Higher-Level Cognition - Computational Explorations in Cognitive Neuroscience - page 382

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something like figure 11.7 when the test is complete —
because it is not possible to label the lines in the simu-
lation, you should rely on the figure to decode what is
going on. The two solid lines represent the color nam-
ing conflict and congruent conditions. The two dotted
lines represent the word reading conflict and congruent
conditions - these lines basically fall on top of one an-
other and so are not labeled separately, but the conflict
condition line is slightly higher at the earliest SOA's.
By comparing the simulation data with the human
data shown in figure 11.4, you can see that the model's
performance shows both commonalities and contrasts
with the behavioral data. We first consider the com-
monalities. The model simulates several important fea-
tures of the behavioral data. Most importantly, the
model shows that word reading is relatively impervious
to color conditions (conflict vs. congruent), even when
the colors precede the word inputs, as indicated by the
similarity of the two dotted lines in the graph. Thus, the
dominant effect in the model is a strength-based compe-
tition — the word reading pathway is sufficiently strong
that even when it comes on later, it is relatively unaf-
fected by competition from the weaker color naming
pathway. Note that there is some evidence of an inter-
ference effect (congruent vs. conflict difference) in the
model for the earliest color SOA's — there is also a hint
of this in the behavioral data.
Another important feature of the human data cap-
tured by the model is the elimination of the interfer-
ence effect of words on color naming when the color
precedes the word by a relatively long time (right hand
side of the graph). Thus, if the color pathway is given
enough time to build up activation, it can drive the re-
sponse without being affected by the word.
There are two important differences between the
model and the human data, however. One difference is
that processing is relatively slowed across all conditions
as the two inputs get closer to being presented simulta-
neously. This is particularly evident in the two word
reading conditions and in the congruent color naming
condition, in the upward slope from -20 to 0 SOA, fol-
lowed by a downward slope from 0 to 20. This effect
can be attributed to the effects of competition — when
inputs are presented together, they compete with one an-
other and thus slow processing. This may be an artifact

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Conflict

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140−

Color Naming

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Congruent

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80−

Word Reading

0123456789 0

−16 −8 0 8 16

SOA (cycles)

Figure 11.7: GraphLog results of the SOA test in the model.
Compare this pattern of results with those for people as shown
in figure 11.4. See text for a discussion of the comparison.

SOA Timing Data

Another important set of data for the model to account
for are the effects of differential stimulus onset times
Glaser and Glaser (1982) discussed previously. To im-
plement this test in the model, we simply present one
stimulus for a specified number of cycles, and then add
the other stimulus and measure the final reaction time
(relative to the onset of the second stimulus). We use
five different SOA (stimulus onset asynchrony) values
covering a range of 20 cycles on either side of the si-
multaneous condition. For word reading, color starts
out preceding the word by 20 cycles, then 16, 12, 8,
and 4 cycles (all indicated by negative SOA), then color
and word are presented simultaneously as in standard
Stroop (0 SOA), and finally word precedes color by
4, 8, 12, 16, and 20 cycles (positive SOA). Similarly,
for color naming, word initially precedes color (nega-
tive SOA), then word and color are presented simulta-
neously (0 SOA), and finally color precedes word (pos-
itive SOA). To simplify the simulation, we run only the
most important conditions - conflict and congruent.

To run the SOA test, first View SOATEST_LOG , and
then press the SOA_Test button. You may want to turn
the network Display off.
You will see that the lower graphlog has been re-
placed with a new one, which displays the reaction time
as a function of SOA on the X axis.

, !

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