Plasticity

1. Plasticity Cognitive Neuroscience

2. Localization of function: Evidence • Cases such as Broca’s • Cortical stimulation • Single cell recording • Functional neuro-imaging • Exquisite intricacy of connectivity (primary sensory areas)

3. Plasticity-the paradox • On the one hand:Remarkable learning capacity, requiring subtle, complex and extensive neural change • On the other hand:Failure to recover function subsequent to damage and early deprivation (critical periods) • Plasticity: • What is it? • How and when does it happen? • What are the limits?

4. Plasticity and its Limitations • Visual system: Limitations-Evidence from critical periods • Visual system: Plasticity-Evidence from Blindness • Somatosensory system: Plasticity • Peripheral manipulations (changing the input) • Deprivation/Amputation • Syndactyly • Enrichment/practice • Central manipulations • Cortical lesions • Mechanisms of plasticity

5. Critical periods “….the elementary qualities of cold, heat, pleasure, pain, red, blue, sound, silence, etc., are original, innate or a priori properties of our subjective nature, even though they should require the touch of experience to waken them into actual consciousness, and should slumber to all eternity without it” (William James, 1890, Vol. 2, p. 618)

6. Plasticity “failures”- Critical PeriodsEffects of Deprivation • Ocular dominance columns

7. A: Right eye of normal monkey was injected with a radio-labeled amino acid; Autoradiograph in V1 taken 10 days later. Radioactivity forms white strips. B: Animal deprived of vision in one eye during the critical period and the eye which had been open was injected. (Hubel & Wiesel, 1977)

8. Plasticity “failures”- Critical PeriodsEffects of Deprivation (2) Binocular cells: at the borders of ocular dominance columns there are cells that get input from both eyes-binocular cells

9. Distribution of V1 neurons as a function of the degree to which they respond to contralateral and ipsilateral stimulation (4=responds equally well to contra and ipsi stimulation

11. Plasticity “failures”- Critical PeriodsEffects of Deprivation • Human strabismus: imbalance in eye muscles upsetting the coordination between the two eyes -> loss of binocular receptive fields -> ambliopia (one eye becomes dominant leading to loss of cortical representations and acuity in the less dominant eye)

13. Plasticity “failures”- Critical PeriodsEffects of Deprivation • Orientation selectivity: Blakemore & Cooper (1970)

16. Plasticity in the visual system Sadato et al (1996) • PET study • Subjects: • Blind (congenital or early onset) • Sighted subjects • 3 tactile tasks presented to index finger • Non-discrimination task: sweep index finger over a rough surface homogeneously covered with Braille dots • Non-Braille discrimination: line angle, line width, character ID • Braille lexical decision (blind Ss only)

17. Sadato et al. (1996) Activation in Primary Visual Cortex (task vs. rest) Non-discrim. non-Braille discrim. Braille LD Sighted Blind Sighted Blind Blind - + - + + non-sig non-sig sig sig sig

18. Plasticity and its Limitations • Visual system: Limitations-Evidence from critical periods • Visual system: Plasticity-Evidence from Blindness • Somatosensory system: Plasticity • Peripheral manipulations (changing the input) • Deprivation/Amputation • Syndactyly • Enrichment/practice • Central manipulations • Cortical lesions • Mechanisms of plasticity

20. Plasticity in the Somatosensory System: Mapping Somatosensory cortex

21. Plasticity in Somatosensory cortex: Amputation Merzenich et al. (1984) • Amputation of digits in owl monkeys • 244-342 penetrations within and bordering 3b hand area • Stimulation skin (fine-tipped glass probe) while recording cortical receptive fields (anesthetized monkeys) What happens to the neurons that used to get input from digit 3?

24. Plasticity in Somatosensory cortex: Amputation Results: • Remapping: • Neurons dedicated to amputated digit 3 respond to stimulation by digits 2 & 4 • Remapping maintains topography (2) Smaller receptive fields in the reorganized area

25. Remapping: What does it ‘feel” like? • Experience of a phantom limb • Under what conditions is it experienced? • Variable, but in some cases there are “referred sensations” Ramachandran et al. • VQ: • 17 year old male • Left arm amputated 6 cm above the elbow (4 weeks before testing) • Testing: • Brushed Q-tip over different skin surfaces • Eyes shut and asked to report perceived location of the sensations

27. Reference fields: • Topographically organized • Sharp boundaries • Stable with repeated testing • Touch and temperature

28. Plasticity in Somatosensory Cortex: Syndactyly Allard et al. (1991) • Skin of digits 3 and 4 in adult monkeys was surgically connected to create artificial syndactyly • -> Increases the amount of simultaneous input from normally separated fingers • Maps were obtained 3-7 months later

29. Results: Normal discontinuity between representations was obliterated -> two-digit receptive fields that are only rarely seen in the normal hand

30. Plasticity in the Somatosensory System: Practice Merzenich et al. • Finger tips are stimulated by being set on a revolving corrugated metal drum for 1 ½ hours per day for 3 months

31. Results: -cortex expands at the expense of adjacent fingers -receptive fields are unusually small

32. Plasticity in Somatosensory Cortex: Removal of Syndactyly Moligner et al (1993) • Using MEG mapped hand area of somatosensory cortex in two human adults with congenital syndactyly • Remapped area 1-5 weeks following surgery

34. Results: -pre-surgical maps were largely non-somatotopic -significant post-surgery reorganization with clear somatotopy -the degree of reorganization correlated with the severity of the syndactyly

35. Plasticity and its Limitations • Visual system: Limitations-Evidence from critical periods • Visual system: Plasticity-Evidence from Blindness • Somatosensory system: Plasticity • Peripheral manipulations (changing the input) • Deprivation/Amputation • Syndactyly • Enrichment/practice • Central manipulations • Cortical lesions • Mechanisms of plasticity

36. Somatosensory system: Plasticity after central lesion Jenkins & Merzenich(1987) A-immediately after lesion B-58 days after lesion, ulnar aspect

37. Somatosensory system: Plasticity after lesionHow might it “feel” Rapp et al. (2002) • 2 Ss with left hemisphere parietal lesions extending into S1 and S2 • Very accurate tactile detection • Tactile localization?

38. Stimulation locations

39. Results: RSB

40. Results: AKH

41. Mechanisms of plasticity • Axonal sprouting • cortical neurons deprived of sensory input secrete neurotrophic factors, causing nearby neurons to sprout new axon terminals • Unmasking of “silent” inputs

42. F F F F skin skin brain brain H H H H Unmasking of silent inputs?-The case of amputation • Assume that sensory cells from face normally project both to cortical face and hand neurons • Assume that the signal from face to hand is normally inhibited by intact connections from hand-hand • Loss of hand-hand input “reveals: hand-fact connection

43. Calford & Tweedale (1990) • Flying foxes • Localized neurons that respond to stimulation • of thumb • Anesthetized thumb • Recorded from “thumb” neurons • Dark areas indicate receptive field location of • these neurons • **notice how rapidly the receptive fields expand • and contract