Why do stimuli in a nonclassical receptive field usually need to be paired with central stimuli?

Stimuli in a nonclassical receptive field typically need to be paired with central stimuli because, by definition, stimuli presented in these peripheral regions are unable to evoke a response from a neuron on their own.1

The nonclassical receptive field—often referred to as the silent surround, modulatory surround, or end-inhibitory zone—consists of areas beyond the classical receptive field. Because these peripheral stimuli are generally subthreshold or inhibitory, they cannot independently change a neuron's spiking activity. Instead, their function is to modulate or add their effect to the responses already being driven by the more sensitive central portion of the receptive field. Consequently, researchers identify these regions by pairing stimuli in the classical receptive field with stimuli in the surrounding region to observe how the latter influences the primary response.1