EEG and MEG source imaging in duration/pitch comparison

N’Diaye K. (speaker), Garnero L., Pouthas V.
CNRS-UPR640 LENA, Paris, France

Recent years have seen a renewed interest in the study of time perception in the range of sub-second durations. Even though a distributed network of brain areas has been repeatedly evidenced in timing tasks, little is known on the chronometry of their respective involvement. To address this issue, we simultaneously recorded electro- and magnetoencephalography (EEG / MEG) as human participants were performing a duration comparison task (600/900 ms empty intervals marked by 20ms brief tones). Moreover, to assess the specificity of the brain response towards timing-related processes we contrasted this task with a non-temporal one (pitch comparison) using the exact same set of stimuli. Delayed motor selection and preparation was enforced by randomizing response mapping. Topography and time-course of regional activity over the cortical surface was reconstructed from scalp signals through minimum L2-norm source estimation. Hit rates over 80% (without significant differences between the two tasks) confirmed that subjects selectively attended to the correct dimension. Both encoding and comparison phases evoked slow potentials/fields distinct from sensory evoked responses. Distributed source analysis revealed an extended network of cortical areas involved at the encoding stage (bilateral premotor cortex), at the comparison one (right prefrontal) or in both (bilateral inferior parietal and superior temporal gyrus). Although both tasks seemed to recruit common areas, they differed in the distribution of activity within this network as right frontal and bilateral parietal regions showed stronger activity when the temporal dimension of a stimulus is to be judged, especially at the comparison stage. The partial interdimensional overlap of evoked responses supports the hypothesis that timing tasks tap both unspecific processes (possibly related to magnitude judgment in common with non-temporal task) and more specific timing-related resources. Time-frequency analyses are now being carried on to assess whether task-related oscillatory responses might differentiate these two types of processes. Oral Communication #SS0306 at the IXth International Conference on Cognitive Neuroscience (ICON9), La Havana, Cuba, September 2005.