Inner speech-related brain areas mapped using a novel rhyming task

The brain functional correlates of phonological encoding have been examined by means of tasks requiring silent articulation, in many cases utilizing judgments of whether two words rhyme or not. However, most studies have employed written stimuli, which may not be appropriate in languages with transparent orthography (e.g. Spanish), and control tasks have often been less than ideal for functional imaging purposes. We aimed to identify brain activations during phonological encoding using a visual rhyming judgment task that minimized reading-related confounds, and we employed a novel control task that engaged presemantic stages of visual object processing.

Twenty-eight healthy, right-handed adults, out of an initial sample of 32, performed a task requiring them to judge whether the names of line drawings of animals and objects rhymed. The control task consisted of judging whether two pictures of animals and objects viewed from different angles were the same or different. Brain activations were examined using fMRI and whole-brain, voxel-based analysis.

The rhyming condition (rhyming > control) elicited increased activation in the left inferior frontal cortex, including Broca's area, as well as the precentral, supramarginal, and superior temporal gyri, plus the supplementary motor area, with subcortical activity being seen in the basal ganglia and left cerebellum. The reverse contrast (control > rhyming) activated occipital and ventral temporal cortical regions.

These results support the validity of this new picture-based rhyming task to study phonological encoding, and they highlight the role of the left inferior frontal cortex including Broca's area, among other regions, in silent articulation. Regions activated by the control task overlapped those known to be involved in visual object processing.