Stereological methods for estimating the total number of neurons and synapses: issues of precision and bias

The emergence of a new generation of stereological techniques for counting objects in histological sections has prompted a debate about whether or not these methods are better than previously available techniques when they ar e used to make estimates of the total numbers of neurons and synapses in a neural structure. During this debate, the concepts of an unbiased estimate and that of a precise estimate have often been confused. A full understandi ng of the distinction between these two separate aspects of an estimate is required in order to be able to appreciate the virtues of these new countin g methods and to apply them correctly. This review intends to make the fund amental issues of this debate more clear, and describes (I) the fundamental differences between the newer design-based counting techniques and previou sly available assumption-based techniques, and (2) the distinction between an unbiased estimate and a precise estimate.