We studied behavioural responses and 32-channel brain potentials to nociceptive stimuli during all-night sleep in 12 healthy subjects, using sequences of thermal laser pulses delivered over the dorsum of the hand. Laser stimuli less than 20 dB over perception threshold had clear awakening properties, in accordance with the intrinsic threatening value of nociceptive signals. Even in cases where nociceptive stimulation did not interrupt sleep, it triggered motor responses in 11% of trials. Only four subjects reported dreams, and on morning questionnaires there was no evidence of incorporation to dreams of nociceptive stimuli. Contrary to previous reports suggesting the absence of cortical nociceptive responses during sleep, we were able to record brain-evoked potentials to laser (LEPs) during all sleep stages. Sleep LEPs were in general attenuated, but their morphology was sleep-stage-dependent: in stage 2, the weakened initial response was often followed by a high-amplitude negative wave with typical features of a K-complex. During paradoxical sleep (PS) LEP morphology was similar to that of waking, but frontal components showed strong attenuation, consistent with the reported frontal metabolic deactivation. A late positive component (450–650 ms) was recorded in both stage 2 and PS, the amplitude of which was significantly enhanced in trials that were followed by an arousal. This response appeared functionally related to the P3 wave, which in waking subjects has been associated to conscious perception and memory encoding.
aINSERM, U 879 (Central Integration of Pain), Bron F-69677, France
bUniversité Claude Bernard, Lyon F-69003, France
cHospices Civils de Lyon, Unité d’Hypnologie, Hôpital Neurologique, 59, Bd Pinel, Bron F-69677, France
dUniversité catholique de Louvain, Unité de Neurosciences Cognitives, Faculté de Psychologie et des Sciences de l’Education, Louvain-la-Neuve B-1348, Belgium
eUniversidad de Malaga, Facultad de Medicina, Dept de Physiologia, Malaga E-29080, Spain
*Corresponding author. Address: Hospices Civils de Lyon, Unité d’Hypnologie, Hôpital Neurologique, 59, Bd Pinel, Bron F-69677, France. Tel.: +33 4 72 35 78 28; fax: +33 4 72 35 73 97.
Submitted May 2, 2007; received in revised form October 2, 2007; accepted October 22, 2007.