1. 4: Neuroplasticity Cognitive Neuroscience David Eagleman
Jonathan Downar

2. Chapter Outline The Brain Dynamically Reorganizes to Match Its Inputs
The Brain Distributes Resources Based on Relevance
The Brain Uses the Available Tissue
A Sensitive Period for Plastic Changes
Hardwiring versus World Experience
The Mechanisms of Reorganization
Changing the Input Channels

3. The Brain Dynamically Reorganizes to Match Its Inputs
Changes to the Body Plan
Changes to Sensory Input

4. Changes in the Body Plan
The brain is constantly changing, reorganizing with each new experience.
Plasticity is the ability to change and to retain that new structure.
Plastic changes must be relevant.
Some systems have a sensitive period early in life when they have greater plasticity.

5. Changes in the Body Plan
The homunculus is the map of the body within the sensory and motor cortices.
FIGURE 4.2 (a) Motor homunculus and (b) sensory homunculus. The body becomes topographically mapped on the precentral gyrus (motor cortex) and postcentral gyrus (somatosensory cortex). Those areas with more sensation, or that are more finely controlled, have larger areas of representation.

6. Changes in the Body Plan
Changes to the body, such as losing a limb, can result in changes to the representation of the body in the brain.
Sensory areas that responded to the damaged part of the body are taken over by adjacent sensory areas.
Phantom limb pain is pain that seems to come from the missing body part.

7. Changes in the Body Plan
FIGURE 4.3 Changes in sensory maps: the brain adapts to changes in incoming activity, even in adulthood. After hand amputation in humans, neighboring cortical territory (purple and green) takes over the territory that previously coded for the hand (orange).

8. Changes to Sensory Input
Removing or altering sensory input, even on a temporary basis, can cause a remapping of the brain.
The speed of this remapping suggests that there are existing connections that can be unmasked.
Such reorganization has been observed in auditory and visual systems.

9. Changes to Sensory Input
FIGURE 4.6 Cortical reorganization. In this fMRI image, auditory and tactile tasks activate the otherwise unused visual cortex of early blind participants. Brain regions activated more in the blind than in the sighted are shown in the orange–yellow spectrum; areas more active in the sighted than in the blind are shown in blue–green. To see the gyri and sulci (the hills and valleys) of the cortex, the brain has been artificially “inflated” using a computer algorithm. Figure from Renier et al. (2010).

10. The Brain Distributes Resources Based on Relevance
The Role of Behavior
The Role of Relevance: Gating Plasticity with Neuromodulation

11. The Role of Behavior
The brain uses adaptive coding, altering the amount of resources assigned to a function depending on how important it is.
Sensory and motor representations will reorganize based on the particular skills and needs of the person.

12. The Role of Behavior
FIGUE 4.7 Functional mapping of primary motor cortex. When a monkey trains on a task that requires fine-digit manipulation (such as grabbing small objects), the cortical representation of digits expands. Shown here is a functional mapping of the primary motor cortex, demonstrating an expansion of the digit representation (purple) and a shrinkage of the forearm representation (green).

13. The Role of Relevance: Gating Plasticity with Neuromodulation
The behavior must be relevant to the organism to result in plasticity.
Plasticity can be turned on or turned off (gated) in particular places at particular times.
Neuromodulators, such as acetylcholine, control this gating.

14. The Role of Relevance: Gating Plasticity with Neuromodulation
FIGURE 4.9 Cholinergic pathways in the brain. Of special importance is the nucleus basalis, which transmits acetylcholine broadly throughout the cortex.

15. The Brain Uses the Available Tissue
Maps Adjust Themselves to the Available Brain Tissue
Cortical Reorganization after Brain Damage

16. Maps Adjust Themselves to the Available Brain Tissue
Maps will make use of the available amount of brain tissue.
Research with the visual system of tadpoles found that the input makes use of the available brain area, whether there is less brain area or more input.

17. Maps Adjust Themselves to the Available Brain Tissue
FIGURE 4.12 Plasticity in the development of the nervous system. (a) Fibers from the tadpole’s eye map retinotopically onto the tectum. (b) If half the tectum is removed, the complete input fits itself onto the smaller available area. (c) If a third eye is transplanted on one side, the tectum reorganizes to accommodate the additional input. (d) If half the retina is removed, the information from the remaining fibers spreads out to cover the available area of the tectum.

18. Cortical Reorganization after Brain Damage
Following injury to the central nervous system, some function tends to be recovered as swelling decreases.
Cortical reorganization can occur over a longer period of time to allow further recovery of function.
The language problems of aphasia tend to decline as the right hemisphere takes over.

19. A Sensitive Period for Plastic Changes
A Window of Time to Make Changes
The Sensitive Period in Language
Neuromodulation in Young Brains

20. A Window of Time to Make Changes
Plasticity is greatest during periods of development known as sensitive periods.
After the sensitive period has passed, plasticity is still possible, but not as easy.
The success of treatment for strabismus (lazy eye) early in life is an example of these sensitive periods.

21. The Sensitive Period in Language
Acquisition of a second language supports the idea of sensitive periods.
If you are exposed to a second language before age 7, you will be as fluent as a native speaker.
If exposed between 8 – 10 years, it will be harder to achieve fluency.
If exposed after age 17, fluency will be low.

22. The Sensitive Period in Language
FIGURE 4.14 Johnson and Newport’s study demonstrated the relationship between age of arrival in the United States and total score correct on a test of English grammar.

23. Neuromodulation in Young Brains
In humans, young people have greater brain plasticity.
There is a tradeoff between plasticity and efficiency, and, as your brain gets better at some tasks, it becomes less able to perform other tasks.
Young animals show widespread plasticity without needing attentional focus.

24. Hardwiring versus World Experience
Aspects of the Brain Are Preprogrammed
Experience Changes the Brain
Brains Rely on Experience to Unpack Their Programs Correctly

25. Aspects of the Brain Are Preprogrammed
We are born with certain reflexes, such as grasping and sucking.
Sperry conducted studies of the newt visual system and developed the chemoaffinity hypothesis.
Connections within the visual system are preprogrammed to follow chemical cues to reach their target.
Chemical cues can be attractive or repulsive.

26. Aspects of the Brain Are Preprogrammed
FIGURE 4.15 How the newt’s optic nerve makes its connections. (a) Fibers from the retina maintain their organized layout when they plug into the optic tectum. (b) To determine how the fibers find their destinations, Sperry cut the optic nerve and rotated the eye upside down. When the fibers regrew, they plugged into the tectum in their original pattern, (c) This led Sperry to conclude that the fibers do not find their destinations by visual experience, but instead by preprogrammed signaling.

27. Experience Changes the Brain
The environment alters the brain and affects the brain’s ability to learn.
Rats in an enriched environment have more extensive dendrites.
Neurons in the language area known as Wernicke’s Area have more elaborate dendrites in college-educated individuals.

28. Experience Changes the Brain
FIGURE 4.16 Neurons in the brain of a rat. (a) A representative neuron in the brain of a rat reared in a normal environment. (b and c) In enriched environments the neurons grow more extensive arborizations. (d, e, and f) In deprived environments the dendrites shrink to the point of total disappearance.

29. Brains Rely of Experience to Unpack Their Programs Correctly
The environment not only influences brain development, but is necessary for development.
The encoding discussed previously is only at a general level.
Experience is required to refine the connections.

30. Brains Rely of Experience to Unpack Their Programs Correctly
Kittens raised with strabismus do not develop binocular vision because they do not get appropriate input from both eyes.

31. Brains Rely of Experience to Unpack Their Programs Correctly
FIGURE 4.17 Kittens raised with artificial strabismus. histograms show the number of cells in the kitten’s visual cortex that respond to input from one eye or the other, along an arbitrary scale of 1 (activity is driven by input to the contralateral eye) to 7 (activity is driven by input to the ipsilateral eye). Neurons in the middle of the distribution (around 4) respond to activity in both eyes equally—in other words, they are binocular. In the kitten reared with strabismus, almost none of the neurons develop binocularly.

32. The Mechanisms of Reorganization
Neurons Compete for Limited Space
Competition for Neurotrophins
Rapid Changes: Unmasking Existing Connections
Slow Changes: Growth of New Connections

33. Neurons Compete for Limited Space
Neurons, axons, and dendrites need to compete for resources to survive.
The initial connections to the muscles and the visual system are refined over time by activity-depended plasticity.

34. Neurons Compete for Limited Space
FIGURE 4.20 Ocular dominance columns in primary visual cortex result from competition for space. (a) At 15 days in the cat, the input layer of primary visual cortex has approximately uniform input from the left and right eyes. (b) As the animal matures, the connectivity comes to reflect alternating input from both eyes equally. (c) When retinal activity is blocked, the segregation does not occur. (d) When one of a young animal is patched, the inputs from the weak eye progressively shrink as the strong inputs from the other eye successfully fight for the territory.

35. Neurons Compete for Limited Space
Pruning is the process of removing neurons and processes that are not needed.
Apoptosis is a form of cell death that is normal in development and enables the cells to die without affecting adjacent neurons.

36. Competition for Neurotrophins
Neurotrophins are chemicals that help to sustain the neurons.
Generally, they are secreted by the target to promote survival in the neurons that reach the target.
They allow the cell to differentiate.
In young cells, they prevent apoptosis in cells that make appropriate connections.

37. Competition for Neurotrophins
FIGURE 4.21 A ganglion of sensory cells from a chick embryo cultured in the (a) absence or (b) presence of nerve growth factor.

38. Rapid Changes: Unmasking Existing Connections
Many existing connections are masked by activity within the nervous system.
Other connections predominate and inhibit the weaker connections that exist.
Following damage or loss of input, this inhibition is lost, unmasking these connections.

39. Rapid Changes: Unmasking Existing Connections
FIGURE 4.22 Due to disinhibition, the widely spread and previously silent projections from the thalamus begin to play a functional role. As a result, the receptive field of downstream neurons can expand to contain neighboring structures.

40. Slow Changes: Growth of New Connections
Longer-term changes, over weeks or months, are likely due to the growth of new connections.
If the short-term changes are advantageous, then growth of new connections will follow.

41. Slow Changes: Growth of New Connections
FIGURE 4.23 Growth of new neurites into a region after loss of previous input. (a) Neuron 1 innervates the target; neuron 2 does not. (b) Loss of input to neuron 1 occurs. (c) Neuron 2 projects to target, replacing the input of neuron 1.

42. Changing the Input Channels
The plasticity of the brain enables new forms of input.
For example, the brain can learn to interpret input from a retinal implant.
The BrainPort enables different sensations to be delivered tactilely to the tongue.

43. Changing the Input Channels
FIGURE 4.24 The bionic retinal implant. A camera mounted in front of the eye sends its video feed to an electrode array at the back of the eye.