To elucidate how the flattening of sensory tuning due to a deficit in tonic inhibition slows motor responses, we simulated a neural network model in which a sensory cortical network (N S) and a motor cortical network (N M) are reciprocally connected , and the N M projects to spinal motoneurons (Mns). The N S was presented with a feature stimulus and the reaction time of Mns was measured. The flattening of sensory tuning in N S caused by decreasing the concentration of gamma-aminobutyric acid (GABA) in extracellular space resulted in a decrease in the stimulus-sensitive N M pyramidal cell activity while increasing the stimulus-insensitive N M pyramidal cell activity , thereby prolonging the reaction time of Mns to the applied feature stimulus. We suggest that a reduction in extracellular GABA concentration in sensory cortex may interfere with selective activation in motor cortex, leading to slowing the activation of spinal motoneurons and therefore to slowing motor responses.