Reading and Phonological Processing After Left Hemispherectomy
Reading in healthy children depends on well-described “building blocks” such as the ability to manipulate phonemes and match them to written letters in a process known as phonological awareness, vocabulary and verbal memory. All these skills are mostly in the left hemisphere. We investigated what happens when the left hemisphere is resected very early in life and concluded that the isolated right hemisphere uses the same “building blocks” as its left counterpart.
Cerebral hemispherectomy, a surgical procedure undergone to control intractable seizures, is becoming a standard procedure with more cases identified and treated early in life. While the effect of the dominant hemisphere resection on spoken language has been extensively researched, little is known about reading abilities in individuals after left-sided resection. Left-lateralized phonological abilities are the key components of reading, i.e., grapheme-phoneme conversion skills. These skills are critical for the acquisition of word-specific orthographic knowledge and have been shown to predict reading levels in average readers as well as in readers with mild cognitive disability. Furthermore, impaired phonological processing has been implicated as the cognitive basis in struggling readers. Here, we explored the reading skills in participants who have undergone left cerebral hemispherectomy.
Seven individuals who have undergone left cerebral hemispherectomy to control intractable seizures associated with perinatal infarct have been recruited for this study. We examined if components of phonological processing that are shown to reliably separate average readers from struggling readers, i.e., phonological awareness, verbal memory, speed of retrieval, and size of vocabulary, show the same relationship to reading levels when they are mediated by the right hemisphere.
We found that about 60% of our group developed both word reading and paragraph reading in the average range. Phonological processing measured by both phonological awareness and nonword reading was unexpectedly spared in the majority of participants. Phonological awareness levels strongly correlated with word reading. Verbal memory, a component of phonological processing skills, together with receptive vocabulary size, positively correlated with reading levels similar to those reported in average readers. Receptive vocabulary, a bilateral function, was preserved to a certain degree similar to that of strongly left-lateralized phonological skills. Later seizure onset was associated with better reading levels.
When cerebral hemispherectomy is performed to control seizures associated with very early (in utero) insult, it has been found that the remaining right hemisphere is still able to support reading and phonological processing skills that are normally mediated by the left hemisphere. Our results also suggest the existence of variability in individuals after hemispherectomy, even within groups having the same etiology and similar timing of insult.
Status: Published in Epilepsy and Behavior. Click here for the publication.
Collaboration: University of South Carolina, University of California at Los Angeles