Now try to suppose that there is no functional difference between the experience of a person with normal vision, and one who has inverted luminance. A scene that appears bright to us will appear dark and murky to him - yet he must report that things are well illuminated and distinct. Where we report a scene as bright and cheerful, he will experience darkness, yet report the same. Surely there is reason to suspect that he cannot be having the very same qualitative experience that I have, when I survey a dark, murky, gloomy scene, if such qualia in him cause reports that the scene is bright, clear, distinct and cheery. Pitch inversion Vision is not the only modality that might have inversions.
Musical sounds have an experienced loudness and pitch.
Taking pitch first, we can then suppose that there is an individual who has inverted pitch, relative to normal humans. What you and I hear as the high pitched screaming alarm of a smoke detector, PI hears as fog horn. We all agree that tubas are capable of much lower notes than are piccolos. Is it possible that PI's inverted qualia have exactly the same functional roles as ours. It must be if qualia are not determined by functional role.