Research on unconscious perceptual processing aims to isolate the early stages of perception. Naturally, this is difficult to demonstrate with behavior-focused psychological research methods.
The issue: How should we best describe our efforts to untangle consciousness to isolate unconscious processes?
The term contamination has been used, starting notably with Jacoby and colleagues in the early 1990s (Debner & Jacoby, 1994; Jacoby et al., 1992). The following is a recent definition offered by Stockart et al. (2025):
Contamination: A confound that occurs when the results of the processing measure can be explained by residual awareness of the critical content, which was not captured by both the objective and subjective measures (e.g., due to insufficient sensitivity).
In other words, research results that seem like unconscious processing, could, in fact, occur from conscious processing. Numerous studies have suggested this possibility. However, the opposite argument has also been made: Consciousness indicators might be influenced by unconscious processes (Debner & Jacoby, 1994; Micher et al, 2024). This suggests that our measurements of consciousness are impure and overestimated.
I'm not a fan of the term contamination.
Contamination has negative connotations. It suggests something terrible or undesirable has happened, like pollution. Neutral language like mixture or leakage would be preferable. This is a minor quibble.
Contamination implies that unconscious perception might exist in pure form. What if this is not the case though? Mixture might be the normal way that perception works. Top-down processes like attention might influence perception all the way down to relatively low levels of processing. This possibility seems likely given anatomy of neural networks in the brain. Every level of the visual system from the thalamus onward has feedforward and feedback connections.
Contamination in pure form might also imply some distinct boundary exists between unconscious and conscious processes. This may not be accurate. While it might feel like a clear boundary exists at a subjective or introspective level, significant research evidence suggests a continuum of awareness states might exist.
So, for multiple reasons I'm not a fan of using the term contamination in describing unconscious perceptual processing research. These scholars are not wrong: "contamination" exists. This is about interpretation. Mixture of conscious and unconscious states seems to be a real phenomenon. Contamination is a feature, not a bug (to borrow a saying from computer science). Perhaps mixture is normal, not undesirable.
We can do better than using this term.
References
Debner, J. A., & Jacoby, L. L. (1994). Unconscious perception: Attention, awareness, and control. Journal of Experimental Psychology: Learning, Memory, and Cognition, 20(2), 304–317. https://doi.org/10.1037/0278-7393.20.2.304
Jacoby, L. L., Lindsay, D. S., & Toth, J. P. (1992). Unconscious influences revealed: Attention, awareness, and control. American Psychologist, 47(6), 802–809. https://doi.org/10.1037/0003-066X.47.6.802
Micher, N., Mazenko, D., & Lamy, D. (2024). Unconscious processing contaminates objective measures of conscious perception: Evidence from the liminal prime paradigm. Journal of Cognition, 7(1), 71. https://doi.org/10.5334/joc.402
Stockart, F., Schreiber, M., Amerio, P., Carmel, D., Cleeremans, A., Deouell, L. Y., Dienes, Z., Elosegi, P., Gayet, S., Goldstein, A., Halchin, A.-M., Hesselmann, G., Kimchi, R., Lamy, D., Loued-Khenissi, L., Meyen, S., Micher, N., Pitts, M., Salomon, R., … Mudrik, L. (2025). Studying unconscious processing: Contention and consensus. Behavioral and Brain Sciences, 1–77. https://doi.org/10.1017/S0140525X25101489