Cortical Plasticity Coursework Materials

Cortical plasticity Plasticity  The ability to change. o Neural plasticity: neurons, or some of their properties, can change. o Cortical plasticity: changes that occur in the cortex (whole regions).  Our brain is shaped by experience, and can learn and remember patterns in the sensory world. This helps to: o Refine movements. o Recover function after injury. Developmental plasticity – visual system  Segregation of geniculo-cortical afferents is an experiencedependent process during the early developmental stage.  Cells within the cortex are supplied by both eyes. o However, usually respond best to one eye or the other.  Cells are arranged into columns, ocular dominance columns, on the basis of the dominant eye.  Process is governed by visual experience. o Usually begin with some overlap between the two eyes. o However, with normal visual experience, there is separation of the axons.  Representing ocular dominance columns:  o Neurons classified on basis of responsiveness to one eye or other. o Those classed as 1 respond exclusively to opposite eye. o Those classed as 7 respond exclusively to same side. o Neurons classed as 4 respond equally well to either eye. When opposite eye is occluded, the other eye becomes dominant.  Ocular dominance is restricted to a sensitive period of development. o If subject is affected early on, normal ocular dominance will be affected. o However, after a certain period (>4 months), little change occurs. Sensory cortex plasticity  Touch, vision and hearing all represent their sensory epithelial topographically.  Somatosensory cortex: somatotopic.  Visual cortex: retinotopic. o Stimulating one part of the retina causes certain cells to fire to one part of the visual cortex. o Moving the stimulation slightly causes cells to fire in adjacent regions.  Auditory cortex: tonotopic. o Frequency-specific organisation. o Neurons which respond best to certain frequencies are grouped together, often in columns. o Adjacent to these columns lie neurons that respond to similar frequencies, but not the same.  These maps are not fixed.  Peripheral manipulations and behaviourally relevant experiences can change them.  Therefore, these are use-dependent plasticities. Plasticity in somatosensory cortex  Removing a digit from a monkey caused reorganisation of the somatosensory cortex.   o Area of cortex normally response to digit 3 now responsive to digits 2 and 4. Plasticity of the somatosensory cortex has also been shown in human subjects: o Amputation 6cm above elbow, however, felt hand was attached just below stump. o Reported simultaneous feeing in face region and tingling in digit. o This has been classed as phantom limb pain. Severity of phantom limb pain varies: o Some experience severe pain on a daily basis. o Others feel a tingly sensation. Plasticity of auditory cortex  Auditory cortex represents frequencies in an orderly manner.  Extensive reorganisation of this ordered representation occurs following hearing loss.  Research has shown the plasticity of the auditory cortex of a cat: o Discrete hearing loss of frequencies above ~18.5 – 19kHz.    o A significant number of neurons that use to be sensitive to higher frequencies are now responsive to frequencies ~18.5 – 19kHz. One characteristic of the auditory system is that, throughout the brainstem, higher frequencies tend to inhibit lower frequencies. Following a hearing loss or lesion, inhibition is lifted and excitability increases in a restricted frequency band. These frequencies may dominate when competing for cortical space.