Review
Empirical support for higher-order theories of conscious awareness

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Higher-order theories of consciousness argue that conscious awareness crucially depends on higher-order mental representations that represent oneself as being in particular mental states. These theories have featured prominently in recent debates on conscious awareness. We provide new leverage on these debates by reviewing the empirical evidence in support of the higher-order view. We focus on evidence that distinguishes the higher-order view from its alternatives, such as the first-order, global workspace and recurrent visual processing theories. We defend the higher-order view against several major criticisms, such as prefrontal activity reflects attention but not awareness, and prefrontal lesion does not abolish awareness. Although the higher-order approach originated in philosophical discussions, we show that it is testable and has received substantial empirical support.

Section snippets

The empirical implications of higher-order theories of conscious awareness

Although by no means unanimously supported, the higher-order view of conscious awareness (see Glossary) is popular among philosophers and scientists alike 1, 2, 3, 4, 5, 6, 7, 8, 9, 10. It is sharply contrasted with first-order theories 11, 12, 13, which hold that a mental state being conscious is determined solely by the neural or representational character of the perceptual state (i.e. first-order states). For instance on a first-order view the conscious experience of seeing a face is

Distinguishing the higher-order view from alternatives

In this article we consider three major alternatives to the higher-order view: neuronal global workspace theory 20, 21, 22, information integration theory [23] and the first-order view 11, 12, 13, 24, 25, 26. These four major theories make different empirical predictions (Table 2).

According to the neuronal global workspace theory 20, 21, 22, neurons with long-range connections in the prefrontal and parietal areas form a network (i.e. workspace) for conscious processing. As with the higher-order

Conscious awareness is associated with specific prefrontal mechanisms

Numerous neuroimaging studies have identified activations in prefrontal and parietal cortices in association with conscious awareness 22, 34. On the face of it, these findings support the higher-order, global workspace and information integration views over the first-order view.

The higher-order view can be distinguished from global workspace theory by conditions in which task performance is matched but conscious awareness differs. One study [35] used visual masking to create conditions in which

The argument from the possible function of conscious awareness

Although studies of subliminal priming demonstrate that unconscious information can influence behavior, such influence has traditionally been thought to be minimal 39, 40. One difficulty in assessing this claim is that the usual method for rendering a stimulus subjectively invisible involves impairing the perceptual signal itself until task performance is at chance [41]. Thus, when subjects perform cognitive tasks poorly with subjectively invisible stimuli, it is often difficult to know whether

Empirically based criticisms of the higher-order view

It could be argued that the prefrontal activity that occurs with conscious awareness 22, 34 reflects something other than awareness itself, perhaps attention or access to the perceptual information. Two studies that have controlled for attention 24, 54 by requiring subjects to perform a task unrelated to the visual target in question would seem to be compatible with this argument. These studies found that visible targets (compared to targets rendered invisible by masking), when unattended, were

Suggestive evidence based on clinical disorders

Clinical cases are often useful in theoretical debates. Consider a neurological condition that might at first glance seem to tell against the higher-order view. In blindsight 3, 66, visual awareness is abolished even though above-chance task performance is retained. Because blindsight is typically due to damage to the primary visual cortex, one might conclude that blindsight supports the first-order rather than the higher-order view. However, damage to an area often has distal effects, and it

Concluding remarks

We have reviewed empirical evidence that supports the higher-order view of conscious awareness and addressed empirically based challenges to the view. We focused on cases in which awareness differed despite matched task performance because these cases most crucially distinguish the higher-order view from its alternatives. Because such matching is experimentally difficult to achieve, the most decisive evidence is only now beginning to emerge. Our arguments are therefore not conclusive, and

Acknowledgments

HL is funded by the Netherlands Organization for Scientific Research (grant number 2300164119) and the Templeton Foundation (grant number 21569). He thanks Joe Lau for first introducing him to the higher-order view, and Mickey Goldberg for pointing out that cases of Charles Bonnet syndrome are relevant. He has benefited greatly from discussions with David Chalmers, Uriah Kriegel, Richard Brown and most importantly Ned Block. DR thanks Larry Weiskrantz, Martin Davies, Ned Block and Josh Weisberg

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