A Light and a Dark Side Cortical

concepts A light and a dark side Cortical

of two fingers can become fused. reorganization Thomas Elbert and Sabine Heim Musicians provided one of the first exam- he gigantic network that makes up the ples of this phenomenon in humans: string “The brain’s representations of the human cerebral cortex is dynamically players usually practise for hours a day over periphery are dynamic and Tand unceasingly reorganizing itself. many years. While playing, the digits of the This continuously changing entity is not left hand are continuously fingering the continuously modified by easy to track, but we can glimpse the living strings, but the right-hand task of manipu- experience.” brain by observing the organization within lating the bow involves much less processing zones representing the first stages of sensory of information from individual fingers. In input from the outside world. Primary string players, the cortical representations of processing levels. After a brain lesion there auditory, visual and somatosensory cortices the somatosensory left-hand regions were are initial deficits in behaviour, perceptual or mirror the spatial arrangement of the found to be expanded compared with those cognitive skills, but often spontaneous relief respective peripheral receptors: tonal fre- of the right hand and compared with the left of symptoms. Cortical reorganization may quency (place in the cochlea), visual space hand of non-musicians. be crucial for such recovery. New interven- (place on the retina) or body surface are What holds for the somatosensory cortex tion strategies in conditions such as hemi- represented in the form of maps imprinted also holds for auditory processing: larger plegia, writer’s cramp, phantom-limb pain, on the cortical sheet. responses are elicited in the auditory cortex aphasia or language-based learning disor- Although genetically encoded programs when musicians hear their own instrument ders provide a ‘bright side’ ready to be control the connections of these maps from played than are elicited by tones from other exploited by neurorehabilitation. the periphery to the cortical destination, their instruments. Perceptual correlates of map The dark side occurs when a peripheral organization ultimately depends on the effi- alteration indicate superior performance — lesion, by itself manageable, is triggered by an cacy of the synapses connecting the nerve cells one ‘bright side’ of cortical plasticity. unusually intense, stressful experience to comprising the network, which is affected by The most remarkable alteration of corti- cause a negative or catastrophic cortical re- environmental experience. For instance, two cal maps has been observed in amputees. The organization, such as that underlying phan- receptors of the same fingertip are more fre- nonstimulated area — for instance the arm tom-limb pain and tinnitus. Fusion of repre- quently activated in sync than are two recep- region — remains actively engaged in infor- sentational zones is at the core of dystonia tors of different digits. According to the heb- mation processing of nearby zones. As a con- (such as musician’s cramp or writer’s cramp); bian learning model, synchronous stimula- sequence, the adjacent face and shoulder Michael Merzenich’s notion that dyslexia tion would lead to connections between the areas invade what was formerly arm territory. and even autism may be a consequence of representations of the same fingertip but to a There is a close association between this inva- inadequate functional organization (of the separation from those of the other digits: sion and adverse symptoms such as phan- phonemotopic representation) stirs an excit- representational zones are shaped by the tem- tom-limb pain and, in the auditory system, ing new avenue of research. And what about poral pattern of coincident experience. An tinnitus. Because treatment designed to extreme experiences, such as traumatic stress alteration in behaviourally relevant input reverse this invasion reduces phantom-limb or childhood abuse? Do they have the power towards the cortex will trigger a reorganiza- pain, the adverse response is probably an to trigger reorganization of maps beyond the tion — alteration — of the map. The repre- unwanted consequence of the dynamic brain. representational cortex, with consequent sentation of a fingertip, for instance, can be This maladaptive disaster reveals the dark secondary disasters? enlarged; representations of adjacent fingers side of cortical reorganization. In a nonlinear, dynamic, self-organizing can invade its territory; or the representation The functional organization of one level of system such as the brain, small causes can the cortex is governed by the interplay of earli- have large effects. The brain should not be er and later representational stages of the studied just in terms of monocausal genetic sensory processing stream. In mammals, the and environmental influences. Learning is thalamus (one of the highest processing stages the adaptation of brain dynamics — when it of the reptilian brain) feeds sensory informa- has been disastrous, behavioural techniques tion into the cortex. The cortex in turn feeds (in conjunction with appropriate medica- processed information back to the thalamus. tion) may be the remedy. This prospect is the When these top-down projections are inacti- bright side of the dark side. ■ BRIDGEMAN ART LIBRARY BRIDGEMAN ART vated, thalamic functional organization is Thomas Elbert and Sabine Heim are in the dramatically degraded. There are nearly ten Department of Psychology, University of Konstanz, times as many fibres projecting top-down Box D25, 78457 Konstanz, Germany. from the representational cortex to the thalamus as there are bottom-up from the FURTHER READING thalamus to the cortex. Through this route, Buonomano, D. V. & Merzenich, M. M. Annu. Rev. top-down processing determines the way in Neurosci. 21, 149–186 (1998). which sensory information is organized, Gilbert, C. D. Physiol. Rev. 78, 467–485 (1998). filtered and processed. What we perceive is van Praag, H., Kempermann, G. & Gage, F. H. Nature not only a matter of the incoming event but Rev. Neurosci. 1, 191–198 (2000). also a highly edited configuration of top- Elbert, T., Heim, S. & Rockstroh, B. Neural plasticity down organization and reorganization. and development. In Handbook of Developmental It is not surprising, then, that lesions in Cognitive Neuroscience. (eds Nelson, C. & Luciana, M.) Cortical representation in musicians, such as the central nervous system can lead to alter- (MIT Press, Cambridge, MA, 2001). Niccolò Paganini, reflects their art. ations in functional organization at all Nudo, R. J. Curr. Opin. Neurobiol. 9, 740–747 (1999).