Recording of intrinsic optical signals from neurons was first proposed in 1986 by Grinvald et al. [5], using potential-dependent dyes; the method was then used for recording the intrinsic electrical signals of nerve cells both in vivo [4] and in brain slices [11]. The optical prop- erties of neurons were found to change in accordance with their activity, allowing their locations to be deter- mined. Experiments of this type generally measure the difference in brightness between active and inactive zones of nerve tissue in reflected light, though whether this change occurs because of changes in the absorption of light, in the fluorescent properties of neurons, or for other reasons thus far remains unclear. The reflective properties of nerve cells can change, for example because of changes in neuron volume [2]. Some authors [11] ascribe these changes to the uptake of Cl ions by astroglial cells, as experimental studies have shown that extracellular chloride blocks the optical signal. The authors believed that chloride uptake leads to increases in the volume of astroglial cells and decreases in their uptake of natural chromophores, such as hemoglobin, cytochromes, and even reduced NAD [13]. Increases in neuron activity are accompanied by increases in their oxygen requirement, and changes in light reflection correspond to the transition between oxy- hemoglobin and deoxyhemoglobin, because the latter absorbs more red light. Thus, an activated zone of cortex is darker than the neighboring area. There is one further possible explanation for the observed optical signal – that it reflects changes in blood flow in the microzone of the brain surface [12]. However, experiments based on block- ade of vasodilatation had no effect on the intrinsic signal of the cortex [6]. This hypothesis also does not corre- spond completely to the fact that optical signals can be obtained in brain slices [11]. Thus, the question of the mechanisms of this phenomenon remains open and it appears that intrinsic optical signal, which is dependent on nerve cell activity, arises from several simultaneous origins. A complex specialized apparatus is used for recording intrinsic optical signals in the infrared region of the spectrum in nerve tissues. However, as first shown by Peterson and Goldreich [13], simpler meth- ods can also be used in the visible part of the spectrum, Neuroscience and Behavioral Physiology, Vol. 31, No. 2, 2001 Use of a Simplified Method of Optical Recording to Identify Foci of Maximal Neuron Activity in the Somatosensory Cortex of White Rats M. Yu. Inyushin, A. B. Vol’nova, and D. N. Lenkov 0097-0549/01/3102-0201$25.00 © 2001 Plenum Publishing Corporation 201 Translated from Rossiiskii Fiziologicheskii Zhurnal imeni I. M. Sechenova, Vol. 85, No. 11, pp. 1385–1390, November, 1999. Original article submitted March 31, 1999. Eight mongrel white male rats were studied under urethane anesthesia, and neuron activity evoked by mechanical and/or electrical stimulation of the contralateral whiskers was recorded in the prima- ry somatosensory cortex. Recordings were made using a digital USB chamber attached to the print- er port of a Pentium 200MMX computer running standard programs. Optical images were obtained in the barrel-field zone using a differential signal, i.e., the difference signal for cortex images in con- trol and experimental animals. The results obtained here showed that subtraction of averaged sequences of frames yielded images consisting of spots reflecting the probable position of activated groups of neurons. The most effective stimulation consisted of natural low-frequency stimulation of the whiskers. The method can be used for preliminary mapping of cortical zones, as it provides for rapid and reproducible testing of the activity of neuron ensembles over large areas of the cortex. KEY WORDS: Optical recording, somatosensory cortex, whiskers. Developmental Neurobiology Laboratory, A. A. Ukhtom- skii Physiological Science Research Institute, St. Peters- burg State University, 7/9 Universitetsksya Bank, 199034 St. Petersburg, Russia.