The hemispheric lateralization of speech processing depends on what “speech” is: a hierarchical perspective

A recurring question in neuroimaging studies of spoken language is whether speech is processed largely bilaterally, or whether the left hemisphere plays a more dominant role (cf., Hickok and Poeppel, 2007; Rauschecker and Scott, 2009). Although questions regarding underlying mechanisms are certainly of interest, the discussion unfortunately gets sidetracked due to the imprecise use of the word “speech”: by being more explicit about the type of cognitive and linguistic processing to which we are referring it may be possible to reconcile many of the disagreements present in the literature.