Dynamics on the double morse potential: a paradigm for roaming reactions with no saddle points

In this paper we analyze a two-degree-of-freedom Hamiltonian system constructed from two planar Morse potentials. The resulting potential energy surface has two potential wells surrounded by an unbounded flat region containing no critical points. In addition, the model has an index one saddle between the potential wells. We study the dynamical mechanisms underlying transport between the two potential wells, with emphasis on the role of the flat region surrounding the wells. The model allows us to probe many of the features of the “roaming mechanism” whose reaction dynamics are of current interest in the chemistry community

Timing and Spatial Distribution of Somatosensory Responses Recorded in the Upper Bank of the Sylvian Fissure (SII Area) in Humans

International audienceWe studied responses of the parieto-frontal opercular cortex to electric stimuli, as recorded by intra-cortical electrodes during stereotactic EEG presurgical assessment of patients with drug-resistant temporal lobe epilepsy. After electrical stimulation of the median nerve at the wrist, we consistently recorded a negative-positive biphasic response peaking at 60 ms (N60) and 90 ms (P90) post-stimulus in the upper bank of the sylvian fissure contralateral to stimulation. Talairach stereotactic coordinates of the electrode contacts recording these responses covered the pre- and post-rolandic part of the upper bank of the sylvian fissure (25<x<55 mm; -27<y<+13 mm; 0<z<21 mm), corresponding to the accepted localization of SII area in man. The sources of these responses were deeply situated in the cortex of the upper bank of the sylvian fissure at approximately 40 mm from the midline sagittal plane, so that some of them could be located in the insular cortex. Moreover this study suggests the existence of dipolar SII sources radial to the scalp surface, which are overlooked in magnetic recordings. Somatosensory evoked potentials (SEPs) recorded in SII are delayed by approximately 40 ms as compared with SEPs generated in the primary somatosensory cortex. This long delay between SI and SII responses is not fully explained though it is coherent with the timing of activation issued from MEG source modeling data

Les réponses operculo-insulaires aux stimulations cutanées nociceptives chez l'homme. Revue de la littérature et données récentes

International audienceCO2 laser stimulation selectively activates the endings of small myelinated A delta fibers, involved with non-myelinated C fibers in the processing of nociceptive information. Thus, potentials evoked by CO2 laser stimulation reflect the activation of cortical areas receiving inputs from the spinothalamic tract. In this article we review data on the early pain-related CO2 laser evoked potentials recorded on the scalp, or by intracortical electrodes, during presurgical assessment of patients with drug-resistant epilepsy. A combination of surface and depth recordings allows the description of early cortical pain responses in terms of latency, polarity and scalp topography. Such a technique also allows the localization of the anatomical generators of these early responses using dipolar source modeling of scalp-recorded evoked potentials, or intracortical recordings, in stereotactical conditions. The earliest response recorded on the scalp to CO2 laser stimulation was an N1-P1 dipolar potential field at a latency of 140-200 ms. The N1 and P1 maximal voltages are recorded in the temporal region contralateral to stimulation and mid-frontal region, respectively. Intracerebral electrodes record an activation of a dipolar cortical source in the same latency range located in the upper bank of the sylvian fissure, corresponding to the second somatosensory (SII) area ipsi- and contralateral to the stimulation and insular cortex. The SII-insular responses ipsilateral to stimulation are likely to be triggered via transcallosal fibers coming from the opposite SII area. The operculo-insular cortex contralateral to stimulation, activated through direct thalamocortical projections, is likely to represent the first step in the cortical processing of peripheral A delta fiber pain inputs

Assessment of intraspinal and intracranial conduction by P30 and P39 tibial nerve somatosensory evoked potentials in cervical cord, brainstem and hemispheric lesions.

In routine recordings of tibial nerve somatosensory evoked potentials (SEPs), a global central conduction time is evaluated by measuring the interval between the segmental spinal N22 potential, recorded in the lumbar region, and the cortical P39 potential. In this study, we tested the reliability of the scalp far-field P30 potential, which originates in the vicinity of the cervico-medullary junction, in order to evaluate separately intraspinal and intracranial conduction in normal subjects and patients with cervical cord and intracranial lesions. P30 and cortical P39 potentials were studied in 23 healthy subjects and in 70 patients with cervical cord (n = 47), brainstem (n = 11) or hemispheric lesions (n = 12) selected on the basis of neuroimaging-computed tomography (CT) or magnetic resonance (MR)-findings. Median nerve SEPs were also recorded in all patients. Of the several montages tested to obtain the P30 potential, the Fpz-Cv6 derivation gave the highest signal-to-noise ratio; it permitted to obtain a P30 potential that peaked at 29.2 ± 1.6 ms in all normal subjects. P30 abnormalities were observed only in patients with cervical or cervico-medullary lesions; these were associated with a normal P39 in only two of 33 abnormal recordings. Conversely, P30 was consistently normal in lesions situated above the cervico-medullary junction whether associated with normal, delayed, or reduced P39. P30 abnormalities were subclinical in 42% of abnormal recordings. All patients with normal tibial and median nerve SEPs on both sides had normal touch, joint, and vibration sensation in the four limbs. There was a strong correlation between tibial nerve P30 and median nerve PI4 data in the whole series of patients; both potentials behaved similarly in all cases of intracranial supramedullary lesions. Combined abnormalities of P30 and P39 potentials thus indicate that conduction is impaired at the spinal level and proved to be particularly informative for detecting spinal cord dysfunction in patients with neuroimaging evidence of a narrowed cervical canal. Recording of abnormal N13, PI 4, or P30 potentials provided evidence of a cervical cord dysfunction in 66% of patients who had a suspected spondylotic myelopathy. Recording of tibial nerve P30 potential has proven to give reliable and useful information when a separate assessment of intraspinal and intracranial somatosensory conduction is needed; it merits inclusion, as does the upper limb N13 potential, in the evaluation of patients whose MR image indicates cervical canal narrowing. © 1995 American Electroencephalographic Society

Early secondary somatosensory area (SII) SEPs. Data from intracerebral recordings in humans

International audienceTo record somatosensory evoked potentials (SEPs) to median nerve stimulation by chronically implanted electrodes in the parieto-rolandic opercular area of 9 epileptic patients, in order to evaluate whether somatosensory evoked responses could be generated in the second somatosensory area (SII) earlier than 40 ms after stimulus

Effect of stimulus rate on the cortical posterior tibial nerve SEPs: a topographic study

We performed topographical mapping of somatosensory evoked potentials (SEPs) in response to posterior tibial nerve stimulation delivered at 2, 5 and 7.5 Hz in 15 healthy subjects. P37 was significantly attenuated at 5 and 7.5 Hz and the N50 component attenuated only at 5 Hz, its amplitude remaining stable for further increases in stimulus frequency. Frontal N37 and P50 potentials showed no significant decrease when the stimulus repetition frequency was changed from 2 to 7.5 Hz. P60 showed an attenuation of the amplitude only at 7.5 Hz. Latency and scalp topographies of all cortical components examined remained unchanged for the 3 stimulus rates tested. The optimal stimulus rate for mapping of tibial nerve SEPs was lower than 5 Hz. The distinct recovery function of the contralateral. N37-P50 and ipsilateral P37-N50 responses suggests that these potentials arise from separate generators