Comparing unconscious processing during continuous flash suppression and meta-contrast masking just under the limen of consciousness

Peremen, Ziv; Lamy, Dominique

doi:10.3389/fpsyg.2014.00969

ORIGINAL RESEARCH article

Front. Psychol., 11 September 2014

Sec. Consciousness Research

Volume 5 - 2014 | https://doi.org/10.3389/fpsyg.2014.00969

This article is part of the Research Topic Invisible, but how? The depth of unconscious processing as inferred from different suppression techniques. View all 13 articles

Comparing unconscious processing during continuous flash suppression and meta-contrast masking just under the limen of consciousness

$\r\nZiv Peremen,$ Ziv Peremen^1,2 $Dominique Lamy,*\r\n$ Dominique Lamy^1,2*

¹The School of Psychology Sciences, Tel Aviv University, Tel Aviv, Israel
²The Sagol School of Neuroscience, Tel Aviv University, Tel Aviv, Israel

Stimuli can be rendered invisible using a variety of methods and the method selected to demonstrate unconscious processing in a given study often appears to be arbitrary. Here, we compared unconscious processing under continuous flash suppression (CFS) and meta-contrast masking, using similar stimuli, tasks and measures. Participants were presented with a prime arrow followed by a target arrow. They made a speeded response to the target arrow direction and then reported on the prime’s visibility. Perception of the prime was made liminal using either meta-contrast masking (Experiment 1) or CFS (Experiments 2 and 3). Conscious perception of the prime was assessed using a sensitive visibility scale ranging from 0 to 3 and unconscious processing was measured as the priming effect on target discrimination performance of prime-target direction congruency when prime visibility was null. Crucially, in order to ensure that the critical stimuli were equally distant from the limen of consciousness, we sought stimulus and temporal parameters for which the proportion of 0-visibility trials was comparable for the two methods. We found that the method used to prevent conscious perception matters: unconscious processing was substantial with meta-contrast masking but absent with CFS. These findings suggest that CFS allows very little perceptual processing, if at all, and that previous reports of high-level and complex unconscious processing during CFS may result from partial awareness.

Introduction

Visual consciousness has been the focus of intense research in the last two decades (Marcel, 1983; Erdelyi, 1986, 2004; Greenwald et al., 1996; Vorberg et al., 2003; Ramsøy and Overgaard, 2004; Lau and Passingham, 2006; Schmidt and Vorberg, 2006; Lamy et al., 2009; Sandberg et al., 2010). The search for the limits of unconscious processing lies at the heart of this research: which processes can unfold in the absence of conscious perception and conversely, for which processes is consciousness essential? In other words, what is the function of consciousness? The most widely used empirical strategy used to address this question is to probe the influence on behavior of stimuli that are barred from conscious access, so as to assess what processes can be performed outside perceptual awareness.

A rather large arsenal of paradigms stand at disposal to prevent a visual stimulus from entering consciousness: pattern masking (e.g., Breitmeyer and Ganz, 1976), meta-contrast masking (Breitmeyer, 1978), object-substitution masking (Di Lollo et al., 2000), continuous flash suppression (CFS; Tsuchiya and Koch, 2005), the attentional blink (Raymond et al., 1992), inattentional blindness (Mack and Rock, 1998), and more (see Kim and Blake, 2005 for a review). The choice of the paradigm used to demonstrate unconscious processing often appears to be arbitrary, despite the fact that the different paradigms are known to affect perceptual processing in qualitatively different ways (e.g., Enns, 2004; Almeida et al., 2010; Kanai et al., 2010; Faivre et al., 2012; Fogelson et al., 2014). On the one hand, one could claim that the method used to prevent conscious perception should not matter as long as unconscious perception is demonstrated. On the other hand, however, it is important to minimize failures to identify processes that can be performed without consciousness. To do that, targeting the procedures that obliterate conscious processing while most minimally impairing unconscious processing would seem to be the most judicious strategy. To illustrate this point bluntly, blindfolding observers to prevent conscious perception would be a bad choice because it would also thoroughly eliminate unconscious processing.

In the present paper, we focused on a paradigm that has become increasingly popular in consciousness research: CFS (henceforth, CFS, Tsuchiya and Koch, 2005), and investigated the extent to which it disrupts unconscious processing. With this method, arrays of randomly generated shapes of different colors (Mondrians) presented successively at ∼10 Hz to one eye can reliably suppress the conscious awareness of an image presented to the other eye. One of the main reasons for the enthusiasm surrounding CFS is that, unlike backward masking, which is effective only when the target is presented very briefly (typically for less that 100 ms), CFS-induced suppression can last very long, on the order of seconds (Shimaoka and Kaneko, 2011; Stein et al., 2011). Based on the premise that high-level computations may require relatively long processing times, CFS should be a particularly well-suited paradigm in order to measure high-level unconscious processing.

Consistent with this conjecture, several studies relying on breaking suppression during CFS have revealed that we are capable of performing complex, high-level cognitive operations without conscious perception (e.g., Jiang et al., 2006; Costello et al., 2009; Mudrik et al., 2011; Sklar et al., 2012; Lupyan and Ward, 2013). In a nutshell, the rationale underlying the use of breaking suppression is that if a stimulus is found to overcome (or break) suppression earlier than another stimulus, then one can conclude that the property on which the two stimuli differ was processed unconsciously. However, there has been no convincing evidence that differences in breaking suppression times reflect genuine unconscious processing rather than processing under partial consciousness (e.g., Stein et al., 2011; see Gayet et al., 2014).

Other CFS studies that either used a traditional dissociation procedure or examined the neural consequences of this method have generated conflicting findings as to whether CFS interferes with low-level or with high-level cognitive processing (see Sterzer et al., 2014 for a review). For instance, some authors showed that subliminal stimuli suppressed using CFS elicit semantic processing (e.g., Almeida et al., 2008, 2010; Bahrami et al., 2010), while others showed that CFS-suppressed stimuli undergo only low-level perceptual processing (e.g., Faivre et al., 2012). Likewise, while several functional MRI studies showed robust suppression of activity in higher visual areas during CFS (e.g., Fang and He, 2005; Jiang et al., 2006; Hesselmann and Malach, 2011) but not in the primary visual cortex (Watanabe et al., 2011), Yuval-Greenberg and Heeger (2013) recently showed that CFS does in fact modulate fMRI responses in the primary visual cortex (see also Kang et al., 2011 for consistent findings using ERP methodology).

Most crucially, however, the very few studies that directly compared the extent of unconscious processing when stimuli are rendered invisible using CFS vs. other methods, suggest that CFS has the lower hand: it actually elicits more restricted unconscious processing (Almeida et al., 2010; Faivre et al., 2012, 2014; Izatt et al., 2014). For instance, Faivre et al. (2012) showed that while emotional face primes biased subsequent preference judgments when suppressed from awareness by gaze-contingent crowding, they did not elicit such emotion-related processing when suppressed by backward masking or CFS. Instead, they only produced an effect akin to low-level perceptual adaptation: responses to a face target were slower following an identical suppressed prime face relative to a suppressed face conveying the same emotional expression but displayed by a different individual. In addition, Almeida et al. (2010) showed that backward-masked primes elicited category- and identity-specific priming both with tool and with animal stimuli, whereas CFS-suppressed primes were associated only with small category-specific priming, and only with tool stimuli.

The foregoing studies relied on an objective measure of conscious perception to ensure that the prime was subliminal (but Izatt et al., 2014 used also a subjective measure). Specifically, using an experimental strategy that has become standard in the study of unconscious processing (Dehaene et al., 1998; Ansorge et al., 2009; Hsieh et al., 2011; Van Opstal et al., 2011) they included experimental trials in which the influence of a subliminal prime on responses to a subsequent target was probed, and prime-awareness test trials in which chance performance at judging the critical property of the prime was demonstrated. Thus, for instance, Almeida et al. (2010) showed that a suppressed prime facilitated response to a categorically congruent target, yet performance at discriminating the category to which this prime belonged was at chance.

It is important to note that with objective measures of conscious perception, it is of tantamount importance to select stimuli that cannot be discriminated above chance: just a few visible trials can jeopardize the success of the whole experiment (e.g., Rouder et al., 2013). Thus, the safest strategy is to select deeply subliminal stimuli at the risk of “overshooting,” that is, of cutting into unconscious processing itself. However, the magnitude of such overshooting cannot be assessed because performance is undiscriminably at chance whether the critical stimulus is just under the limen or completely hidden from view (see Figure 1). As a consequence, finding that unconscious processing occurs using one method but not using another, may not necessarily reflect that these methods constitutively disrupt different stages of processing: instead, it might simply indicate that the stimulus parameters selected to ensure chance objective performance pushed perceptual processing further from the limen with one method relative to the other.

FIGURE 1

FIGURE 1. Illustration of the ideas that (1) to maximize sensitivity for measuring unconscious processing, the critical stimulus must be as close as possible to the limen of consciousness and (2) chance performance at discriminating the critical stimulus is not informative with regard to this stimulus’ distance from the limen of consciousness.

The objective of the present research was to assess the extent of unconscious processing using liminal stimuli instead of subliminal stimuli. We compared CFS with meta-contrast masking, a method that is thought to interfere with perceptual processing at a relatively late stage (e.g., Del Cul et al., 2007; Enns, 2004) and has been associated with robust priming (e.g., Vorberg et al., 2003; Kentridge et al., 2008; Peremen and Lamy, 2014). We assessed conscious perception of the prime using a sensitive subjective visibility scale akin to the Perceptual Awareness Scale (e.g., Ramsøy and Overgaard, 2004)¹. One of the main advantages of using this measure in the present context is that it allows using liminal stimuli, that is, stimuli that are subjectively invisible on some proportion of the trials and perceived at various degrees of clarity on other trials. In this way, one can prevent conscious processing while minimally encroaching on unconscious processing. In addition to minimizing the distance of the critical stimulus perception from the limen of consciousness, visibility scales allow one to measure this distance – a feature that is particular useful when comparing different methods for preventing conscious perception: similar proportions of invisible trials should indicate similar distances from the limen.

In the present study, we compared the extent of unconscious processing of a prime arrow direction when this arrow was rendered invisible using meta-contrast masking (Experiment 1)², or CFS (Experiments 2 and 3). In all three experiments, participants were presented with a liminal prime arrow followed by a clearly visible target arrow, the direction of which was either congruent or incongruent with the prime arrow direction. On each trial, participants first made a speeded forced-choice discrimination response to the direction of the target arrow and then rated the visibility of the prime on a scale ranging from 0 to 3. Unconscious processing of the prime arrow direction was measured as the performance difference between the congruent and incongruent conditions on trials in which participants reported their subjective visibility of the prime to be null.

Experiment 1

Methods

Participants

Twenty two right-handed undergraduate students from Tel Aviv University (13 women), age 22–28 years (M = 24.9, SD = 1.9) were tested in one session for course credit. All subjects reported normal or corrected-to-normal vision.

Apparatus and stimuli

Sample displays are presented in Figure 2. The stimuli were presented on a 17-inch 85-Hz CRT monitor. The fixation display consisted of a plus sign (0.2°× 0.2° of visual angle). The prime display consisted of a small arrow (1.6°× 0.8°) and the target-mask display consisted of a larger arrow, 2.1° in width and 1.1° in height. Both arrows were gray (RGB 127, 127, 119) against a black background (RGB 0, 0, 0), were centered at fixation and pointed either leftward or rightward. Thus, the prime arrow either pointed in the same direction as the target arrow (congruent trials) or in the opposite direction (incongruent trials).

FIGURE 2

FIGURE 2. Sequence of events in Experiment 1. In this example, the directions of the prime and target arrows are congruent. Participants were required to make a speeded response to the target mask arrow direction (left or right) and then rate subjective visibility of the prime.

Procedure and design

Each trial began with a 500-ms presentation of the fixation display. The prime display then appeared for 24 ms, followed after a variable SOA (0, 24, 47, 71, 94, or 118 ms) by the target-mask display. Then, a blank screen appeared until subjects provided the first response or after 2,000 ms had elapsed, followed by a question mark in the middle of the screen, which prompted the subjects to provide the second response. A new trial began immediately after second response.

On each trial, subjects provided two responses: they first made a speeded response to the target-mask arrow direction by pressing designated keys as fast as possible on the numerical keypad with their right hands (“1” when the arrow pointed to the left and “3” when it pointed to the right). Then, they provided a subjective report of the prime visibility using a scale ranging from 0 (“I saw nothing at all”) to 3 (“I saw the arrow clearly”) by pressing designated keys (“z,” “x,” “c,” and “v” which were covered with stickers labeled 0, 1, 2, and 3, respectively) on the keyboard with their left hands. Five percent of the trials were catch trials: the target was presented alone, without a prime. The purpose of introducing catch trials was to anchor 0-visibility judgments to situations in which no prime appeared. On 10% of the trials (no-go trials) the target arrowheads were truncated and observers had to press the space-bar instead of providing the two responses pertaining to the prime³. Each subject completed 500 trials divided into ten blocks and following two practice blocks of 50 trials each. Before practice, the observers viewed the sequence of events at a very slow pace that enabled them to clearly distinguish between the prime and target.

All combinations of the prime and target arrow directions were equiprobable and randomly mixed. They were equally likely to be congruent or incongruent. Prime-to-target SOAs were equiprobable and randomly mixed.

Results

The data from two participants were excluded from analysis: one because his mean RTs were slower than the group’s by more than 3 SDs and the other, because of a technical error. Prime-absent (or catch) trials as well as no-go trials were excluded from all analyses. In all RT analyses, trials in which responses to the target direction were inaccurate were excluded (2.3%) and so were trials in which the RT exceeded the mean of its cell (resulting from crossing the factors included in the relevant analysis) by more than 2.5 SDs (fewer than 1% of the trials). An ANOVA with SOA as a within-subject factor and mean visibility as the dependent measure revealed that mean visibility followed the U-shaped pattern characteristic of type-B meta-contrast masking (Kolers and Rosner, 1960) and was lowest at the 47-ms SOA (this trend did not reach significance after Huynh–Feldt correction, F(5,75) = 2.69, p < 0.09). The mean proportion of trials per visibility for each SOA is shown in Figure 3⁴.

FIGURE 3

FIGURE 3. Proportion of trials in each level of prime visibility (0–3) as a function of the SOA between the prime and target, in Experiment 1.

Reaction times

A linear mixed-effects model with visibility (0, 1, 2, or 3) and congruency (congruent vs. incongruent) as within-subject factors was performed on the mean RTs. Mean RT and accuracy data are presented in Table 1 and the mean congruency effect at each visibility level is shown in Figure 4⁵. The main effect of congruency was significant, F(1,19) = 86.94, p < 0.0001, with faster RTs when the prime and target arrows were congruent than when they were incongruent. The main effect of visibility was also significant, F(3,53) = 32.16, p < 0.0001, with longer RTs as visibility increased. The interaction between the two factors was significant, F(3,53) = 3.7, p < 0.02. Further analyses revealed that the congruency effect was larger for visibility three than for all other levels, all ps > 0.03 and that the congruency effects for visibility levels 0, 1, and 2 did not differ from each other, all Fs < 1. Crucially, the congruency effect was significant when visibility was null, 49 ms F(1,53) = 14.82, p < 0.001.

TABLE 1

TABLE 1. Mean reaction times and accuracy on congruent and on incongruent trials in Experiment 1 as a function of visibility rating.

FIGURE 4

FIGURE 4. Mean response-priming effect in milliseconds in Experiment 1, as a function of visibility rating. Priming was significant for visibility 0–3. Error bars represent standard errors. *p < 0.002.

Accuracy

Similar analyses were conducted on the accuracy data pertaining to the responses to the target arrow. They showed similar trends, thus ensuring that speed-accuracy trade-off was not a concern. The main effect of visibility was significant, F(3,53) = 6.81, p < 0.001, the main effect of congruency approached significance F(1,19) = 3.75, p < 0.07 and the interaction between the two factors was not significant, F < 1.