The organization of behaviour revisited.

Abstract Donald Hebb pioneered many current themes in behavioural neuroscience. He saw psychology as a biological science, but one in which the organization of behaviour must remain the central concern. Through penetrating theoretical concepts, including the "cell assembly," "phase sequence," and "Hebb synapse," he offered a way to bridge the gap between cells, circuits and behaviour. He saw the brain as a dynamically organized system of multiple distributed parts, with roots that extend into foundations of development and evolutionary heritage. He understood that behaviour, as brain, can be sliced at various levels and that one of our challenges is to bring these levels into both conceptual and empirical register. He could move between theory and fact with an ease that continues to inspire both students and professional investigators. Although facts continue to accumulate at an accelerating rate in both psychology and neuroscience, and although these facts continue to force revision in the details of Hebb's earlier contributions, his overall insistence that we look at behaviour and brain together -- within a dynamic, relational and multilayered framework --remains. His work touches upon current studies of population coding, contextual factors in brain representations, synaptic plasticity, developmental construction of brain/behaviour relations, clinical syndromes, deterioration of performance with age and disease, and the formal construction of connectionist models. The collection of papers in this volume represent these and related themes that Hebb inspired. We also acknowledge our appreciation for Don Hebb as teacher, colleague and friend. The importance of population coding (dynamic "ensembles" of neural activity) is demonstrated by Airikian and Georgopoulos in studies of the direction of reach in primate movement, and movement planning. Behaviourally relevant contextual modulations of receptive field properties are demonstrated through shifting patterns of somatosensory representations in the striatum by Lidsky and Brown. Foehring and Lorenzon examine current data related to the issues of neuronal plasticity in terms of Hebb's early and insightful speculations of necessary congruence of presynaptic/postsynaptic events. Johnson demonstrates Hebb's view that learning must be studied in the context of developmental change, and that by combining animal (imprinting) and human (face recognition) studies, common themes related to the historical nature/nurture issue can be clarified. Kolb then demonstrates a similar importance for developmental considerations in analyses of recovery of function, with analyses that combine careful measures of behaviour and anatomical analyses of dendritic structure. Next, Jacobs and Nadel pursue questions of plasticity into issues of human representations that may follow dramatic and incompletely assimilated early experiences, revealed by panic attacks and phobias. Salmon, Butters and Chan extend developmental analyses into later life, with a particular focus upon the deterioration of representations in Alzheimer's Disease. Wickelgren applies formal neo-associationist models to dynamic properties of representation in humans, thus rounding out the picture of modeling that can help guide future neurobehavioural research. It is clear from these papers of current research that Donald Hebb's legacy remains rich, multifaceted, and insightful. His work continues to inspire a growing body of research in both psychology and neuroscience, at both conceptual and empirical levels. Most importantly, his insights and dissatisfaction with either behaviouristic approaches that ignore brain, or brain studies that fail to refer to behaviour, have forged an interdisciplinary field of behavioural neuroscience, a true biological psychology, that continues to excite and challenge us today. Donald Hebb: Science and the Man AN OVERVIEW When, as then-chair of psychology at Dalhousie, I had the opportunity to invite Donald Hebb to join our university, after he retired from McGill in 1977, it was an enormous personal pleasure. …