John P Donoghue. Brown University, Providence, USA.
Plasticity of sensory and motor cortical and subcortical representations in the adult brain appears to be a general phenomenon in animals that has now been extended to humans. There is a growing understanding of the mechanisms and rules that regulate the form and extent of reorganization; these appear to include activity-dependent control of synaptic efficacy, details of circuit arrangements, and growth of new axonal arbors. Of particular relevance to plasticity of cerebral cortical sensorimotor representations is recent evidence for the participation of intracortical horizontal pathways. These fibers provide a substrate for reorganization and contain mechanisms for increases or decreases in synaptic efficacy that depend on particular spatiotemporal activation patterns. Functional reorganization of adult sensory and motor pathways, once an uncommon notion, is now a widely documented phenomenon. A substantial body of information (which will be described below), particularly from the past 10-15 years of research, has demonstrated that neuronal properties in auditory, somatic sensory, visual and motor circuits cm be reorganized in the adult mammalian CNS, either in response to lesions of peripheral or central structures or by experience alone. A general phenomenon that has repeatedly been described is that neurons in one area assume properties of neurons in another, usually adjacent, area. Simply stated, representations in a particular sensory or motor realm are said to expand (or contract) so that they occupy a larger (or smaller) plot of neural ‘real estate’, an effect that is termed ‘representational plasticity’ (see [1*,2*,.3] for recent examples). A number of excellent reviews of these plasticity effects are available (see e.g. [3-.5]). The aim of this review is to discuss recent findings confirming representational plasticity and showing the extent location and mechanisms of plasticity in sensorimotor circuits.
Evidence for plasticity in the human CNS
A number of new methodologies allowing non-invasive evaluation of brain function and organization have made it possible to investigate the potential for sensorimotor reorganization in the human CNS. These studies have generally confirmed results obtained from earlier experimental investigations in animals. Consistent with the idea of expansion of cortical representations, positron emission tomography (PET) studies have shown that arm amputees activate a wider than normal expanse of sensorimotor cortex when making shoulder movements [C,], suggesting that the remaining arm muscles have an expanded representation. Transcranial magnetic stimulation, which allows mapping of movements evoked by stimulation using a coil placed above the region of the motor cortex, has also identified enlarged representations of proximal muscles in amputees [7-9]. These observations have now been more directly examined using invasive mapping techniques in a tumor patient who was also an amputee [10*]. Magnetic field recordings of somatic sensory potentials in human arm amputees indicate an expansion of the face representation of the primary somatic sensory cortex (SI) into the SI arm area, whereas in normal subject, the representation of sentient fingers expanded when the other fingers were transiently anesthetized [ 12]. Studies in human have also provided valuable new information concerning plasticity by showing that stimulation of peripheral nerves in amputees or nerve-transected patients continuous to elicit sensation in the deafferented tissue [3*,14]. This type of finding would have been difficult to obtain in non-human investigations. Because perception of missing body parts persists in these patients, central representations cannot be entirely absent. It would be of interest to know what maintains representations devoid of their input. A further implication of these findings is that a reduced extent of a sensory representation can still provide a sensory percept. Whether or not this percept is normal remains to be determined.