Physiology unit, Department of Neuroscience, Box 593, Uppsala, Sweden. goran.sperber@neuro.uu.se|Physiology unit, Department of Neuroscience, Box 593, Uppsala, Sweden. goran.sperber@neuro.uu.se|Physiology unit, Department of Neuroscience, Box 593, Uppsala, Sweden. goran.sperber@neuro.uu.se|Physiology unit, Department of Neuroscience, Box 593, Uppsala, Sweden. goran.sperber@neuro.uu.se|Physiology unit, Department of Neuroscience, Box 593, Uppsala, Sweden. goran.sperber@neuro.uu.se