An fMRI study of whispering: the role of human evolution in psychological dysphonia

Humans are the only species known to use whispering for communication, and humans are also the only species known to suffer from functional (psychological) aphonia, a condition in which only whispered and not vocalized speech can be produced. Thus, whispering is uniquely a human function and psychological aphonia is uniquely a human dysfunction; both can be considered to result from human evolution. We hypothesize that the human brain has developed one or more identifiable switching mechanisms that overlay whispering onto the mechanisms of normal speech production. Thus, for whispering the speech production system operates as for normal speech production, but the switch to whispering activates the laryngeal abductor muscle (and possibly the inferior pharyngeal constrictor) so that the larynx does not completely adduct and turbulence noise is produced at the larynx. We hypothesize that psychological aphonia results from involuntary or subconscious activation of this switching mechanism. Preliminary research leads us to hypothesize that there are two switching mechanisms. This research used functional magnetic-resonance imaging to contrast brain activity for both normal speech and whispering (versus silence) for normally phonic subjects in an imagined social situation. One pattern of results involved a decrease of overall activity for whispering, with possible activation of the corpus callosum, and possible crossover of the main locus of activation from the dominant to the nondominant side. The other involved increased overall activity for whispering versus normal voice, accompanied by an increase of activity in the frontal cortex. More participants exhibited the former pattern than the latter one. These results may be related to the fact that there are actually two types of psychological aphonia, one that occurs after bouts of severe coughing associated with lower-respiratory disorders and the other due to purely psychological factors. We hypothesize that these are associated with the two different switching mechanisms, an upper one associated with the conscious brain and a lower one associated with more basic functions such as protecting the vocal folds. Understanding brain mechanisms for voluntary whispering and relating them to the occurrence of dysphonia has obvious implications for improving clinical assessment and treatment of aphonia, and would also contribute to improved understanding of human function in general.