Past events

Over the past 65 million years, the evolution of mammals led - in several lineages - to a dramatic increase in brain size. During this process, some neocortical areas, including the primary sensory ones, expanded by many orders of magnitude. The primary visual cortex, for instance, measured about a square millimeter in late cretaceous stem eutherians but in homo sapiens comprises more than 2000 mm2. If we could rewind time and restart the evolution of large and large brained mammals, would the network architecture of neocortical circuits take the same shape or would the random tinkering process of biological evolution generate different or even fundamentally distinct designs? In this talk, I will argue that, based on the consolidated mammalian phylogenies available now, this seemingly speculative question can be rigorously approached using a combination of quantitative brain imaging, computational, and dynamical systems techniques. Our studies on visual cortical circuit layout in a broad range of eutherian species indicate that neuronal plasticity and developmental network self-organization have restricted the evolution of neuronal circuitry underlying orientation columns to a few discrete design alternatives. Our theoretical analyzes predict that different evolutionary lineages adopt virtually identical circuit designs when using only qualitatively similar mechanisms of developmental plasticity.

The tactile sensations mediated by the whisker-to-barrel cortex system allow rodents to efficiently detect and discriminate objects and surfaces. The temporal structure of whisker deflections and the temporal correlation between deflections occurring on several whiskers simultaneously vary for different tactile substrates. We hypothesize that tactile discrimination capabilities rely strongly on the ability of the system to encode different levels of inter-whisker correlations. To test this hypothesis, we generated complex spatio-temporal patterns of whisker deflections during electrophysiological recordings in the barrel cortex, the ventro-posterior medial (VPM) nucleus of the thalamus and the trigeminal ganglion. A piezoelectric-based stimulator featuring 24 independent and fully adjustable whisker actuators was built for this purpose (Jacob et al., 2010). Using this stimulator in anesthetized rats, we have previously shown that cortical neurons exhibit direction selectivity to the apparent motion of a multivibrissal stimulus (i.e. an emerging property of the global stimulus), uncorrelated to the local direction of individual whiskers (Jacob et al. 2008). Since a certain level of multiwhisker integration has been reported in the VPM, the nucleus relaying tactile information to the barrel cortex, we showed that emergent properties of multiwhisker stimulations are already coded by VPM neurons although to a lesser degree than in cortex (Ego-Stengel et al., 2012). Finally, we applied a reverse correlation approach to this problem by using Gaussian white noise stimulation on 24 whiskers and progressively varying the level of temporal correlation among them. Based on spike-triggered analysis for various levels of inter-whisker correlation, our recent findings (Estebanez et al., 2012) show that neuronal cortical networks implement coexisting coding schemes to cope with the varying statistics of the tactile sensory world. We propose a simple and comprehensive framework that not only accounts for most of the previous reported phenomenology of multiwhisker interactions but also provides a physiological role for this functional selectivity in terms of local contrast and global motion detection.