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Abstract: Little is known about the mechanisms of spatial integration in the cortex. In the barrel cortex the response to simultaneous activation of multiple whiskers is sub-linear and even suppressive. The important role of inhibition in suppressing integrated responses was reveled by blockade of inhibition using bicucullin. However, the exact mechanism that leads to sub-linear spatial integration remains unclear. Here we explored the mechanism of spatial integration by recording the membrane potential and studying the integration of excitatory and inhibitory components arriving to the cell by stimulating two neighboring whiskers. Conductance measurements and intracellular blockade of inhibition demonstrated that excitatory components arriving from the two pathways are summed almost linearly, indicating that sub-linear summation is not caused by reduction of excitation. On the other hand, inhibitory components are summed supralinearly: inhibition was on average ~50% larger than predicted. These findings suggest that sub-linear spatial integration arises from nonlinear properties of the cortical network leading to activation of inhibitory inputs that are not activated by either stimulus alone.