P Youell

Caudal cingulate cortex involvement in pain processing: an inter-individual laser evoked potential source localisation study using realistic head models

&NA; Electrophysiological studies have revealed a source of laser pain evoked potentials (LEPs) in cingulate cortex. However, few studies have used realistically shaped head models in the source analysis, which account for individual differences in anatomy and allow detailed anatomical localisation of sources. The aim of the current study was to accurately localise the cingulate source of LEPs in a group of healthy volunteers, using realistic head models, and to assess the inter‐individual variability in anatomical location. LEPs, elicited by painful CO2 laser stimulation of the right forearm, were recorded from 62 electrodes in five healthy subjects. Dipole source localisation (CURRY 4.0) was performed on the most prominent (P2) peak of each LEP data set, using head models derived from each subjects structural magnetic resonance image (MRI). For all subjects, the P2 LEP peak was best explained by a dipole whose origin was in cingulate cortex (mean residual variance was 3.9±2.4 %). For four out of five subjects, it was located at the border of the caudal division of left anterior cingulate cortex (area 24/32′) with left posterior cingulate cortex (area 23/31). For the fifth subject the dipole was centred in right posterior cingulate cortex (area 31). This study demonstrates that the location of the cingulate source of LEPs is highly consistent across subjects, when analysed in this way, and supports the involvement of caudal cingulate regions in pain processing.

Differential effects on the laser evoked potential of selectively attending to pain localisation versus pain unpleasantness.

OBJECTIVE To determine the effects on the laser evoked potential (LEP) of selectively attending to affective (unpleasantness) versus sensory-discriminative (localisation) components of pain. METHODS LEPs, elicited by painful CO2 laser stimulation of two areas of the right forearm, were recorded from 62 electrodes in 21 healthy volunteers, during three tasks that were matched for generalised attention: Localisation (report stimulus location), Unpleasantness (report stimulus unpleasantness), Control (report pain detection). LEP components are named by polarity, latency, and electrode. RESULTS N300-T7 peak amplitude was significantly greater during Localisation than Unpleasantness. The difference in N300-T7 amplitude between Localisation and Control approached significance, suggesting an increased amplitude in Localisation compared with Control, rather than a reduced amplitude in Unpleasantness. Peak amplitude, latency, and topography of N300-FCz, P450, P600-800 (early P3) and P800-1000 (late P3) did not differ significantly between tasks. CONCLUSIONS These results suggest that the N300-T7 LEP peak reflects the activity of cerebral generators involved in the localisation of pain. The topography of N300-T7 is consistent with a source in contralateral secondary somatosensory cortex/insula and maybe primary somatosensory cortex. SIGNIFICANCE This study confirms a role of the lateral pain system in the localisation of pain, and distinguishes it from stimulus novelty or attention.

Anatomical localization and intra-subject reproducibility of laser evoked potential source in cingulate cortex, using a realistic head model

OBJECTIVES To (i) accurately localize the cingulate source of late laser evoked potentials (LEPs) using a realistic head model incorporating the individuals anatomy and (ii) assess the within-subject reproducibility of this source. METHODS Late LEPs, elicited by painful CO2 laser stimulation of the right forearm, were recorded from 62 electrodes in one healthy subject. This was repeated 9 times, over 3 different days. Dipole source localization (CURRY 4.0) was performed on the most prominent (P2) peak of each LEP data set, using a head model derived from the subjects structural magnetic resonance image. RESULTS In all cases the P2 LEP peak was best explained by a dipole located close to the border of the caudal division of left anterior cingulate cortex with left posterior cingulate cortex (mean residual variance was 1.7+/-0.4%). The maximum standard deviation from the mean dipole location was 3.2 mm. CONCLUSIONS This study demonstrates that the location of the cingulate source of late LEPs is highly reproducible within this subject, when analyzed in this way, and suggests involvement of caudal cingulate regions in pain processing.

Source localisation of 62-electrode human laser pain evoked potential data using a realistic head model

Laser evoked potentials (LEPs), elicited by painful laser stimulation of the right forearm, were recorded from 62 electrodes in a single healthy subject. The positions of the electrodes on the scalp were co-registered with the subjects structural magnetic resonance image (MRI) of the brain. Spatio-temporal dipole modelling, using a head model derived from the MRI, estimated sources in left posterior cingulate, posterior parietal and anterior insular cortices. The parietal source peaked in activity at 260 ms, which explained the N1/N2 peaks of the LEPs. The cingulate source was the most strongly activated, at 400 ms, and accounted for the P2 LEP component. The insular source showed late, prolonged activation, peaking in magnitude at 850 ms. This is the first study to report scalp-recorded LEP generators in posterior parietal and insular cortices. Although these sources require replication, they are consistent with other functional imaging studies.

Thermal measurements in a soft tissue model during irradiation by a high-power semiconductor diode laser

The temperature of a soft tissue model was measured during laser irradiation. A diode laser with a continuous wave output power of up to 10 W and a wavelength of 990 nm was used to heat and ablate samples of agar gel doped with haemoglobin. The internal temperature of the tissue was measured at depths of 2 - 5 mm below the surface using a thermocouple. The temperature at the surface was measured remotely using an infra-red sensor (over an area 1.4 mm in diameter at the center of the interaction). This method of measurement provides an inexpensive alternative to thermal imaging cameras. Temperature changes in time during the interaction both at the tissue surface and as a function of tissue depth are presented. At the onset of tissue surface rupturing and subsequent tissue ablation the temperature at a depth of 2 mm was found to be 75 +/- 6 degree(s)C. At this time a temperature of 60 +/- 2 degree(s)C, high enough to cause tissue coagulation, had been reached to a depth of 3 mm. After 10 s of continuous tissue ablation, the temperature at a depth of 3.5 mm and beyond had not reached coagulation temperature. The surface temperature rose steadily during irradiation and reached a temperature of 100 degree(s)C at the time of rupturing of the tissue surface. During the subsequent tissue ablation the measured temperature increased rapidly, reaching a maximum of 250 +/- 30 degree(s)C within a further 10 s.

Investigations into the interaction of a high-power semiconductor diode laser with biological tissue

Preliminary investigations of the interaction between a continuous wave diode laser and biological tissue are presented. The laser used has a wavelength of 980 nm and a maximum output power of 10 W. The tests were carried out on porcine liver samples. The interaction was seen to develop over three different stages during laser irradiation. The initial stage of tissue coagulation was followed by a distinct tissue surface eruption, followed by the ablation of a crater into the tissue. Measurements of the time evolution of these stages are presented over a range of incident spot diameters, and related to theoretical expectations. Measurements taken from the resultant crater are presented, including the extent of residual tissue damage around the crater, over a range of applied laser exposure times.

Intra-subject reproducibility of laser evoked potential dipole source locations, estimated using a realistic head model

Introduction: Dipole source localisation is a means of identifying possible cerebral generators of scalp-recorded evoked potential signals with millisecond temoral resolution. Previous studies on the reproducibility of source locations of laser evoked potentials (LEPs) have used spherical head models and relatively few recording electrodes. The aim of the present study was to test the reproducibility of LEP source locations using a greater number of recording electrodes and a realistically shaped head model in the source analysis.

High-power diode laser system near 1 μm and comparative tissue interaction studies

Due to their high reliability, modest electrical and cooling requirements, and their compact size, diode laser systems are attractive high power, fiber-coupled laser sources for surgical and therapeutics procedures. We describe Applied Optronics Corporations LM series of portable air-cooled diode laser systems delivering 25 W or 50 W of cw power from the distal end of a disposable, 0.37 NA optical fiber with a core size of 600 micrometers or 1 mm respectively. In comparative tissue interaction studies using the 980 nm laser source, three laser interaction regimes are identified and characterized for laser interactions with six types of cadervic soft tissue.