Sensory Learning and the Brain's Body Map

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 decades ¹ 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 reflexes ³ 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 184mammalian brain area when it comes to synaptic modification mechanisms, it remains difficult to pin down its involvement in any well-defined learning task.