Translating Time

The Model

Based on: Workman AD, Charvet CJ, Clancy B, Darlington RB, Finlay BL. 2013. Modeling transformations of neurodevelopmental sequences across mammalian species. J Neurosci. 33: 7368-7383


Basic Model 

A general model of neural development is derived to fit 18 mammalian species, including humans, macaques, several rodent species, and 6 marsupial mammals. The empirical data generating the model comprises 271 developmental events. These events include measures of initial neurogenesis, axon extension, establishment and refinement of connections, as well as later events such as myelin formation, brain growth and early behavioral milestones. Our model now predicts events during postnatal development, which occurs up to the third year of human postnatal life. The present model was fitted to the data with the “quasi-Newton” form of optimization, which is discussed in  Workman et al. 2013,. This method accounts for the fact that brains of different species mature at different rates by assigning a slope and a constant to each species. The interaction terms account for the fact that there are heterochronic shifts in a subset of events across species.

The regression predicts post conception (PC) dates for each species, which has the form:

Y = onset + slope*eventscale  + interaction term

This quasi-newton method has been fitted simultaneously to all cells with data, but it also generates estimates for the cells with no empirical data. The correlation between predicted dates and empirical dates is r=0.993 (Workman et al., 2013). Please note that for the data in this model, the values are measured from a species typical estimate of conception (PC 0). The day of birth is considered postnatal day 0 (PN0).

Although we are confident the predictions and comparisons for the neural events that are included in our database will be of value, we suggest careful interpretation. Please consult the confidence interval of the predicted dates. The model will not be accurate for any dates that lie outside the range of empirical events in this database. The user can translate timing of events that fall within the range of data for any of the 18 species in the model. 


Understanding the interaction terms:

Our interaction terms reflect heterochronic changes in developmental schedules between species. Specifically, we have an interaction term for cortical neurogenesis for all examined species (primates, carnivores, ungulate, marsupials ) but the rodents and rabbits. That is, cortical neurogenesis is relatively delayed in all species but the rodents and rabbits. Also, retinal neurogenesis is delayed in the cat with respect to all examined species and has its interaction term (see Workman et al., 2013).