Thu, 21 Aug 2008 11:05:09 BST CiteULike: xllis Rodriguez http://www.citeulike.org/user/xlli/author/Rodriguez CiteULike.org en-gb Copyright © 2004-2008 citeulike.org http://www.citeulike.org/user/xlli/article/440657 <i>Hum Brain Mapp, Vol. 8, No. 4. (1999), pp. 194-208.</i><br /><br />This article presents, for the first time, a practical method for the direct quantification of frequency-specific synchronization (i.e., transient phase-locking) between two neuroelectric signals. The motivation for its development is to be able to examine the role of neural synchronies as a putative mechanism for long-range neural integration during cognitive tasks. The method, called phase-locking statistics (PLS), measures the significance of the phase covariance between two signals with a reasonable time-resolution (<100 ms). Unlike the more traditional method of spectral coherence, PLS separates the phase and amplitude components and can be directly interpreted in the framework of neural integration. To validate synchrony values against background fluctuations, PLS uses surrogate data and thus makes no a priori assumptions on the nature of the experimental data. We also apply PLS to investigate intracortical recordings from an epileptic patient performing a visual discrimination task. We find large-scale synchronies in the gamma band (45 Hz), e.g., between hippocampus and frontal gyrus, and local synchronies, within a limbic region, a few cm apart. We argue that whereas long-scale effects do reflect cognitive processing, short-scale synchronies are likely to be due to volume conduction. We discuss ways to separate such conduction effects from true signal synchrony. Measuring phase synchrony in brain signals. JP Lachaux E Rodriguez J Martinerie FJ Varela Hum Brain Mapp, Vol. 8, No. 4. (1999), pp. 194-208. 2005-12-17T15:51:41-00:00 1999 Hum Brain Mapp 1065-9471 8 4 194 208 phase synchronization http://www.citeulike.org/user/xlli/article/256111 <i>Eur J Neurosci, Vol. 12, No. 7. (July 2000), pp. 2608-2622.</i><br /><br />This paper studies gamma-band responses from two implanted epileptic patients during a simple visual discrimination task. Our main aim was to ascertain, in a reliable manner, whether evoked (stimulus-locked) and induced (triggered by, but not locked to, stimuli) responses are present in intracranial recordings. For this purpose, we introduce new methods adapted to detect the presence of gamma responses at this level of recording, intermediary between EEG-scalp and unicellular responses. The analysis relies on a trial-by-trial time-frequency analysis and on the use of surrogate data for statistical testing. We report that visual stimulation reliably elicits evoked and induced responses in human intracranial recordings. Induced intracranial gamma activity is significantly present in short oscillatory bursts (a few cycles) following visual stimulation. These responses are highly variable from trial to trial, beginning after 200 ms and lasting up to 500 ms. In contrast, intracranial-evoked gamma responses concentrate around 100 ms latencies corresponding to evoked responses observed on the scalp. We discuss our results in relation to scalp gamma response in a similar protocol [Tallon-Baudry et al. (1996) J. Neurosci., 16, 4240-4249] and draw some conclusions for bridging the gap between gamma oscillations observed on the scalp surface and their possible cortical sources. A quantitative study of gamma-band activity in human intracranial recordings triggered by visual stimuli. JP Lachaux E Rodriguez J Martinerie C Adam D Hasboun FJ Varela Eur J Neurosci, Vol. 12, No. 7. (July 2000), pp. 2608-2622. 2005-07-14T16:04:43-00:00 2000 Eur J Neurosci 0953-816X 12 7 2608 2622 analysis eeg statitistical