Can a non-conscious stimulus modulate executive control?

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The terms central executive and executive control originate from the work of Broadbent (1950), Posner & Snyder (1975), Shallice (1978) and Baddeley (1986). These terms refer to the set of processes underlying the planning, initiation, execution and supervision of voluntary, goal-directed behavior, as well as flexibility in the design of new or non-routine strategies. Executive control processes include goal maintenance, selection of goal-relevant perceptual representations and actions, inhibition of inappropriate actions, and error detection and correction.

A long tradition, which can be traced back to René Descartes' reflections on the nature of human consciousness, associates the flexibility of executive control with a necessarily conscious level of processing. However, if we reject Cartesian dualism, even these very high-level operations must well result from the functioning of an assembly of cerebral systems, and it's not clear why they couldn't be triggered in the absence of consciousness. Are executive functions performed by processors like any others, whose execution speed and decisions are likely to be influenced by non-conscious cues? This question, crucial to the development of a theory of consciousness, is above all an empirical one, and several researchers have recently posed it using the subliminal masking paradigm.

Uwe Mattler (2003) presents a series of experiments in which a subliminal cue appears to influence a high-level executive process: the decision to switch cognitive tasks. However, these experiments are flawed in that they fail to distinguish how much of the acceleration in reaction time might be due to a visual priming recognition step - a step that should logically precede task preparation and execution. Lau and Passingham's (2007) experiment is more conclusive: using functional MRI, they demonstrate that an entire network of brain areas partially switches, in preparation for the task ahead, in response to a subliminal cue to change task.

Similarly, Van Gaal et al (2008) demonstrate that a process of action inhibition, associated with the prefrontal cortex, is partially triggered when a visual cue, previously associated by conscious instruction with response inhibition, is presented below the threshold of consciousness. However, these behavioral effects are very weak and emerge only after considerable routinization of the task, casting doubt on their association with a level of executive processing.

The clearest results are obtained in the field of error detection. When we make an error in a motor response, and this error is easily detectable, an evoked potential called "error negativity", presumably originating in the anterior cingulate cortex, appears on fronto-central electrodes a few tens of milliseconds after the erroneous response. This error detection can be triggered without the subject's knowledge (Nieuwenhuis, Ridderinkhof, Blom, Band & Kok, 2001). In an anti-saccade task, the eyes may deviate from the required trajectory even though the subject denies having made a mistake. In this case, error negativity is present, indicating that certain brain areas have non-consciously identified the deviation of the eye trajectory. However, there are several important differences between conscious and non-conscious errors. Only the former lead to a PE wave, similar to the P300 wave, which immediately follows the negativity to the error. What's more, a slowdown on the next trial is only observed if the subject reports awareness of the error.

This recent and still embryonic work leads us to a nuanced conclusion. Certain executive control processes can clearly be partly triggered by subliminal stimuli - such as strategy choice, action inhibition or error detection. However, the heterogeneity of the results is significant: non-conscious error detection in eye movements is a rapid and massive process, whereas non-conscious influences on strategy choice and especially action inhibition are tiny effects. What's more, in no case is there any lasting non-conscious alteration of executive control extending beyond a few seconds.