Is digital dexterity really related to corticospinal projections?: a re-analysis of the Heffner and Masterton data set using modern comparative statistics.

Using a data set of 69 different mammalian species, Heffner and Masterton propose that the longer and deeper the fibres of the corticospinal tract, the greater an animal's digital dexterity. Because of the effects that phylogeny may have upon the extant phenotype of a given species, however, data from a wide range of species can rarely be considered to represent fully independent data points. Using modern comparative statistics, which incorporate phylogenetic information, we reanalysed their data set such that the assumption of independence was not violated. If Heffner and Masterton's hypothesis is correct, then one would expect evidence of strong correlated evolution between corticospinal tract anatomy and digital dexterity once the effects of the phylogenetic relationships between the species in the data set have been removed. The results show that a distinct bias in the number of primate species sampled by Heffner and Masterton significantly affected their findings. Furthermore, once phylogeny has been taken into account, only the length of the corticospinal tract fibres showed a significant relationship with two out of the four behaviours analysed, digital dexterity and hand-eye coordination. Based upon our results we recommend the use of modern comparative statistics for synthesising neural structure and behaviour, rather than examining structure function relationships in an ahistorical context. It is also evident that there is a need for data on the length and depth of the corticospinal fibres for a greater range of species so that the relationship between the corticospinal tract structure and motor behaviour for mammals as a whole can be more readily interpreted.