Developmental surface dyslexias - Naama Friedmann

cortex 44 (2008) 1146–1160 1155
The first subtype of developmental surface dyslexia, input
surface dyslexia, was a deficit that related to the orthographic
input lexicon. As a result, the eight individuals who had this
type of developmental surface dyslexia were forced to read
via the grapheme-to-phoneme conversion route, and therefore
made regularization and potentiophone errors in reading.
Because the orthographic input lexicon was inaccessible to
them, they also failed on lexical decision, and given that the
orthographic input lexicon was inaccessible, they could not
reach the semantic system from reading, and hence made errors
in comprehension of homophones and potentiophones.
Frequency effects on reading aloud indicate that the deficit
of the participants in this group was in the orthographic input
lexicon rather than in the access to it. For one of the participants
in this group, this conclusion was supported by his
poor performance also when the words were spelled aloud
to him. 9
The second subtype of developmental surface dyslexia
that was witnessed in the current study, orthographic lexicon
output surface dyslexia, was characterized by unimpaired
orthographic input lexicon, but impaired connection from
it to the next stages: phonological output lexicon and the
semantic system. As a result, the three individuals who
had this surface dyslexia subtype read aloud via the sublexical
route, and hence made regularization and potentiophone
errors, but still could identify the correct spelling
of written words, and choose between the correct spelling
of a word and its pseudohomophone. However, because
the access from the orthographic input lexicon to the semantic
system was impaired, they could not use their intact
lexical knowledge to access the correct meaning of
homophones and potentiophones, and had to access meaning
in an indirect way: they read the words via the sublexical
route, and the input to semantics was the phonological
result of this conversion. This led to failure in the homophone/potentiophone
comprehension task.
The third subtype of developmental surface dyslexia
identified in this study, interlexical surface dyslexia, resulted
from disconnection between the orthographic input lexicon
and the phonological output lexicon. Because of this disconnection,
the six individuals with this subtype of developmental
surface dyslexia had to read via the sublexical
route, which caused regularization and potentiophone errors
in reading aloud, but when they did not have to reach
the phonological output lexicon, in tasks of lexical decision
and comprehension, they performed at a normal level. That
is to say, because they had access to the orthographic input
lexicon, they could identify the correct spelling of written
words, and because they had access from the orthographic
input lexicon to the semantic system, they could also access
the meaning of lexical items from the orthographic input
lexicon, and therefore their comprehension was intact
9 When discussing developmental surface dyslexia, it is hard to
imagine how an intact orthographic input lexicon would develop
when the access to it from visual analysis is impaired. Thus, it is
unlikely to find such a deficit in developmental surface dyslexia,
of impaired access to the orthographic input lexicon, but with intact
orthographic input lexicon. Such a deficit might be more
clearly manifested in acquired dyslexia.
even for homophones and potentiophones. Their impairment
was in the connection between the orthographic
and the phonological lexicons rather than in the phonological
output lexicon itself, as indicated by their good naming
performance.
One other pathway could theoretically be employed by the
participants with the interlexical disconnection – because the
semantic system is accessible to them, they could have proceeded
to oral reading via the pathway from semantics to the
phonological output lexicon. However, the finding that none
of these participants made even a single semantic or morphological
paralexia indicates that they did not use this route
for oral reading. A possible conclusion is that the route for
reading aloud via the semantic system is only employed as
a last resort, when neither the direct route nor the sublexical
route is available for reading. This is the case in deep dyslexia,
where this is the only route available, and reading via
it yields many semantic and morphological errors.
The fact that this distinction was found in developmental
dyslexia suggests that the orthographic input lexicon
can develop even when its output is impaired, and that developmental
dyslexias can exhibit selectivity in impairment,
similar to acquired dyslexias. It is interesting to note that
the selective impairment of the participants reported in
the current study is even more selective than that reported
for adults with acquired surface dyslexia. First, whereas for
some participants with acquired surface dyslexia the sublexical
route was not completely intact (Coltheart, 2006),
the five participants in the current study who were tested
in nonword reading read them well, indicating a good sublexical
route.
Furthermore, apart from surface dyslexia that results
from a deficit at the orthographic input lexicon, the two subtypes
that were reported in the literature included either
a deficit to the phonological output lexicon or a semantic
deficit (Ellis et al., 2000). Whereas the individuals reported
in studies of acquired surface dyslexia (Bub et al., 1985; Graham
et al., 1994; Howard and Franklin, 1987; Kay and Ellis,
1987; Kay and Patterson, 1985; McCarthy and Warrington,
1986; Patterson and Hodges, 1992; Shallice et al., 1983) had
general lexical or semantic–conceptual deficits, not only in
reading, the participants in the current study had no aphasia,
and only one of them had mild naming difficulties.
This leads to an important difference between the subtypes
of surface dyslexia described in the literature until now, and
the subtypes we describe in the current study. In the current
study two new subtypes were found, which involved the
connections between components rather than a deficit to
the components themselves. 10 One subtype results from
a deficit in the connections between the orthographic input
lexicon and both the semantic system and the phonological
10 We do not think that the fact that subtypes of acquired surface
dyslexia reported until now involved damage to the lexicon or the
semantic system whereas the subtypes in the current study of
developmental surface dyslexia did not involve lexical and semantic
impairment, relates to some deep difference between acquired
and developmental dyslexia. There is no reason to assume
that the two types of surface dyslexia we described here that result
from damage to the connections from the orthographic input
lexicon cannot occur in acquired surface dyslexia as well.