Content area
Full Text
PersPecTives
o p i n i o n
A little more conversation, a little less action candidate roles for the motor cortex in speech perception
Sophie K. Scott, Carolyn McGettigan and Frank Eisner
Abstract | The motor theory of speech perception assumes that activation of the motor system is essential in the perception of speech. However, deficits in speech perception and comprehension do not arise from damage that is restricted to the motor cortex, few functional imaging studies reveal activity in the motor cortex during speech perception, and the motor cortex is strongly activated by many different sound categories. Here, we evaluate alternative roles for the motor cortex in spoken communication and suggest a specific role in sensorimotor processing in conversation. We argue that motor cortex activation is essential in joint speech, particularly for the timing of turn taking.
Spoken communication relies on the listener rapidly decoding the signal that is produced by the speaker. The apparent ease of speech production and perception processes belies the complexity of the motor acts that are necessary to produce speech and of the resultant acoustic signal that the listener processes. Indeed, specific speech sounds (phones)can be hard to separate and identify from the speech signal. This is because individual speech sounds can be produced in a variety of ways. The way that sounds are produced varies with their position in a word: in British English, the phone /p/ in port is quite unlike the /p/ sound in sport for example, in the former it is aspirated (produced with a puff of air) and in the latter it is not. Speech sounds also vary according to the surrounding
phonemes, so the /s/ at the start of sue is
acoustically different from the /s/ at the start of see, as the position of the lips anticipates the following vowel. In continuous speech, we can consider the sounds of speech to run into each other and influence each other, similar to letters in cursive handwriting as compared with printed letters1.
Experimental evidence of the variability in speech sounds was very striking to the first researchers who were able to investigate
the structure of speech in spectrograms2
(BOX 1), and it was proposed that the listener
tracks the articulatory gestures (that is,...