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 p ropose that the longer and deeper the fibres of the corticospinal tract, th e greater an animal's digital dexterity. Because of the effects that phylog eny may have upon the extant phenotype of a given species, however, data fr om a wide range of species can rarely be considered to represent fully inde pendent data points. Using modern comparative statistics, which incorporate phylogenetic information, we reanalysed their data set such that the assum ption of independence was not violated. If Heffner and Masterton's hypothes is is correct, then one would expect evidence of strong correlated evolutio n between corticospinal tract anatomy and digital dexterity once the effect s of the phylogenetic relationships between the species in the data set hav e been removed. The results show that a distinct bias in the number of prim ate species sampled by Heffner and Masterton significantly affected their f indings. 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-ey e coordination. Based upon our results we recommend the use of modern compa rative statistics for synthesising neural structure and behaviour, rather t han examining structure-function relationships in an ahistorical context. I t 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 relatio nship between the corticospinal tract structure and motor behaviour for mam mals as a whole can be more readily interpreted. (C) 1999 Elsevier Science B.V. All rights reserved.