Derrick to Nigel: "I don't follow why you think that step 2 is based on the implicit premise that vision is representational"

I also raised the point that Derrick's argument simply assumes that visual experience is consequent upon brain activity.  Suppose, to be a devil's advocate, I were to maintain that visual experience is directly events in the world beyond the body and what we measure in the brain is further processing of  these events. The time lag arguments are then a non-sequitur. Yes, there might be time lags in the brain but these would have nothing to do with experience.

If I were a Direct Realist I would agree that neural time-lags are consistent with a form of Representationalism but they are also consistent with Direct Realism, after all, the body must process the light to react to it.  I am not a Direct Realist which is why I proposed you might use an optical gate to make it clear that there is a non-neural time lag in your argument. If the flash of light is entirely isolated from its source when it is seen then it is difficult to argue that it is the source rather than a signal from the source that enters visual experience.