Stochastic resonance in human electroencephalographic activity elicited by mechanical tactile stimuli E. Manjarrez a, * , O.Diez-Martı´nez b , I. Me ´ ndez a , A. Flores a a Instituto de Fisiologı ´a, Beneme ´ rita Universidad Auto ´noma de Puebla, 14 Sur 6301, Col. San Manuel. Apartado Postal 406. Puebla, Pue. CP 72570, Me ´ xico b Departamento de Psicologı ´a, Universidad de las Ame ´ ricas-Puebla, Cholula, Pue. CP 72820, Me ´ xico Received 29 January 2002; received in revised form 22 February 2002; accepted 22 February 2002 Abstract Stochastic resonance (SR) is a phenomenon in which the response of a non-linear system to a weak input signal is optimized by the presence of noise. The aim of this study was to demonstrate the experimental occurrence of SR in electroencephalographic (EEG) activity elicited by mechanical tactile stimuli. Our experiments show that EEG responses evoked by mechanical tactile stimuli in the region overlying the somatosensory cortical area were optimized by the addition of certain noise amplitudes. All subjects showed distinct SR behavior. The signal-to-noise ratio (SNR) of the response evoked by mechanical indentations of the skin was an inverted U-like function of the input noise. As the noise amplitude increased, SNR values became larger. A maximum value was reached with a particular noise amplitude value. Beyond such peak, with higher noise amplitudes, the curve subsided gradually. To our knowledge, this is the first documented evidence that such remarkable phenomenon embodies electrical processes of the human brain. Such behavior might explain related findings described in psychophysical studies. q 2002 Elsevier Science Ireland Ltd. All rights reserved. Keywords: Noise; Stochastic resonance; Somatosensory cortex; Background activity; Evoked potentials; Signal-to-noise ratio Stochastic resonance (SR) refers to the increase of the signal-to-noise ratio (SNR) on the output of certain non- linear systems, obtained through an increase of the noise level on the input [7,16]. Many physical and biological systems exhibit this non-linear phenomenon [7,13]. The essential feature is that the application of a particular level of noise may optimize the output SNR. Hence, typically the plot of SNR versus input noise is an inverted U-like function characterized by maximal enhancement of SNR at a specific noise amplitude value. The descriptions of such behavior include both experimental [2–4,6,9,14,15,19,20] and theo- retical [8,10] studies in a large variety of neural systems. Psychophysical experiments in humans show that the presence of a particular non-zero level of noise may signifi- cantly enhance the ability of an individual to detect a sub- threshold tactile stimuli [2,4,14]. The interpretation of these findings suggests that the increased detection capability is a form of SR-like phenomena [2]. However, it is not clear from these experiments whether the electrical activity of cortical neurones participating in the process also exhibits SR-like behavior. In this context, the purpose of the present study was to substantiate whether SR-type behavior occurs in somatosensory cortical evoked potentials elicited by mechanical tactile stimuli. We performed experiments in a sample of 15 healthy young subjects (ten males and five females; 18–32 years of age). All the procedures conformed to the Declaration of Helsinki (1964) established by the World Medical Asso- ciation. Subjects lacked any evidence of previous or current serious medical disease or detectable neurological disor- ders. A closed-loop mechanical stimulator-transducer [1] allowed measures of the force and displacement of applied stimuli. The output of two independent function generators provided input to the stimulator-transducer. One of these (Tektronix CFG 253 together with a Master-8 AMPI), generated the pulse-like signal while, the other (Wavetek Model 132), supplied the superimposed noise. The ampli- tude of the test stimulus signal was 3 £ 10 23 N, and the range of the stimulus noise was from 0 to 3 £ 10 23 N. The left hand of each subject was held in a fixed position. A 1- mm diameter flat cylindrical probe of acrylic was used to Neuroscience Letters 324 (2002) 213–216 0304-3940/02/$ - see front matter q 2002 Elsevier Science Ireland Ltd. All rights reserved. PII: S0304-3940(02)00212-4 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).