Background. Capacity limitation theories have proved to
be surprisingly resilient in characterizing
some of the cognitive deficits in schizophrenia. However, this perspective
has not generally been
applied to short-term verbal memory tasks. We explored this issue by
first attempting to ascertain
if gross misallocations of processing resources might explain
impairments in short-term memory in
schizophrenia on a classic digit span task and in a second study
by attempting to determine what
effects delay and memory set size had on a divided attention short-term
verbal memory paradigm.
Methods. In the first study 16 patients with schizophrenia
and 21 normal controls received 40 trials of
a three digit task and 20 trials of a six digit span task. As the absolute
number of digits presented
and duration of presentation in two conditions were identical, subjects
thus had equivalent
‘opportunities’ to make errors if distraction, in the
sense of misallocation of cognitive resources,
were at the root of poor performance. In the second study 15 patients with
schizophrenia and 15
normal controls were tested in conditions in which two, four or six words
were presented and in
which rehearsal was prevented by an interference task (colour naming)
for delays of 5, 10 or 15 s.
Results. Patients had disproportionate difficulty on the
six digit rather than the three digit condition,
suggesting that deficits in the verbal working memory short-term store
may not be the result of
attentional factors. In the second study, patients' performance
was differentially worsened by the
interference task, by memory set size (i.e. a capacity limitation) and
by delay, a measure of decay rate.
Conclusions. In concert, these studies demonstrate that
schizophrenia patients have difficulties on
verbal short-term memory span tasks not because of misallocation of
resources, but rather because
of limitations in ‘representational capacity’ and
maintenance of information over delays.