Auditory processing and physiological factors
Thus far, we have looked at the case for an auditory disturbance based on disrupted laterality. However, there is limited evidence that stuttering may also be associated with differences at a more peripheral level. In an often quoted study, Shearer and Simmons (1965) compared electromyographic (EMG) activity in the reaction of the stapedius muscle to vocalization between a small group of people who stutter and control speakers. This tiny muscle contracts as a reflex action at the onset of vocalization (Borg & Zarkrisson, 1975).2 Shearer and Simmons noted that while the timing of
stapedius contraction was consistent within the control group, the group who stuttered showed a greater delay in timing onsets. Further research has proved equivocal though: Hall and Jerger (1978) found no difference between experimental and control two groups with regard to the timing of stapedius contraction, but did find that the scale of contraction was reduced amongst those who stuttered. Subsequently, Hannley and Dorman (1982) found no differences between their two groups, although it is possible that this may be due to a different method used to activate the laryngeal nerves to those used in earlier experiments. A problem with all of these studies is that findings were based on a small numbers of subjects and this, coupled with the range of findings, leaves many questions as to a possible physiological basis to auditory disturbance unanswered.
Another source of data lies in the study of air and bone conduction, and the scientific phenomena that if two pure tones of identical pitch (frequency) and loudness (amplitude) are presented in opposite phase (180 degrees), then they will cancel each other out, and no sound will be heard. An early study (Stromstra, 1957) had a group of people who stutter and a control group subjected to two such tones, one presented through air and the other through bone (via the teeth). The subjects were then asked to manually vary the amplitude and phase until they no longer heard any sound. A significant difference was found between the two groups in the relative phasing of the air and bone conducted tones at 2000 Hz. In a related experiment, Stromstra (1972) had similar groups of subjects adjust amplitude and phase of two air conducted tones presented at either ear until the sounds cancelled themselves out. The phase disparity at several frequencies was found to be twice as wide for the people-who-stutter group as for the controls. Again, small subject numbers call for cautious interpretation, but there is tentative support here for the notion that people who stutter have a reduced capacity to control and manipulate auditory signals
Further investigation of our companystuttering and expressive language
moments of stuttering
circumlocution in stuttering
prevalence of stuttering
stuttering siblings
stuttering blocks
stuttering examples
stuttering test
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