Introduction

One of the many ambitious goals set in our era of neuroscience is to understand the brain modifications that underlie learning — first, to identify the relevant neuronal circuits and second, to describe the cellular changes occurring within those circuits. For nearly three decades1 long-term potentiation and long-term depression have been investigated, and synaptic plasticity of this sort is now believed to constitute the cellular change underlying most types of learning.2 But progress in the first of the two issues has been slower: we still know relatively little about the locus of brain modifications during learning of even the simplest tasks. Exceptions do exist: the cerebellar cortex and interpositus nucleus clearly form a locus of plasticity during certain precisely-timed conditioned reflexes3 and the amygdala, a critical locus during certain types of emotional conditioning.4 Yet, consider how little is known about what parts of the brain are involved when a monkey learns to remember visual stimuli across a delay. Lesion experiments point to an important role of the hippocampus,5 but even the claim of the hippocampal locus can be called into question.6 Thus, while the hippocampus is the most systematically studied mammalian brain area when it comes to synaptic modification mechanisms, it remains difficult to pin down its involvement in any well-defined learning task.

Here, we will not address the problem of what learning consists of at the cellular level. Our purpose is another one, namely to consider where the neuronal modifications that underlie learning are stored and accessed in cerebral cortex. Our focus is on sensory learning, the improvement in sensory-perceptual ability that occurs with training in a specific task. Since the question "How are learning-related modifications distributed?" is poorly posed, we need to make the problem much more precise to have any hope of arriving at a concrete conclusion. To this end, we have focused on tactile learning, and have expressed the question in a more specific way: "Does the somatotopic organization that characterizes sensory cortex constitute a structural framework for perceptual learning and memory?"

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