Non-conscious prioritization speed is not explained by variation in conscious cognitive speed, decision thresholds, short-term visual memory, and by the three networks of attention (alerting, orienting and executive)

Sklar, Asael, Ariel Goldstein, Yaniv Abir, Ron Dotsch, Alexander Todorov, and Ran Hassin. 2020. “Did You See It? Robust Individual Variance in the Prioritization of Contents to Conscious Awareness.” PsyArXiv. January 20. doi:10.31234/osf.io/hp7we

Abstract: Perceptual conscious experiences result from non-conscious processes that precede them. We document a new characteristic of the human cognitive system: the speed with which the non-conscious processes prioritize percepts to conscious experiences. In eight experiments (N=375) we find that an individual’s non-conscious prioritization speed (NPS) is ubiquitous across a wide variety of stimuli, and generalizes across tasks and time. We also find that variation in NPS is unique, in that it is not explained by variation in conscious cognitive speed, decision thresholds, short-term visual memory, and by the three networks of attention (alerting, orienting and executive). Finally, we find that NPS is correlated with self-reported differences in perceptual experience. We conclude by discussing the implications of variance in NPS for understanding individual variance in behavior and the neural substrates of consciousness.


NPS=non-conscious prioritization speed


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And then, you suddenly become aware: it might be of a child running into the road in front of your car, your friend walking on the other side of the street, or a large spider in your shoe. On the timeline that stretches between non-conscious processes and the conscious experiences that emerge from them, this paper focuses on the moment in which your conscious experiences begin: just when you become aware of the child, your friend or the spider. Before this point in time processing is strictly non-conscious, after this moment conscious processing unfolds.

For many, the idea that non-conscious processing generates visual awareness is unintuitive. Imagine suddenly finding yourself in Times Square. You may imagine opening your eyes and immediately experiencing busy streets, flashing ads and moving people, all at once. Intuitively, we feel our experience of the world is immediate and detailed. Yet this intuition is wrong; the literature strongly suggests that conscious experiences are both limited in scope (e.g., Cohen, Dennett, & Kanwisher, 2016; Elliott, Baird, & Giesbrecht, 2013; Wu & Wolfe, 2018) and delayed (e.g., Dehaene, Changeux, Naccache, & Sergent, 2006; Libet, 2009; Sergent, Baillet, & Dehaene, 2005). The feeling that we consciously experience more than we actually do is perhaps the most prevalent psychological illusion, as it is omnipresent in our every waking moment (e.g., Cohen et al., 2016; Kouider, De Gardelle, Sackur, & Dupoux, 2010)1.

Measurements of the “size” or “scope” of conscious experience indicate a rather limited number of objects can be experienced at any given time (e.g., Cohen, Dennett, & Kanwisher, 2016; Elliott, Baird, & Giesbrecht, 2013; Wu & Wolfe, 2018). Other objects, the ones not consciously experienced, are not necessarily entirely discarded. Such objects may be partially experienced (Kouider et al., 2010) or integrated into a perceptual ensemble (Cohen et al., 2016) yet neither constitutes fully conscious processing.

Considerable research effort has identified what determines which visual stimuli are prioritized for conscious experience. This work found that both low-level features (e.g. movement, high contrast) and higher-level features (e.g., expectations, Stein, Sterzer, & Peelen, 2012; emotional value, Zeelenberg, Wagenmakers, & Rotteveel, 2006) influence the prioritization of stimuli for awareness.

Evidently, the process that begins with activation patterns in the retina and ends with a conscious percept has a duration (e.g., Dehaene, Changeux, Naccache, & Sergent, 2006; Libet, 2009; Sergent, Baillet, & Dehaene, 2005). Considering the above examples, clearly this duration may have important consequences. If you become aware quickly enough, you are more likely to slam the brakes to avoid running over the child, call out to your friend, or avoid a painful spider bite.

Here, we focus on this aspect of how conscious experiences come about using a novel perspective. Specifically, we examine individual variability in the speed with which our non-conscious processes prioritize information for conscious awareness (i.e., do some individuals become aware of stimuli more quickly than others?). Examination of individual differences provides rich data for psychological theories (for a recent example see de Haas, Iakovidis, Schwarzkopf, & Gegenfurtner, 2019), an acknowledgement that has recently gained renewed interest (e.g., Bolger, Zee, Rossignac-Milon, & Hassin, 2019). We report 8 experiments documenting robust differences, and examine possible mechanisms that may bring these differences about.

To examine non-conscious prioritization speed (NPS) we use two long-duration masking paradigms. The main paradigm we employ is breaking continuous flash suppression (bCFS; Tsuchiya & Koch, 2005). In bCFS, a stimulus is presented to one eye while a dynamic mask is presented to the other eye (see Figure 1). This setup results in long masking periods, which may last seconds. Participants are asked to respond when they become conscious of any part of the target stimulus. This reaction time, the duration between the initiation of stimulus presentation and its conscious experience, is our measure of participants' NPS.

Like many others (e.g., Macrae, Visokomogilski, Golubickis, Cunningham, & Sahraie, 2017; Salomon, Lim, Herbelin, Hesselmann, & Blanke, 2013; Yang, Zald, & Blake, 2007), we hold that bCFS is particularly suited for assessing differences in access to awareness for two reasons. First, CFS allows for subliminal presentations that can last seconds. Thus, unlike previous masking techniques, it allows for a lengthy non-conscious processing. Second, bCFS allows one to measure spontaneous emergence into awareness, focusing on the moment in which a previously non-conscious stimulus suddenly becomes conscious2.

Crucially, to overcome the limitations associated with using just one paradigm, we use another long duration masking technique that has the same advantage, Repeated Mask Suppression (RMS; Abir & Hassin, in preparation). Using two different paradigms allow us to generalize our conclusions beyond the specific characteristics and limitations of each of the paradigms.

In eight experiments, we document large differences between individuals in NPS. Across experiments, we show that some people are consistently faster than others in becoming aware of a wide variety of stimuli, including words, numbers, faces, and emotional expressions.

Moreover, this individual variance is general across paradigms: Participants who are fast prioritizers in one paradigm (CFS; Tsuchiya & Koch, 2005) are also fast when tested using a different suppression method (RMS; Abir & Hassin, in preparation; see Experiment 3), a difference which is stable over time (Experiment 7). We extensively examined possible sources of this individual trait. Our experiments establish that NPS cannot be explained by variation in conscious cognitive speed (Experiment 4), detection threshold (Experiment 5), visual short-term memory (Experiment 6), and alerting, orienting and executive attention (Experiment 7). Finally, we find that differences in NPS are associated with self-reported differences in the richness of experience (Experiment 8). Based on these results we conclude that NPS is a robust trait and has subjectively noticeable ramifications in everyday life. We discuss possible implications of this trait in the General Discussion.


Discussion

Overall, the current findings paint a clear picture. In eight experiments we discovered a highly consistent, stable and strong cognitive characteristic: NPS. NPS manifested in a large variety of stimuli – from faces and emotional expressions, through language to numbers. It was stable over time (20 minutes) as well as measurement paradigm (bCFS vs. bRMS). We additionally found NPS to be independent of conscious speed, short-term visual memory, visual acuity and three different attentional functions and largely independent of conscious detection thresholds.

In previous research differences in suppression time between stimuli (e.g. upright faces, Stein et al., 2012; primed stimuli, Lupyan & Ward, 2013) have been used as a measure of stimuli’s priority in access to awareness. In such research, individual variance in participants' overall speed of becoming aware of stimuli is treated, if it is considered at all, as nuisance variance during analysis (e.g., Gayet & Stein, 2017). A notable exception to this trend is a recent article (Blake, Goodman, Tomarken, & Kim, 2019) that documented a relationship between individual differences in the masking potency of CFS and subsequent binocular rivalry performance. Here, we greatly extend this recent result as we show that individual variance in NPS is highly consistent across stimuli and time, generalizes beyond bCFS, and is not explained by established individual differences in cognition.

Because of its effect on conscious experience, it is easy to see how NPS may be crucial for tasks such as driving or sports, and in professions such as law enforcement and piloting, where the duration required before conscious processing initiates can have crucial and predictable implications. In fact, NPS may be an important factor in any task that requires both conscious processing and speeded reaction. Understanding NPS, its underlying processes and downstream consequences, is therefore a promising avenue for further research.

Another promising direction would be to examine NPS using neuroscience tools, especially with respect to the underpinnings of conscious experience. First, understanding what neural substrates underpin individual differences in NPS may shed new light on the age-old puzzle of what determines our conscious stream. Second, understanding NPS may shed new light on some of the currently intractable problems in the field of consciousness research, such as separating neural activity that underlies consciousness per se, from neural activity that underlies the non-conscious processes that precede or follow it (De Graaf, Hsieh, & Sack, 2012). Thus, understanding NPS may provide missing pieces for many puzzles both in relation to how conscious experience arises and in relation to how it may differ between individuals, and what the consequences of such differences might be.