Please cite this article in press as: Will B, et al., The concept of brain plasticity—Paillard’s systemic analysis and emphasis on structure and function (followed by the translation of a seminal paper by Paillard on plasticity), Behav Brain Res (2008), doi:10.1016/j.bbr.2007.11.008 Available online at www.sciencedirect.com Behavioural Brain Research xxx (2008) xxx–xxx Research report The concept of brain plasticity—Paillard’s systemic analysis and emphasis on structure and function (followed by the translation of a seminal paper by Paillard on plasticity) Bruno Will a,* , John Dalrymple-Alford b , Mathieu Wolff b , Jean-Christophe Cassel a a LINC, UMR 7191, ULP-CNRS, IFR des Neurosciences 037, GDR 2905 CNRS, 12 rue Goethe, F-67000 Strasbourg, France b Van der Veer Institute for Parkinson’s and Brain Research, and Department of Psychology, University of Canterbury, Private Bag 4800, Christchurch 8140, New Zealand Received 21 October 2007; accepted 6 November 2007 Available online 5 December 2007 Abstract Although rejected for the most part of the 20th Century, the idea of brain plasticity began to receive wide acceptance from the 1970s. Yet there has been relatively little theoretical comment on the definition and use of “plasticity” in the field of neurobiology. An early exception to this lack of critical reflection on neural plasticity was provided by Jacques Paillard in a seminal paper that he published in 1976 [Paillard J. R´ eflexions sur l’usage du concept de plasticit´ e en neurobiology. J Psychol 1976;1:33–47]. As this valuable contribution was published in French, the present authors provide an English adaptation to help convey his ideas to an international audience, together with a contemporary commentary on this paper. Paillard’s definition of the term “plasticity” is probably as pertinent today as it was 30 years ago, especially in terms of its relevance to multiple levels of analysis of brain function (molecular, cellular, systemic). Sadly, Jacques Paillard died in 2006; our comments therefore also include a brief biographical tribute to this outstanding neuroscientist. Keywords: Brain; Concept; Elasticity; Flexibility; Function; Nervous system; Structure 1. Introduction The principle of brain plasticity is readily acknowledged in contemporary neuroscience, but its general acceptance is rela- tively recent, beginning in the 1970s. Notions of neuroplasticity had certainly existed previously (e.g., [5]), but the broad concept became current only after the early findings on enriched envi- ronments (e.g., [36]) and visual deprivation (e.g., [18]) had been established. Resistance against the principle of brain plasticity was probably mainly due to the influence of the great Span- ish neuroanatomist, Santiago Ramon y Cajal, who had firmly postulated that neural connections in the adult brain are fixed and immutable [35]. From this perspective, it is intriguing that Ramon y Cajal himself had speculated that mental exercise, such as learning a musical instrument, might be associated with an increase in the growth of new axon collaterals and new termi- * Corresponding author. Tel.: +33 390 242 007; fax: +33 390 241 958. E-mail address: bruno.will@linc.u-strasbg.fr (B. Will). nal dendrites [34]. This conjecture, which was made prior to the use of the word “synapse” by Sherrington in 1897, was a forerunner to the more recent speculation on “cell assemblies” [17]. Of course, previously neglected concepts of plasticity are universally endorsed in contemporary neuroscience (e.g., for plasticity of spinal neural circuitry [10]; for discussions of the functional properties of neurogenesis, see [2,14,21]. One chal- lenge facing contemporary neuroscience is, however, the almost unbridled proliferation of examples of “brain plasticity”. This apparently simple and attractive concept is instead an extraor- dinary complex and elusive issue, exacerbated by the fact that the idea is conveyed differently by different subdisciplines and often at multiple levels of analysis (from genetic to behavioural). This important issue was encapsulated by an early theoretical paper on this topic [31]. As many of the inherent problems with plasticity remain unanswered today, the current paper provides a contemporary perspective on Paillard’s ideas, accompanied by an English translation of his original article. Many of Pail- lard’s comments are perhaps as relevant today as they were at the original time of writing, because they challenge researchers 0166-4328/$ – see front matter doi:10.1016/j.bbr.2007.11.008