Mr. Chips: An Ideal-Observer Model of Reading


Legge, Klitz & Tjan (Psychological Review, 104, 524-553, 1997) have described an ideal-observer model for a simple reading situation. An ideal-observer model is an algorithm that yields optimal performance, given the constraints of the task. Mr. Chips is the name of a computer simulation that implements this ideal-observer model. 

The purpose of the model is to examine how basic visual, motor and cognitive constraints influence reading behavior in people with normal and low vision. Legge, Klitz & Tjan (1997) show how this model provides novel accounts of characteristic patterns of normal eye movements in reading, including the presence of regressive saccades, word skipping, and a preferred viewing position in words. The paper also describes how the model provides insights into the reading difficulties encountered by people with visual-field loss resulting from eye disease. 

We provide here a brief overview of the model followed by three examples of its behavior. Details can be found in Legge, Klitz & Tjan (1997).

Mr. Chips diagram
Cartoon of the ideal reader, illustrating the sources of information available and the optimizing principle. (Figure 1, Legge, Klitz & Tjan, 1997)

Mr. Chips reads texts that consist of words drawn at random from the dictionary. The model makes no use of syntax or semantics (yet). 

Mr. Chips's task is to read through the text in the minimum number of saccades, identifying all the words sequentially without error. 

Mathematically, Mr. Chips uses an entropy minimization principle (upper right in cartoon above). At any point, he may have partial information about the current word (some of the letters and/or word length information). He "makes the saccade that minimizes uncertainty (bits of information) about the current word." He can't leave the current word until he has unambiguously identified it (i.e. its uncertainty is zero.)

You can view four examples of short eye-movement (saccade) sequences from the model below. The first example illustrates the behavior of Mr. Chips with normal vision. The last three examples illustrate the behavior of Mr. Chips when his "retina" contains blind spots (scotomas). These animations were prepared by Steve Mansfield.

Normal Vision Example

Video file
Animation by J. Stephen Mansfield

This is an example of an ideal observer reading with normal, intact central vision. The white circle represents the reader’s visual span, which is 9 characters in width, while the grey ring represents 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. As he identifies words, they appear in italics below his moving retina. In this example, you are seeing a sequence of 6 fixations.

Central-Scotoma Example (Legge, Klitz & Tjan, 1997, Fig. 3)

The pattern, which starts in the video on the left, is a schematic of the retina. Similar to the example above, the visual span has a width of 9 letters, but with a three-letter-wide scotoma at the center. Like the example above, 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. As he identifies words, they appear in italics below his moving retina. Asterisks indicate the presence of unrecognized letters. You are seeing a sequence of 8 fixations.

Video file
Animation by J. Stephen Mansfield

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. 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.

Alcatraz Retina Example (Legge, Klitz & Tjan, 1997; Fig. 6)

This example uses a retina in which islands of vision alternate with scotomas. In 2-D, we could call it a Swiss cheese retina, but we refer to this 1-D version as the Alcatraz retina.

Video file
Animation by J. Stephen Mansfield


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.

Alcatraz Retina Example with Noisy Saccades (Legge, Klitz & Tjan, 1997; Fig. 14) 

Video file
Animation by J. Stephen Mansfield


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 Alcatraz 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.

These last three 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 [email protected]