The discovery of Mirror Neurons in Macaque monkeys has lead to theories of the neurophysiological substrate of speech perception being grounded in mirror neurons. This is also relevant to the evolution of speech as if ability to perceive a rapid stream of phonemes is present in species such as macaques then this provides a foundation on which other linguistic abilities could have been built to form language.
A recent paper by Rogalsky et al. (2011) explores these theories by testing the hypothesis that damage to the human mirror system should cause severe deficits in speech perception. This is due to there being a number of recent studies which explore whether the areas of motor neurons are activated during speech perception but these do not address the prediction that patients with lesions in the motor regions (left posterior frontal lobe and/or inferior partiental lobule) should lack an ability to perceive speech.
Patients with Broca’s aphasia are well documented as having severe speech perception and Broca’s area is known to be an area of motor speech perception. This sets up a link between a lesions involving Broca’s area and a difficulty in speech perception. However, despite these problems in speech perception, it has been shown that Broca’s aphasics are quite capable of processing speech sounds. This creates a problem for motor theories of speech perception as it would predict the ability to percieve speech sounds when the lesion lies in Broca’s area. Rogalsky et al. (2011) states that this conclusion may not be so reliable as a lot of the group based studies which these conclusions have been drawn from do not present detailed lesion information but instead rely on clinical diagnosis of Broca’s aphasia to infer lesion location.
Rogalsky et al. (2011) present 5 cases of people with lesions which effect areas of mirror neurons.