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The human visual system is remarkably good at categorizing objects even in challenging visual conditions. Here we specifically assessed the robustness of the visual system in the face of large contrast variations in a high-level categorization task using natural images. Human subjects performed a go/no-go animal/nonanimal categorization task with briefly flashed grey level images. Performance was analysed for a large range of contrast conditions randomly presented to the subjects and varying from normal to 3% of initial contrast. Accuracy was very robust and subjects were performing well above chance level (approximately 70% correct) with only 10-12% of initial contrast. Accuracy decreased with contrast reduction but reached chance level only in the most extreme condition (3% of initial contrast). Conversely, the maximal increase in mean reaction time was approximately 60 ms (at 8% of initial contrast); it then remained stable with further contrast reductions. Associated ERPs recorded on correct target and distractor trials showed a clear differential effect whose amplitude and peak latency were correlated respectively with task accuracy and mean reaction times. These data show the strong robustness of the visual system in object categorization at very low contrast. They suggest that magnocellular information could play a role in ventral stream visual functions such as object recognition. Performance may rely on early object representations which lack the details provided subsequently by the parvocellular system but contain enough information to reach decision in the categorization task.

International audience; The generators and functional correlates of gamma oscillations within the sensorimotor cortex remain unclear. With the goal of locating the oscillations' sources precisely and then studying the relationship between oscillatory reactivity and ongoing movement, we recorded stereoelectroencephalograms with intracerebral electrodes in eight epileptic subjects awaiting surgical treatment. The sensorimotor cortex was free of lesions and was exhaustively explored with the electrodes. Subjects were asked to perform various self-paced movements contralateral to the exploration zone, brief and sustained, distal movements and a pointing movement. We used the event-related desynchronization method to quantify the reactivity of the 40-60-Hz band before, during and after the performance of movement. A very focused, event-related synchronization of gamma rhythms was found in all subjects. It was predominantly observed in the primary sensorimotor area and its distribution was consistent with the functional map established using electrical stimulations. Two different temporal patterns were observed, the event-related synchronization of gamma rhythms was related either to movement onset or to movement offset but was never recorded before movement. This observation suggests that gamma oscillations are more probably related to movement execution than to motor planning. The different patterns argue in favour of multiple functional roles; it has been shown that gamma oscillations may support the efferent drive to the muscles and here we show that they are also likely to be related to somatosensory integration. We therefore suggest that gamma oscillations in the 40-60-Hz band may support afferent sensory feedback to the sensorimotor cortex during the performance of movement.

International audience; The mRNAs encoding the synaptic vesicle proteins syntaxin 1B and synapsin I were measured using in situ hybridization in several brain regions--the dentate gyrus, CA3 and CA1 of the hippocampus, the parietal, the motor and prefrontal cortices and the core and shell of the accumbens--of rats that were learning a spatial reference or working memory task on a radial arm maze. The mRNA encoding syntaxin 1B was significantly increased in all hippocampal regions in rats learning the working memory task, whereas it was increased in the prelimbic area of the prefrontal cortex and the shell of the accumbens in rats learning the spatial reference memory task. No change in mRNA encoding syntaxin 1B was observed in the motor and parietal and cortices or the core of the accumbens, and the mRNA encoding synapsin I was not significantly different from that of naive caged controls or rats running the maze for continuous reinforcement in any of the brain structures examined. These results demonstrate that the gene encoding a key member of synaptic vesicle function is up-regulated in a task- and brain-specific manner during learning. They are discussed in terms of the potential role this protein may play in trans-synaptic propagation of plasticity within specific neural networks as a function of the information required in the laying down of different types of memory.