The Transition from Unconscious to Conscious Visual Perception: Gradual or All-or-None?
Bachelor Project Brain & Cognition Maud Fliers (Student ID 10448594) Supervisor dr. T. Stein
Abstract
An important question about the elusive concept of consciousness is whether the transition from unconscious to conscious is gradual or all-or-none. The current opinion in consciousness research is divided. Both gradual and all-or-none consciousness views are supported. This may partly be the result of marked variety in methodology. Therefore, the present study aimed to compare outcomes of four different consciousness methods. Twenty-eight Dutch subjects participated in this experiment, consisting of three techniques (Backward Masking, Attentional Blink, Continuous Flash Suppression) and one subjective measure (the Perceptual Awareness Scale). Results showed that with longer stimulus presentation, both objective and subjective consciousness increased gradually. Therefore, the present study supports the notion that the transition from unconsciousness to
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1. Introduction
The understanding of what exactly consciousness is, and what processes underlie it, remains a holy grail of both philosophy and science (Michael S. Gazzaniga, 2014). Is
consciousness a device for thoughts, a sensation we experience, a state we are in? When trying to answer these questions, we are immediately faced with a great deal of complexity. Intuitively it feels as if we are consciously in control of our actions. But brain research has shown that there are thousands of processes guiding our actions unconsciously, outside of conscious awareness (Michael S. Gazzaniga, 2014). This makes both consciousness and unconsciousness difficult to grasp. Great thinkers have devoted themselves to the
understanding of the brain processes underlying consciousness for decades. In spite of that, many aspects of consciousness remain unclear. Overall, neither science nor philosophy have thus far been able to fully tackle the elusive phenomenon that consciousness is (Lamme, 2010).
One of the aspects of consciousness that needs more clarity is the transition from unconsciousness to consciousness. How exactly does this transition take place? The first part of this introduction summarizes the divided status quo in the field of consciousness research on this topic. The second part elaborates on how this divided status quo has come about, and why consciousness is difficult to measure. The third part sums up the three lab
techniques for measuring consciousness that are of interest in the present study. The fourth part focuses on the scale for measuring subjective consciousness that is of interest in the present study. In the fifth and final part, the present study’s aim and expectations are stated.
1.1 In the field of cognitive neuroscience it remains heavily debated whether conscious awareness is a gradual or an all-or-none phenomenon (Sergent & Dehaene,
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2004a). Surprisingly, both these opposing views are well supported scientifically. On the one hand, several studies propose that the emerging of conscious awareness is gradual
(Nieuwenhuis & de Kleijn, 2011; Overgaard, Rote, Mouridsen, & Ramsoy, 2006). According to this gradual view, conscious perception arises when the strength of a present stimulus crosses a threshold toward the higher end of a consciousness continuum (Nieuwenhuis & de Kleijn, 2011). When visiting a movie theatre for example, it isn’t hard to image that your consciousness establishes itself in different degrees. You may be fully conscious watching the movie, while you still remain faintly aware of the whispered conversations next to you or the popcorn smell in the air. Many streams of perceptual information reach your senses at any given time (Windey, Vermeiren, Atas, & Cleeremans, 2014). Intuitively, this results in gradual levels of awareness of certain stimuli (Windey et al., 2014).
An influential theory that underlies this gradual view of consciousness is the Signal Detection Theory (SDT) (first introduced by Green & Swets, 1966). According to this theory, we are able to make correct judgments about stimuli, even though we are uncertain that we have seen them. Additionally, the SDT proposes that there is a grey area between
unconsciousness and consciousness. In this grey area visual information reaches us both increasingly and unknowingly. This supports the notion that consciousness is established through varying degrees, and that consciousness awareness is a gradual phenomenon.
On the other hand, the view that conscious awareness is not gradual but dichotomous is also well supported. Several studies advocate that the transition from unconscious to conscious awareness is either all or none (Asplund, Fougnie, Zughni, Martin, & Marois, 2014; Sergent & Dehaene, 2004a). According to this view, there is a sharp line between unconscious and conscious states (Windey & Cleeremans, 2015). Brain research in
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monkeys, for example, has shown that changes in their neural activity when seeing an object is either all or none (Super, Spekreijse, & Lamme, 2001).
The most influential theory that underlies this all-or-none view of consciousness is the Global Workspace Theory (GWT) (first introduced by Baars, 1993). In the GWT it is proposed that we have a global workspace, comparable with working memory, with a spotlight of selective attention. In short, the stimuli that are shone upon with this attention spotlight are processed consciously, and the stimuli that are in the dark remain unconscious. The GWT is considered the predominant view on consciousness. It is the precursor of the more modern Global Neuronal Workspace Theory (GNWT) (Sergent & Dehaene, 2004b), which looks at conscious processing on a neural level. The GNWT proposes that the basis of consciousness is a sudden burst of neuronal activity, establishing global activity patterns in the brain. According to the GNWT, this brain activity underlying consciousness is non-linear. In sum, both the GWT and the GNWT support that the transition from unconscious to consciousness is all-or-none.
1.2 Thus there are two opposing views on consciousness being gradual or all-or-none. Both are supported scientifically, but seem contradictory. To reach a more thorough
understanding of consciousness, it seems necessary to understand this divided status quo in the field. This would allow us to make progress in capturing consciousness in all its aspects. What makes up the divided current opinion in consciousness research?
Firstly, this division may be the result of indistinct terminology use (Windey &
Cleeremans, 2015). Previous research on consciousness has not yet reached a consensus on how to define different consciousness concepts (Kunimoto, Miller, & Pashler, 2001; Windey & Cleeremans, 2015). “Consciousness”, “gradual consciousness”, “all-or-none
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consciousness” and “awareness” are generic terms. The use of indistinct terminology makes it hard to decipher which aspect of consciousness is being addressed (Windey & Cleeremans, 2015). This, at least in part, may contribute to why consciousness is difficult to measure.
Secondly, these opposing views may be the result of the challenges that measuring consciousness poses (Kunimoto et al., 2001). Generally, there is a choice between
subjective- and objective measures of consciousness. Both have advantages, but also some important disadvantages.
Subjective measures of consciousness aim to measure the degree in which the stimuli that were rendered hard to see by the techniques, are perceived consciously by participants. This relies on the participant reporting on what he or she experiences. Consequently, this gives access to subjective experiences that would otherwise be unknown, which is a great advantage. However, subjective measures depend heavily on the participants’ interpretation of what consciousness constitutes and their capability to reflect on their own experience. So when subjective measures are used, inherently the results are prone to response biases. This is the case when, for example, participants actually processed the stimuli consciously, but fail to report this (Ramsøy & Overgaard, 2004). On the whole, participants are notoriously bad at self-reporting accurately (Mook, 2001). As a matter of fact, the form that subjective measures take actually shape the way conscious experience of certain stimuli is reported (Szczepanowski, Traczyk, Wierzchon, & Cleeremans, 2013). This could pose problems for results obtained with subjective measures, because they may be distorted and unreliable.
Therefore, the choice for using objective measures of consciousness instead of subjective ones might seem like an obvious alternative. In particular because objective measures don’t rely on self-report, and therefore are less prone to response biases.
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However, the use of objective measures poses problems of its own. Objective measures may eliminate the problem of response biases, but in doing so they also eliminate valuable information about the subjective experience of consciousness. One may argue that consciousness is in itself a partly subjective phenomenon. A substantial part of it may be overlooked as soon as the subjective side of it is discarded. All in all, the disadvantages that both subjective and objective measures of consciousness may pose, seem to contribute to the difficulty of measuring consciousness.
Thirdly, the opposing views on consciousness may be the result of the large variety in existing consciousnesses research methods. The methodology for studying consciousness is still under development (Sandberg, Bibby, Timmermans, Cleeremans, & Overgaard, 2011). Existing methods differ to a great extent in the way that they approach and measure consciousness. Even within each method, a large number of different statistical approaches are used (for examples, see Anzulewicz et al., 2015; Dehaene, Changeux, Naccache, Sackur, & Sergent, 2006; Fleming & Lau, 2014; Koch & Tsuchiya, 2007; for a review, see Sandberg, Timmermans, Overgaard, & Cleeremans, 2010; Szczepanowski et al., 2013). None of the many existing methods have yet become the golden standard. The consequence seems to be that the outcome of consciousness research is strongly influenced by the method used (Sandberg et al., 2010). This influence is not ideal. Conclusions of consciousness research should preferably not be influenced by the method used, but only be derived from
consciousness itself. On the whole, both the large variety in methodology and the seeming dependency of outcome on method of choice, seems to contribute to why consciousness is difficult to measure.
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Therefore, it seems relevant to look deeper into how different methods relate to gradual vs. all-or-none consciousness precisely. There are four methods for measuring consciousness that are of interest for the present study. Among these four methods are three lab techniques, involving the rendering of stimuli on the verge of unconsciously and consciously visible. What is remarkable about all three techniques mentioned, is that they can provide an objective measure of consciousness. In the following paragraph all three will be presented and briefly evaluated on their strengths and weaknesses.
1.3 The first lab technique of interest in the present study is Backward Masking (BM) (first introduces by Exner, 1868). BM seems relevant when addressing the transition from unconsciousness to consciousness because it more or less allows to establish where the threshold for conscious awareness lies. In this technique, a brief target stimulus is flashed on a screen and then followed shortly by a masking stimulus, (almost) overwriting the first one. Participants are only able to react to the target stimulus when the interval between the target and the mask is just long enough. The threshold for perceiving the target consciously usually lies around dozens of milliseconds. When the interval between the target and the mask is not long enough, the mask prevents further processing of the target, thereby making it impossible to reach consciousness (Kim & Blake, 2005) What is striking about BM, apart from providing a handle to establish conscious awareness thresholds, is that it has been shown that below-threshold stimuli can still be processed outside of awareness (Del Cul, Baillet, & Dehaene, 2007). For example, a target masked to invisibility does not reach consciousness, but can nonetheless facilitate the recognition of a following target of the same color (Kim & Blake, 2005). In regard to consciousness being gradual or all-or-none, previous results from BM have shown that awareness emerges gradually (Sandberg et al., 2011).
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The second lab technique of interest in the present study is the Attentional Blink paradigm (AB) (first introduced by Raymond, Shapiro, & Arnell, 1992). AB is relevant when addressing the transition from unconsciousness to consciousness because it provides one account of physically identical stimuli sometimes being consciously perceived and
sometimes not (Nieuwenhuis & de Kleijn, 2011). In this technique, a series of visual stimuli is presented to the participant, each following shortly after the other. The attentional blink consists of the inability to consciously perceive every second of two targets in the series (Asplund et al., 2014). This inability is due to an engagement of attention with the first target, leaving no attention left for the second target. So, the most important implication of the AB is that attention is intrinsically related to consciousness. Attention-focus has a facilitating effect at both earlier- and postperceptual stages of information processing, and modulates conscious perception (Asplund et al., 2014). What is also striking about the AB paradigm is that stimulus rendered invisible, and thus impossible to process consciously, can still impact visual processing (Kim & Blake, 2005). In regard to consciousness being gradual or all-or-none, previous AB studies have shown that awareness emerges dichotomously (Sandberg et al., 2011).
The third lab technique of interest in the present study is the breaking continuous flash suppression paradigm (B-CFS) (Jiang, Costello, & He, 2007; Stein, Hebart, & Sterzer, 2011). B-CFS is relevant when addressing the transition from unconsciousness to
consciousness because it is often used to measure differences in the potency of stimuli to gain access to awareness (Stein et al., 2011). Furthermore, is considered a highly sensitive technique. Since the introduction of B-CFS, more evidence has amounted to the notion that high-level processing of certain properties of stimuli can occur before fully conscious
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consciousness being gradual or all-or-none, previous B-CFS studies have shown that awareness emerges gradually, with some stimulus properties arising to consciousness before other properties do (Gayet & Stein, 2017, in review; Stein et al., 2011).
1.4 Besides the three lab techniques mentioned previously, there is one subjective scale of interest in the present study: the Perceptual Awareness Scale (PAS) (Overgaard et al., 2006). The PAS is a widely used and prominent subjective scale of consciousness. It is a direct measure of how clear participants’ subjective experience of a certain stimuli was, after completing a task. What is remarkable about the PAS is that it was not created by an
experimenter. Instead, a group of participants were asked to design an awareness scale they felt fitted their experience best, and PAS was the consenting result. So far, the PAS has been proven to be an intuitive measure, making it relatively easy for participants to report their own experience (Overgaard et al., 2006). More importantly, the PAS allows for both gradual and all-or-none reporting of conscious awareness. Participants are given four options, varying from ‘no experience’ to ‘a weak experience’ to ‘an almost clear experience’ to ‘a clear experience’. This could mean that results from the PAS give insight to whether subjective experience of consciousness is gradual or all-or-none. Gradual consciousness would be marked by outcomes entailing all four PAS options, indicating that participants experience varying degrees of consciousness. All-or-none consciousness would be indicated only two of the PAS outcomes; ‘no experience’ or ‘a clear experience’, corresponding to a dichotomous experience. Adding to the advantage of being an intuitive measure, the allowance for both a gradual and an all-or-none interpretation makes the PAS a suitable measure for subjective consciousness.
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1.5 To summarize, it seems that gradual or all-or-none consciousness outcomes depend on the use of different psychophysical techniques (Sandberg et al., 2011). A
conclusive answer about how the transition from unconscious to conscious visual perception takes place is yet to be formed. Multiple consciousness methods have been compared in earlier studies (see Sandberg et al., 2010; Sergent & Dehaene, 2004a). Gradual
consciousness is expected in BM and B-CFS (Sandberg et al., 2010; Stein et al., 2011), whereas all-or-none consciousness is expected in AB (Sandberg et al., 2010). However, the simultaneous application of these psychophysical techniques (BM, AB, B-CFS) plus one subjective scale (PAS) in one experiment has not been reported. The aim of the present study was to compare the outcomes of three objective and one subjective consciousness parameter in one experiment. Doing so could provide valuable new information about the nature of the transition between unconscious and conscious visual perception. Additionally, it could add to knowledge about the relation between objective and subjective
consciousness. Tasks used in the present study deviated slightly from former studies, therefore outcome expectancies are not yet clear.
2. Methods
2.1 Participants
A total of 28 native Dutch speakers participated in our study (15 men, 13 woman, mean age = 25.4, SD = 11.1). Seventeen participants were students of the University of Amsterdam (UvA) and participated in exchange for course credits. These participants were recruited via www.lab.uva.nl, the enrolment website of the UvA lab. The other participants were not students and did not receive compensation for their participation. These
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inclusion criterion for participation was to have normal or corrected-to-normal vision. There were no formal exclusion criteria.
2.2 Experimental Procedure
Participants were invited to the University of Amsterdam Psychology lab during office hours. In the lab an experimenter welcomed them, handed them printed instructions (see supplement 1-3) and explained the procedure of the experiment. The experiment consisted of the fulfillment of three tasks: the BM task, the AB task and the B-CFS task (explained further below). After having made sure the participants fully understood the procedure, the participants singed the informed consent form (see supplement 4) stating that they were well informed and participated voluntarily. Subsequently, participants started the
experiment. All three tasks were run on a computer, using the Psychtoolbox functions of the program MATLAB r2016b. The computer was situated in a lab room with dimmed light and no acoustic distractions. Communication with the experimenter during the tasks was strongly discouraged, to guarantee a minimum amount of distraction. To make sure that participants understood each task before it began they were asked to carry out a practice version at first, which took approximately 2 minutes per task. Next, participants carried out task 1 through 3. The order of the sequence of the tasks varied consistently between two experimenters, to establish counterbalancing. Between each task participants were allowed 5 minute-breaks. The entire protocol lasted approximately 2.5 hours.
2.3 Materials
2.3.1 Backward Masking Task
The BM task (Exner, 1868) consisted of 384 trials. Each trial consisted of a sequence of stimuli followed by two tasks (see Figure 1). First, a brief target stimulus was flashed on
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the screen. The target stimulus consisted of either an upright or inverted face, presented after either 10, 20, or 20+10 ms to either the left or right side of the screen. This was followed, either immediately or after a delay of 10 ms, by three masks. The order in which different durations were used was randomized over all the trials. The three masks appeared for 100 ms each, possibly overwriting the target stimulus. They consisted of ‘Mondrian-like masks’: grey black and white circles in different sizes all over (see Figure 1). At the end of each trial participants completed two tasks. The first task was a face localization task. Participants were asked to indicate whether they had seen the target stimulus on the left or right side of the screen, by pressing the left or right arrow of their keyboard. This resulted in an accuracy score, which varied from 0 to 1.00. An accuracy score of 0 means zero face localization accuracy, 0.50 means face localization accuracy at chance level, 1.00 means a correct face localization in all trials. The accuracy score is an indication of overall objective consciousness of the visual stimuli presented. The second task was to fill out the Perceptual Awareness Scale (PAS), which is explained further below. Participants had 10 second breaks after every 128 trials.
Figure 1. Example of a trial in the Backward Masking task. Each trial consists of a sequence of stimuli (face, 3 x Mondrian-like mask) followed by two tasks (face localization task and the PAS). In this example, note the upright face on the right side of the screen.
12 2.3.2 Attentional Blink Task
The AB task (Raymond et al., 1992) consisted of 384 trials. Each trial consisted of a rapid serial visual presentation of 24 stimuli, presented for 100 ms each, followed by three tasks (see Figure 2). At T1, after anywhere between 8-12 stimuli, a brief target stimulus flashed on the screen, consisting of either diamonds or circles. This was followed by a second stimulus at T2, which was either an upright or inverted face, presented after a lag of either 2, 3 or 8 stimuli to either the left or right side of the screen. The lag refers to the number of stimuli between T1 and T2. The order in which different lags were used, and the number of stimuli after which T1 was presented, was randomized over all trials. At the end of each trial participants completed three tasks. The first task was a target discrimination task.
Participants were asked to indicate whether they had seen either diamonds or circles, by pressing the left or right arrow of their keyboard. This resulted in a T1 accuracy score, which varied from 0 to 1.00. An accuracy score of 0 means zero target discrimination accuracy, 0.50 means target discrimination accuracy at chance level, 1.00 means a correct target
discrimination in all trials. Accuracy score is an indication of overall objective consciousness of the visual stimuli presented. The second task was a face localization task. Participants were asked to indicate whether they had seen the face stimulus on either the left or right side of the screen, by pressing the left or right arrow of their keyboard. This resulted in a T2 accuracy score, which varied from 0 to 1.00. An accuracy score of 0 means zero face
localization accuracy, 0.50 means face localization accuracy at chance level, 1.00 means a correct face localization in all trials. The accuracy score is an indication of overall objective consciousness of the visual stimuli presented. The third task was to fill out the PAS, which is explained further below. Participants had 10 second breaks after every 128 trials
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Figure 2. Example of a trial in the Attentional Blink task. Each trial consists of a rapid serial visual presentation (Mondrian-like faces, either diamonds or circles, face) followed by three tasks (discrimination task, face localization task and the PAS). In this example, note the upright face on the left side of the upper right panel and the circles in the upper middle panel.
2.3.3 Breaking Continuous Flash Suppression Task
The B-CFS task (Jiang et al., 2007) was conducted in a setting that provoked binocular rivalry. In this paradigm, different stimuli are presented to each eye of the participant and only the information presented to the dominant eye is processed consciously (Kim & Blake, 2005). The task consisted of 640 trials. Each trial consisted of a sequence of stimuli followed by two tasks (see Figure 3). At first, CFS Mondrian-like masks were flashed on the screen to both the dominant and non-dominant eye. They showed grey black and white circles in different sizes (see Figure 3) and were updated every 100 ms. This was followed shortly by the target stimulus, presented only to the dominant eye. The target stimulus consisted of either an upright or inverted face, presented for either 0.2, 0.4, 0.8, 1.6 or 2.4 seconds to the left or right side of the screen. The order in which different durations were used was
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randomized over all the trials. At the end of each trial participants completed two tasks, presented only to the dominant eye. The first task was to indicate whether they had seen the target stimuli on either the left or right side of the screen, by pressing the left or right arrow of their keyboard. This resulted in an accuracy score, which varied from 0 to 1.00. An accuracy score of 0 means zero face localization accuracy, 0.50 means face localization accuracy at chance level, 1.00 means a correct face localization in all trials. The accuracy score is an indication of overall objective consciousness of the visual stimuli presented. The second task was to fill out the PAS, which is explained further below. Participants had 10 second breaks after every 128 trials.
Figure 3. Example of a trial in the B-CFS task in a binocular rivalry setting. Each trial consists of a sequence of stimuli (CFS Mondrian-like masks, face) followed by two tasks (face
localization task and the PAS). In this example, note the dominant eye is the right one, hence the face stimuli and tasks are only presented to the right eye.
2.3.4 Perceptual Awareness Scale
After completing each trial of the previous three tasks, participants were asked to fill out the PAS (Overgaard et al., 2006). The PAS indicates how clear the subjective experience of the previously shown stimulus is. It consists of four options, varying from ‘no experience’ via ‘a weak experience’ and ‘an almost clear experience’ to ‘a clear experience’. This
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task, or presentation time, give an indication of overall subjective consciousness of the visual stimuli presented.
2.4 Statistical Analysis
For all three tasks (BM, AB, B-CFS) the following analyses were conducted using IBM SPSS Statistics 23, considering p<0.05 as statistically significant. 1. Mean PAS ratings for different presentation times, followed by repeated measures ANOVA. 2. Mean target localization accuracy for different presentation times, followed by repeated measures ANOVA. 3. The distribution of PAS scores for different presentation times, followed by repeated measures ANOVA comparing proportion of PAS presses for different presentation times. Additionally, a mixed ANOVA for experiment (BM, AB, B-CFS) x presentation time x visibility on proportions was conducted to compare the distribution of PAS scores across all techniques. In case Mauchly’s test for sphericity was significant, indicating that the
assumption of sphericity was violated, the Huynh-Feldt correction was applied. 3. Results
3.1 Backward Masking 3.1.1 Subjective Performance
Mean PAS scores for the different presentation times were 1.34 (SD = 0.450) for 10 ms, 1.58 (SD = 0.551) for 20 ms and 2.06 (SD = 0.674) for 20 + 10 ms, and are presented in Figure 4. In the repeated measures ANOVA, Mauchly’s test indicated that the assumption of sphericity was violated, χ2(2) = 12.39, p = 0.002. Therefore, degrees of freedom were
corrected using Huynh-Feldt estimates of sphericity (ε = .60). The analysis showed that presentation time had a significant effect on mean PAS scores, F(1.20,11.96) = 47.49, p < .001.
16 3.1.2 Objective Performance
Mean face localization accuracy for different presentation times was 0.67 (SD = 0.114) for 10 ms, 0.79 (SD = 0.142) for 20 ms and 0.90 (SD = 0.108) for 20 + 10 ms, and is presented in Figure 5. The increase in mean face localization accuracy with higher
presentation time was significant, F(2,20) = 50.91, p < .001.
3.1.3 Distribution of PAS scores
Proportion of PAS scores depending on presentation time are displayed in Figure 6. There was a significant interaction between proportion of PAS scores and presentation time, F(6,60) = 17.37, p < .001. As can be seen in Figure 6, distribution of PAS presses was not bimodal but skewed to the right, showing a graded distribution of PAS scores. With
presentation time, the proportion of low PAS scores lessened and the proportion of higher PAS scores increased. More specifically, the proportion of the PAS score of 1 (‘no
experience’) decreased strongly. The proportion of both PAS scores 2 and 3 (‘a weak experience’ and ‘an almost clear experience’) increased strongly. Finally, the proportion of the PAS score of 4 (‘a clear experience’) increased slightly.
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Figure 4. Mean PAS score as a function of presentation time in the Backward Masking task. Note the significant increase (p<.001) in mean PAS score with increasing presentation time.
Figure 5. Mean face localization accuracy as a function of presentation time in the Backward Masking task. Note the significant increase (p<.001) in mean face localization accuracy with increasing presentation time.
1,34 1,58 2,06 1,00 2,00 3,00 4,00 10 20 20 + 10 Me an P As Score
Presentation Time (milliseconds)
Mean PAS Score
in BM
0,67 0,79 0,90 0,00 0,10 0,20 0,30 0,40 0,50 0,60 0,70 0,80 0,90 1,00 10 20 10 + 20 Me an Fa ce Lo calis at ion Acc u ra cyPresentation Time (milliseconds)
Mean Face Localisation Accuracy
in BM
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Figure 6. Proportion of PAS scores depending on presentation time in the Backward masking task. The proportion of PAS pressed changed significantly (p<.001) with presentation time. Each color represents a different presentation time (grey = 10 ms, green = 20 ms, dark green = 20 + 10 ms).
3.2 Attentional Blink
3.2.1 Subjective performance
Mean PAS scores for different presentation times were 1.50 (SD = 0.424) for lag 2, 1.58 (SD = 0.476) for lag 3 and 1.73 (SD = 0.386) for lag 8, and are presented in Figure 7. In the repeated measures ANOVA, Mauchly’s test indicated that the assumption of sphericity had been violated, χ2(2) = 7.00, p = 0.030. Therefore, degrees of freedom were correcting
using Huynh-Feldt estimates of sphericity (ε = .69). The analysis showed that presentation time had a significant effect on mean PAS scores, F(1.37,11.37) = 10.45, p = .004, ω2 = .24.
0 0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9 1 1 2 3 4 1 2 3 4 1 2 3 4 Pro p o rtio n o f PAS P re ss ed PAS Score
Distribution of PAS scores
in BM
19 3.2.2 Objective performance
Mean face localization accuracy for different presentation times were 0.68 (SD = 0.141) for lag 2, 0.752 (SD = 0.150) for lag 3 and 0.799 (SD = 0.120) for lag 8, and are presented in Figure 8. In the repeated measures ANOVA, Mauchly’s test indicated that the assumption of sphericity had been violated, χ2(2) = 9.87, p = 0.007. Therefore, degrees of
freedom were correcting using Huynh-Feldt estimates of sphericity (ε = .62). The analysis showed that presentation time had a significant effect on mean face localization accuracy, F(1.24,11.15) = 6.50, p = .022, ω2 = .21.
3.2.3 Distribution of PAS scores
Proportion of PAS scores depending on presentation time are shown in figure 9. There was a significant interaction between proportion of PAS scores and presentation time, F(6,54) = 8.63, p < .001. As can be seen in Figure 9, the distribution of PAS presses was not bimodal but skewed to the right, showing a graded distribution of PAS scores. With
presentation time, the proportion of low PAS scores lessened and the proportion of higher PAS scores increased. More specifically, the proportion of the PAS score of 1 (‘no
experience’) decreased slightly. The proportion of the PAS score of 2 (‘a weak experience’) increased strongly. The proportion of the PAS score of 3 (‘an almost clear experience’) increased slightly. Finally, the proportion of the PAS score of 4 (‘a clear experience’) also increased slightly.
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Figure 7. Mean PAS score as a function of presentation time in the Attentional Blink task. Note the significant increase (p<.001) in mean PAS score with increasing presentation time.
Figure 8. Mean face localization accuracy as a function of presentation time in the Attentional Blink task. Note the significant increase (p<.001) in mean face localization accuracy with increasing presentation time.
1,50 1,58 1,73 1,00 2,00 3,00 4,00 2 3 8 Me an P AS score
Lag (number of stimuli)
Mean PAS score in AB
0,68 0,75 0,80 0,00 0,10 0,20 0,30 0,40 0,50 0,60 0,70 0,80 0,90 1,00 2 3 8 Me an Fa ce Lo calis at ion Acc u ra cy
Lag (number of stimuli)
Mean Face Localisation Accuracy
in AB
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Figure 9. Proportion of PAS scores depending on presentation time in the Attentional Blink task. The proportion of PAS pressed changed significantly (p<.001) with presentation time. Each color represents a different lag (grey = lag 2, blue = lag 3, dark blue = lag 8).
3.3 Breaking Continuous Flash Suppression 3.3.1 Subjective Performance
Participants with a mean PAS score of 4, indicating a clear experience at all times, were discarded. Mean PAS scores for different presentation were 1.28 (SD = 0.212) for 0.2, 1.49 (SD = 0.395) for 0.4, 1.79 (SD = 0.489) for 0.8, 2.00 (SD = 0.571) for 1.6 and 2.16 (SD = 0.626) for 2.4, and are presented in Figure 10. Presentation time had a significant effect on mean PAS scores, F(4,16) = 13.96, p < .001.
3.3.2 Objective Performance
Participants with a mean face localization accuracy of 1.00, indicating a 100 % correctness at all times, were discarded. Mean face localization accuracy for different
0 0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9 1 1 2 3 4 1 2 3 4 1 2 3 4 Pro p o rtio n o f PAS p re ss ed PAS score
Distribution of PAS scores
in AB
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presentation times were 0.62 (SD = 0.112) for 0.2, 0.69 (SD = 0.105) for 0.4, 0.76 (SD = 0.067) for 0.8, 0.82 (SD = 0.116) for 1.6 and 0.86 (SD = 0.07) for 2.4 seconds, and are presented in Figure 11. Presentation time had a significant effect on mean face localization accuracy, F(6,36) = 6.56, p < .001.
3.3.3 Distribution of PAS scores
Proportion of PAS scores depending on three of five presentation times (0.2, 0.8, 2.4) are presented in figure 12. There was a significant interaction between proportion of PAS scores and presentation time, F(6,24) = 12.06, p < .001. As can be seen in Figure 12, distribution of PAS presses was not bimodal but skewed to the right, showing a graded distribution of PAS scores. With presentation time, the proportion of low PAS scores lessened and the proportion of higher PAS scores increased. More specifically, the
proportion of the PAS score of 1 (‘no experience’) decreased strongly. The proportion of the PAS score of 2 (‘weak experience’) increased slightly. Finally, the proportion of both PAS scores 3 and 4 (‘an almost clear experience’ and ‘a clear experience’) increased strongly.
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Figure 10. Mean PAS score as a function of presentation time in the B-CFS task. Note the significant increase (p<.001) in mean PAS score with increasing presentation time.
Figure 11. Mean face localization accuracy as a function of presentation time in the B-CFS task. Note the significant increase (p<.001) in mean face localization accuracy with increasing presentation time. 1,28 1,49 1,79 2,00 2,16 1,00 2,00 3,00 4,00 0.2 0.4 0.8 1.6 2.4 Me an P AS Score
Presentation Time (seconds)
Mean PAS Score
in B-CFS
0,62 0,69 0,76 0,82 0,86 0,00 0,10 0,20 0,30 0,40 0,50 0,60 0,70 0,80 0,90 1,00 0.2 0.4 0.8 1.6 2.4 Me an Face Localis at ion Ac curacyPresentation Time (seconds)
Mean Face Localisation Accuracy
in B-CFS
24
Figure 12. Proportion of PAS scores depending on presentation time in the B-CFS task. The proportion of PAS pressed changed significantly (p<.001) with presentation time. Each color represents a different lag (grey = 0.2 seconds, red = 0.8 seconds, brown = 2.4 seconds).
3.4 Comparison across techniques
There was a significant technique x presentation time x proportion of visibility interaction, F(12, 138) = 4.45, p = <.001, indicating that the visibility of target stimuli
differed with presentation time and between the three different techniques (BM, AB, B-CFS).
4. Discussion
In the present study, the transition from unconscious to conscious visual perception was explored. The following general observations were made for all methods included (BM, AB, B-CFS, PAS): 1) Longer stimulus presentation coincided with a gradual increase of both subjective and objective consciousness. 2) Longer stimulus presentation coincided with a change in PAS score distribution. The longer the stimulus presentation, the less 1, and the more 2, 3, and 4 was pressed. This indicates a shift from a predominantly nonconscious
0 0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9 1 1 2 3 4 1 2 3 4 1 2 3 4 Pro p o rtio n o f PAS p re ss ed PAS Score
Distribution of PAS scores
in B-CFS
25
experience to a gradually conscious experience. Altogether, the present study indicates that consciousness is gradual rather than all-or-none.
The simultaneous application of different psychophysical techniques and subjective scales in one experiment allowed for a more substantial insight regarding the divided status quo in the field of consciousness research than available until the present time. For the first time, both BM, AB as well as B-CFS and the PAS outcomes regarding visual consciousness were compared. Two observations were striking when making this comparison.
First, objective accuracy, indicating overall objective consciousness of the visual stimuli presented, was high in all task, for all presentation times. The lowest mean accuracy was 0.62, which is still well above chance. In contrast, mean PAS score, indicating overall subjective consciousness of the visual stimuli presented, was very low in all task, for all presentation times. This is exemplified by the observation that the highest mean PAS score was 2, which indicates only ‘a weak experience’. This contradiction between good objective performance and weak subjective experience suggests that participants were either
conservative in their answering and/or that participants were able to perceive stimuli more accurately than their subjective experience indicated. The latter would support the notion that visual processing can occur outside of conscious awareness (Lamme, 2010).
The second striking observation is that results of all methods used indicated a gradual transition from unconscious to conscious, even though for some of these methods, previous studies have found predominantly different outcomes. Therefore, outcomes of each
methods used (BM, AB, B-CFS, PAS) will be evaluated separately in the context of previous research.
26
Results from the BM task showed that consciousness emerged gradually, both objectively and subjectively. This is in line with previous research, predominantly showing that the BM paradigm indicates a gradual emerging of consciousness (Sandberg et al., 2011). However, BM has an important limitation when used for measuring conscious awareness. In some cases, it is not clear what the boundary between seen and unseen targets is. For example, observers can be aware that the target appeared, without being aware of any specifics of the target (Kim & Blake, 2005). In the context of the present study this would mean that although participants were able to localize the face stimuli better the longer it was presented, this does not necessarily translate to gradual emerging of complex visual information in real life.
Results from the AB task showed that consciousness emerged gradually, both objectively and subjectively. Notably, this is in contrast with previous research, predominantly showing that the AB paradigm indicates an all-or-none emerge of
consciousness (Sandberg et al., 2011). This conflicting outcome may be due to the limitations of the AB paradigm when used for measuring conscious awareness. In the AB paradigm it is presumed that focused attention leads to conscious awareness. But attention-focus cannot be interpreted interchangeably as awareness, since attention and conscious experience are grounded in different neural activity (Lamme, 2003). In spite of this, attention-focus and states of consciousness resulting from attentional selection remain intertwined in the AB (Kim & Blake, 2005). This could lead to mixed results concerning gradual or all-or-none emerging of consciousness, since attention presumably establishes itself slightly differently than consciousness.
27
Results from the B-CFS task showed that consciousness emerged gradually, both objectively and subjectively. This is in line with previous research, predominantly showing that the B-CFS paradigm indicates a gradual emerge of consciousness (Sandberg et al., 2011). However, B-CFS effects in previous research could potentially have been affected by lower uncertainty level response biases between conditions (Gayet & Stein, 2017, in review). The present study ruled out this potential bias by using fixed stimuli presentation times.
Results from the PAS showed that consciousness emerged gradually. This is in line with previous research, predominantly showing that the PAS indicated a gradual emerge of consciousness (Overgaard et al., 2006). Although the found results of the present study seem to reflect an unambiguously gradual emerging of consciousness, a number of limitations should be considered.
Firstly, it can be argued that PAS is prone to the same biases as any introspective report, as is mentioned in the introduction of the present study. An alternative subjective awareness measure that partly overcomes this disadvantage is for example Confidence Rating (CR) (first introduced by Bernstein & Eriksen, 1965). CR is a self-report measure whereby participants express their confidence in having perceived the stimuli of interest, after having completed a task. In contrast to the PAS, with CR the participants are not asked to introspect directly on how well they perceived a certain stimulus. Consequently, an important advantage of CR is that results do not rely on participants’ introspection qualities (Sandberg et al., 2010). However, an important disadvantage of the CR is that any two given participants may vary greatly in the extent to which they are inclined to feel confident about a given answer, even when they perceived the stimulus just as clearly.
28
Secondly, it is debatable whether PAS allows for both gradual and all-or-none reporting of conscious experience. In the present study it was presumed that gradual consciousness would be indicated by PAS outcomes consisting of all four PAS options. All-or-none consciousness would be indicated by PAS outcomes consisting of either ‘no experience’ or ‘a clear experience’. However, it can be argued that PAS in fact shows dichotomous
subjective consciousness in all cases. A PAS score of 1 would indicate no consciousness, but anything above, i.e. 2, 3 and 4, would indicate full consciousness. That would mean that PAS results in the present study in fact show all-or-none consciousness. However, the inbetween PAS presses such as ‘a weak experience’ and ‘an almost clear experience’, cannot be seen interchangably with full consciousness, as all-or-none views imply. Only ‘a clear experience’ should be considered the same as full consciousness.
In future studies, some limitations of the PAS could possibly be overcome by
choosing a different subjective awareness measure, such as a continuous scale. A continuous scale consists of a slider, which participants can use to indicate the visibility of the stimulus previously shown. This allows for participants to score anywhere between 0 and 100, or any grading the experiment leader chooses. With such a refined scale, participants have very detailed answering possibilities. Therefore, can be argued that gradual versus all-or-none consciousness would be a lot more apparent from continuous scale results. This might be a good alternative subjective consciousness scale to the use of the PAS in future experiments. On the other hand, continuous scales might not have the advantage of being as intuitive as the PAS.
In summary, results from the present study are mostly in line with previous studies on the transition from unconsciousness to consciousness. Overall, objective and subjective
29
measures indicated a graded emerging of visual consciousness. By inference, this study supports the notion that the elusive concept of consciousness is gradual.
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Supplement 1 Instructies Backward Masking
In dit experiment krijg je een reeks van beelden te zien. Ieder beeld begint met een gezicht dat heel kort verschijnt. Dit gezicht kan zowel rechtop als ondersteboven in beeld verschijnen. Het gezicht kan zowel aan de linker- als rechterkant vertoond worden, en het is belangrijk dat je dit probeert te onthouden. Soms wordt het gezicht maar heel kort geflitst, waardoor hij misschien zelfs niet te zien is. In dit geval vragen wij je te gokken of hij zich links of rechts bevond.
Voorbeeld 1: gezichten
Nadat het gezicht vertoond is, zal je nog een paar tellen een warrige achtergrond te zien krijgen. Hier hoef je verder niets mee te doen.
Voorbeeld 2: warrige achtergrond Vervolgens heb je twee taken:
Taak 1 is om aan te geven of het gezicht LINKS of RECHTS van het midden te zien was. Dit geef je aan door op het linker (bij links) of rechter (bij rechts) pijltje van het toetsenbord te drukken.
Taak 2 is om aan te geven hoe goed je het gezicht gezien denkt te hebben. Dit geef je aan door één van de vier onderstaande opties te kiezen:
1 = no experience (je hebt het gezicht niet gezien)
2 = brief glimpse (je denkt iets van het gezicht waargenomen te hebben) 3 = almost clear experience (je hebt het gezicht gezien, maar niet volledig) 4 = clear experience (je hebt het gezicht volledig waargenomen)
Supplement 2
Instructies Attentional Blink
In dit experiment krijg je een reeks van beelden te zien. Ieder beeld bestaat uit een drietal plaatjes naast elkaar. Deze plaatjes zijn neppe gezichten, zie voorbeeld 1.
Voorbeeld 1: drie neppe gezichten
In totaal krijg je zo’n twintig beelden direct achter elkaar te zien. Alle opvolgende beelden bestaan ook uit drie neppe gezichten, maar met een andere samenstelling, zie voorbeeld 2.
Voorbeeld 2: nog meer neppe gezichten, maar net even anders
Na enkele beelden komt er in het midden geen nep gezicht, maar een groen gekleurd vlak met ófwel
circles, ófwel diamonds, zie voorbeeld 3.
Voorbeeld 3: links is diamonds, rechts is circles.
Taak 1: Onthoud of dit diamonds of circles zijn: hier wordt je later naar gevraagd Vervolgens gaat de reeks neppe gezichten door, zoals omschreven bij voorbeeld 1 & 2. Echter: in één van de beelden van de reeks is een ECHT gezicht verstopt, zie voorbeeld 4 (dit gezicht kan ook op z’n kop staan). Het is aan jou om dit gezicht te zien
Voorbeeld 4: hier zie je een gezicht (rechtop), aan de linkerkant van het midden
Taak 2 is om aan te geven of dit echte gezicht LINKS of RECHTS van het midden te zien was Je hoeft niet op te letten of het gezicht rechtop staat, daar vragen we niet naar. Alleen de locatie
(rechts/links) is belangrijk. Aangezien we je bewustzijn proberen te manipuleren, zal de helft van alle rondes het gezicht niet bewust te zien zijn. Dan weet je niet wat je moet aangeven. In dat geval vragen we je om op taak 2 te gokken waar het gezicht te zien was.
Ten slotte krijg je een schaal in beeld.
Taak 3 is aangeven hoe zeker je ervan bent, dat je het gezicht gezien hebt. 1 = niets gezien
2 = in een flits iets langs zien komen 3 = bijna volledig kunnen zien 4 = volledig gezien
Supplement 3
Instructies Breaking-Continuous Flash Suppression
In dit experiment zie je telkens een vierkant met een warrig patroon. In dit patroon verschijnt op een gegeven moment een gezicht. Dit gezicht kan zowel rechtop als ondersteboven in beeld verschijnen. Het gezicht kan daarnaast zowel aan de linker- als rechterkant vertoond worden, en het is belangrijk dat je dit probeert te onthouden. Het kan heel goed dat je het gezicht vaak helemaal niet ziet. In dit geval vragen wij je te gokken of hij zich links of rechts bevond.
Voorbeeld 1: gezichten
Voorbeeld 2: warrig patroon
Vervolgens heb je twee taken:
Taak 1 is om aan te geven of het gezicht LINKS of RECHTS van het midden te zien was. Dit geef je aan door op het linker (bij links) of rechter (bij rechts) pijltje van het toetsenbord te drukken.
Taak 2 is om aan te geven hoe goed je het gezicht gezien denkt te hebben. Dit geef je aan door één van de vier onderstaande opties te kiezen:
1 = no experience (je hebt het gezicht niet gezien)
2 = brief glimpse (je denkt iets van het gezicht waargenomen te hebben) 3 = almost clear experience (je hebt het gezicht gezien, maar niet volledig) 4 = clear experience (je hebt het gezicht volledig waargenomen)
De bijbehorende toetsen zijn de toetsen 1, 2, 3 en 4 van het toetsenbord.
Supplement 4
Informed Consent
Ik verklaar hierbij op voor mij duidelijke wijze te zijn ingelicht over de aard en methode van het onderzoek, zoals uiteengezet in de bovenstaande informatie brochure “Spot het gezicht”, behorend bij het onderzoek “Consciousness: Is it gradual or all-or-none”. Mijn vragen zijn naar tevredenheid beantwoord.
Ik stem geheel vrijwillig in met deelname aan dit onderzoek. Ik behoud daarbij het recht deze instemming weer in te trekken zonder dat ik daarvoor een reden hoef op te geven en besef dat ik op elk moment mag stoppen met het experiment. Indien mijn onderzoeksresultaten gebruikt zullen worden in wetenschappelijke publicaties, dan wel op een andere manier openbaar worden gemaakt, zal dit volledig geanonimiseerd gebeuren. Mijn persoonsgegevens zullen niet door derden worden ingezien zonder mijn uitdrukkelijke toestemming.
Als ik nog verdere informatie over het onderzoek zou willen krijgen, nu of in de toekomst, kan ik me wenden tot de onderzoekers (email: emergenceofconsciousness@gmail.com).
Voor eventuele klachten over dit onderzoek kunt u zich wenden tot het lid van de Commissie Ethiek van de afdeling Psychologie van de Universiteit van Amsterdam, de heer doctor Hans Phaf (e-mail: R.H.Phaf@uva.nl).
Aldus in tweevoud getekend:
……… ……… Naam proefpersoon Handtekening
Datum: ….. - ….. - 2017 Plaats: Amsterdam
‘Ik heb toelichting verstrekt op het onderzoek. Ik verklaar mij bereid nog opkomende vragen over het onderzoek naar vermogen te beantwoorden.’
……… ……… Naam onderzoeker Handtekening
