Developmental surface dyslexias - Naama Friedmann

cortex 44 (2008) 1146–1160 1147
the current study we focus on developmental surface dyslexia
and show that at least three different loci on the lexical
route can be impaired, causing three different subtypes
of developmental surface dyslexia, which differ in the patterns
of performance in various tasks.
When the lexical route is unavailable, readers may be
forced to rely on the grapheme-to-phoneme route for oral
reading. Looking more closely into what can cause the lexical
route to be unavailable, several possible loci of impairment
emerge: one might be the orthographic input lexicon
or the access to it ( in Fig. 1). Another possibility is that
the orthographic input lexicon itself is intact and accessible,
but its output is damaged: either its output both to
the phonological output lexicon and to the semantic system
(marked in Fig. 1), or only the output to the phonological
lexicon (marked in Fig. 1), with intact access to the semantic
system.
Grapheme-to-phoneme reading can also result from an
impairment to the semantic system, to the phonological output
lexicon, or its access to the phonemic output buffer (Jackson
and Coltheart, 2001) but in these cases, surface dyslexia is
part of a more general language or semantic impairment, and
is not restricted to reading, and we will not explore these subtypes
in the current study.
The three impairments (1–3 in Fig. 1) are expected to
yield a similar pattern of reading aloud. Since all three impairment
loci cause reading via conversion rules, all three
impairments should result in regularizations in reading
aloud. However, importantly, the three impairments are
expected to differ with respect to their effect on lexical decision
and comprehension. The first impairment – involving
lack of access to the orthographic input lexicon, or impaired
orthographic input lexicon 1 – would result both in
difficulty in lexical decision and in impaired comprehension.
Namely, this deficit will result in inability to determine
whether a letter string forms an existing word,
especially when it can be sounded out via grapheme-tophoneme
conversion as an existing word. The comprehension
in this type of impairment will rely solely on the
phonological output of the grapheme-to-phoneme conversion.
Thus, words that are regular and nonhomophonous might
be understood correctly, but irregular words and homophones
would either not be recognized as a word and
thus not be understood, or be recognized as a different
word and misunderstood. For example, if a word like
‘‘yacht’’ would be read as yakt, the reader might say that
she does not recognize the word. A word like ‘‘sale’’,
when identified solely on the basis of the phonological lexicon,
might be defined as ‘‘To move along the sea with
a boat’’, and a word like ‘‘too’’ might be defined as ‘‘The
number after one’’.
Individuals with an impairment of the second subtype –
impaired connection from the orthographic input lexicon to
both the phonological output lexicon and the semantic
1 Ascribing a deficit to the orthographic input lexicon still leaves
the question open whether the deficit is in processes operating in
and on the lexicon, or whether the representations in the lexicon
are impaired (for example, ‘‘faded’’, and require additional activation
to be accessed).
semantics
orthographic-visual analysis:
letter identification letter position letter-word binding
2
orthographic
input lexicon
2
1
3
phonological
output lexicon
phonemic buffer
grapheme-to-phoneme
conversion
Fig. 1 – A model of single word reading. The numbers
indicate possible loci of impairment that lead to
developmental surface dyslexia.
system – are expected to be able to decide whether a letter
sequence is a word or not, even when it is a pseudohomophone,
as they have access to the orthographic input lexicon.
Because they do not have access to semantics, they
will fail in comprehension in much the same way as the
first subtype.
Finally, individuals with an impairment of the third subtype
– whose impairment results from a disconnection between
the orthographic input lexicon and the phonological
output lexicon – are expected to perform well both in lexical
decision tasks and in comprehension tasks, when these tasks
do not involve oral reading. This is because they have intact
access to the orthographic input lexicon and from it to the
semantic system. Only when they read aloud will their surface
dyslexia be manifested, because they will be forced to
use the grapheme-to-phoneme route. 2
Thus, in order to determine the locus of impairment for
each individual with surface dyslexia, reading aloud is not
enough. The performance in lexical decision, and specifically
the ability to reject pseudohomophones (nonwords
that can be sounded out like real words, such as fone), would
indicate whether the orthographic input lexicon is accessible.
Comprehension tasks of homophones might speak for
whether or not there is access to the semantic system
from the orthographic input lexicon (Marshall, 1984a).
In a seminal study of acquired surface dyslexia, Coltheart
and Funnell (1987) identified seven loci that might
lead to acquired surface dyslexia. They showed that their
2 Theoretically, when the pathway between the lexicons is impaired,
one can also use the route from semantics to the phonological
output lexicon in order to read aloud. However, whereas
this route is the natural route for word retrieval, it does not
seem to be a natural route for reading, and this is why the arrow
appears dashed in Fig. 1. It seems to be a last resort, used only
when no other route is available for reading, as is the case in
deep dyslexia. We will return to this point in the Discussion, on
the basis of our results.
1