Elsevier

Consciousness and Cognition

Volume 18, Issue 4, December 2009, Pages 939-951
Consciousness and Cognition

Very brief exposure: The effects of unreportable stimuli on fearful behavior

https://doi.org/10.1016/j.concog.2009.08.001Get rights and content

Abstract

A series of experiments tested the hypothesis that very brief exposure to feared stimuli can have positive effects on avoidance of the corresponding feared object. Participants identified themselves as fearful of spiders through a widely used questionnaire. A preliminary experiment showed that they were unable to identify the stimuli used in the main experiments. Experiment 2 (N = 65) compared the effects of exposure to masked feared stimuli at short and long stimulus onset asynchronies (SOA). Participants were individually administered one of three continuous series of backwards masked or non-masked stimuli: unreportable images of spiders (25-ms SOA), clearly visible images of spiders (500-ms SOA), or unreportable images of trees (25-ms SOA). Immediately thereafter, they engaged in a Behavioral Avoidance Test (BAT) with a live, caged tarantula. Exposure to unreportable images of spiders resulted in greater approach towards the tarantula than unreportable neutral images. A post-hoc comparison with clearly visible exposure to these same images approached significance. These effects were maintained at a 1-week follow-up (N = 57). In Experiment 3 (N = 26), participants engaged in the BAT 1 week prior to the exposure manipulation in order to provide a baseline measurement of their avoidant behavior, and again immediately after the exposure manipulation. Exposure to unreportable images of spiders reduced avoidance of the tarantula. Similar exposure to trees did not. Implications for the non-conscious basis of fear are discussed.

Introduction

A preponderance of evidence indicates that in order to diminish fear of a stimulus, one must directly confront it with full conscious awareness, or undergo exposure (Anton and Swinson, 2000, Barlow, 2002, Emmelkanp, 2004, Foa, 1997). Yet this robust finding of clinical research is at odds with recent basic scientific research, which views fear as primarily mediated by non-conscious processes that are relatively impenetrable to conscious, cognitive control (e.g., Öhman & Mineka, 2001). Mounting behavioral and neuroscientific studies have shown that fear responses can be activated and learned outside of awareness (Carlsson et al., 2004, Katkin et al., 2001, LeDoux, 1996, Morris et al., 1998, Phelps, 2005, Öhman and Soares, 1993, Öhman and Soares, 1994, Öhman and Soares, 1998). Some studies have showed that the emotional experience of fear can be similarly evoked and influence behavior in expected ways, even when the individual is unaware of what elicited the experience (Ruys and Stapel, 2008, Öhman and Soares, 1994).

Such findings converge on an intriguing question: if fear can be induced, acquired and experientially evoked outside of awareness, is it possible to bypass direct confrontation of a feared object in exposure? That is, is it possible to enhance approach to the feared object by exposure to the feared stimulus without full conscious awareness? This article presents two experiments that addressed this question.

The hypothesis that exposure in the absence of full awareness can have positive effects on avoidance of a feared object is a direct extension of contemporary research and theory on the non-conscious basis of fear. LeDoux (1996) identified parallel neural circuits for processing threatening stimuli in the rat brain, one that is associated with consciousness and another that is not. In the latter circuit, or “low-road”, the amygdala receives stimulation directly from the thalamus, generating an immediate, autonomic response to a stimulus faster than consciousness would require. Neuroscientific research across mammal species, including case studies of human brain damage and neuroimaging studies of the normal brain, has similarly identified a dual neural architecture of fear, implicating structures that are dissociable in terms of consciousness (Bechara et al., 1995, Carlsson et al., 2004, Etkin et al., 2004, Fendt and Fanselow, 1999, LaBar et al., 1995, Lang et al., 2001, Morris et al., 1998, Phelps, 2005, Williams et al., 2004). Interestingly, some studies have found greater amygdala activation to non-conscious than to conscious stimuli (e.g., Carlsson et al., 2004).

This body of research provides a biological basis for Öhman and Mineka’s (2001) theory of the fear module: a specialized neural system for automatically processing stimuli that posed threats in the evolutionary history of mammals. Ohman and colleagues have shown that fear responses can be non-consciously activated (Esteves et al., 1994, Öhman and Soares, 1993, Öhman and Soares, 1994) and conditioned (Esteves et al., 1994, Öhman and Soares, 1998) by a variety of fear-relevant stimuli. In a study of fearful participants, when fear-relevant images were presented non-consciously, they activated equivalent or even stronger galvanic skin responses than when they were presented consciously (Öhman & Soares, 1994). On the basis of such findings, Öhman and Mineka propose that the fear module is mostly not under voluntary control, but rather activated extremely rapidly and outside of awareness. Indeed, Weins and Öhman (2007) conclude that “...in fear, unconscious processing might dominate conscious processing” (p. 67).

If fear responses can be activated and acquired outside of awareness, it may be that they can be similarly diminished. Although no study has directly tested this hypothesis, at least one suggested it as a viable possibility. Tyrer et al., 1978, Lee et al., 1983 tested what they called a subliminal exposure technique by presenting a brief film related to agoraphobia (e.g., crowded markets; busy roads; public transportation). Both subliminal and supraliminal exposure to the film reduced phobic symptoms and physiological responses in treatment-resistant agoraphobics. Interestingly, participants who received subliminal exposure evidenced the greatest reduction in physiological measures of anxiety.

This study did not assess the impact of exposure without awareness on avoidance of the feared object. Weinberger, Siegel and Siefert (submitted for publication) conducted such a study. They exposed spider-fearful and non-fearful participants to a series of masked images of either spiders or outdoor scenes (control stimuli) that they evidenced no awareness of, and then asked them to approach a live, caged tarantula. The fearful participants got closer to the tarantula following exposure to the images of spiders. Because this study compared the effects of masked feared and control stimuli presented at the same stimulus onset asynchrony (SOA; ∼20-ms), it is possible that the experimental manipulation had an effect on approaching the feared object because of the content of the stimuli, rather than the brief duration of exposure.

The purpose of this article is to present a series of experiments that build on the initial finding of Weinberger et al. (submitted for publication). Experiment 2 addresses the alternative explanation of this finding just noted above by comparing the effects of exposure to feared stimuli at very brief (25-ms) and long (500-ms) SOAs on approach towards the actual feared object. It also builds on the initial finding of Weinberger et al. by assessing the delayed effects of such exposure (1-week later). The overwhelming majority of studies have assessed the immediate rather than the delayed effects of unreportable stimuli on behavior (e.g., Bargh, 1997, Bornstein and Pittman, 1992, Greenwald et al., 1996, Kihlstrom, 1987), presumably because of the assumption that such stimuli only have short-lived effects on behavior (cf. Greenwald et al., 1996). However, prior studies have tended to employ cognitively-mediated rather than affectively charged stimuli. Sohlberg and Birgegard (2003) found long-term effects of multiple 5-ms exposures to an emotionally-evocative phrase in five experiments. This suggests that very brief exposures to affectively charged stimuli may have more enduring effects on behavior.

Like Weinberger et al., Experiment 2 only tests whether very brief exposure to feared stimuli can influence approach towards the actual feared object – not whether it reduces avoidance of it from a previous baseline. The latter would constitute a stricter test of very brief exposure because it involves prior, actual exposure to the feared object, which may eradicate the effects of the subsequent, virtual and much briefer exposure. The purpose of Experiment 3 was to conduct this stricter test of very brief exposure.

The reader will notice the use of the phrase “very brief exposure.” Since its inception, psychology has been plagued by the problem of measuring awareness. The literature is rife with controversy over this topic. Some have advocated the use of so-called “objective” measures of awareness, which typically require participants to discriminate between categories of masked stimuli (e.g., Cheesman and Merikle, 1984, Holender, 1986). Countering this view, Bowers’ (1984) emphasized that awareness is phenomenologically subjective, and thus cannot be captured with objective measures. The subjective nature of awareness is especially relevant to the current study. As Tyrer et al. (1978) showed, if participants are exposed to feared stimuli that they are subjectively unaware of, they will likely not experience distress. Thus, prior to proceeding with the main experiments, an initial experiment was conducted to establish conditions for preventing subjective awareness of the exposure images.

However, any subjective measure of awareness has an inherent limitation. Because people may differ in terms of their willingness to self-report on awareness of masked stimuli (or exhibit response bias, e.g., Wiens, Emmerich, & Katkin, 1997), one cannot definitively determine just how aware/unaware a person is of a particular stimulus. Because of this limitation, and in light of the long-standing controversy over awareness, we do not refer to the exposure technique used in the experiments below as “subliminal” or “non-conscious”. We simply call it what it is: very brief exposure. Similarly, we refer to the masked stimuli employed as “unreportable” images, emphasizing the ability of participants to report their identity, rather than “subliminal”, which implies an objective threshold of awareness. The issue of awareness of the exposure stimuli used in the main experiments is taken up in the General Discussion.

Section snippets

Preliminary experiment: masking the stimuli

We employed a backward masking procedure to render participants subjectively unaware of the target stimuli (cf., Breitmeyer, 1984). Öhman and Soares, 1993, Öhman and Soares, 1994 found that when the SOA between a fear-relevant target stimulus and a suitable backwards masking stimulus was 30-ms or less, fearful and non-fearful participants were unable to recognize the target stimulus.

Experiment 2

The purpose of this experiment was to determine whether exposure to the unreportable images of spiders from the preliminary experiment would prompt spider-fearful participants to approach a live, caged tarantula (relative to control exposure). We compared the potential effects of exposure to these stimuli at very brief and long SOAs. Following the empirical argument presented earlier, there were two main hypotheses:

  • (1)

    Those exposed to unreportable images of spiders would approach a live, caged

Experiment 3

This experiment tested the hypothesis that exposure to unreportable images of spiders would reduce avoidance of a live tarantula. There were two differences between this experiment and Experiment 2. The first was that spider-fearful participants engaged in the BAT with the live tarantula prior to the exposure manipulation in order to provide a baseline measurement of their avoidant behavior. The second difference was that this experiment did not include a group who were exposed to clearly

General discussion

Taken together, the results of Experiments 2 and 3 suggest that very brief exposure to feared stimuli can have positive effects on avoidance of the corresponding feared object. Experiment 2 showed that participants who rated themselves as fearful of spiders got closer to a live, caged tarantula following exposure to unreportable images of spiders (relative to control exposure). Experiment 3 showed that an independent sample reduced their avoidance of the tarantula as a result of such exposure.

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