Neurons throughout the visual system respond only to local patches of the visual field. This schematic diagram shows an experimental protocol in which the subject’s eyes are fixated on the center X while investigators show circles at different positions (at each moment in time, there is only one circle on the screen). Above each circle, the ticks indicate the activity of a hypothetical neuron. This neuron fires vigorously when the circle appears in the lower left position, defining its receptive field (arrow). Note that there is spontaneous activity in other locations, but only one location elicits a vigorous response. 0.25 deg, 0.25 degree of visual angle.