Frontal lesion effects on
verbal fluency in a network model.
Previous work on neural network modeling of frontallesion
effects in neuropsychological tests is extended to the verbal fluency test.
In this test, the subject is asked to say all the words he or she can think
of beginning with a specified letter in a minute, and the test runs over
three minutes with three different initial letters. Damage to the
left frontal cortex interferes with this task in three ways: reducing the
attention the subject pays to the task as a whole; increasing the likelihood
that the subject will violate the rules, for example by repeating words
or using proper names; and increasing the number of perseverative errors,
which consist in this case of the subject saying words that started with
a previously used but currently incorrect letter. A simple neural
network, whereby frontal damage is mimicked by lowered gain of reinforcement
signals, is constructed that reproduces all these effects.