Scalp topography of ultralate C-®bres) evoked potentials following thulium YAG laser stimuli to tiny skin surface areas in humans E. Opsommer a, * , T. Weiss b , W.H.R. Miltner b , L. Plaghki a a Faculty of Medicine, Universite  catholique de Louvain, Avenue Mounier 53/READ 5375, B-1200 Brussels, Belgium b Department of Biological and Clinical Psychology, Institute of Psychology, Friedrich Schiller University, Am Steiger 3, Haus 1, D-07743, Jena, Germany Accepted 11 June 2001 Abstract Aim:Toinvestigate1)thescalptopographyofultralatelaserevokedpotentialsLEPs)relatedtoC-®breactivation,whichcandirectlybe obtained by thulium YAG Tm YAG) laser stimulation of tiny skin surface areas about 0.23 mm 2 ) and 2) the in¯uence of the performance of a motor task on ultralate LEPs. Methods: Laser stimuli were applied to the dorsum of the left hand. LEPs were recorded with 58 scalp electrodes from 9 healthy subjects in two different conditions, with and without a reaction time RT) task press a button upon detection). Results: On high resolution electroenchephalogram recordings, ultralate LEPs were characterized by a broad positive component peak latency: 1133 ^ 91 ms) with maximum amplitude about the vertex. Moreover, the performance of a RT task had no in¯uence on latency, amplitude and topographical patterns of two maps chosen at the positive peak latency in ultralate LEPs. Nevertheless, a negative in¯exion latency 1300 ms) appeared after the positive component in the task condition possibly re¯ecting movement-related potentials. Conclusion: Tm YAG laser stimulation of tiny skin surface areas allows recording the dynamic scalp topography of ultralate C-®bres) LEPs, with or without the performance of a RT task. q 2001 Elsevier Science Ireland Ltd. All rights reserved. Keywords: Nociception; Tm YAG laser; Laser evoked potentials; Scalp topography; Reaction time; C-®bres 1. Introduction High power laser heat stimuli selectively activate cuta- neousAd-andC-®breterminalsDevoretal.,1982;Bromm and Treede, 1983). Until recently, studies using laser stimu- lation were mainly focussed on the properties of cutaneous Ad-®bre afferents and related brain evoked potentials called late laser evoked potentials LEPs) as, with usual stimula- tionparameters,theevokedbrainresponserelatedtoC-®bre activation is rarely observed Bromm and Treede, 1987; Kakigi et al., 1989; Garcia-Larrea et al., 1997). Therefore, there is a need for a non-invasive, selective and quantitative method for fundamental and clinical investigations of C- ®bre function. A few such methods intended to activate selectively afferent C-®bres following laser stimulation are available. In chronological order, 4 methods are accessible: 1) ischemic A-®bre nerve conduction block Bromm et al., 1983); 2) selective pathological impairment of A-®bre function Treede et al., 1988a; Lankers et al., 1991); 3) stimulation of tiny skin surface areas based on the higher distribution density of C-®bres as compared to Ad-®bres Bragard et al., 1996) and 4) temperature controlled laser heat stimuli based on the lower temperature activation threshold of C-®bres as compared to Ad-®bres Magerl et al., 1999). All these methods have in common the suppres- sion of transmission or a severe reduction of activation of Ad-®bres related to the ®rst pain i.e. a short latency, brief, sharp and pricking sensation), and the concomitant appear- ance of ultralate LEPs possibly related to the activation of afferent C-®bres held responsible for the second pain i.e. a long latency, prolonged, diffuse and burning sensation). The major aim of the present investigations was to exam- inethescalptopographyofultralateLEPswithalargearray of scalp electrodes following laser stimulation applied to a tiny skin surface area ,0.25 mm 2 ). By this method of stimulation, ®rst described by Bragard et al. 1996), it is possible to directly and selectively activate C-®bre afferents based on the fact that cutaneous C-®bres have a higher distribution density and lower activation threshold as compared to Ad-®bres. This method, using conventional time domain-averaging, revealed the occurrence of ultralate LEPs with latencies of about 1 s post-stimulus related to the activation of afferents C-®bres as the peripheral conduction Clinical Neurophysiology 112 2001) 1868±1874 1388-2457/01/$ - see front matter q 2001 Elsevier Science Ireland Ltd. All rights reserved. PII:S1388-245701)00622-8 www.elsevier.com/locate/clinph CLINPH 2000727 * Corresponding author. Tel.: 132-2-764-53-75; fax: 132-2-764-53-60. E-mail address: opsommer@read.ucl.ac.be E. Opsommer).