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4: Neuroplasticity Cognitive Neuroscience David Eagleman Jonathan Downar 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 2 The Brain Dynamically Reorganizes to Match Its Inputs Changes to the Body Plan Changes to Sensory Input 3 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. 4 Changes in the Body Plan The homunculus is the map of the body within the sensory and motor cortices. 5 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. 6 Changes in the Body Plan 7 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. 8 Changes to Sensory Input 9 The Brain Distributes Resources Based on Relevance The Role of Behavior The Role of Relevance: Gating Plasticity with Neuromodulation 10 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. 11 The Role of Behavior 12 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. 13 The Role of Relevance: Gating Plasticity with Neuromodulation 14 The Brain Uses the Available Tissue Maps Adjust Themselves to the Available Brain Tissue Cortical Reorganization after Brain Damage 15 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. 16 Maps Adjust Themselves to the Available Brain Tissue 17 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. 18 A Sensitive Period for Plastic Changes A Window of Time to Make Changes The Sensitive Period in Language Neuromodulation in Young Brains 19 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. 20 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. 21 The Sensitive Period in Language 22 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. 23 Hardwiring versus World Experience Aspects of the Brain Are Preprogrammed Experience Changes the Brain Brains Rely on Experience to Unpack Their Programs Correctly 24 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. 25 Aspects of the Brain Are Preprogrammed 26 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. 27 Experience Changes the Brain 28 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. 29 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. 30 Brains Rely of Experience to Unpack Their Programs Correctly 31 The Mechanisms of Reorganization Neurons Compete for Limited Space Competition for Neurotrophins Rapid Changes: Unmasking Existing Connections Slow Changes: Growth of New Connections 32 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. 33 Neurons Compete for Limited Space 34 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. 35 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. 36 Competition for Neurotrophins 37 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. 38 Rapid Changes: Unmasking Existing Connections 39 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. 40 Slow Changes: Growth of New Connections 41 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. 42 Changing the Input Channels 43