Differences Between Children with Dyslexia and Typically Developing
Readers in the Use of Executive Functions During the Reading of Isolated Words
and Sentences
“Bottom-up processes” are cognitive processes driven by the physical
stimulus. Decoding a word is an example of a bottom-up process, because it
begins with the stimulus itself. “Top-down processes” originate in the brain
and influence the way physical stimuli are analyzed. Reading a sentence
involves top-down processes. When we read the beginning of a sentence, we
generate expectations about how the sentence will continue, both in terms of
content and in terms of parts of speech, for example, whether a verb or an
adjective should come next. These expectations facilitate the technical process
of reading.
To what extent is reading comprehension related to top-down and
bottom-up processes? Kintsch’s
model, presented in 1988, proposed that reading comprehension begins with
bottom-up processing and is strongly influenced by top-down processes. Reading
comprehension includes two stages. The first is a “construction stage,” during
which the reader creates alternative mental representations of the meaning of
the text. The reader creates these representations using linguistic knowledge
and world knowledge. In the second stage, the integration stage, the reader
uses executive functions to integrate elements of the text.
People with dyslexia have impairments in executive functions such as
working memory, inhibition, and shifting. These impairments are evident both at
the behavioral level, in tests assessing these executive functions, and at the
neurological level. Individuals with dyslexia show reduced activation in the
anterior cingulate cortex when reading words. This region is associated with
executive functions. The frontoparietal system, which is also associated with
executive functions, has been implicated in improvements in reading among
children with dyslexia following an executive-function-based intervention.
Raya Meri, Rola Farah, and Tzipi Horowitz-Kraus, Israeli researchers
from the Technion who conducted this study, examined 20 children with dyslexia
between the ages of 8 and 12, with no history of another neurological or
psychiatric diagnosis such as ADHD, and 19 typically developing readers within
the same age range. The children were asked to read isolated words and
sentences during fMRI brain scanning, and they also completed reading tests and
tests assessing executive functions.
The executive-function tests included phonemic fluency, digit span, the
Stroop test, a symbol search test, the Wisconsin Card Sorting Test, and the
BRIEF questionnaire. Phonemic fluency, in which the child must say as many
words as possible beginning with a particular letter within one minute,
assesses executive functions because it requires a controlled search of the
knowledge store for words that meet the criteria. The digit span test assesses
executive functions in terms of working memory. The Stroop test assesses
executive functions in terms of inhibition. In this test, the child is required
to rapidly name, in sequence, the color in which words are printed. The words
themselves are names of colors that do not correspond to the colors in which
they are printed. To name the color, the child must suppress reading the word,
which is an automatic process. The Wisconsin Card Sorting Test assesses
executive functions in terms of shifting. In this test, the child is placed
into a “set,” that is, the child becomes adjusted to applying a rule according
to which the cards are sorted. The rule is then changed without warning, and
the psychologist assesses how quickly the child can adapt his or her responses
to the new rule. It is not clear to me why the symbol search test was included
among the tests of executive functions. It may have been included as a measure
of inhibition, because it requires rapid identification of the correct symbols
while suppressing responses to similar but incorrect symbols.
During the brain scans, the researchers examined the degree of
functional connectivity among four brain networks: two “top-down” networks
associated with executive functions and two “bottom-up” networks associated
with the orienting of attention. Functional connectivity refers to coordinated
activation between distant brain regions that share common functions.
Functional connectivity reflects the behavior of the brain network underlying
higher cognitive functions.
During the brain scan, isolated words were presented to the children. In
each trial, six words were displayed, and the child had to decide whether two
of the six words were repetitions of one another. The children were also
presented with meaningful and nonsensical six-word sentences. They were asked
to decide whether each sentence made sense.
The researchers’ assumption was that word recognition is associated with
bottom-up processing, whereas sentence comprehension is associated with
top-down processes.
Meri and her colleagues found that children with dyslexia read words and
sentences less accurately and more slowly than typically developing readers.
Children with dyslexia scored lower in several areas of executive functioning,
including phonemic fluency, shifting, and inhibition, compared with typically
developing readers. Among
the children with dyslexia, a significant correlation was found between oral
reading fluency and phonemic fluency.
During sentence
reading, children with dyslexia showed lower functional connectivity between
brain networks involved in executive functions and those involved in attention
orienting than did typically developing readers. The children with
dyslexia used the brain networks associated with executive functions and
attention orienting to a similar extent when reading sentences and when reading
words.
The findings from the tests and the brain scans indicate that children
with dyslexia have impairments in both bottom-up and top-down processing.
Together, these two impairments lead to difficulties in reading comprehension. Impaired reading of isolated
words, a bottom-up process, leads to difficulty understanding the sentence as a
whole. However, even when all the words in a sentence are decoded correctly,
impairment in executive functions, a top-down process, can also lead to
difficulty comprehending the sentence.
Typically developing readers read sentences more quickly and accurately
than isolated words. It is possible that sentence context facilitates reading
in typical readers. The brain scans showed that typical readers make greater
use than readers with dyslexia of the combination of top-down and bottom-up
processes during sentence reading, but not during the reading of isolated
words. Typical readers rely more heavily on top-down processes during sentence
comprehension than do readers with dyslexia
Meri, R., Farah, R., & Horowitz-Kraus, T. (2020). Children with
dyslexia utilize both top-down and bottom-up networks equally in contextual and
isolated word reading. Neuropsychologia, 147, 107574.
Comments
Post a Comment