Mirror neurons, the mirror system, and human language. Tim Miller NSCI5202 University of Minnesota

Transcription

1 Mirror neurons, the mirror system, and human language Tim Miller NSCI5202 University of Minnesota

2 A quick note on theory Dr. Redish has lamented the lack of theory in neuroscience In the meantime, there is a glut of experimental evidence without theory Mirror neuron research turns this upside down: Lots of speculation about what they do, how they work Proving these claims can be troublesome

3 Outline Introduction Background Definition Discovery Representation Putative functions of mirror neurons Imitation Mind reading Empathy Speech and language Conclusion

4 Relevant Neuroanatomy Premotor cortex Lateral part of frontal cortex, in front of primary motor cortex Stimulation of premotor areas elicits complex motor movements Projects to M1 and spinal cord Inputs: Area 46: Working memory of object location (PFC) Areas 5 & 7: Integrating sensory modalities in motor planning (also projects to M1) (Posterior parietal areas) Loops with basal ganglia and cerebellum via thalamus Kandel, Schwartz, and Jessel 2000

5 Definitions of mirror neurons Mirror neurons:...neurons that discharge when an individual performs an action, as well as when he/she observes a similar action done by another individual. (Rizzolatti 2005)...cells with extraordinarily complex response characteristics, closely linked to visual observation of goal-directed actions. (Miall 2003) To motor system, they look like any other PMc neuron

6 Discovery of mirror neurons Discovered serendipitously by researchers looking at F5 neurons Originally aimed to separate stimulus responses from motion responses F5 cells responded when the experimenter picked up or placed food in box Monkey was not making any overt moves purely responding to visual stimulus Formal experiments 2 neurons displayed (one excitatory, one inhibitory) out of 12 claimed (and 184 neurons studied) Rizzolatti et al. 1992

7 Where in the monkey brain is F5? Rizzolatti et al., 1996, Cog. Brain Res.

8 Experimental notes of interest Grasping with tools did not elicit activity Moving hand alone did not elicit activity Threatening actions did not elicit activity Neuron activity stronger with proximity to the animal EMG recording showed no muscle activity during observed movements

9 Contradictory results? Decety et al. '94 (PET) No change in premotor areas during observation of virtual reaching movements Virtual objects, virtual hand - remember what VR looked like in '94? Non-biological stimuli cannot excite F5 Future work: Does 12 years make a difference? Is the biological response graded?

10 This is

11 Mirror neurons in humans Doing electrophysiology in humans is problematic As a result, one cannot say for certain that mirror neurons exist Experiment by Fadiga et al. (1995) using TMS and MEPs Recorded MEPs in hand muscles while observing 4 conditions During observation of actions, MEP pattern similar to executing actions Referred to in the lit. as mirror system

12 Representation Issues What is it that these neurons encode? Action/Object coincidence Motor actions Intentions How do these differ? How could you test this experimentally?

13 Experiments in representation (Iacoboni '05) Studied response of putative mirror system in humans using fmri Benefits/drawbacks of using humans? 4 conditions: Context only Action only Action within context (2)

14 (Iacoboni 2005) Figure 2

15 (Iacoboni 2005) Figure 3

16 Results and Conclusions Found highest response in action with context condition Conclusion: Mirror system responds to intentions, not just actions Speculation: Logically related neurons fire activates a chain of motor acts Intention is coded by such a chain of related motor acts

17 Functions of mirror neurons Imitation Mind reading Empathy Speech and language?

18 Imitation Easy to imagine abstractly (?) This leads to 2 specific questions: How specifically could it work (model/theory) Does the system actually work this way? Miall (2003) reports on the connection between MNs and forward models Review: Forward models predict the sensory consequences of a motor action using efference copy Inverse models output motor commands given desired sensory state

19 Imitation contd. Hypothesis: F5 mirror neurons form interface between forward and inverse models Issues: Cerebellum? Multi-level control?

20 Mind reading AKA Intuitive psychology, Folk psychology The ability to infer states of mind in others is an important social ability This ability is often lacking in people with autism Mirror neurons provide a possible mechanism for this ability

21 Gallese and Goldman '98 Posit that mirror neurons are used for mind reading, not imitation Promoting simulation theory of mind reading (vs. theory theory ) Argue that mind reading has selection advantages Review/theoretical paper no experimental results What does this tell us? Mirror neurons could be used for mind reading

22 Empathy Can be thought of as a step beyond mind reading Not only understanding mental state, but coexperiencing emotions Example: Reacting to observation of disgust in others Experiment: Wicker et al. (2003) study examining activation patterns in observing vs. experiencing disgust Both of us disgusted in my insula: The common neural basis of seeing and feeling disgust

23 Here comes the science... fmri study of 14 male subjects Subjects had two conditions: Inhaling odorants producing disgust/pleasure/neutral Viewing video of others showing facial expression of disgust/pleasure/neutral reactions Same areas in insula showed activation Discussion: Authors speculate that this is a separate mirror system Unconscious (Automatic) response to facial configuration

24 Where is the insula? Harvard Medical School Center for Morphometric Analysis

25 Wicker et al. (2003) results

26 Mirror neurons and language There are two distinct threads to this section 1) Evolution of language and speech through the mechanism of the mirror system 2) Development of speech sounds in infants and the relation to the mirror system

27 Models of speech development Rizzolatti and Arbib (1998) argue for the mirror system's involvement in language development Argument outline: Communication requires sender-receiver parity Mirror neuron system could be a precursor, since it can already recognize intention in actions Proto-speech developed from action recognition and evolution of novel structures Modern language developed culturally from protospeech

28 Evaluation? Difficult to experimentally test ideas about how certain mechanisms evolved Arbib (2005) article in Behav. Brain Sci. expands these ideas plenty of critics This seems very speculative and I'm going to just whoosh on by it

29 Speech background Spectrogram Plot of frequency vs. time (vs. amplitude) Frequencies Y- axis Time X - axis Formants Thick dark bands in the spectrum ih ss k ss neh iy It's the yellow box next to the green box

30 Neural networks review Use Hebbian learning to associate correlating stimuli Remember basins of attraction? Stable points represent points of interest in the map Map is defined in representation space

31 Phoneme learning model Two maps One sensory, one motor Sensory map: Represented by two formants Motor map: Represented by articulator parameters A map consists of a number of neural units with Gaussian receptive fields Input is computed as a population code (?) Connections develop (persist) through experience Babbling (and hearing one's babbling) Hearing external sounds

32 Hebbian Learning During babbling Motor command and resulting sound affect maps simultaneously Units co-activated most strongly will become preferred Preferred to be defined later

33 Simulation setup Babbling phase: Randomly varied articulatory parameters generated (2 or 6) Parameters go into a physical model of vocal system which produces sounds First 2 formants of sounds are extracted -> Auditory map 1 st Simulation: Babbling only 2 nd Simulation: Babbling with external input

34 Simulation 1 results Babbling produced preferred responses Areas of the maps with roughly linear mapping were preferred Correlated small changes lead to strengthened connections With these maps, system can imitate external sounds

35 6 motor parameters (instead of 2) 10% babbling, 90% external vowels Squares: French vowels Circles: German vowels Results: Exposure to external input moved clusters How can this be? Simulation 2

36 Experimental validation Motor practice should improve perception (Check) In the absence of babbling, production and perception will suffer (Unknown) Difficulty in learning new phones (Check) Why Not a loss of plasticity, but everything gets sent into a perceptual cluster Perceptual commitment rather than neural commitment New predictions?

37 Paper discussion How does the time course stand up to real life (Is it really simultaneous?) How does this relate to learning in birdsong? How could you make this dynamic (i.e. To handle words)? What causes disparity in vowel locations between babbling and actual spoken languages? (How) could you test this model experimentally?

38 But weren't we talking about mirror neurons? Yes! Mirror neurons are an emergent property of this model The neurons in the motor map have similar properties to mirror neurons Of course the model requires the existence of something much larger the associated motor/auditory maps Suggest a visual map also correlated with motor map to create visual mirror neurons

39 Conclusions Mirror neurons are cells with interesting and complex properties Many putative functions for mirror neurons Lots of hype! The model of phoneme acquisition suggests it may be overblown

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