Consciousness is at the heart of our every experience but still poorly understood scientifically. New research offers insights into how weak and ambiguous stimuli can be consciously perceived through repetition and offers evidence for a new type of memory—a subliminal sensory buffer store (SSBS)—which has been predicted by one of the leading theories of consciousness. Experimental data also shows that visual masking, a widely used technique in psychology and neuroscience, does not erase or overwrite information as was previously thought but merely limits conscious access to it. These findings have a profound impact on consciousness studies and cognitive science in general.
Consciousness is at the same time what we know best and a complete mystery. Every experience we ever had is the result of our own consciousness (Pang, 2023a). Yet, despite our intimate familiarity with it, consciousness is difficult to define and poorly understood scientifically (Pang, 2023b). We know more about distant galaxies and the deepest regions of the oceans than about the basis for our own experiences. A leading neuroscientist wagered in 1998 that the neural mechanisms of consciousness would be discovered within 25 years. He recently conceded his bet lost (Lenharo, 2023). While a complete understanding of consciousness remains elusive, tremendous progress has still been made in this field. As recently as 1996, the International Dictionary of Psychology's entry on consciousness stated that "nothing worth reading has been written about it" (Sutherland, 1996). Now, there are countless books, journals, laboratories, and entire research departments dedicated to this field of enquiry. One fruitful avenue has been to study perception and explore the difference between consciously perceived stimuli and those that remain unconscious. A common way to control the visibility of stimuli is through masking.
2. Visual Masking
A briefly shown image that is normally consciously perceived can be rendered unconscious by showing a distracting image—called a mask—in close spatiotemporal proximity (Breitmeyer & Öǧmen, 2006). This method is known as visual masking and is widely used in research on perception and consciousness (Bachmann & Francis, 2014; Breitmeyer & Öǧmen, 2006). Traditional models have explained this phenomenon as the mask erasing or overwriting memory representations of the original stimulus (Averbach & Coriell, 1961). Many researchers have questioned this explanation (Breitmeyer & Öğmen, 2000; Losier et al., 2017; Vogel, Woodman, & Luck, 2006) but without any empirical evidence, the erasure hypothesis remained dominant (Pang & Elntib, 2021).
3. Unconscious Memory
Memory is one of the most important cognitive functions and memory disorders like amnesia have devastating consequences and impede many aspects of human functioning (Allen, 2018). Memory has been divided into long- and short-term stores. A second division is between explicit memories that can be consciously accessed and described and implicit memories, which function without conscious awareness. Traditional models only accounted for two types of implicit memories: Procedural memory and priming (Irvine, 2013; Squire & Dede, 2015; Soto & Silvanto, 2014). Procedural memory includes habits, skills (like riding a bicycle), and simple forms of conditioning (Squire & Dede, 2015). Priming occurs when a briefly presented stimulus affects subsequent behaviour without entering awareness (Bargh, 2016; Francken et al., 2011). A typical example is the word of a colour being shown briefly and masked. Despite being unaware of this masked word, participants have been shown to detect the colour of an image shown afterwards much faster if it is congruent with the primed word and slower if it is incongruent (Cheesman & Merikle, 1984).
More recent research has challenged this traditional categorisation into mostly conscious and only limited unconscious forms of memory. There are indications that subliminal stimuli may be maintained for some time and be used after a delay period to solve behavioural tasks (Soto et al., 2011; Pan et al., 2014; Samaha et al., 2016; Henke et al., 2013). Neural signatures related to memory encoding have also been described in the absence of conscious awareness (Degonda et al., 2005; Duss, et al., 2014; Bergström & Eriksson, 2018). Other studies have directly related neural data to behavioural outcomes in further support of memory encoding of subliminal information (Dutta et al., 2014; Trübutschek et al., 2017; Züst et al., 2014; Reber et al., 2012). Although the broader idea of unconscious memory is becoming more widely accepted, the detailed functions, classifications, and purposes of these memory stores remain controversial. Despite a large research effort, implicit memory remains very poorly understood (Persuh et al., 2018; Stein et al., 2016).
4. A New Type of Memory: Subliminal Sensory Buffer Store (SSBS)
My research partner and I have recently uncovered evidence for a new type of implicit memory that plays a key role in understanding how weak and ambiguous stimuli are perceived, how visual masking works, and how weak and ambiguous external inputs are turned into conscious experiences (Pang, 2021a;2021b). Our findings were published in the Nature journal Scientific Reports (Pang & Elntib, 2021) and the Elsevier journal Consciousness and Cognition (Pang & Elntib, 2023). A single strongly masked visual stimulus cannot be consciously perceived. However, we found that when a masked stimulus is repeated within short succession, it enteres conscious awareness and suddenly becomes visible (Pang & Elntib, 2021;2023). This indicates that masked information is not erased or overwritten but retained for short periods of time in what we called a subliminal sensory buffer store (SSBS). This buffer store is not accessible by conscious awareness but if confirmatory evidence is obtained, information accumulates and is transformed into a more durable and consciously accessible form of storage (Pang & Elntib, 2021;2023).
Although consciousness is still poorly understood, several theories have been proposed to explain crucial aspects of it. One of the leading theories is the global workspace theory (GWT) of consciousness, which describes conscious experiences as the result of integrating unconscious processes (Baars, 1988; Dehaene et al., 2006; Kouider & Dehaene, 2007). GWT suggests that subliminal information competes for conscious awareness, which is like a spotlight. What enters this spotlight depends on the stimulus strength (bottom-up strength) as well as top-down factors like attention, expectations, and prioritisation (Kouider & Dehaene, 2007). For this process to work, subliminal information needs to be briefly stored while it competes with other stimuli for conscious awareness, and then either be discarded or be consciously perceived. Several researchers explicitly predicted the existence of a non-conscious buffer store that retains information for at least a few hundreds of milliseconds purely on theoretical grounds (Kouider & Dehaene, 2007; Smith et al., 2011; Smithson & Mollon, 2006). However, there was no direct empirical evidence in support of this (Pang & Elntib, 2023). Our research bridged this gap and offered direct experimental evidence for the existence of such a memory store as well as providing some further information, like its duration and decay.
We used a repeated-measures, double-blind, controlled experimental study to probe perception, awareness, and memory across three experiments. For Experiment 1, we recruited 40 participants (16 male, 24 female) aged between 18 and 66 (M = 38.5, SD = 13.08). Experiments 2 and 3 consisted of 15 participants (7 male, 8 female) aged between 20 and 61 (M = 34.6, SD = 12.96).
In all experiments, participants were shown a series of presentations on a computer screen and were asked to report what they saw in a blank field as a content report (CR) or guess if unsure. After entering an answer, participants were asked to rate how clear their experience was on the four-point perception-awareness scale (PAS; Romsøy & Overgaard, 2004), using the Peremen and Lamy (2014) adjustment (starting the scale at zero to reflect the absence of experience to make the scale more intuitive). The final part of the evaluation phase asked participants to identify the correct target from four options in a forced-choice task (FCT).
The presentation sequence consisted of a focus signal, followed by a series target images and masks, as shown in Fig 1. Experiment 1 used three different conditions that showed the target stimulus 10 times, five times, or only once (as a control condition). The target stimulus was replaced by blank frames when shown five times or only once. Additionally, Experiment 1 used different repetition intervals by extending the length of time the mask was shown. Experiment 2 only used three repetition intervals (35 ms, 70 ms, and 139 ms plus the blank frames). Experiment 3 only used 70 ms plus the blank frames but used two control conditions, one where the target stimulus was shown at the beginning and one where it was displayed at the end. For more details on the method, please refer to Pang and Elntib (2021; 2023).
Fig 1: Presentation sequence used in the experiment: A focus signal was shown, followed by a repeated sequence of masks (M) and targets (T). Experiment 1 used three conditions, showing a target 10 times, 5 times, or only once, while using different repetition intervals (longer timings were achieved by extending the mask duration). Experiment 2 and 3 added blank frames before and after the target to increase masking effects.
We tested the impact repetition of a strongly masked visual stimulus had on how they were perceived using both subjective and objective measures. We also examined the impact of repetition interval timings across a broad range (from 7 ms to 8,337 ms) in Experiment 1. As expected, masking severely limited the perception of the target stimulus in the control condition, eliciting only minimal conscious awareness, in line with previous findings on masking (Bachmann & Francis, 2014; Breitmeyer & Öğmen, 2000; 2006). However, when repeated at short intervals, the very same masked stimulus was consciously perceived. This increase in perception was significant across all the measures we used, as shown in Fig 2: Mean PAS results increased from 0.28 (SD = 0.50) without repetition to 0.89 (SD = 0.92) when repeated five times and 1.05 (SD = 0.98) when repeated 10 times. A factorial ANOVA showed that this was statistically significant with extremely strong effect F(1.48, 57.79) = 148.75, p < 0.01 ηp2 = .79 (ε = .74). In fact, the p value was below the 5-sigma requirement used in physics to validate a new finding (giving it a 1 in 3.5 million chance of results merely being a statistical anomaly; Lamb, 2012). Psychology and neuroscience have a lot more variables that can affect results and usually do not require such stringent standards. Our actual p value was in the order of 10-13, indicating that this is highly unlikely to be a statistical fluke but most probably represents a real phenomenon. These ratings are averages across all repetition interval times and were much higher at shorter intervals, suggesting an even larger effect size for those conditions. Objective measures had similar results with the percentage of correct content reports (CR) rising from 11.9% in the control condition to 43.1% with five presentations and 48.9% with 10 presentations, F(1.98, 77.25) = 149.59, p < 0.01, ηp2 = .79 (ε = .99). Percentage of correct responses in the forced-choice task (FCT) also rose from 34.8% in the control condition to 60.8% and 65.9% for five and 10 presentations respectively, F(1.93, 75.25) = 96.70, p < 0.01, ηp2 = .71 (ε = .97). Both objective measures had p values in the order of 10-13, again several orders of magnitude below the .05 and 0.01 thresholds usually used in the field to denote statistical significance.
While the number of presentations impacted perception, this was only the case at short repetition intervals. At long repetition intervals, repetition had a negligible effect on perception but at short intervals, it made masked stimuli clearly visible. We did not observe a clear cut-off point in the data but rather a gradual fading. Extraction of meaningful information was severely impaired after about 300 ms and most data was lost after 700 ms. The impact of repetition intervals was also statistically significant for all measures: PAS F(8.92, 347.70) = 52.17, p < 0.01, ηp2 = .57 (ε = .47; Supplementary Table 1), CR results F(12.58, 490.78) = 22.88, p < 0.01, ηp2 = .37 (ε = .66; Supplementary Table 2), and FCT performance F(12.41, 483.95) = 11.34, p < 0.01, ηp2 = .23.
Fig 2: Results of subjective (PAS, image a) and objective measures (CR and FCT, images b and c) from Experiment 1. Results for all measures were significantly higher when masked stimuli were repeated but decreased as repetition intervals lengthened.
Fig 3: Results of Experiment 2, which used more data points per participant but tested a narrower range to confirm the broader previous findings.
Experiment 2 confirmed these two main findings by using more data points for each condition-repetition interval pair per participant but testing only two conditions (10 presentation and one presentation as the control condition) and only three repetition intervals. All results were statistically significant with large effect sizes (see Pang & Elntib, 2021). Results for Experiment 2 are shown inf Fig 3.
Since the time from target presentation to evaluation was longer in the control condition, where the target was shown towards the beginning of the presentation sequence, we designed a new experiment (Experiment 3) where the target was shown at the end in a revised control condition. However, this did not change results and validated our previous findings: The number of presentations still had a significant impact on perception while the target position in the control (and the revised control condition) did not significantly impact perception (for more details, see Pang & Elntib, 2023).
Many previous studies have found that masking renders a visual stimulus invisible (Bachmann & Francis, 2014; Breitmeyer & Öǧmen, 2006). Our results show that when the exact same presentation sequence is repeated, this previously invisible stimulus can be consciously perceived. This seemingly simple result has profound implications. Here, we will discuss three of the main implications: Evidence that masking does not erase or overwrite information from the target stimulus, evidence for a new type of memory, and how this memory relates to consciousness.
As discussed above (2. Visual Masking), masking has traditionally been explained as the mask erasing or overwriting memory traces of the target stimulus (Averbach & Coriell, 1961; Pang, 2017). This hypothesis has been questioned extensively (Breitmeyer & Öğmen, 2000; Losier et al., 2017; Vogel, Woodman, & Luck, 2006) and several researchers predicted the existence of a subliminal memory store on theoretical grounds (Kouider & Dehaene, 2007; Smith et al., 2011; Smithson & Mollon, 2006) but empirical support for this notion was lacking. Our research indicates that masking does not erase or overwrite the memory trace of the target stimulus, otherwise repetition would have no effect as each repetition instance would start without any previous information. Our data also shows that this effect cannot simply be attributed to repetition increasing the number of detection opportunities or an overall increase in detection probability (see Atas et al., 2013) because long repetition intervals did not significantly improve perception despite having the same detection opportunities (i.e. number of target presentations) as shorter intervals. Perception only occurred clearly when repeated at short intervals. Another alternative explanation is that fragments of the target survived the masking and can be put together through repetition. However, this too is highly unlikely for several reasons: Firstly, again this hypothesis cannot account for why fragment integration did not occur at longer intervals. Secondly, the mask was the same each time, which would result in identical fragments that cannot be put together into a whole as key pieces would be missing regardless of the number of repetitions. Thirdly, participants not only showed better objective target identification but reported improved subjective perceptual awareness. Finally, this hypothesis would suggest that the mask and target would directly compete against each other as only the fragments that were not overwritten survive. This in turn would imply that the better the target was perceived, the poorer the mask would be experienced. This is not congruent with our own experience of trying the experiment ourselves where the mask was clearly perceived and the target suddenly appears during the repetition sequence nor is it congruent with informal participant reports—however, further research is needed to obtain data and do a formal analysis on this (see Pang & Elntib, 2023). Overall, our data offers empirical support against the erasure and overwriting hypotheses and provides evidence in support of those who have questioned and suggested other explanations. Visual masking is among the most widely used psychophysics tools with applications across a broad range of research areas. Our findings directly challenge the commonly held explanation for this phenomenon and instead offer insight into an alternative explanation in the form of a brief memory buffer store that has been previously predicted but not empirically found.
If memory traces of the target stimuli are not erased or overwritten, they must be retained in a way that is not directly accessible to conscious awareness for masking to work. Information without conscious access is a widely accepted notion (see for example Del Cul et al., 2007). Our data shows that masked stimuli were poorly perceived when shown by themselves. However, at short repetition intervals, they became visible and identifiable. These findings indicate that information was briefly retained in an unconscious—or subliminal—memory store, where information could accumulate through repetition and that such confirmatory evidence was then able to enter conscious awareness. We called this memory a subliminal sensory buffer store (SSBS)—since it contains sensory data and buffer because it is very brief. Given that awareness was highest at repetition intervals below 300 ms and that most meaningful information was lost when repetition intervals exceeded 700 ms, information within the SSBS seems to be retained for short periods and then fade rapidly but in gradual way rather than in a binary way (i.e. being turned off abruptly), which is similar to known supraliminal short-term memory stores (Yi et al., 2018). SSBS is different from supraliminal short-term memory stores, like iconic memory, in that it is not accessible to conscious awareness. It is also different from previously described subliminal memory stores, such as priming, because it does not merely influence subsequent behaviour without being detected but allows for information to accumulate and be made consciously accessible. While our research method may result in some priming effects to occur, our findings show something distinct from pure priming and point to a taxonomic independence of SSBS.
Empirical evidence about a subliminal sensory memory has wide-ranging implications in understanding consciousness. As mentioned above (4. A New Type of Memory: Subliminal Sensory Buffer Store (SSBS)), our findings directly support one of the predictions made by a leading theory of consciousness: Kouider and Dehaene (2007) explicitly predicted a memory store with strikingly similar properties to SSBS and lack of empirical evidence for such a store has been used as an argument in debates related to consciousness and cognition (Block, 2011). While evidence for an unconscious memory store described here can inform existing debates on consciousness, it also offers new insights and opens up new areas of enquiry. For example, when using electroencephalogram (EEG) to measure brain activity, a distinct measurable wave that is seen as a key signature of conscious awareness—the P300—occurs roughly 250 to 500 ms after stimulus onset (Polich, 2007). Given that backward masking, where the mask is shown after the target stimulus, is highly effective at eliminating perception of what was shown prior to the mask (Breitmeyer & Öğmen, 2006), it is possible that all visual stimuli pass through SSBS before being consciously perceived as a temporary placeholder where the brain evaluates its importance. This could explain the delayed onset of the P300 wave component and other findings that point towards conscious experience being delayed. While this is speculative, SSBS could potentially play a crucial factor in consciousness more generally. The difference between unconscious SSBS representations and memory traces with conscious access offers a new way to investigate what specific brain activity is being consciously perceived and the method we pioneered together with Atas et al. (2013) is likely to prove useful in a range of different research settings as it allows precise control over the visibility of identical stimuli based solely on repetition timing.
The experiments presented here did not include any brain imaging or neural measurement techniques. For this reason, there is no empirical data on the neural mechanisms behind SSBS or what brain areas are involved. While the explanatory framework described here is compelling, alternative interpretations of the data cannot be ruled out. For example, although highly unlikely given the data and also what is generally known about consciousness and perception, it cannot be ruled out that masked stimuli briefly entered conscious awareness but were forgotten before the evaluation phase and thus, participants were unable to report them. Direct measures of brain activity would be needed to demonstrate that brain states differed throughout the different conditions. Finally, we did not assess whether priming effects occurred and if they influenced the objective measures used here (CR and FCT). Although subliminal priming effects may have impacted responses, such effects would not change our overall conclusions that include robust results in objective measures as well as subjective reports, which point to a phenomenological change in experience. Further research is needed to answer these questions conclusively and to uncover more about SSBS, for example to test whether it contains pre-categorical or already processed information.
Consciousness remains a mystery, but new research approaches are generating a deeper understanding of some of the aspects involved, especially in relation to conscious perception. In this paper, evidence for a subliminal sensory buffer store (SSBS) is shown, which differs from previously described memory stores. This store briefly holds sensory information without conscious access before gradually fading. This fast but gradual decay starts after around 300 ms and most information is lost after around 700 ms. SSBS provides empirical support for a key concept of the global workspace theory (GWT)—one of the leading explanatory frameworks for consciousness. It further demonstrates that visual masking does not erase or overwrite memory traces of a target stimulus but rather stores it in a way that cannot be consciously accessed and may be crucial for conscious perception in general, as a place where information is confirmed and evaluated in terms of salience and importance (which could explain the general delay in conscious awareness observed by many researchers). Further research based on these findings, especially when measuring and imaging brain activity directly, could help us understand more about the neural mechanisms that turn perceptual inputs into conscious experiences. Consciousness remains enigmatic, but research is slowly shining light into some aspects of this mystery and the studies presented here help uncover a small piece in this giant puzzle.
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Mr Damian K F Pang, MSc
Mr Damian K F Pang, MSc, is an aviator, entrepreneur, scientist, and philosopher. His primary research focus is on consciousness, perception, and memory, and he has uncovered a subliminal sensory buffer store (SSBS), which plays a key role in the conscious perception of vague and ambiguous stimuli. His other research interests include the philosophy of mind and the differences and similarities between human cognition and artificial intelligence (AI).