Cortical circuits for silent speechreading in deaf and hearing people

Abstract

This fMRI study explored the functional neural organisation of seen speech in congenitally deaf native signers and hearing non-signers. Both groups showed extensive activation in perisylvian regions for speechreading words compared to viewing the model at rest. In contrast to earlier findings, activation in left middle and posterior portions of superior temporal cortex, including regions within the lateral sulcus and the superior and middle temporal gyri, was greater for deaf than hearing participants. This activation pattern survived covarying for speechreading skill, which was better in deaf than hearing participants. Furthermore, correlational analysis showed that regions of activation related to speechreading skill varied with the hearing status of the observers. Deaf participants showed a positive correlation between speechreading skill and activation in the middle/posterior superior temporal cortex. In hearing participants, however, more posterior and inferior temporal activation (including fusiform and lingual gyri) was positively correlated with speechreading skill. Together, these findings indicate that activation in the left superior temporal regions for silent speechreading can be modulated by both hearing status and speechreading skill.

Fig. 1

Activation for speechreading relative to the baseline task for each group. (A) Deaf group (top) and (B) hearing group (bottom); 13 participants per group; voxel-wise p-value = 0.05, cluster-wise p-value = 0.0025. Activations on lateral renderings are displayed up to 15 mm beneath the cortical surface. Five sequential axial sections, showing activation in superior temporal regions, including the planum temporale (PT) and Heschl's gyrus (HG) are also displayed.

Fig. 2

Activation for the speechreading condition as a function of hearing status. The region indicated was more active for deaf than hearing participants, when performance on the TAS (z-score) was entered as a covariate (voxel-wise p-value = 0.05, cluster-wise p-value = 0.01). No regions were more active for hearing than deaf participants. No right hemisphere regions were significantly different across the groups. Five sequential axial sections, showing activation in superior temporal regions, including the planum temporale (PT) and Heschl's gyrus (HG) are also displayed.