Movement nulling: for heterochromatic photometry and isolating channels for 'real' and 'apparent' motion

Null measurements given by cancelling forces, voltages, or whatever, are used in physics for gaining 'objectivity'--by avoiding 'subjective' perceptions; but, somewhat paradoxically, null methods can be useful for studying perception itself. Here we consider cancelling opposed movements for photometry with coloured lights, and some recent experiments, carried out with John Harris, on nulling 'real' against opposed 'apparent' motion for teasing out some neural movement channels.