ReviewUnconscious vision and executive control: How unconscious processing and conscious action control interact
Introduction
Since long it has been argued that not all visual processing is conscious (Münsterberg, 1910), but especially the recent decades have seen a tremendous number of articles concerning the capabilities of unconscious vision (for topical reviews, see Ansorge et al., 2011, Dehaene et al., 2006, Dehaene and Naccache, 2001, Kiefer et al., 2012, Kunde et al., 2012, Lamme, 2003). The current article focuses on one very fruitful method in the area of unconscious vision, namely masked priming (Greenwald et al., 1996, Marcel, 1983). There are other methods to study unconscious vision, like investigating visual capabilities after brain lesions (Goodale, Milner, Jakobson, & Carey, 1991) or priming during continuous flash suppression (Almeida et al., 2008, Tsuchiya and Koch, 2005). In comparison to brain lesion studies, however, masked priming allows studying unconscious processing also in healthy participants, therefore avoiding interpretational difficulties due to neuro-plastic changes of processing after brain damage. Furthermore, masked priming has been applied in a greater variety of studies compared with the more recent method of continuous flash suppression. Finally, methods in which attention is directed away from a stimulus or in which binocular rivalry is used to lower conscious perception of a stimulus are not entirely convincing in terms of the claimed invisibility of the stimuli (Blake, 1998, Holender, 1986), and therefore are also only occasionally discussed in the present review. As masked priming research provides an important window on unconscious vision for several decades, our portrait therefore almost naturally relies mostly on research in this area, although we will sometimes refer to related findings with other methods as well.
The focus of the current review is on the connection between unconscious vision and executive functions. These functions encompass the setting up and representation of goals and the operations needed to achieve these goals (Baddeley and Hitch, 1974, Miller and Cohen, 2001, Miyake et al., 2000). In this context, task-control representations, sometimes simply called ‘task sets’, denote representations specifying preceding conditions that have to be met for the execution of an action (e.g., ‘It turns dark, so I should switch on the light.’) or an operation (e.g., ‘I have to get out of the airport, so I should search for an exit sign.’) the corresponding action (e.g., ‘Switch on the light.’) or operation (e.g., ‘Attend to the signs under the ceiling.)’ itself, as well as optional intended consequences of these actions, such as outcomes (e.g., ‘Light is on.’, or ‘There is an exit sign.’). Finally, executive control includes the processes necessary for securing the success of the actions and operations. Among these supporting processes are the shielding of goal representations against conflicting goals, the monitoring of the outcomes of actions and operations, and the registration and correction of errors during the execution of the operations (Norman & Shallice, 1986). The latter are typically involved when an operation is repeatedly performed, as in a computer experiment consisting of many trials.
Intuitively, there seems to be a tight connection between the activity of executive functions and consciousness or awareness. For example, when I decide to buy an apartment because I do no longer want to pay the rent, I have the strong intuition that I am fully aware of setting up the task set representation of how I go about buying an apartment, including the visual information that I have taken into account while deciding. Accordingly, early theories equated conscious vision with top-down controlled processing and unconscious vision with so-called automatic processing (Norman and Shallice, 1986, Posner and Snyder, 1975). In this context, automatic processing means that unconscious vision would run off independently of an agent’s own intentions, being entirely stimulus triggered, and being even uncontrollable – that is, not modifiable by a currently opposing intention or task set. In fact, until today, many theories take this stance on unconscious vision (Mulckhuyse & Theeuwes, 2010).
As we will delineate, however, much research on masked priming supported a different view (Ansorge and Neumann, 2005, Dehaene and Naccache, 2001, Kiefer and Martens, 2010, Neumann, 1990). Specifically, our review has three aims. First, we look at the most important strands of research that led to the conclusion that unconscious vision depends on top-down control. This will be done in part 2. Second, we will review different theories of masked priming and point out some surprising communalities between these theories (mostly in part 2), as well as the differences between them (in parts 2, 3, and 4). Finally, we will detail the limits of top-down control of unconscious vision in part 3, and the limits of unconscious vision in general in part 4. The latter concerns the very limited power of unconscious vision to modify or set up task sets in the first place.
In masked priming, a visual prime is presented followed by a visual mask at the same position or surrounding the same position. Typically, the interval between prime and mask is short (about a few tens of milliseconds). This procedure is called ‘backward masking’ because the mask follows the prime (Breitmeyer, 1984). Backward masking can lead to the complete absence of the prime’s visibility. Sometimes this subjective lack of awareness is additionally reflected in objective chance performance when the participant is asked to discriminate the prime stimulus (Dagenbach et al., 1989, Hines et al., 1986, Klotz and Neumann, 1999, Marcel, 1983). Its invisibility notwithstanding, a masked prime influences overt behavior. For example, using visible boys and girls names as targets that had to be discriminated by their gender, with a masked gender-congruent prime presented before the target (e.g., a male prime before a male target) responses were facilitated as compared to a masked incongruent prime (e.g., a male prime before a female target) (Greenwald et al., 1996). This influence was found although the participants could not reliably discriminate between the prime’s gender per se.
Clearly, such experimental tasks like to press one button for a girl’s name and another button for a boy’s name require setting up and execution of a task set. On the side of the participants, such a task requires connecting two arbitrary responses as the to-be-executed operations with the two different classes of inputs of boys’ versus girls’ names. Although the fact that such a task set needs to be implemented to get going is sometimes not considered important by researchers using masked priming to address questions of semantic memory, lexical access and so on, implementing the task set on the side of the participants means we have entered the realm of executive control. Masked priming must therefore have something to say about the interactions between unconscious vision and executive functions, which will be highlighted in the next sections of this article.
Section snippets
Unconscious vision and executive control
Theories on action control traditionally assume a strong link between cognitive control and consciousness. For example, Jack and Shallice (2001) make a clear distinction between conscious action where control is possible and automatic action where it is not. This distinction echoes the traditional dichotomy between unconscious automatic processing that is independent of executive control and controlled processing in the conscious domain (Posner and Snyder, 1975, Shiffrin and Schneider, 1977).
Masked priming during task execution: Is it all top-down?
The review so far shows that masked priming effects contradict two of the three traditionally proposed hallmarks of automatic processing, namely (1) attention-independence, (2) intention-independence if not intention-resistance, and (3) awareness-independence (Posner & Snyder, 1975). According to this view, an effect that is independent of the participants’ awareness about the stimulus being processed should at the same time not require the participant’s deliberate control about the processing
Influences of unconscious stimuli during the setting up of task-control representations and goal selection
Although the setting up of a task-control representation and the representation of a goal both logically have to precede the execution of a task set and the pursuit of a goal, we have postponed the discussion of the influence of masked priming on these processes until the end. The reason for this is that we think that to select a goal and to set up task sets, awareness of the reasons for doing so is much more relevant than during the execution of a task set. To understand this conclusion, we
Conclusion
Our review has revealed that unconscious or unaware visual processing serves particular, already intended actions and cognitive operations (Ansorge and Neumann, 2005, Dehaene and Naccache, 2001, Kiefer and Martens, 2010, Kunde et al., 2003). By contrast conscious or aware processing seems to be necessary for setting up a task set in the first place (Dehaene and Naccache, 2001, Kunde, 2003, Mayr, 2004, Neumann, 1990), and conscious vision seems to promote the global availability of mental
Acknowledgments
This research was supported by Grants of the German Research Foundation within the Research Network “Neuro-Cognitive Mechanisms of Conscious and Unconscious Visual Perception” (PAK 270/1 and 2) to WK (Ku 1964/7-1) and MK (DFG Ki 804/3-2). We also thank Bruno Breitmeyer, Tom Carr, Thomas Schnidt, and an anonymous reviewer for helpful comments on an earlier draft of this manuscript.
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