Finally, some numbers are the result of estimations based on correlations observed between number of cortical neurons and brain mass within closely related taxa. This is in part why it may be considered to be less reliable. A variation of the optical fractionator was responsible for the previous total human brain neuron count of 100,000,000,000 neurons, which has been revised down to 86,000,000,000 by the use of the isotropic fractionator. Most numbers in the list are the result of studies using the newer isotropic fractionator. These are the optical fractionator, an application of stereology and the isotropic fractionator, a recent methodological innovation. The elephant depends on its exceptionally large cerebellum, while birds make do with a much smaller one.Дiffering methods have been used to count neurons and these may differ in degree of reliability. It accounts most accurately for variations in dependence on the cerebellum. The current best predictor is number of neurons in the forebrain, based on Herculano-Houzel's improved neuron counts. One early speculation was brain size (or weight, which provides the same ordering.) A second proposal was brain-to-body-mass ratio, and a third was encephalization quotient, sometimes referred to as EQ. The question of what physical characteristic of an animal makes an animal intelligent has varied over the centuries. List of animal species by forebrain (cerebrum or pallium) neuron number

How many brains does an octopus have