Proxies for animal intelligence have varied over the centuries. One early suggestion 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 current best predictor is number of neurons in the forebrain, based on Herculano-Houzel's improved neuron counts. This accounts for variation in the number of neurons in the rest of the brain, for which no link to intelligence has been established. Elephants, for example, have an exceptionally large cerebellum, while birds make do with a much smaller one. Differing methods have been used to count neurons, and these may differ in degree of reliability. The primary methods are the optical fractionator, an application of
stereology and the isotropic fractionator, a recent methodological innovation. Most numbers in the list are the result of studies using the newer isotropic fractionator. 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. This is in part why it may be considered to be less reliable. Finally, some numbers are the result of estimations based on correlations observed between number of cortical neurons and brain mass within closely related taxa. The following table gives information on the number of neurons estimated to be in the
sensory-associative structure: the
cerebral cortex (aka
pallium) for mammals, the dorsal ventricular ridge ("DVR" or "hypopallium") of the pallium for birds, and the
corpora pedunculata ("mushroom bodies") for insects. == See also ==