From single to multiple deficit models of developmental disorders

402 B.F. Pennington / Cognition 101 (2006) 385–413
4.2. Interactive processing
Another multiple deWcit view of dyslexia comes from connectionist models of
single word reading. These models have certainly produced surprises for single
deWcit theories of dyslexia syndromes. When Hinton and Shallice (1991) modeled
deep dyslexia, the a priori prediction was that semantic errors would be
produced by damage to a particular component of the network. Instead, it turned
out that damage practically anywhere in the network produced such errors because
of the interactive nature of the processing involved in going from print to
semantics.
A similar conclusion was reached for errors in reading pseudowords and exception
words, respectively, in the application of connectionist models to dyslexia subtypes
(Manis, Seidenberg, Doi, McBride-Chang, & Petersen, 1996). Each error type
is the hallmark of a diVerent dual-process subtype of developmental dyslexia (i.e.,
phonological vs. surface) and is thought to reXect damage to distinct mechanisms
(e.g., Temple, 1985a, 1985b). What Manis et al. (1996) demonstrated was that
although the network could be altered in diVerent ways to produce each subtype,
these alterations did not produce clean dissociations. An alteration that mainly
aVected exception word reading also impacted the accuracy of reading pseudowords
and vice versa. Once again the interactive nature of network dynamics is
responsible.
Drawing on this connectionist work, Snowling and colleagues (Snowling, 2002;
Snowling et al., 2000) have proposed a multiple deWcit model that has many similarities
to the multiple deWcit model proposed here. Based on the connectionist
models of normal and abnormal reading developed by Seidenberg and colleagues
(Harm & Seidenberg, 1999; Plaut, Seidenberg, McClelland, & Patterson, 1996;
Seidenberg & McClelland, 1989), these researchers emphasized the interactive
nature of reading development. In their view, a strength in semantic codes may
oVset a phonological deWcit, so it is the interaction of multiple cognitive factors
that determines whether one has dyslexia. They also used this model to understand
poor comprehenders, who are normal at decoding single printed words but
impaired in reading comprehension, and to criticize the severity hypothesis discussed
earlier because it only focuses on one component of the several that interact
in reading.
4.3. Unitization and automaticity
The double deWcit model (Bowers, 1993; Wolf & Bowers, 1999) proposes a nonphonological
cause of RD. Similar to other subtyping models, this model proposes
two single cognitive deWcit subtypes of RD, the familiar phonological deWcit subtype
and a new, naming-speed deWcit subtype. Dyslexics with both deWcits have a “double
deWcit” and are predicted to have more severe reading problems than those found in
either single deWcit subtype, although the model is not explicit about whether the
joint eVects of the two deWcits are additive or interactive. It is well documented that
groups with RD perform poorly on measures of rapid serial naming and that such