Speech shadowing is a psycholinguistic experimental technique in which subjects repeat speech at a delay to the onset of hearing the phrase. The time between hearing the speech and responding, is how long the brain takes to process and produce speech. The task instructs participants to shadow speech, which generates intent to reproduce the phrase while motor regions in the brain unconsciously process the syntax and semantics of the words spoken. Words repeated during the shadowing task would also imitate the parlance of the shadowed speech.
The reaction time between perceiving speech and then producing speech has been recorded at 250 ms for a standardised test. However, for people with left dominant brains, the reaction time has been recorded at 150 ms. Functional imaging finds that the shadowing of speech occurs through the dorsal stream. This area links auditory and motor representations of speech through a pathway that starts in the superior temporal cortex, extends to the inferior parietal cortex and ends with the posterior and inferior frontal cortexes, specifically in Broca's area.
The speech shadowing technique was created as a research technique by the Leningrad Group led by Ludmilla Chistovich and Valerij Kozhevnikov in the late 1950s. In the 1950s, the Motor theory of speech perception was also in development through Alvin Liberman and Franklin S. Cooper. It has been used for research on stuttering and divided attention, with focus on the distraction of conversational audio while driving. Speech shadowing also has applications for language learning, as an interpretation method and in singing.
History
The Leningrad group was interested in the time difference between the articulation and perception of speech. The speech shadowing technique was formulated to measure this difference. To measure the initiation of speech, an artificial palate was placed in the speaker's mouth. When the tongue moved to begin pronunciation and touched the palate, the measurement of reaction time began. The experiment concluded that the reaction time for consonants was consistently shorter than the reaction time to any vowel. The reaction time to a vowel depended on the consonant that came before it. This supported the phoneme as being the most basic unit of speech registered by the brain, rather than a syllable. The phoneme is the smallest distinguishable unit of sound, but the smallest unit that has assigned meaning is a consonant-vowel syllable.
Ludmilla Chistovich and Valerij Kozhevnikov focused on research of the mental processes that stimulate the functions of perception and production of speech in communication. In linguistics, speech perception was the chronological process that analysed steadily paced and similar sounding words but Chistovich and Kozhevnikov found speech perception to be the staggered integration of syllables known as non-linear dynamics. This refers to the diversity of tones and syllables in speech, which is perceived without a conscious detection of delay and forgotten with the limited working memory capacity. This observation developed research towards the speech shadowing technique for research in psycholinguistics.
Shadowing was used to measure the reaction time taken to repeat consonant-vowel syllables. Alveolar consonants were measured when the tongue first touched an artificial palate and labial consonants were measured by the contact of metal pieces when the upper and lower lips pressed together. The participant would begin to mimic the consonant as the speaker finished the utterance of the consonant. This consistent rapid response shifted research focus towards close speech shadowing.
Close speech shadowing is when the technique requires an immediate repetition, at the fastest pace a person is able to achieve. It does not allow people to hear the entire phrase beforehand or to understand the words vocalised until the end of a sentence. It was found that close speech shadowing would occur at the shortest delay of 250 ms. It has also been found to occur with a minimum delay between 150 m/s in left-hemisphere dominant brains. The left hemisphere is associated with enhanced performance with linguistic skill and information processing. It engages with analytic patterns of thought and experiences ease with the speech shadowing task.