Spatial-temporal distribution of whisker-evoked activity in rat somatosensory cortex and the coding of stimulus location

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 popula tions of cortical neurons. Microelectrode arrays, arranged in a geometric 1 0 X 10 grid, were inserted into the thalamo-recipient layers of "barrel cor tex" (the vibrissal region of somatosensory cortex) in urethane-anesthetize d rats, and neuronal activity across large sets of barrel-columns was measu red. Typically, 5 msec after deflection of a whisker a 0.2 mm(2) focus of a ctivity emerged. It rapidly expanded, doubling in size by 7 msec, before re tracting and disappearing 28-59 msec after stimulus onset. The total territ ory engaged by the stimulus ranged from 0.5 to 2.9 mm(2) (2-11 barrels). St imulus site dictated the domain of activity. To quantify the coding of whis ker location, we applied the population d' measure of discriminability. Act ivity patterns elicited by two whiskers were highly discriminable at the in itial 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 respon se 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 resul ts show that single-whisker deflections are represented in a multicolumn re gion constrained by barrel cortex map topography. The nature of this coding allows information about stimulus location to be coded extremely rapidly a nd unambiguously by one to two spikes per neuron.