Prosodic and narrative processing in American Sign Language: an fMRI study

Abstract

Signed languages such as American Sign Language (ASL) are natural human languages that share all of the core properties of spoken human languages but differ in the modality through which they are communicated. Neuroimaging and patient studies have suggested similar left hemisphere (LH)-dominant patterns of brain organization for signed and spoken languages, suggesting that the linguistic nature of the information, rather than modality, drives brain organization for language. However, the role of the right hemisphere (RH) in sign language has been less explored. In spoken languages, the RH supports the processing of numerous types of narrative-level information, including prosody, affect, facial expression, and discourse structure. In the present fMRI study, we contrasted the processing of ASL sentences that contained these types of narrative information with similar sentences without marked narrative cues. For all sentences, Deaf native signers showed robust bilateral activation of perisylvian language cortices as well as the basal ganglia, medial frontal, and medial temporal regions. However, RH activation in the inferior frontal gyrus and superior temporal sulcus was greater for sentences containing narrative devices, including areas involved in processing narrative content in spoken languages. These results provide additional support for the claim that all natural human languages rely on a core set of LH brain regions, and extend our knowledge to show that narrative linguistic functions typically associated with the RH in spoken languages are similarly organized in signed languages.

Figure 1

Activation elicited by each ASL sentence type, relative to its matched control condition (movies of 3 ASL sentences played backward and overlaid). Z statistic images were thresholded using clusters determined by z > 2.3 and a (corrected) cluster significance threshold of p = .05. Subsequent to thresholding, these maps were masked to include only voxels that showed greater activation for ASL sentences than the low-level baseline (a still image of the signer). The maps for each condition have been overlaid to show areas of conjunction (i.e., significant activation in each condition) in purple, and areas of disjunction in blue (activation only for narrative sentences) and red (activation only for non-narrative sentences).

Figure 2

Areas showing greater activation for narrative than non-narrative sentences. Z maps were thresholded at z > 1.96 (uncorrected) with a minimum cluster size of 0.3 mL, and were masked to include only voxels significantly activated in the comparison between that sentence type and its backward-layered control condition as well as greater activation for ASL sentences than the low-level baseline.