title: Do we activate specifically somatosensory thin fibres with the concentric planar electrode? A scalp and intracranial EEG study.
abstract: Laser-evoked potentials (LEPs) are acknowledged as the most reliable laboratory tool for assessing thermal and pain pathways. Electrical stimulation with a newly developed planar concentric electrode, delivering stimuli limited to the superficial skin layers, has been suggested to provide selective activation of Aδ fibres without the inconveniences linked to laser stimulation. The aim of our study was to compare the scalp and intracranial responses to planar concentric electrode stimulation (CE-SEPs) with those of LEPs and standard somatosensory-evoked potentials (SEPs). Sixteen healthy subjects, 6 patients with intracortical electrodes, and 2 patients with selective lesions of the spinothalamic pathway were submitted to Neodymium:Yttrium-Aluminium-Perovskite laser stimulations, and electrical stimulations using standard electrodes or planar concentric electrodes (CE). In both healthy controls and epileptic implanted patients, CE- and standard SEPs showed significantly shorter latencies than LEPs. This is consistent with Aβ-fibre activation, peripheral activation time being unable to account for longer LEP latencies. In the patients with spinothalamic lesions, LEPs were absent after stimulation of the affected territory, while CE-SEPs were still present. For these 2 reasons, we conclude that the planar CE does not selectively activate the Aδ and C fibers, but coexcites a significant proportion of large myelinated Aβ fibres that dominate the ensuing cortical response. The use of CE-SEPs for the detection of spinothalamic system lesions is therefore not warranted; the planar electrode can, however, represent a useful tool to study nociceptive reflexes, which can be reliably elicited even in the presence of Aβ coactivation.
authors.affiliation: Central Integration of Pain, Center for Neuroscience of Lyon, Inserm U1028 & University Claude Bernard Lyon 1, Bron 69677, France Servicio de Neurofisiologia Clinica, Hospital Universitari de Bellvitge, Barcelona S-08907, Spain Laboratoire d'étude des mécanismes cognitifs, Univ Lyon 2, Bron F-69676, France.
authors.forename: Caroline
authors.initials: C
authors.lastname: Perchet
authors.forename: Maud
authors.initials: M
authors.lastname: Frot
authors.forename: Audran
authors.initials: A
authors.lastname: Charmarty
authors.forename: Cecilia
authors.initials: C
authors.lastname: Flores
authors.forename: Stephanie
authors.initials: S
authors.lastname: Mazza
authors.forename: Michel
authors.initials: M
authors.lastname: Magnin
authors.forename: Luis
authors.initials: L
authors.lastname: Garcia-Larrea
mesh_terms: D004566:Electrodes / Q000706:statistics & numerical data; D004567:Electrodes, Implanted / Q000706:statistics & numerical data; D004569:Electroencephalography / Q000295:instrumentation / Q000379:methods* / Q000592:standards; D005073:Evoked Potentials, Somatosensory / Q000502:physiology*; D005260:Female; D006801:Humans; D053844:Lasers, Solid-State; D008297:Male; D008875:Middle Aged; D009413:Nerve Fibers, Myelinated / Q000502:physiology*; D010146:Pain / Q000175:diagnosis / Q000503:physiopathology*; D012535:Scalp / Q000294:innervation; D013003:Somatosensory Cortex / Q000166:cytology / Q000502:physiology*; D013133:Spinothalamic Tracts / Q000166:cytology / Q000502:physiology*; D055815:Young Adult