Subcortical Contributions to Massive Cortical Reorganizations

TL;DR: Evidence from noninvasive is capable of extensive reorganization came from a reimaging of evoked activity in the brains of humans with port of a single raccoon that had lost a forearm at some arm amputations that cortex formerly devoted to the unknown time prior to its capture.

Abstract: on the face, much as in the normal cortical representaThe dynamic nature of sensory representations in the tions of the face. The reactivated cortex extended some adult brain gives us the lifelong potential to adapt to 10–14 mm mediolaterally and 9–11 mm rostrocaudally, changes in our environment and to compensate for ina much greater extent than found previously after more jury. Reorganizations in sensory systems typically inlimited nerve damage (Merzenich et al., 1984). Moreover, volve relatively limited shifts in the topography of the similar reactivations undoubtedly took place in higherfunctional representations, and in most reorganizations level somatosensory representations such as the secof the somatosensory cortex only neurons within 1–2 ond somatosensory area, S2, and the parietal ventral mm of the borders of the affected zone in cortex acquire area, PV, since these areas depend on inputs from antenew or altered receptive fields (Merzenich et al., 1984). rior parietal cortex. In fact, for many years, it was assumed that this was The report of extensive reactivation of deprived cortex the maximal distance for plasticity in the adult central in these monkeys was soon followed by a description nervous system. Yet, the adult brain is capable of much of a patient with an arm amputation who felt light touch more extensive changes, involving much larger extents to the face as being both on the face and on the fingers of the nervous system. Large-scale peripheral deafferof the missing arm (Ramachandran et al., 1992). This entations, such as limb amputation or spinal cord damobservation suggested that the brains of such individuage, lead to extensive reactivations of the large regions als reorganize after amputation, as in the monkeys with of somatosensory cortex deprived by the injury. These arm deafferentation, and that touching the face leads unusually large changes in cortical organization are not to activation of both face and reactivated hand zones easily explained by cellular mechanisms involving the of cortex, hence the misplaced and double sensations. potentiation of previously existing connections. HowThis supposition was soon given further credibility when ever, there is now evidence that new connections may it became possible to record from the cortex of three grow into regions deprived of primary afferent inputs as monkeys with long-standing therapeutic amputations of a result of injury. Subcortical sensory representations the hand or forelimb. Recordings from area 3b in these are smaller than their cortical counterparts; therefore, monkeys revealed a reactivation of the deprived hand even limited new growth subcortically can lead to masand forelimb cortex by inputs from the stump of the arm sive cortical reactivations. and the face (Figure 1; see also Florence and Kaas, The first suggestion that the somatosensory system 1995). In addition, there is evidence from noninvasive is capable of extensive reorganization came from a reimaging of evoked activity in the brains of humans with port of a single raccoon that had lost a forearm at some arm amputations that cortex formerly devoted to the unknown time prior to its capture (Rasmusson et al., missing hand comes to be activated when the face is 1985). The large representation of the forepaw in primary stimulated (e.g., Flor et al., 1995). Finally, after deactivasomatosensory cortex of this animal had been comtion of large extents of somatosensory cortex by tranpletely reactivated by an expanded representation of section of the dorsal columns of the spinal cord at a high the stump. However, the finding received little attention, cervical level (Jain et al., 1997), the large unresponsive perhaps because the age of the animal when the injury zones of cortex in areas 3a, 3b, and 1 became responoccurred was not known, so it was not conclusive that sive to inputs from the face and the few dorsal column the extensive change reflected mechanisms of adult afferents from the anterior arm that entered the spinal plasticity. Several years later, a subsequent report of cord above the cut (Figure 1). These related findings demextensive cortical change convinced the research comonstrate beyond any doubt that large extents of somatomunity that the adult brain is capable of enormous sensory cortex, when deprived of normal sources of