Role of the Right Hemisphere Sentence Comprehension

Researcher Gillian King

Supervisors Associate Professor Jeff Coney

Date: Completed 2012

Ethics Approval Number: 2005/139

It is commonly accepted that the left cerebral hemisphere is dominant for language and represents the most well known instance of hemispheric asymmetry in humans. Emerging evidence accumulated over the last 30 years however has indicated that the RH also plays a role in language comprehension.The current investigations focus specifically on sentence comprehension since normal-behavioural investigations into hemispheric sentential processing are relatively under-represented. In addition, many of these studies are fraught with methodological deficiencies which may, in part, explain inconsistent findings, particularly with regards to RH performance. To address this possibility, the current five investigations sought to fine-tune the selection of stimuli as well as existing normal-behavioural methodology with specific focus on removing some of the variables known to disadvantage RH processing. Furthermore, a disparity between outcomes generated from the normal-behavioural divided field methodology and other methodologies also suggest that previous normal-behavioural studies may have under-estimated RH performance. Indeed, researchers using other paradigms to explore sentence processing in the hemispheres, such as neuro-imaging and electrophysiological techniques as well as the study of clinical populations, provide more convincing and consistent evidence of strong, if not specialised RH involvement in language processing. Design parameters were therefore tightened with the aim of highlighting the RH’s particular role in sentence comprehension and to discern any differentiating processing mechanisms between the hemispheres. A computer-based lexical decision task was used to record reaction time and error rates. Right handed participants from Murdoch University, Western Australia, viewed a centrally positioned sentence fragment that was followed by a laterally presented word or non-word target. Results from the first four experiments suggested that such an approach was highly successful as the strength of RH responses was unprecedented when compared to previous normal-behavioural studies (with the exception of Gouldthorp& Coney, 2011; Gouldthorp& Coney, 2009a; Gouldthorp& Coney, 2009b; Gouldthorp& Coney, 2009c, who adopted a similar experimental methodology in our laboratory). Indeed, the short, simple sentences used in this current series of studies were not able to separate the hemispheres as both performed reasonably equitably under most conditions, providing strong evidence for a RH message-level processing capacity. A fifth final experiment was designed to confirm that the visual half-field paradigm effectively isolated RH responses, thereby providing veridicality to the strong right hemispheric responses observed throughout the current research program. This was achieved reservedly when weaker and stronger imager groups performed significantly differently on the basis of hemispheric responses to imagery. It was concluded that the short and simple nature of the sentences used throughout the program of research together with the fine-tuned stimuli and experimental methodology, resulted in authentic and strong RH responses that were indeed comparable to those observed in the LH. In addition, the final study highlighted the possibility that individual differences in patterns of cerebral lateralization in general sentence comprehension may have also contributed to the paucity of hemispheric interactions in the present research program and may well be an important factor that needs further consideration in future hemispheric studies.