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Gary McGraw, John Rehling & Robert Goldstone. (1994) Letter Perception:
Toward a conceptual approach. In the Proceedings of the Sixteenth Annual
Conference of the Cognitive Science Society, pages 613-618, Atlanta, GA,
Author of the summary: Patrawadee Prasangsit, 1999, firstname.lastname@example.org
The actual paper is online.
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Conceptual-level representations of letter parts, namely roles, are important
in human's process of letter recognition.
Analogical reasoning occurs at roles and spatial relations among them.
This paper shows that roles play an important part in letter perception.
It presents the results of a simple experiment in lowercase letter recognition.
The concept, for instance, lowercase-'a' can be thought of as a component
composed of two smaller ideas: (1) the idea of a curved umbrella-handle-like
bar on the right, and (2) the idea of a small c-like curve on the left.
These two conceptual components, which are called roles, are not
explicit shapes per se but are ideas about what the acceptable bounds for
letter-part shapes are, how far such shapes can be stretched before they
lose their interpretation, and how they interact with other roles to form
a complete object. Roles and relationships between roles make up
the internal structure of a letter category. Category membership
at the whole-letter level is determined by category membership at the lower
level of roles.
Slippage involves allowing certain descriptions in a mental representation
to "slip, " or be replaced by related descriptions according to contextual
pressures brought to bear by the situation.
Each letter is formed by a set of short line segments, called quanta,
on a fixed grid of dimension 3x7. See figure 3.
Subjects are presented with a number of lowercase letters, one at a
time, both in normal and styled shapes (called NORMALS and FONTS, respectively),
and asked to identify them. Based on the results, the authors make
Average reaction time increases while accuracy decreases, which can be
explain as follows. Difficulty is directly determined by how easy
a given letterform can be broken into parts that fill roles. Complicated
letterforms are ambiguous (thus accuracy decreases) as well as take longer
to parse into roles (thus reaction time increases).
NORMALS letterforms are far more easily recognized than FONTS. Figure
4 and 5 show results in terms of reaction time and accuracy.
The number of quanta does not alone determine its difficulty.
"Blurred prototypes" of letters are created by averaging the quanta lists
of NORMALS and FONTS letters in a given category together.
A hierarchical cluster (figure 7) is created using the Euclidean distances
between the blurred prototypes.
When compare this cluster with the errors made by human, they do
not agree. For instance, 'e' and 's' are very closely related in
the hierarchical cluster (hence probably have higher chance of being mistaken
as one another) but errors made by human mistaking one as another is only
This contrary suggests that human's process of recognition has little to
to do with low-level information such as quanta. Literal physical
properties are often less important in similarity judgments than are higher-level
Some pair of letters look similar but their higher-order concepts are too
different for people to miscategorize one as another. On the other
hand, some pair of letters have similar concepts that people tend to slip
them them since human perception is flexible. In fact, all of the
top human errors in the experiments can be explained by conceptual slippage
(such as, unclear role fillers).
Conclusion: Results from the experiments provide evidence for the existence
of conceptual-level representations of letter parts, namely roles.
The authors intend to incorporate such representations into a computer
model of letter recognition.
A plot of the number of categories in which each token was found versus
its average reaction time. They strongly correlate. As the
number of categories increase, so does the reaction time.
This result can be used to classify the tokens into two sets: those with
ambiguous role fillers (i.e., few possible categories) and those with strange
role fillers (i.e., large number of possible categories). The latter
takes longer reaction time. When a token is ambiguous, its parts
can fill two or more sets in a fairly reasonable fashion and hence takes
subjects some time to decide. As more choices are considered, it
takes longer. When a token is strange, its parts do not really fit
any sets of roles well, so subjects make a guess after an extended period
Summary author's notes:
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Last modified: Wed May 12 01:02:17 EDT 1999