Conscious and unconscious proportion effects in masked priming☆
Introduction
Many theories of human cognition postulate that unconscious cognitive processes, such as unconscious priming, are automatic and independent of attention. According to these traditional accounts, unconscious processing is confined to a passive spreading of activation, inflexible and independent of the subject’s attention and strategies. Recently, Dehaene and colleagues (Dehaene, Changeux, Naccache, Sackur, & Sergent, 2006) defined subliminal processing as “a condition of information inaccessibility where bottom–up activation is insufficient… for global ignition” (p. 3). However, they stress that even though the bottom–up strength of subliminal stimuli is limited, top–down manipulations can have an effect on subliminal processing, indicating that subliminal processing is not merely a matter of automatic activation spreading. Indeed, a number of experimental reports using the masked priming paradigm have revealed that unconscious processing is affected by several top–down effects. Unconscious priming has been shown to be influenced by temporal attention (e.g., Naccache, Blandin, & Dehaene, 2002), spatial attention (e.g., Sumner, Tsai, Yu, & Nachev, 2006) and strategies, such as target set (e.g., Greenwald, Abrams, Naccache, & Dehaene, 2003) and target notation (Kunde, Kiesel, & Hoffmann, 2003). For instance, this latter top–down effect of target notation was investigated by Kunde and colleagues (2003, experiment 4). They showed that priming effects elicited by unconscious Arabic number primes (i.e. 1, 4,…) and number word primes (i.e. One, Four,…) were dependent on the target notation. If all targets were Arabic numbers, only Arabic number primes elicited a congruency effect. Similarly, if the targets were number words, only number word primes evoked an effect. According to the authors, subjects create intentions based on the stimulus characteristics and apply these intentions to the processing of the primes, which is clearly a top–down influence.
Although several studies provide strong evidence in favour of top–down influencing of unconscious priming by several factors, the conducted research is less clear on whether unconscious primes can themselves be used strategically and result in a bottom–up effect. Theoretically, the possibility of such a bottom–up effect of unconscious stimuli has been refuted. The definition of Dehaene et al. (2006), stating that unconscious primes have insufficient strength to result in a bottom–up effect, is situated within a broader framework proposed by Dehaene and Naccache (2001). They attempted to formulate a theoretical framework on consciousness based on a cognitive neuroscience approach. Their global neuronal workspace theory distinguishes two computational spaces in the brain (Dehaene & Changeux, 2004): (1) the network of processors, which is a collection of specialized processors all dedicated to the processing of very specific information and (2) the global neuronal workspace (GNW) which, contrary to the local and modular processors, allows many different processors to exchange information in a global and flexible way. The GNW is assumed to play a crucial role in accessing information into consciousness. A mechanism called top–down attentional amplification, situated in the GNW, can activate local processors in the global workspace. This top–down process is necessary for information to access consciousness. However, to enter consciousness, a processor must not only be activated in the GNW, this activity must also be amplified and maintained over a minimal duration to reach the consciousness threshold. This implies that there are two thresholds in human information processing: (1) the minimal duration to reach any neural activity at all, and (2) the significantly longer duration to reach the consciousness threshold. Stimuli that fall in between these two thresholds can cause unconscious processing, but can never reach a conscious state. A longer or stronger stimulation is required to reliably activate the GNW, necessary to reach consciousness. Contrarily, a masked stimulus, which reaches the first threshold, but not the second one, can result a burst of activity, but only a weak and short-lived activation of the GNW is seen. In a realistic situation however, unconscious processing does not have to be as rigid as described above and context effects come into play. Based on this theoretical framework, the GNW model makes two basic assumptions about the possibility of conscious and unconscious (context) stimuli to result in a top–down and/or a bottom–up effect. First, explicit context stimuli (e.g., task instructions or target manipulations as described at the beginning of the introduction) can induce a reorganization of the GNW through top–down amplification. The GNW theory predicts that such reorganization can effect the processing of unconscious information, as long as the context stimuli which guide this process themselves are conscious. Thus, when a task is prepared consciously, it can be applied unconsciously. This first theoretical assumption has been confirmed by a number of research results, as we described above: several explicit top–down manipulations have proven to influence unconscious processing.
Second, if initially unattended stimuli carry enough strength, they can cause a reorienting of top–down amplification towards them, which is called a bottom–up effect. However, since for such a bottom–up effect to take place conscious access to the information is necessary, a stimulus that never reaches consciousness (e.g., an unconscious prime), would be unable to result in such a bottom–up effect. This second assumption of the GNW model is more controversial and has never been explicitly investigated. However, some indirect evidence contradicting the GNW hypothesis can be mentioned. For example, in a series of studies Bodner and Masson, 2001, Bodner and Masson, 2003 were able to demonstrate that the proportion of related trials mediates priming effects in a masked priming task. What is different about these studies is that they did not use an explicit target manipulation, but an implicit prime manipulation. More specifically, they manipulated the proportion of trials with a related prime. According to the GNW model it would be impossible for such an implicit context factor to exert a bottom–up impact, since this stimulus does not reach consciousness. Still, Bodner and Masson observed an increase in priming when the proportion of related versus unrelated prime trials was higher.
The present study aims to explicitly investigate the two assumptions of the GNW theory (Dehaene & Naccache, 2001) by manipulating a context factor (the proportion of Arabic numbers versus number words) both consciously (experiment 1) and unconsciously (experiment 2). By controlling all other task conditions (task instructions, procedures, stimuli,…) and only varying whether the context factor is manipulated consciously (at the target level) or unconsciously (at the prime level), it becomes possible to directly compare the observed results for the two experiments. Based on the GNW theory we hypothesize that a fully consciously perceived context stimulus would be able to establish a reorganization of the workspace and therefore could be used strategically to enhance task performance resulting in differences in priming effects between conditions (top–down effect). Contrarily, it would be impossible for an unconscious context stimulus to establish a reorganization of the workspace and therefore it could not be used strategically to respond faster and/or more accurate to the target (bottom–up effect).
Section snippets
Experiment 1
In experiment 1 number targets needed to be categorized as smaller or larger than five. The targets were always preceded by novel primes (i.e. primes that were never presented as targets). This experiment was used to investigate the top–down influence of conscious stimuli by using a manipulation of the targets. We manipulated the proportion Arabic targets (e.g., “1”, “4”, etc.) versus number word targets (e.g., “one”, “four”, etc.). This manipulation created three conditions: in the first
Experiment 2
The second experiment was designed to investigate the bottom–up influence of unconscious stimuli by using a prime manipulation. Experiment 2 resembles experiment 1 very closely. However, in this experiment, the between-subject manipulation was altered. Now, we manipulated the proportion of Arabic primes (e.g., “2”, “3”, etc.) versus number word primes (e.g., “two”, “three”, etc.) between subjects. This manipulation again created three conditions: in the first (baseline) condition subjects were
General discussion
The goal of this study was to clarify whether a conscious (target) and unconscious (prime) manipulation of the proportion of Arabic numbers versus number words had an effect on prime processing. Regarding this issue, the global neuronal workspace theory (Dehaene & Naccache, 2001) made two basic assumptions. First, according to this model, it would be impossible for an unconscious stimulus to be used strategically to enhance task performance (bottom–up effect). Contrarily, a fully consciously
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Eva Van den Bussche and Bert Reynvoet are affiliated to the Laboratory of Experimental Psychology of the Department of Psychology, University of Leuven and to the Subfaculty of Psychology and Educational Sciences, University of Leuven–Campus Kortrijk. We thank three anonymous reviewers for their helpful comments and suggestions.