Here is a YouTube video of a Hubel & Wiesel experiment from 1965 in the visual cortex. The video shows the experimenters outlining the receptive field of a simple cell in the first part of the video using symbols. Crosses are used for excitatory responses, and triangles for inhibitory responses.
As shown in the video, neuron firing rate increased when the edge of a bar of light touched the crosses present on the top left. It is not just the orientation and location of light bar, but it seems that the light bar's edge is defining the firing rate, and thus determines the receptive field. So, the oriented receptive field of the cell seems to be defined w.r.t to the stimulus edges, rather than the bright and dark regions stimulating the on-center and off-surround parts of the receptive field. It seems that the simple cell was getting triggered when the edge of the light bar touched the three lines made by cross marks and triangles in that particular orientation. So, the cell under consideration was responding to edges at three separate locations.
Is my understanding correct ? Please help me in understanding how oriented receptive fields made up with center-surround concept explain the excitation at the edge observed. Perhaps it can be seen as the derivative of a box-like receptive field, or an oriented receptive field.