Neural responses to grammatically and lexically degraded speech

Linguistic stimuli that are degraded in various ways have been used in neuroimaging studies to uncover distinct roles for different brain regions involved in processing language. In order to identify brain regions differentially involved in grammatical and lexical processing, we spectrally rotated specific morphemes and manipulated morpheme order to create speech stimuli that were degraded either grammatically or lexically, yet were matched in intelligibility. Twelve healthy participants were scanned with functional MRI as they listened to the grammatically and lexically degraded stimuli, interspersed with clear stimuli in the context of a familiar narrative. Contrary to our expectations, we did not find any brain regions that were selectively sensitive to grammatical or lexical degradation. However, compared to previous studies, there was less signal reduction than anticipated in response to degradation of either type. These findings may reflect increased attention to the degraded stimuli due to the familiar narrative context, since attention to degraded speech has been shown to attenuate the signal decreases typically associated with reduced intelligibility.