Brain Function I: Evidence from Linguistics

1. Brain, Mind, and Belief: The Quest for Truth Brain Function I: Evidence from Linguistics The Neurological Basis of Language and Thought Language, for all its seeming complexity, is more amenable to analysis than other cognitive structures, so that investigation of language is one of the best ways to proceed to an understanding of human mind and nature. Tim Pulju, 1990

2. Agenda for TodayThe Problem: How does the brain work? • History of the study of brain function • Common errors to be avoided • Tool-driven inquiry • The misapplied metaphor • Help from the study of linguistic structure • Claim: As language works in the brain, so the brain works in general • Therefore, If we can understand how language works, we will know how the brain works • Relational networks • Cortical columns of neurons

3. History of the study of brain function I • Early investigators in the 19th century came up with the idea of locationism:Local areas of the cortex have specific functions • Franz Joseph Gall (1758-1828) • promoted the idea • His followers took it too far • As a result, the idea of locationism was widely discredited

4. History of the study of brain function II • Locationism:Local areas of the cortex have specific functions • Paul Pierre Broca (1824-1880) • Stroke patient with impaired speech • Autopsy after patient’s death • Damage in lower left frontal lobe • Area now known as Broca’s area • Responsible for articulation of speech

5. History of the study of brain function III • The origin of Connectionism • Like locationism but more sophisticated:Local areas of the cortex have highly specific simple functions, and complex functions are carried out by multiple interconnected areas • Carl Wernicke (1848 - 1906) • Stroke patient unable to comprehend speech • Damage in upper left temporal lobe • Area now known as Wernicke’s area • Responsible for speech comprehension

6. Connectionism • Connectionism includes a version of locationism • But is more sophisticated • Each local area performs a very specific simple function • Complex functions require multiple local areas acting together • They can act together because they are interconnected • CONNECTIONS RULE!

7. Two basic language areas Primary Somato- sensory Area Leg Primary Motor Area Trunk Arm Hand Wernicke’s area Fingers Mouth Phonological Recognition Phonological Production Broca’s area Primary Auditory Area Primary Visual Area

8. Arcuate Fasciculus (from langbrain website) Connects Wernicke’s area to Broca’s area

9. History of the study of brain function IV • The decline and revival of Connectionism • As with Gall, followers of Wernicke were too speculative and went too far, and the whole idea was discredited for several decades • Finally revived in the 1960’s by Norman Geschwind (1926-1984) • Now widely accepted by neurologists (but criticized by some psychologists)

10. History of the study of brain function V • Two major methods of investigation • Lesion studies • E.g., Broca and Wernicke and Geschwind • If area A is damaged and function F is impaired, then A must have function F (or at least contribute to F) • Functional Brain Imaging • A recent innovation • Made possible by technological advances • Now very widely used • Location-based but sometimes without the sophistication of connectionism

11. Functional Brain Imaging • Electro-Encephalography (EEG) • Excellent temporal resolution • Very poor spatial resolution • Positron Emission Tomography (PET) • Poor spatial resolution • Very poor temporal resolution • Functional Magnetic Resonance Imaging (fMRI) • Spatial resolution better than PET • Temporal resolution a little better than PET • Magneto-Encephalography (MEG) • Excellent temporal resolution • Spatial resolution not so good

12. Positron emission tomography (PET) • PET shows areas of increased cortical metabolism • Spatial resolution: 5-10 mm • How good is that? • Under one sq mm of cortical surface, 130,000 neurons • Temporal resolution: “…on the order of minutes…” (A. Papanicolaou, Fundamentals of Functional Brain Imaging (1998), p. 14)

13. Functional Magnetic Resonance Imaging (fMRI) • Measures the amount of oxygenated blood supplied to different areas of the brain • Common abbreviation: rCBF (regional cerebral blood flow) • When a group of neurons increases its signaling rate, its metabolic rate increases

14. An fMRI example Areas of the brain used in working memory www.firstscience.com/ SITE/ARTICLES/love.asp

15. Properties of fMRI • Temporal resolution: • Not very good • Image reflects the increase in oxygenated blood that occurs 5 to 8 seconds after the neurons fire • Spatial resolution: • Better than PET • But it is unclear whether the imaged area is precisely the area involved in the activity • The flow of oxygenated blood into the depleted area may also flow into neighboring vessels in areas where neural firing did not occur

16. Magnetoencephalography (MEG) MEG (MagnetoEncephaloGraphy) measures the magnetic field around the head Magnetoencephalography magnetic brain picture production of

17. How MEG works An electric current is always accompanied by a magnetic field perpendicular to its direction MEG records the magnetic flux or the magnetic fields that arise from electric currents in neurons Magnetic flux lines are not distorted as they pass through the brain tissue because biological tissues offer practically no resistance to them Therefore, MEG is more accurate than EEG

18. Magnetic flux from source currents Magnetometer Magnetic flux Source current

19. Recording of Magnetic Signals

20. Temporal Resolution of MEG Excellent – unlike fMRI and PET Therefore, it is possible to discern the temporal orderof activation of cortical areas MEG has potential to detect the activation of several brain regions as they become active from moment to moment during a complex function such as recognition

21. A major challenge of MEG The cortex is a parallel processor Hundreds or thousands of dipoles can be active simultaneously Multiple dipoles make comprehensive inverse dipole modeling virtually impossible Hence, compromises are necessary Sample larger time spans (up to 500 ms) Sample larger areas (up to several sq cm)

22. Some MEG results: Speech recognition Hemispheric Asymmetry Wernicke's Area

23. Wernicke’s area in Spanish-English bilinguals From MEG lab, UT Houston

24. Spatial Resolution • How accurately is location determined? • EEG: Poor • PET: Fair – 4-5 mm • fMRI: Fair – 4-5 mm • MEG: Fairly good – 3-4 mm or less • Under good conditions • How good are these figures? • under 1 sq mm of cortical surface,140,000 neurons

25. Temporal resolution Temporal resolution Key neural events can occur within 5 ms Terrible: PET 40 seconds and up Pretty bad: fMRI 10 seconds or more Excellent: MEG and EEG Instantaneous Theoretically it is possible to do ms by ms tracking, to follow time course of activation But such tracking is usually difficult or impossible The inverse problem Too many dipoles at each point in time

26. Sensitivity of Imaging Methods All of the methods have limited sensitivity MEG 10,000 dendrites in close proximity have to be active to detect signal PET and fMRI Similar limitations Any activation that involves fewer numbers goes undetected

27. Faulty thinking in neuroscience I:Tool-driven inquiry • Tool-driven inquiry: letting the available tools shape the investigation • Like looking for the lost car keys under the street light instead of where they got lost • The available tools: Brain imaging machines • What they are good for: determining locations of brain activity • Therefore, what do they investigate? • Locations of brain activity • The question being asked: Where? • The questions not being asked: What?, How? • What is going on? • How does the brain work?

28. More on the lack of interest in what/how • There are no available machines for investigating the what/how question • Experiments have not been devised for investigating the what/how question • It is necessary to rely on thinking • Scientists believe that doing science is conducting experiments and using high-tech machinery • Thinking is done by theoreticians • Akin to philosophers and poets

29. A mitigating circumstance? • Many have not realized that the what/how question is important • They may assume it is already known: • The brain is assumed to work like a computer • A symbol-manipulating device • This assumption is unwarranted

30. Faulty thinking in neuroscience II:The misapplied metaphor • The brain is assumed to work like a computer • This assumption is unwarranted • The computer is a symbol-manipulating device • Not a connectionist system • An example of faulty thinking: • The misapplied metaphor • The brain works by means of connectivity and operations upon its connectivity

31. The Cortex is a NetworkEntirely different structure than that of computers • Connectivity as key property of brain structure • Symbol-manipulation is the key property of computers • The cortex operates by means of connections • Transmission of activation along neural pathways • Changes in connection strengths

32. Computers Exact, literal Rapid calculation Rapid sorting Rapid searching Faultless memory Do what they are told Predictable Brains Flexible, fault tolerant Slow processing Association Intuition Adaptability, plasticity Self-driven activity Unpredictable Self-driven learning Computers and Brains: Different Structures, Different Skills

33. Things that brains but not computers can do • Acquire information to varying degrees • “Entrenchment” • How does it work? • Variable connection strength • Connections get stronger with repeated use • Perform at varying skill levels • Degrees of alertness, attentiveness • Variation in reaction time • Mechanisms: • Global neurotransmitters • Variation in blood flow • Variation in available nutrients • Presence or absence of fatigue • Presence or absence of intoxication

34. How to study the what/how question • You have to think harder • Also, we can make use of findings from structural linguistics • Language as the key to unlock mysteries of the mind • If cortical structures for language are like those for other high-level skills • Then if we figure out language, we also have the answer to how other high-level intellectual processing works

35. Thinking harder • Avoid metaphorical thinking • The brain is not a computer • Not like a human being with paper & pencil & books • In fact it is not like anything else • It is itself: the brain

36. How does your brain tell your fingers what to do? • Question to daughter (age 6): • How does your brain tell your finger to hit that key on the piano? • Sarah: • Well my brain writes a little note and sends it down to my finger, … • What really happens? • Neurons in the motor cortex send activation (nerve impulses) down (through subcortical structures and the spinal cord) to neurons that activate the muscles that make my finger move

37. Auditory Imagery • Auditory images of words, music, etc. • We can hear things in our heads • What is an auditory image? • What does it consist of? • Sound? • There is no air inside the head to vibrate • What hears it? • There are no little ears inside the head

38. Visual Imagery • Visual images of people, buildings, etc. • What is a visual image? • What does it consist of? • Is it a little picture? • If so, where are the eyes to see it? • What is it drawn on? • Where is the visual perception system to interpret it? • If not, what?

39. Vision • When you see something.. • A picture on your retina? • Something in your brain looks at it? • Are there a couple of little eyes inside? • A picture somewhere inside your brain? • Same problems: no eyes inside • And if there were, they would have to be supported by a visual perception system

40. Compare the TV set • Does it have little people inside? • Similarly, no pictures inside the brain • No sounds inside the brain • No words or other symbols inside the brain

41. How vision really works • There is no picture on the retina, just neurons that get activated by light • Some of them get activated by color • The visual perception system learns during early childhood to integrate configurations of these little dots into larger units • Next higher level: larger configurations • Many levels up, recognition of objects • The brain goes through a long process of learning, to build these many levels • More on this next week!

42. The Nature of Language • Some history • Louis Hjelmslev • Prolegomena to a Theory of Language (1943/60) • Linguistic structure is a system of relationships • ”The postulation of objects as something different from the terms of relationships is a superfluous axiom and con-sequently a metaphysical hypothesis from which linguistic science will have to be freed.”

43. Understanding linguistic units as purely relational I dog d - o - g Symbols? Objects?

44. Understanding linguistic units as purely relational II Seems to be one unit Three phonemes (or graphemes), in sequence dog d o g

45. Understanding linguistic units as purely relational III DOG Noun Grammatical properties The meaning of dog – a concept dog The object we are considering d o g

46. Understanding linguistic units as purely relational IV DOG Noun dog d o g

47. Understanding linguistic units as purely relational V DOG Noun We can remove the symbol with no loss of information. Therefore, it is a connection, not an object d o g

48. Another way of looking at it DOG Noun dog d o g

49. Another way of looking at it DOG Noun d o g

50. The phonological (or graphemic) segments DOG Noun What about these segments? Are they objects? d o g