The filling-in process proposed as a cover up for the existence of the blind spot has some conceptual similarities to blindsight. The perceptual operation of a hypothetical mechanism responsible for filling in represents a logical paradox. The apparent indeterminacy of the percept in the optic-disc region can be tested experimentally by viewing the grating test pattern below.

Palmer's attempt to dust off Locke's construct of “inverted spectrum” is discussed here to examine its plausibility. Perceptual inversion could be fulfilled by adopting the notion of “inverted trichromacy” rather than by the proposed existence of “red-green reversed trichromats.” Although the former alternative conforms to a hypothetical world of vampires, it fails to conform to the realities of genetics and neuroscience.