Stochastic resonance in the spinal cord and somatosensory cortex of the cat E. Manjarrez*, G. Rojas-Piloni, H. Pérez, I. Méndez, Z. Hernández-Paxtián and A. Flores Instituto de Fisiología, Benemérita Universidad Autónoma de Puebla. 14 Sur 6301, Col. San Manuel. Apartado Postal 406. Puebla, Pue. CP 72570, México. ABSTRACT The aim of this study was to demonstrate the occurrence of stochastic resonance (SR) in spinal and cortical potentials elicited by periodic tactile stimuli in the anaesthetised cat. The periodic tactile stimuli were applied on the central pad of the hindpaw and the noisy tactile stimuli on the glabrous skin of the third hindpaw digit. This protocol allowed that the signal and noise were mixed not in the skin but in the somatosensory regions of the central nervous system. The results show that a particular level of tactile noise can increase the amplitude of the spinal and cortical potentials elicited by periodic tactile stimuli. The topographical distribution of evoked potentials indicates that the effects of noise were spatially restricted. All cats showed distinct SR behavior at the spinal and cortical stages of the sensory encoding. Such SR was abolished in the cortical but not in the spinal recording after the sectioning of the ascending pathways. This suggests that the spinal neurones may also contribute to the SR observed at the cortical level. The present study documents the first evidence that the SR phenomenon occurs in the spinal and cortical somatosensory system itself and not only in the peripheral sensory receptors. Key words: Noise, tactile, information capacity, stochastic resonance, somatosensory, evoked potentials, spectrum map. 1. INTRODUCTION Tactile information is transmitted from skin to the brain via relays within the central nervous system (CNS). Several lines of evidence suggest that in cats (and to some extent in monkeys) the dorsal columns (DC) and the dorsolateral funiculus (DLF) both contribute to tactile conditioned reflexes, two-point discrimination, tactile and proprioceptive placing, roughness discrimination, and size discrimination 1 . In cats and monkeys, tac- tile deficits are generally made much worse when combined le- sions are made in the DC and the DLF than when a lesion is placed in either alone 1 . Inputs to the DC system enter the spinal cord and ascend to the medulla, where they synapse at the gracile and cuneate nuclei. The axons of postsynaptic cells form a fiber bundle that crosses in the brain stem to the opposite side and ascends to the thalamus. Out- puts of the thalamus are directed to postcentral cortical regions referred to as the somatosensory cortex. The spino-cervical tract is one of the major somatic sensory pathways located in the DLF in the cat spinal cord 1 . It transmits information from cutaneous receptors to the cerebral cortex via relays in the lateral cervical nucleus and the contralateral thalamus. Figure 1 illustrates a highly schematic representation of the DC and spino-cervical ascending pathways. * emanjar@siu.buap.mx; phone: +5222 22 295500 (ext 7326); fax: 5222 22 334511 Fig. 1. General view of two major somatic sensory pathways in the cat spinal cord: The dorsal columns (DC) system and the spino-cervical pathway (located in the dorsolateral funiculus, DLF). Spino-Cervical tract Gracile and Cuneate nuclei Lateral Cervical nucleus Thalamus Somatosensory Cortex DC DLF Peripheral Sensory Receptors Manjarrez et al. (2003) Proc. of SPIE Vol. 5110, pp: xx-xx