he is trying to resolve the ambiguity in "wo.l." with his Swiss Cheese retina, he needs to identify the 'r' to distinguish "world" from "would". He hunts around rather frenetically for several fixations trying to resolve the ambiguity, but the saccade noise keeps thwarting him. Eventually, he gets the word and moves on. He runs into the same problem at the end of the line when he needs to identify the 'd' to distinguish "any" from "and" when he has the ambiguity "an.". Mr. Chips is a little bit luckier in this case and gets the 'd' in relatively few saccades.
These four examples show the emergence of regressions, abnormal saccade strategies, and hunting saccades. Analogous behavior is sometimes observed in people with visual-field loss. Is this behavior maladaptive for people with low vision? Not necessarily. The performance of Mr. Chips indicates that such behavior may represent a good, or even ideal eye-movement strategy, linked to the pattern of field loss.
Send questions about Mr. Chips to legge@umn.edu
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Last updated Monday, August 17, 1998.
©Copyright 1997. Minnesota Laboratory for Low-Vision Research.
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