Is bigger always better? A critical appraisal of the use of volumetric analysis in the study of the hippocampus

摘要

A well-developed spatial memory is important for many animals, but appears especially importantfor scatter-hoarding species. Consequently, the scatter-hoarding system provides an excellent para-digm in which to study the integrative aspects of memory use within an ecological and evolutionaryframework. One of the main tenets of this paradigm is that selection for enhanced spatial memoryfor cache locations should specialize the brain areas involved in memory. One such brain area is thehippocampus (Hp). Many studies have examined this adaptive specialization hypothesis, typicallyrelating spatial memory to Hp volume. However, it is unclear how the volume of the Hp is relatedto its function for spatial memory. Thus, the goal of this article is to evaluate volume as a mainmeasurement of the degree of morphological and physiological adaptation of the Hp as it relatesto memory. We will briefly review the evidence for the specialization of memory in food-hoardinganimals and discuss the philosophy behind volume as the main currency. We will then examinethe problems associated with this approach, attempting to understand the advantages and limit-ations of using volume and discuss alternatives that might yield more specific hypotheses.Overall, there is strong evidence that the Hp is involved in the specialization of spatial memory inscatter-hoarding animals. However, volume may be only a coarse proxy for more relevant andsubtle changes in the structure of the brain underlying changes in behaviour. To better understandthe nature of this brain/memory relationship, we suggest focusing on more specific and relevant fea-tures of the Hp, such as the number or size of neurons, variation in connectivity depending ondendritic and axonal arborization and the number of synapses. These should generate more specifichypotheses derived from a solid theoretical background and should provide a better understandingof both neural mechanisms of memory and their evolution.