Retrieval of words from the lexicon in people with aphasia: an information-theoretic approach.

Retrieving words from the lexicon by people with aphasia: an information
theoretic approach.
E. van Ewijk, MSc (Universiteit Utrecht)

Background
Word finding difficulties are a common and very frustrating element of aphasia.
Research over the last thirty years has investigated which categories of words
are more difficult or easier for people with different types of aphasia. For
people with Broca*s aphasia verbs (in addition to words from functional
categories) are particularly difficult (Berndt et al, 1997; Kim and Thompson,
2000). Recent research shows that this difficulty with verbs may not be
restricted to patients with Broca*s aphasia: patients with Wernicke*s patients
have been shown to have similar difficulties. When verbs are produced by
patients with aphasia, they often lack inflection (e.g. he walk, instead of he
walks). Interestingly, not all types of inflections are equally difficult to
produce. What is still unclear, is why some verb forms are more difficult than
others. Some researchers claim that in aphasia grammatical knowledge is lost
due to damage to the brain (Friedmann and Grodzinsky, 1997), whilst others
suggest grammatical knowledge is intact but is difficult to access and activate
due to reduced processing capacity in these patients.
Recent research in healthy adults has shown that the complexity of words and
word families is an important factor in the processing time required to
retrieve these words from the lexicon (Moscose del Prado Martin, Kostic and
Baayen, 2004). This research uses statistical measures from information theory
to calculate complexity of specific word forms. For each inflected verb form a
complexity measure can be created. The higher the value of the complexity
measure, the longer it takes for normal adults to process the verb form. In
healthy adults this additional processing time is very small and goes unnoticed
in conversation. The capacity of the normal brain is by far sufficient to
process a verb of any complexity measure without errors. However, aphasic
patients are claimed to have reduced processing capacity. It could therefore be
suggested that those verb forms with higher complexity values are the verb
forms that are more difficult (and sometimes impossible) for these patients to
produce.

Goal
This research project aims to we examine possible reduced processing capacity
in people with aphasia. We will investigate whether complexity of word forms,
as calculated by information theoretical measures, can provide an explanation
for the unequal difficulties patients experience with different verb forms. In
addition to investigating language skills, we will also use a visual task to
look at whether this reduced processing capacity is also visible in other
cognitive modalities.

Procedure
For this project we will recruit people with aphasia (Wernicke*s and Broca*s).
In addition to these two patient groups we will recruit a group of healthy
controls. The participants will take part in 3 experiments. Experiment 1
involves an auditory lexical decision task. In this task participants listen to
verbally presented words and have to decide whether the word they hear is a
real word or not. In experiment 2 participants are asked to name a series of
pictures depicting verbs. Experiment 3 is a visual task in which patients are
asked to search for a pre-specified goal in a visual display.
All experiment will be programmed on a touch screen laptop, which means the
whole experimental set up is mobile and participants can be tested on any quiet
location.

Possible hazards for patients
There is no risk involved for the participants. The experiments are cognitive
and absolutely non-invasive. There are two sessions of up to an hours conducted
in two weeks.

Final comments
These experiments will provide more insight into the question of whether
complexity of words (expressed in a quantitative measure) plays an important
role in aphasic patients* difficulties to retrieve word forms from the lexicon.
This information will lead to better understanding of aphasia and has
implications for theories of aphasia as well as clinical application.