Abstract
To investigate the impact of sensory-motor systems on the neural organization for language, we conducted an H215O-PET study of sign and spoken word production (picture-naming) and an fMRI study of sign and audio-visual spoken language comprehension (detection of a semantically anomalous sentence) with hearing bilinguals who are native users of American Sign Language (ASL) and English. Directly contrasting speech and sign production revealed greater activation in bilateral parietal cortex for signing, while speaking resulted in greater activation in bilateral superior temporal cortex (STC) and right frontal cortex, likely reflecting auditory feedback control. Surprisingly, the language production contrast revealed a relative increase in activation in bilateral occipital cortex for speaking. We speculate that greater activation in visual cortex for speaking may actually reflect cortical attenuation when signing, which functions to distinguish self-produced from externally generated visual input. Directly contrasting speech and sign comprehension revealed greater activation in bilateral STC for speech and greater activation in bilateral occipital-temporal cortex for sign. Sign comprehension, like sign production, engaged bilateral parietal cortex to a greater extent than spoken language. We hypothesize that posterior parietal activation in part reflects processing related to spatial classifier constructions in ASL and that anterior parietal activation may reflect covert imitation that functions as a predictive model during sign comprehension. The conjunction analysis for comprehension revealed that both speech and sign bilaterally engaged the inferior frontal gyrus (with more extensive activation on the left) and the superior temporal sulcus, suggesting an invariant bilateral perisylvian language system. We conclude that surface level differences between sign and spoken languages should not be dismissed and are critical for understanding the neurobiology of language.
Highlights
Evidence from lesion-based, neuroimaging, and neurophysiological studies has revealed that the same left perisylvian regions are recruited during the production and comprehension of both spoken and signed languages
As expected from previous studies, sign production was associated with greater activation in parietal cortices compared to speaking, while speaking resulted in greater activation in bilateral superior temporal cortices, which is most likely due to the auditory feedback that occurs during speaking
Listening to speech, including self-produced speech, activates right inferior frontal cortex, whereas self-produced signing does not result in a visual signal that is parallel to perceiving sign language produced by another person (Emmorey et al, 2009a,b), and self-produced signing does not strongly activate right frontal cortices (e.g., Emmorey et al, 2003; Hu et al, 2011)
Summary
Evidence from lesion-based, neuroimaging, and neurophysiological studies has revealed that the same left perisylvian regions are recruited during the production and comprehension of both spoken and signed languages (for reviews see Emmorey, 2002; MacSweeney et al, 2008a; Corina et al, 2013). In the experiments presented here, we endeavor to identify the specific sensory- and motor-related systems that are differentially recruited for spoken and signed languages within the same individual: hearing bilinguals who are native users of American Sign Language (ASL) and English. We turn to language comprehension and report the results of an fMRI study that directly contrasts sentence comprehension in ASL and English in hearing bilinguals. We present data from this study that reveals for the first time the neural conjunction for visual sign comprehension and audiovisual speech comprehension
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