Acknowledgements
The authors would to thank Pascal Fossat and Frédéric Nagy for providing the nociceptive flexion reflex data used in the regression analysis. The first author was supported by the Centro de Matemática da Universidade do Porto, financed by FCT through the programmes POCTI and POSI, with Portuguese and European Community structural funds, and by a FCT Grant PTDC/SAU-NEU/68929/2006.