Plastic changes can also occur after injury to the brain in adults. Sprouting of new axons does occur in the damaged CNS; however, the sprouts do not necessarily restore normal function, and many neural pathways do not appear to produce sprouts. Additional knowledge concerning neural plasticity in the adult CNS is vital if medical therapy is to be improved for many diseases of the CNS and after neural trauma. Research is currently being conducted to explore •Fig. 10.10 Plasticity in the Visual Pathway as a Result of Sensory Deprivation During Development. Theoculardominancecolumnsaredemonstratedbyautoradiographyafterinjectionofaradioactivetracerintooneeye.Thetraceristransportedtothelateralgeniculatenucleusandthentransneurallytransportedtothestriatecortex.Thecortexislabeledinbandsthatalternatewithunlabeledbandswhoseinputisfromtheuninjectedeye.A, Normalpattern.B, Changedpatterninananimalraisedwithmonocularvisualdeprivation.Theinjectionwasmadeintothenondeprivedeye,andtheoculardominancecolumnsforthiseyewereclearlyexpanded.Otherexperimentsshowedthattheoculardominancecolumnsforthedeprivedeyecontracted.(A, FromHubelDH,WieselTN.Proc R Soc Lond B. 1977;198:1.B, FromLeVayS,etal.J Comp Neurol. 1980;191:1.) the potential of human embryonic stem cells for restoring CNS function.
Phantom limb sensation is an example of neural plasticity in adults. A patient whose limb has been amputated often perceives sensations on the missing limb when stimulated elsewhere on the body. Functional imaging studies suggest that this is a result of the spread of connections from the surrounding cortical territories into the cortical region that had served the amputated limb.
Itwastraditionalpolicytodelaycorrectivesurgeryforachildbornwithacongenitalcataractuntilthechildwasolderandmoreabletocopewiththestressofsurgery.However,ifthecorrectionisdeferreduntilafterthe“criticalperiod,”fullrecoveryoffunctionisunlikely.Similarly,childrenbornwithamblyopia, aconditioncharacterizedbystrabismus(crosseye)becauseofrelativeweaknessofoneoftheextraocularmuscles,tendtousetheunaffectedeyeinpreference.Inbothcases,earlysurgeryisnowcommonpracticesothatthecorticalcircuitrycanbecorrectlysculptedbybalancedinputfromthetwoeyes.
Such remapping can also occur after surgical amputation of the second and third digits of the hand. Before surgery, each of the digits was represented in discrete and somatotopically organized areas of the postcentral gyrus (SI cortex). After surgery, the area that represented the amputated digits is now mapped with an enlarged representation of the adjacent digits (
Fig. 10.11 ). Conversely, individuals born with syndactyly (fusion of two or more digits of the hand) have a single or mostly overlapping representation of these digits in the SI cortex. After corrective surgery, the independent digits come to have distinctive representations. Even more remarkable is that monkeys that were trained on a sensory discrimination task requiring repeated daily use of their fingertips showed cortical differences after training. Not only were the SI cortical territories of their fingertips larger than before training but also the number of cortically recorded receptive fields on the fingertips was likewise increased.