fill in the missing word

[Michael Thomas]

> In the Thomas et al. paper, we were purposely replicating other work
> to make a direct comparison. But we did use two different tasks to
> compare the results.  There is also the general issue about what lack
> of generalisation to nonce terms in atypical populations really means.
> Annette

Just to reinforce Annette's comments, in our work with children and adults
with Williams syndrome, we were concerned that aspects of the task demands
in the past tense elicitation task might differentially affect our
atypical and control populations.

Our initial aim was to replicate work by Harold Clahsen and Mayella
Almazan (Cognition, 2000) who used a past tense elicitation task on a
small sample of children with WS. Our aim was to replicate these findings
with a larger sample size, so we stuck to their stimuli and procedures. To
address our concerns about task demands, we also we included a past tense
elicitation procedure developed by Lorraine Tyler and William
Marslen-Wilson for use with adults with aphasia. In contrast to the "fill
in the missing word" procedure, this task required no repetition of
previous words, and participants were prompted in their response by the
provision of the initial sound of the word. We called this procedure
"finish the word I started":

The participant was told "I'm going to say a sentence, and then I'll start
another one and stop in the middle.  Your job is to finish off the word
that I've started."

For example: "The bull sometimes kicks. Yesterday, it k ___"

This is similar to the procedure used by Jean Berko Gleason, (although
compared to her procedure, we omit an intervening question, i.e. The bull
sometimes kicks. What did the bull do yesterday? Yesterday it ___ ) As
Jean comments, putting the required word at the end of the sentence may
provide a context which helps to constrain the response to the inflected
form of the verb.

By incorporating a second task, we were then able to examine how the
respective task demands affected the responses, and whether any effects
differed between atypical and control groups. The results showed a slight
advantage in performance on existing regular and irregular verbs in the
"finish the word I started" task over the "fill in the missing word" task,
but no interaction with participant group.

The interesting result here was in the pattern of errors, which showed up
most clearly on irregular verbs. For both the participants with Williams
syndrome and the typically developing children (our sample here was 6 year
olds through to 10 year olds), the "fill in the missing word" procedure
produced predominantly zero marking errors, whereas the "finish the word I
started" procedure produced predominantly over-regularisation errors. Thus
the task demands appeared to alter the pattern of errors from one of
omission to commission, while the level of correct performance was broadly
similar between the two tasks.

This is consistent with Brian's suspicion that the form of the task itself
may encourage zero marking errors. As Brian points out, the level of zero
marking errors is of theoretical importance in studying disorders such as
Williams syndrome and Specific Language Impairment, so evidence of the
role of task demands is significant here. However, currently we are not
clear whether the higher level of zero marking errors in the "fill in the
missing word" task was due to that task's greater memory load (i.e.,
repetition of a sentence fragment prior to offering the verb form), or due
to a context that constrained a past tense response less strongly than in
the "finish the word I started" procedure.

In addition, we found that this task demand worked *differentially* on our
atypical and control groups. The shift from zero marking to
over-regularisation errors was characteristic of the younger control
children. However, participants with Williams syndrome persisted in
showing this pattern at verbal mental ages at which one would expect it to
disappear. That is, they persisted with an immature pattern of response to
the task demands.

Unfortunately, we weren't able to compare task effects on the production
of nonce terms, since our second elicitation task comprised a larger set
of regular and irregular verbs to allow examination of frequency and
imageability effects. We do think it is important to collect data on the
effect of task demands on nonce terms, because we believe it is still an
open question about how one interprets failure to generalise to nonce
terms in atypical populations, and in particular, atypical populations
with lower IQs.

We also agree with Brian's suggestion that a comparison of question
answering and sentence completion paradigms would offer an important
clarification, especially in the light of ours and Brian's initial
evidence for the effect of task demands in these elicitation tasks.

If anyone would like further details of the error data produced by our
Williams syndrome and control populations, I'd be happy to provide them.
(Incidentally, if you're trying to replicate these results, it's worth
noting the statistical difficulties that arise in comparing
non-independent proportions of error types!)

cheers,

Michael


~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Dr Michael Thomas
Senior Research Fellow
Neurocognitive Development Unit
Institute of Child Health
30 Guilford Street
London WC1N 1EH,  U.K.
tel:      +44 (0)20 7905 2747
fax:     +44 (0)20 7242 7717
http://www.ich.ucl.ac.uk/units/ncdu/NDU_homepage.htm
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