Language in the right hemisphere

Language in the right hemisphere

I am going to write two posts: this one on the right hemisphere and prosody in language, and a later one on the left hemisphere and motor control of language. Prosody is the fancy word for things like rhythm, tone of voice, stress patterns, speed and pitch. It is not things like individual phonemes, words or syntax. In order to properly understand language, we need both.

A recent paper (Sammler, Grosbras, Anwander, Bestelmeyer, and Belin; Dorsal and Ventral Pathways for Prosody; Current Biology, Volume 25, Issue 23, p3079–3085, 7 December 2015) gives evidence of the anatomy of the auditory system in the right hemisphere that is like that in the left. Of course the two hemispheres collaborate in understanding and producing language but the right side processes the emotional aspects while the left processes the literal meaning.

Here is the abstract:

Our vocal tone—the prosody—contributes a lot to the meaning of speech beyond the actual words. Indeed, the hesitant tone of a “yes” may be more telling than its affirmative lexical meaning. The human brain contains dorsal and ventral processing streams in the left hemisphere that underlie core linguistic abilities such as phonology, syntax, and semantics. Whether or not prosody—a reportedly right-hemispheric faculty—involves analogous processing streams is a matter of debate. Functional connectivity studies on prosody leave no doubt about the existence of such streams, but opinions diverge on whether information travels along dorsal or ventral pathways. Here we show, with a novel paradigm using audio morphing combined with multimodal neuroimaging and brain stimulation, that prosody perception takes dual routes along dorsal and ventral pathways in the right hemisphere. In experiment 1, categorization of speech stimuli that gradually varied in their prosodic pitch contour (between statement and question) involved (1) an auditory ventral pathway along the superior temporal lobe and (2) auditory-motor dorsal pathways connecting posterior temporal and inferior frontal/premotor areas. In experiment 2, inhibitory stimulation of right premotor cortex as a key node of the dorsal stream decreased participants’ performance in prosody categorization, arguing for a motor involvement in prosody perception. These data draw a dual-stream picture of prosodic processing that parallels the established left-hemispheric multi-stream architecture of language, but with relative rightward asymmetry.

The ventral and dorsal pathways are also found in both hemispheres in vision. The ventral is often called the ‘what’ pathway and identifies objects and conscious perception while the dorsal is called the ‘where’ pathway and is involved in spatial location for motor accuracy. The auditory pathways appear to also have the dorsal path going to motor centers and the ventral to perceptual centers. And although they deal with different processing functions the pair of auditory pathways appear in both hemispheres, like the visual ones.

 

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