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Rao and Ballard (1999) start their paper by stating that extra-classical receptive fields, which exhibit the phenomenon of end-stopping are difficult to explain. They show that in an idealized predictive coding model trained on natural images, the "neurons" at a higher level in the model have a receptive field that exhibits end-stopping; therefore, if neurons in the brain are implementing predictive coding, they would exhibit this phenomenon.

What I don't understand is this - why is it difficult to explain extra-classical receptive fields? After all, retinal ganglion neurons that are on-center will also exhibit "end-stopping" if a stimulus extends outside of the center, and the explanation for that receptive field is straightforward: the photoreceptors at the periphery hyperpolarize the ganglion cell, whereas those at the center depolarize it. Why can't a similar story be told about cortical neurons? Why was the predictive coding framework needed to explain end-stopping?

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The difference is that ganglion neurons are responding directly to phenomena in their receptive field. That is, the on-center, off-surround ganglion cells are directly connected to the photoreceptors at the center and in the surround. The explanation for off-surround is that photoreceptors at the periphery will directly hyperpolarize the ganglion cell.

Extraclassical receptive field effects are about explaining how a neuron (e.g., in V1) responds to phenomena outside its receptive field. That is, it is possible to map the entire portion of the visual field that it is hooked up to, e.g., by passing a stimulus across the visual field and noting all regions where spike trains are evoked by the stimulus. This portion of the visual field is known as the classical receptive field. It turns out that V1 neurons can be influenced by stimuli outside their classical receptive field. Unlike "end-stopping" in retinal ganglion neurons, this cannot be explained by hyperpolarizing input at the periphery of the receptive field, because the stimulus causing the end stopping is not in its receptive field in the first place.

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