1. Department of Neuroscience and Physiology, the Sahlgrenska Academy at the University of Gothenburg, Box 434, SE-405 30 Göteborg, Sweden, Mathematical Statistics, Chalmers University of Technology, Mathematical Statistics, University of Gothenburg, SE-412 96 Göteborg, Sweden, and Department of Zoology, University of Gothenburg, SE-405 30 Göteborg, Sweden