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Subliminal or not?: An appraisal of semantic processing in the near absence of visual awareness
Stockholm University, Faculty of Social Sciences, Department of Psychology.
2016 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Stimuli that cannot be perceived (i.e., that are subliminal) can still elicit neural responses in an observer, but can such stimuli influence behavior and higher-order cognition? Empirical evidence for such effects has periodically been accepted and rejected over the last six decades. Today, many psychologists seem to consider such effects well-established and recent studies have extended the power of subliminal processing to new limits. In this thesis, I examine whether this shift in zeitgeist is matched by a shift in evidential strength for the phenomenon.

This thesis consists of three empirical studies involving more than 250 participants, a simulation study, and a quantitative review. The conclusion based on these efforts is that several methodological, statistical, and theoretical issues remain in studies of subliminal processing. These issues mean that claimed subliminal effects might be caused by occasional or weak percepts (given the experimenters’ own definitions of perception) and that it is still unclear what evidence there is for the cognitive processing of subliminal stimuli. New data are presented suggesting that even in conditions traditionally claimed as “subliminal”, occasional or weak percepts may in fact influence cognitive processing more strongly than do the physical stimuli, possibly leading to reversed priming effects. I also summarize and provide methodological, statistical, and theoretical recommendations that could benefit future research aspiring to provide solid evidence for subliminal cognitive processing.

Place, publisher, year, edition, pages
Stockholm: Department of Psychology, Stockholm University , 2016. , 102 p.
Keyword [en]
Subliminal priming, Signal detection theory, Bayesian statistics, Visual masking, Consciousness, Awareness, Unconscious processing, Semantic priming
National Category
Psychology (excluding Applied Psychology)
Research subject
Psychology
Identifiers
URN: urn:nbn:se:su:diva-132211ISBN: 978-91-7649-454-7OAI: oai:DiVA.org:su-132211DiVA: diva2:951753
Public defence
2016-09-23, David Magnussonsalen (U31), Frescati Hagväg 8, Stockholm, 13:00 (English)
Opponent
Supervisors
Note

At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 1: Manuscript. Paper 4: Manuscript.

 

Available from: 2016-08-31 Created: 2016-08-01 Last updated: 2016-08-31Bibliographically approved
List of papers
1. Semantic priming goes both ways: Semantic content of subsequent target stimuli interferes with the discrimination of preceding priming stimuli
Open this publication in new window or tab >>Semantic priming goes both ways: Semantic content of subsequent target stimuli interferes with the discrimination of preceding priming stimuli
(English)Manuscript (preprint) (Other academic)
Abstract [en]

Claims of subliminal semantic processing are commonly supported by experiments using stimuli that participants cannot discriminate between and therefore, it is said, cannot perceive. Critical to this operationalization of lack of perception is that discrimination performance is measured under optimal conditions and is not impaired by factors unrelated to perception. Here we report that in the standard paradigm of subliminal processing, prime-stimulus discrimination was underestimated due to the semantic content of the subsequent target stimulus: Just as the prime stimuli interfered with target-discrimination responses (leading to slower reaction times and more errors), the semantic content of target stimuli interfered with participants’ prime-discrimination performance. This is a hitherto neglected factor that may interfere with participants’ ability to report fleeting percepts of the prime stimuli, leading to underestimation of perception and potentially to false support of subliminal semantic processing.

Keyword
Subliminal priming, Visual masking, Consciousness, Awareness, Unconscious processing, Semantic priming
National Category
Psychology (excluding Applied Psychology)
Research subject
Psychology
Identifiers
urn:nbn:se:su:diva-132207 (URN)
Funder
Lars Hierta Memorial Foundation, FO2015-0940
Available from: 2016-08-01 Created: 2016-08-01 Last updated: 2016-08-10Bibliographically approved
2. Subliminal or not?: Comparing null-hypothesis and Bayesian methods for testing subliminal priming
Open this publication in new window or tab >>Subliminal or not?: Comparing null-hypothesis and Bayesian methods for testing subliminal priming
2016 (English)In: Consciousness and Cognition, ISSN 1053-8100, E-ISSN 1090-2376, Vol. 44, 29-40 p.Article in journal (Refereed) Published
Abstract [en]

A difficulty for reports of subliminal priming is demonstrating that participants who actually perceived the prime are not driving the priming effects. There are two conventional methods for testing this. One is to test whether a direct measure of stimulus perception is not significantly above chance on a group level. The other is to use regression to test if an indirect measure of stimulus processing is significantly above zero when the direct measure is at chance. Here we simulated samples in which we assumed that only participants who perceived the primes were primed by it. Conventional analyses applied to these samples had a very large error rate of falsely supporting subliminal priming. Calculating a Bayes factor for the samples very seldom falsely supported subliminal priming. We conclude that conventional tests are not reliable diagnostics of subliminal priming. Instead, we recommend that experimenters calculate a Bayes factor when investigating subliminal priming.

Keyword
subliminal priming, Bayes factor, signal detection theory, null-hypothesis significance testing
National Category
Psychology (excluding Applied Psychology)
Research subject
Psychology
Identifiers
urn:nbn:se:su:diva-132185 (URN)10.1016/j.concog.2016.06.012 (DOI)27351780 (PubMedID)
Available from: 2016-07-28 Created: 2016-07-28 Last updated: 2016-08-10Bibliographically approved
3. Reversed Priming Effects May Be Driven by Misperception Rather than Subliminal Processing
Open this publication in new window or tab >>Reversed Priming Effects May Be Driven by Misperception Rather than Subliminal Processing
2016 (English)In: Frontiers in Psychology, ISSN 1664-1078, E-ISSN 1664-1078, Vol. 7, 198Article in journal (Refereed) Published
Abstract [en]

A new paradigm for investigating whether a cognitive process is independent of perception was recently suggested. In the paradigm, primes are shown at an intermediate signal strength that leads to trial-to-trial and inter-individual variability in prime perception. Here, I used this paradigm and an objective measure of perception to assess the influence of prime identification responses on Stroop priming. I found that sensory states producing correct and incorrect prime identification responses were also associated with qualitatively different priming effects. Incorrect prime identification responses were associated with reversed priming effects but in contrast to previous studies, I interpret this to result from the (mis-)perception of primes rather than from a subliminal process. Furthermore, the intermediate signal strength also produced inter-individual variability in prime perception that strongly influenced priming effects: only participants who on average perceived the primes were Stroop primed. I discuss how this new paradigm, with a wide range of d′ values, is more appropriate when regression analysis on inter-individual identification performance is used to investigate perception-dependent processing. The results of this study, in line with previous results, suggest that drawing conclusions about subliminal processes based on data averaged over individuals may be unwarranted.

Keyword
subliminal priming, unconscious processing, perception, stimulus strength, signal detection theory, trial-based analysis
National Category
Psychology
Research subject
Psychology
Identifiers
urn:nbn:se:su:diva-127669 (URN)10.3389/fpsyg.2016.00198 (DOI)000370555800002 ()
Note

This research was supported by a grant from Stiftelsen Lars Hiertas Minne (award FO2015-0940) and the Department of Psychology, Stockholm University.

Available from: 2016-03-09 Created: 2016-03-09 Last updated: 2016-08-10Bibliographically approved
4. When Perception Trumps Reality: Perceived, not Objective, Meaning of Primes Drive Stroop Priming
Open this publication in new window or tab >>When Perception Trumps Reality: Perceived, not Objective, Meaning of Primes Drive Stroop Priming
(English)Manuscript (preprint) (Other academic)
Abstract [en]

Is semantic priming driven by the objective or perceived meaning of the priming stimulus? This question is relevant as many studies suggest that the objective meaning of invisible stimuli can influence cognitive processes and behavior. In an experiment involving 66 participants, we tested whether the perceived meaning of misperceived stimuli may influence response times. Stroop priming, i.e., longer response times for incongruent versus congruent prime–target pairs, was observed in trials in which the prime was correctly identified. However, reversed Stroop priming was observed when the prime stimulus was incorrectly identified. Even in trials in which participants reported “no” prime percept and identified the primes at close to chance level (i.e., fulfilling both a subjective and objective definition of “subliminal”), Stroop priming corresponded to perceived, not objective, congruency. Under conditions traditionally claimed to be “subliminal,” this allows occasional weak percepts and mispercepts to be intermixed with no percepts.

National Category
Psychology (excluding Applied Psychology)
Research subject
Psychology
Identifiers
urn:nbn:se:su:diva-132208 (URN)
Funder
Lars Hierta Memorial Foundation, FO2015-0940
Available from: 2016-08-01 Created: 2016-08-01 Last updated: 2016-08-10Bibliographically approved

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