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On insular responses and laser-evoked potentials
International Journal of Psychophysiology 43 Ž2002. 197᎐198
Letter to the Editor
On insular responses and laser-evoked potentials Sir, I have read with interest the article from Bentley et al. ‘Source localisation of 62-electrode human laser pain evoked potentials’, recently published in Int. J. Psychophysiol. Ž2001, 41: 187᎐193.. Although in general terms, the authors’ approach is very valuable and their results of indubitable importance for future developments, we should like to react to what they claim to be ‘the first report’ of an insular contribution to laser evoked potentials ŽLEPs.. Indeed, the authors incorrectly state Žp. 191. that ‘neither parietal nor insular sources have been previously reported in the scalp-recorded LEP literature’. Our group was among the first to propose a BESA-based dipolar solution for generators of laser-evoked potentials ŽValeriani et al., 1996.. This study, which is adequately quoted in the article of Bentley and colleagues, found two putative tardive sources located quite medially within the spherical three-shell volume used to approximate the head. Valeriani et al. Ž1996. stated at that time that such generators might correspond to sources located in the medial temporal lobes. As Bentley and colleagues correctly say, such localisation was estimated from dipole coordinates within the spherical head model and with no actual anatomical information. Later on, we developed and published a simplified system of projection of dipole sources onto brain anatomy ŽMerlet et al., 1998, 1999.. After its validation with localisation of epileptic spikes of known origin ŽMerlet et al., 1998., this system served in turn to project the LEP dipolar co-ordinates obtained in Valeriani et al. Ž1996. onto both the Talairach and Tournoux stereotactic brain atlas,
and a set of axial MRI slices normalised to match this atlas. The results were published in an international journal ŽGarcia-Larrea, 1998.. After anatomical projection, the two first sets of sources were found to correspond to the parietal operculum ŽSII area. and the anterior cingulate gyrus ŽBA 24. as had been previously proposed on the basis of their spherical co-ordinates. Conversely, at least one of the two late dipoles was found to project onto the contralateral anterior insular cortex in both MRI and anatomical atlas slices, and accordingly, an insular generator of laserevoked potentials was postulated ŽGarcia-Larrea, 1998.. Unfortunately, this publication, which to my knowledge was the first to consider such an insular generator, was not acknowledged in the article from Bentley et al. Ž2001.. In view of the growing interest on the insular cortex as a putative region involved in pain processing and control ŽFrot et al., 2001., we believe it is important to correctly retrace the history of attempts to localise insular activities in response to painful laser stimulation. It appears also important to stress the fact that the use of anatomical projection systems coupled to careful dipole modelling may provide useful estimations of source localisation even with the use of relatively low electrode density.
0167-8760r02r$ - see front matter 䊚 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 8 7 6 0 Ž 0 1 . 0 0 1 8 9 - 1
Luis Garcia-Larrea, MD, PhD, CERMEP, Hopital Neurologique, 59 Bd. Pinel, 69003 Lyon, France.
198
L. Garcia-Larrea r International Journal of Psychophysiology 43 (2002) 197᎐198
References Bentley, D.E., Youell, P.D., Crossman, A.R., Jones, A.K.P., 2001. Source localisation of 62-electrode human laser pain evoked potential data using a realistic head model. Int. J. Psychophysiol. 41, 187᎐193. Frot, M., Garcia-Larrea, L., Guenot, M., Mauguiere, ´ ` F., 2001. Operculo-insular responses to painful and innocuous stimuli. A study using intracerebral recordings in humans. Pain Žin press.. Garcia-Larrea, L., 1998. Multimodal approaches to laser evoked potentials generators: with a little help from our friends. Pain Forum 7, 216᎐220.
Merlet, I., Garcia-Larrea, L., Ryvlin, P., Isnard, J., Sindou, M., Mauguiere, ` F., 1998. Topographical reliability of mesiotemporal sources of interictal spikes in temporal lobe epilepsy. Electroencephal. Clin. Neurophysiol. 107, 206᎐212. Merlet, I., Garcia-Larrea, L., Froment, J.C., Mauguiere, ` F., 1999. Simplified projection of EEG dipole sources onto human brain anatomy. Neurophysiol. Clin. 29, 39᎐52. Valeriani, M., Rambaud, L., Mauguiere, ` F., 1996. Scalp topography and dipolar source modelling of potentials evoked by CO 2 -laser stimulation of the hand. Electroencephal. Clin. Neurophysiol. 100, 343᎐353.
Letter to the Editor
On insular responses and laser-evoked potentials Sir, I have read with interest the article from Bentley et al. ‘Source localisation of 62-electrode human laser pain evoked potentials’, recently published in Int. J. Psychophysiol. Ž2001, 41: 187᎐193.. Although in general terms, the authors’ approach is very valuable and their results of indubitable importance for future developments, we should like to react to what they claim to be ‘the first report’ of an insular contribution to laser evoked potentials ŽLEPs.. Indeed, the authors incorrectly state Žp. 191. that ‘neither parietal nor insular sources have been previously reported in the scalp-recorded LEP literature’. Our group was among the first to propose a BESA-based dipolar solution for generators of laser-evoked potentials ŽValeriani et al., 1996.. This study, which is adequately quoted in the article of Bentley and colleagues, found two putative tardive sources located quite medially within the spherical three-shell volume used to approximate the head. Valeriani et al. Ž1996. stated at that time that such generators might correspond to sources located in the medial temporal lobes. As Bentley and colleagues correctly say, such localisation was estimated from dipole coordinates within the spherical head model and with no actual anatomical information. Later on, we developed and published a simplified system of projection of dipole sources onto brain anatomy ŽMerlet et al., 1998, 1999.. After its validation with localisation of epileptic spikes of known origin ŽMerlet et al., 1998., this system served in turn to project the LEP dipolar co-ordinates obtained in Valeriani et al. Ž1996. onto both the Talairach and Tournoux stereotactic brain atlas,
and a set of axial MRI slices normalised to match this atlas. The results were published in an international journal ŽGarcia-Larrea, 1998.. After anatomical projection, the two first sets of sources were found to correspond to the parietal operculum ŽSII area. and the anterior cingulate gyrus ŽBA 24. as had been previously proposed on the basis of their spherical co-ordinates. Conversely, at least one of the two late dipoles was found to project onto the contralateral anterior insular cortex in both MRI and anatomical atlas slices, and accordingly, an insular generator of laserevoked potentials was postulated ŽGarcia-Larrea, 1998.. Unfortunately, this publication, which to my knowledge was the first to consider such an insular generator, was not acknowledged in the article from Bentley et al. Ž2001.. In view of the growing interest on the insular cortex as a putative region involved in pain processing and control ŽFrot et al., 2001., we believe it is important to correctly retrace the history of attempts to localise insular activities in response to painful laser stimulation. It appears also important to stress the fact that the use of anatomical projection systems coupled to careful dipole modelling may provide useful estimations of source localisation even with the use of relatively low electrode density.
0167-8760r02r$ - see front matter 䊚 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 8 7 6 0 Ž 0 1 . 0 0 1 8 9 - 1
Luis Garcia-Larrea, MD, PhD, CERMEP, Hopital Neurologique, 59 Bd. Pinel, 69003 Lyon, France.
198
L. Garcia-Larrea r International Journal of Psychophysiology 43 (2002) 197᎐198
References Bentley, D.E., Youell, P.D., Crossman, A.R., Jones, A.K.P., 2001. Source localisation of 62-electrode human laser pain evoked potential data using a realistic head model. Int. J. Psychophysiol. 41, 187᎐193. Frot, M., Garcia-Larrea, L., Guenot, M., Mauguiere, ´ ` F., 2001. Operculo-insular responses to painful and innocuous stimuli. A study using intracerebral recordings in humans. Pain Žin press.. Garcia-Larrea, L., 1998. Multimodal approaches to laser evoked potentials generators: with a little help from our friends. Pain Forum 7, 216᎐220.
Merlet, I., Garcia-Larrea, L., Ryvlin, P., Isnard, J., Sindou, M., Mauguiere, ` F., 1998. Topographical reliability of mesiotemporal sources of interictal spikes in temporal lobe epilepsy. Electroencephal. Clin. Neurophysiol. 107, 206᎐212. Merlet, I., Garcia-Larrea, L., Froment, J.C., Mauguiere, ` F., 1999. Simplified projection of EEG dipole sources onto human brain anatomy. Neurophysiol. Clin. 29, 39᎐52. Valeriani, M., Rambaud, L., Mauguiere, ` F., 1996. Scalp topography and dipolar source modelling of potentials evoked by CO 2 -laser stimulation of the hand. Electroencephal. Clin. Neurophysiol. 100, 343᎐353.