Visual search: magno- and parvocellular systems or color and luminance processes?

It has been found that visual search is slower with isoluminant color stimuli than with luminance stimuli. This has been interpreted to mean that the magnocellular system plays a vitally important role in visual search. We here propose that this observation can be more parsimoniously understood in terms of color and luminance rather than in terms of the magno-and parvocellular systems. According to this interpretation, the slower search under isoluminant condition is a consequence of the fact that visual processing in general is slower, and that spatial vision is poorer, under isoluminant color conditions relative to luminance conditions. We also point out that in order to draw meaningful conclusions with regard to the underlying neural substrate, one needs to equate the luminance and color stimuli for discriminability.