Reading by One Isolated Hemisphere
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Abstract The left hemisphere dominance for nearly all processes associated with reading has been firmly established for both mature and young readers (Vigneau et al., 2006; Lindenberg & Scheef, 2007; Fiebach et al., 2002). Involvement of the RH in reading is traditionally associated with populations exhibiting difficulties with reading (Gazzaniga, 1983; Weekes, Coltheart & Gordon, 1997; Patterson et. al., 1988; Coltheart, 2000), although the most recent work suggests that the right hemisphere may participate in reading in normal adults who reach high proficiency (Ettinger-Veenstra et al., submitted). However, the question remains how the two hemispheres work together during reading and to what extent each isolated hemisphere can support reading and the decoding processes associated with reading (e.g., phonological awareness, phonological memory, rapid naming and vocabulary retrievals). In this study we investigated the latter issueis issue by assessing reading abilities in twenty three children and young adults (age range 6-25 years) who have undergone left or right cerebral hemispherectomy for intractable catastrophic epilepsy. All participants had either complete removal of one hemisphere, i.e. anatomical hemispherectomy, or partial removal and complete disconnection of the affected hemisphere in a modified functional hemispherectomy. The goal of our study was to assess reading abilities including word recognition and oral comprehension. While reading is an extremely complicated cognitive task including a variety of knowledge areas and metacognitive skills we focused on those components of reading that Chall (1983) called “learning to read” steps (in contrast to “reading to learn” which comes later in life). We investigated whether phonological awareness and memory, rapid naming and vocabulary size predict reading capacity similar to neurologically-intact readers. About 60% of all the participants had average reading abilities. Similar to our previous results (Smets, 2010), we documented that side of the resection by itself was not a meaningful predictor of reading and its underlying components. Instead, side and etiology (developmental vs. acquired pathology) together was a better predictor of reading capacity. As with general language outcomes following hemispherectomy (Liegeous et al., 2008a; b) we saw both functional lateralization and equipotentiality of both hemispheres for reading. Participants with pathology acquired after a period of normal development showed a predicted pattern of reading outcomes reflecting progressive language lateralization, i.e. participants with left hemisphere removal had the worst outcomes while those who had the right hemisphere removed uniformly scored within the normal range. In contrast, participants with developmental pathology did not exhibit any clear hemispheric advantage. We had few cases of developmental pathology with a right-sided resection, commanding caution in our conclusions, with 2 out of 3 participants failing to reach average levels. However, in sharp contrast to the acquired group, 6 out of 9 participants with the developmental pathology and left resection were average readers. Surprisingly, there was no straightforward correlation between reading components and reading capacity in this population. Phonological processing and rapid naming presented significant difficulty for almost all participants including those with average reading skills. As a group, only in participants with the remaining right hemisphere phonological awareness, phonological memory and vocabulary predicted reading capacity. No such correlations were found for individuals with the remaining left hemisphere. Furthermore, rapid naming did not correlate with reading in any group.