What changes in spike rate are used to identify a classical receptive field?

A classical receptive field is identified by observing deviations in the action potential (or "spike") discharge rate of a neuron away from its background activity level when discrete stimuli are presented in the sensory periphery.1

Specifically, this region is defined by the following changes in spiking activity:
* Excitatory and Inhibitory Responses: The classical receptive field includes areas where stimuli can elicit either excitatory or inhibitory responses. For instance, in centre-surround antagonism, stimuli in the central region may increase the spike rate, while stimuli in the surrounding region elicit inhibitory responses (or evoke responses when the stimulus is turned off).1
* Stimulus Sensitivity: By definition, the classical receptive field is the area where stimuli are capable of changing the neuron's spiking activity. Conversely, stimuli presented outside of this classical region do not, by themselves, change the cell's spiking activity.1