Threshold assessment for unconscious stimulus presentation using hierarchical drift diffusion models

Background: Humans often visually perceive emotional cues without cognitive awareness. Even though many studies have examined unconscious processing, they apply different timing conditions for stimulus presentation ranging from 8-30ms. However, the exact presentation span at which an emotional expression is perceived unconsciously is still to be found. This prospective study aims to tested which presentation time can lead to an unaware processing of the emotion-specific stimulus information. We further aimed to understand the decision pro-cess of classifying emotional material depending on the presentation time and potential influ-ence factors. Methods: A cohort of 40 healthy participants rated images of emotional faces (whether the faces showed a sad, neutral or happy expression) presented for 8.3ms, 16.7ms and 25ms. Sub-jective stimulus awareness as well as objective measures of stimulus awareness, such as per-formance accuracy within the task, were evaluated. In addition, response biases and perfor-mance precision (drift rates) were analyzed via hierarchical drift diffusion models. Results: Subjectively, the participants reported stimulus awareness in 65% of the 25ms trials, in 36% of 16.7ms trials, and in 2.5% of 8.3ms trials. Performance accuracy and response preci-sion increased with longer presentation time. Emotion-dependent response precision was re-flected in decreased performance (drift rates) in sad trials and a bias during happy trials, in all timing conditions except 8.3ms estimated with the hierarchical drift diffusion models. The detection rate (probability of making a correct response) during 8.3ms was at 14.6% and still below chance level for 16.7ms trials (42.2%). Conclusion: Taken together, the experiments suggest a presentation time of 16.7ms as optimal for subconscious priming. During 25ms trials, subjective rating of stimulus awareness and high-performance accuracy indicate a conscious stimulus perception. While 8.3ms could be too short to distinguish between emotions, an emotion classification was still possible during 16.7ms while the performance precision and probability of making correct responses tends to indicate an unconscious processing.