Rats use their facial vibrissae ("whiskers") to locate and identify objects. To learn about the neural coding of contact between whiskers and objects, we investigated the representation of single-vibrissa deflection by populations of cortical neurons. Microelectrode arrays, arranged in a geometric 10 x 10 grid, were inserted into the thalamo-recipient layers of "barrel cortex" (the vibrissal region of somatosensory cortex) in urethane-anesthetized rats, and neuronal activity across large sets of barrel-columns was measured. Typically, 5 msec after deflection of a whisker a 0.2 mm(2) focus of activity emerged. It rapidly expanded, doubling in size by 7 msec, before retracting and disappearing 28-59 msec after stimulus onset. The total territory engaged by the stimulus ranged from 0.5 to 2.9 mm(2) (2-11 barrels). Stimulus site dictated the domain of activity. To quantify the coding of whisker location, we applied the population d' measure of discriminability. Activity patterns elicited by two whiskers were highly discriminable at the initial cortical response; peak discriminability typically occurred within 16 msec of stimulus onset. To determine how widely information about stimulus location was distributed, we measured population d' while excluding response data from the on-center electrodes of the two tested whiskers. Response patterns remained discriminable, indicating that information about stimulus location was distributed across barrel cortex. Taken together, these results show that single-whisker deflections are represented in a multicolumn region constrained by barrel cortex map topography. The nature of this coding allows information about stimulus location to be coded extremely rapidly and unambiguously by one to two spikes per neuron.