Cognitive penetration: Would we know it if we saw it?

How can the impenetrability hypothesis be empirically tested? We comment on the role of signal detection measures, suggesting that context effects on discriminations for which post-perceptual cues are irrelevant, or on neural activity associated with early vision, would challenge impenetrability. We also note the great computational power of the proposed pre-perceptual attention processes and consider the implications for testability of the theory.