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.
Participants took part in 4 word comprehension tests and 4 speech discrimination tests.
In the comprehension tests, subjects were presented with a spoken word and asked to point to the matching picture out of an array of four. In different variants of the test, the distracter pictures where either all phonemically related, all semantically related, all unrelated, or contained a mixture of distracter types. In the discrimination tasks, subjects heard pairs of syllables and were asked to decide whether the two items were the same or not. The four versions of this test were as follows. One test used real words drawn from the comprehension test items. Another used non-words that were created by changing the vowel of the words used in the words discrimination test. These two tests were matched for onset and offsets of the speech stimuli but were not matched for phonotactic probability (how common a given sequence of sounds is) or neighborhood density (how many similar sounding words there are to a stimulus item). Therefore another word discrimination and non-word discrimination test was created in which words and non-words were matched on these variables.
As can be seen from this graph all of the patients with damage affecting the presumed human mirror system exhibited high-levels of performance on all of the speech tasks. This follows from previous studies which assert that within Broca's aphasia the motor speech system is not necessary for speech perception. There is a week trend for patients with lesions which affectected the temporal lobe for performance to worsen but still remained above chance. This is true even of one of the patients with complete destruction of the left superior temporal lobe indicating that speech perception is not even strongly left dominant.
- The human mirror system/motor speech system is not critical for speech perception.
- Temporal lobe structures, rather than motor structures, are the primary substrate for speech perception
- This system is bilaterally organized
Implications within Language Evolution?
These conclusions hold problems for theories of the evolution of speech but do not necessarily rule out theories that mirror neurons may have had a hand in the emergence of language had it, as proposed by people such as Michael Arbib, initially emerged as gestural language. It would be interesting to see how people with legions such as those used in this study faired in the perception of sign language.
Mirror neurons are also thought to contribute to our theory of mind and our ability to empathise, this also has implications within the evolution of language as this is presumably quite a key pre-adaptation needed for language to be utilised at all.
Rogalsky, C., Love, T., Driscoll, D., Anderson, S., & Hickok, G. (2011). Are mirror neurons the basis of speech perception? Evidence from five cases with damage to the purported human mirror system Neurocase, 1-10 DOI: 10.1080/13554794.2010.509318