Internal stochastic resonance in the coherence between spinal and cortical neuronal ensembles in the cat E. Manjarrez * , J.G. Rojas-Piloni, I. Me ´ndez, L. Martı´nez, D. Ve ´ lez, D. Va ´ zquez, A. Flores Institutode Fisiologı´a, Beneme ´ rita Universidad Auto ´noma de Puebla, 14 Sur 6301, Col. San Manuel, Apartado Postal 406, Puebla, Pue. CP 72570, Me ´ xico Received 18 February 2002; received in revised form 11 March 2002; accepted 19 March 2002 Abstract Internal stochastic resonance is a phenomenon in which the coherence of a non-linear system is enhanced by the presence of a particular, non-zero level of noise generated by internal or external sources without a periodic input signal. The aim of this study was to demonstrate the experimental occurrence of internal stochastic resonance in the coherence between spinal and cortical neuronal ensembles. Simultaneous recordings of spinal and cortical evoked potentials were made in the somatosensory system of the anaesthetized cat. Evoked potentials were produced by input noise introduced in the tactile stimulation of the hindpaw skin. Coherence between the spinal and cortical evoked activity recorded during different levels of input noise was calculated. All animals showed distinct internal stochastic resonance like behavior. We found that the mean coherence was an inverted U-like function of the level of input noise with a mean coherence peak of 0.43. To our knowledge, this is the first documented evidence of such phenomenon in an in vivo preparation of the central nervous system. q 2002 Elsevier Science Ireland Ltd. All rights reserved. Keywords: Noise; Tactile; Stochastic resonance; Somatosensory cortex; Evoked potentials; Coherence; Dorsal horn; Indentations Stochastic resonance is a phenomenon wherein the response of a non-linear system to a weak periodic input signal is optimized by the presence of a particular, non-zero level of noise [2,4,8,12]. This phenomenon appears when a non-linear system is simultaneously driven by external noise and an external periodic input signal. However, non- linear systems in the presence of internal or external noise can also display stochastic resonance like behavior, even without external periodic signal. This phenomenon has been called autonomous stochastic resonance [9], coherence resonance [10,14], or internal stochastic resonance [15,20]. To our knowledge, although the stochastic resonance beha- vior in neural systems has been studied extensively [4– 8,16,19], there have been no experimental studies concern- ing the existence of internal stochastic resonance in an in vivo preparation of the central nervous system. This kind of studies could be relevant because the most important sources of noise are present in vivo and are those not active in an in vitro preparation [5,7,19]. The purpose of the present study was to provide experimental evidence of the internal stochastic resonance phenomenon in the coherence between spinal and cortical neurones in the somatosensory system of the anaesthetized cat. Disclosure of this phenom- enon could be important, because a function of the somato- sensory spino-cortical pathway is the signal detection in a noisy environment, partly generated by dorsal horn spinal neurones producing the spontaneous cord dorsum potentials [11]. Guidelines contained in NIH publication on the principles of laboratory animal care 85-23 revised in 1985 were followed throughout. Briefly, experiments were carried out in adult cats (2.0–3.5 kg) initially anaesthetized with pentobarbitone (35 mg kg 21 of weight, i.p.). Most proce- dures were reported previously [11] and are briefly described here. The right surface of the skull was exposed and a trephine hole, 7 mm diameter, was made over the SI somatosensory cortex, contralateral to the site of stimula- tion. The microelectrode holder (piezoelectric Burleigh, 6000 ULN) allowed fast penetration of micropipettes with- out dimpling of the cortex. The lumbo-sacral and low thor- acic spinal segments were exposed. Pools were formed with the skin around the exposed tissues, filled with mineral oil Neuroscience Letters 326 (2002) 93–96 0304-3940/02/$ - see front matter q 2002 Elsevier Science Ireland Ltd. All rights reserved. PII: S0304-3940(02)00318-X www.elsevier.com/locate/neulet * Corresponding author. Tel.: 15222-22-44-1657; fax: 15222- 22-33-4511. E-mail address: emanjar@siu.buap.mx (E. Manjarrez).