Higher-Level Cognition - Computational Explorations in Cognitive Neuroscience - page 383

of the kWTA form of competition, as it was not found
in the original Cohen et al. (1990) model.
Another difference, which was present in that model,
is the increasingly large interference effect for earlier
word SOA's on color naming in the model, but not in
people. It appears that people are somehow able to re-
duce the word activation if it appears sufficiently early,
thereby minimizing its interfering effects. Cohen et al.
(1990) suggested that people might be habituating to the
word when it is presented early, reducing its influence.
However, this explanation appears unlikely given that
the effects of the early word presentation are minimal
even when the word is presented only 100 msec early,
allowing little time for habituation. Further, this model
and other models still fail to replicate the minimal ef-
fects of early word presentation even when habituation
(accommodation) is added to the models.
An alternative possibility is that the minimal effects
of early word presentation reflect a strategic use of per-
ceptual (spatially mediated?) attentional mechanisms
(like those explored in chapter 8) that can be engaged
after identifying the stimulus as a word. According to
this account, once the word has been identified as such,
it can be actively ignored, reducing its impact. Such
mechanisms would not work when both stimuli are pre-
sented together because there would not be enough time
to isolate the word without also processing its color.

Reduce the pfc_gain parameter from .8 to .75,
and do a Test .
You should see that the model is now much slower
for the conflict color naming condition, but not for any
of the other conditions. This is exactly the same pat-
tern of data observed in frontal and schizophrenic pa-
tient populations (Cohen & Servan-Schreiber, 1992).
Thus, we can see that the top-down activation coming
from the PFC task units is specifically important for the
controlled-processing necessary to overcome the prepo-
tent word reading response. Note that to fit the model
to the actual patient reaction times, one must adjust for
overall slowing effects that are not present in the model
(see chapter 8 for a discussion of how to compare model
and patient data).
Although we have shown that reducing the PFC gain
can produce the characteristic behavior of frontal pa-
tients and schizophrenics, it is still possible that other
manipulations could cause this same pattern of behavior
without specifically affecting the PFC. In other words,
the observed behavior may not be particularly diagnos-
tic of PFC deficits. For example, one typical side ef-
fect of neurological damage is that overall processing is
slower — what if this overall slowing had a differential
impact on the color naming conflict condition? To test
this possibility in the model, let's reduce the dt_vm pa-
rameter in the control panel, which determines the over-
all rate of settling in the model.

, !

Restore pfc_gain to the default value of .8, and
then reduce dt_vm from .01 to .008, and do a Test .

Effects of Frontal Damage

, !

Now that we have seen that the model accounts for sev-
eral important aspects of the normal data, we can as-
sess the importance of the prefrontal (PFC) task units
in the model by weakening their contribution to bias-
ing the posterior processing pathways (i.e., the hidden
layer units in the model). The strength of this con-
tribution can be manipulated using a weight scaling
( wt_scale ) parameter for the connections from the
PFC to the Hidden layer. This parameter is shown as
pfc_gain in the control panel, with a default value of
.8. Because the model is relatively sensitive, we only
need to reduce this value to .75 to see an effect. Note
that this reduction in the impact of the PFC units is func-
tionally equivalent to the gain manipulation performed
by Cohen and Servan-Schreiber (1992).

Question 11.3 Compare the results of this overall
slowing manipulation to the PFC gain manipula-
tion performed previously. Does slowing also pro-
duce the characteristic behavior seen in frontal and
schizophrenic patients?

Go to the PDP++Root window. To continue on to
the next simulation, close this project first by selecting
.projects/Remove/Project_0 . Or, if you wish to
stop now, quit by selecting Object/Quit .

, !

11.3.3 Summary and Discussion

The Stroop model demonstrates how the frontal cortex
might contribute to controlled processing by providing