The local field potential (LFP) is generated by large populations of neurons, but unitary contribution of spiking neurons to LFP is not well characterized. We investigated this contribution in multi-electrode array recordings from the cerebral cortex of human and monkey by calculating the spike-triggered LFP average (st-LFP). The resulting st-LFPs were dominated by broad spatio-temporal components due to on-going activity, synaptic inputs and recurrent connectivity. To observe the local field of a single spike we applied spatial filtering. The filtered st-LFPs were limited to an area of 800 μm from the neuron, and propagated at axonal speed, which is consistent with their unitary nature. In addition, we discriminated between putative inhibitory and excitatory neurons and found that the former dominated this unitary LFP contribution, consistently with previous findings in hippocampal slices. Thus, in human and monkey cortex, the LFP may primarily reflect inhibitory neuron activity.