A study was undertaken to investigate the response of the rodent somatosensory barrel cortex to single-whisker, near-threshold vibrissal stimuli. Cortical responses to controlled whisker deflections were recorded by (i) conventional multi-unit extracellular recording within the cytochrome oxidase rich barrels centers and the interbarrel septa, and (ii) intrinsic signal optical imaging, a technique that provides a spatial view of cortical activation thought to be related to the deoxygenation of

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... emoglobin in activated areas. Barrel cortex neurons responded weakly to whisker deflections of 0.04°. Their response to a series of small stimuli of increasing amplitude was well-fitted by a logarithmic function. Responses to larger stimuli declined monotonically with distance from the center of the barrel column, and were characterized by greater onset and offset firing rates, by greater post-excitatory reduction of firing to below spontaneous levels, and by shorter response latency. In comparison to measurements taken previously from primary vibrissal afferent fibers, we conclude that cortical cells can respond to activity in a very small fraction of first-order sensory neurons. Vibrissa Def lection A solenoid-like stimulator with negative feedback from a linear variable differential transformer (LVDT) was used to provide quantitative vibrissa def lections (Developed by M. Fong, University of California, San Francisco, Department of Otolaryngology, 1993). The LVDT provides an on-line signal of the actual position of the stimulus probe (Fig. 1) . Thus, we were able to measure the actual stimulus applied to the whisker under study. A small plastic rectangle (approximate dimensions 1 × 5 mm) attached to the probe tip rested gently on the caudal aspect of the D1 whisker on the right side of the face, holding it slightly rostral of resting position. Stimulus onset was in the rostral direction, roughly orthogonal to the longitudinal axis of the whisker. Four different near-square wave stimuli were applied to whisker D1 at ∼14 mm from the snout (Fig. 1) . Stimulus amplitudes were 9, 17, 34 and 68 µm, delivered at 4 Hz. These amplitudes were chosen based on preliminar y observations from multi-unit recordings that a 10 µm