Effects of sleep on pain-related somatosensory evoked potentials in humans Xiaohong Wang a,b, , Koji Inui a , Yunhai Qiu a,b , Minoru Hoshiyama a , Tuan Diep Tran a , Ryusuke Kakigi a,b a Department of Integrative Physiology, National Institute for Physiological Sciences, Myodaiji, Okazaki 444-8585, Japan b Department of Physiological Sciences, School of Life Sciences, The Graduate University for Advanced Studies, Hayama, Kanagawa, Japan Received 2 August 2002; accepted 19 September 2002 Abstract We investigated effects of sleep on pain-related somatosensory evoked potentials (SEP) following painful electrical stimulation of the left index finger. The biggest advantage of this method is that signals ascending through both A-beta fibers relating to touch and A-delta fibers relating to pain can be recorded simultaneously. While the subject was awake, non-painful stimulation evoked early- and middle latency components, N20, P30 and N60, at the C4 electrode, and painful stimulation evoked not only early- and middle latency components at the C4 but also later pain-specific components, N130 and P240, at the Cz electrode. During sleep, N20 and P30 did not show a significant change in amplitude, N60 showed a slight but significant amplitude reduction, and N130 and P240 significantly decreased in amplitude or disappeared, as compared with those while awake. Therefore, we speculate on the mechanisms generating each component as follows; (1) N20 and P30 are the primary components generated in SI ascending through A-beta fibers. (2) N60 is the secondary component generated in SI involving cognitive function to some degree. (3) N130-P240 are the pain-specific components ascending through A-delta fibers, and closely related to cognitive function, because they were much affected by consciousness, different from the components ascending through A-beta fibers. # 2002 Elsevier Science Ireland Ltd and the Japan Neuroscience Society. All rights reserved. Keywords: Electroencephalography (EEG); Pain; Primary somatosensory cortex; Secondary somatosensory cortex; Somatosensory 1. Introduction Cerebral activities in humans, particularly responses to various kinds of sensory stimuli, should change during sleep. There have been several reports on the waveform changes of somatosensory evoked potentials (SEP), recorded by averaging electroencephalography (EEG), during sleep in humans (Yamada et al., 1988; Addy et al., 1989; Nakano et al., 1995; Noguchi et al., 1995). In these studies, a decreased amplitude and prolonged latency of early cortical potentials were reported. By applying painful stimuli such as high-intensity electrical pulses and a CO 2 laser, pain-related SEP and somatosensory evoked magnetic fields (SEF), recorded by averaging magnetoencephalography (MEG), can be recorded (see recent reviews by Kakigi et al., 2000a,b). However, to our knowledge, only one paper has system- atically reported the effects of the state of arousal on pain-related SEP using a CO 2 laser (Beydoun et al., 1993), and it found that the amplitude of pain-related SEP remarkably decreased during sleep. The main advantage of using painful electrical stimulation is that signals ascending through both fast A-beta fibers relating to touch and slow A-delta fibers relating to pain can be recorded simultaneously, and repeated stimuli can be easily applied without causing skin damage, unlike CO 2 laser stimulation.  Corresponding author. Tel.:  /81-564-55-7769; fax:  /81-564-52- 7913 E-mail address: wangxh@nips.ac.jp (X. Wang). Neuroscience Research 45 (2003) 53  /57 www.elsevier.com/locate/neures 0168-0102/02/$ - see front matter # 2002 Elsevier Science Ireland Ltd and the Japan Neuroscience Society. All rights reserved. PII:S0168-0102(02)00198-0