Opinion
Attention and consciousness: two distinct brain processes

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The close relationship between attention and consciousness has led many scholars to conflate these processes. This article summarizes psychophysical evidence, arguing that top-down attention and consciousness are distinct phenomena that need not occur together and that can be manipulated using distinct paradigms. Subjects can become conscious of an isolated object or the gist of a scene despite the near absence of top-down attention; conversely, subjects can attend to perceptually invisible objects. Furthermore, top-down attention and consciousness can have opposing effects. Such dissociations are easier to understand when the different functions of these two processes are considered. Untangling their tight relationship is necessary for the scientific elucidation of consciousness and its material substrate.

Introduction

Commonly used in everyday speech and in the scholarly literature, ‘attention’ and ‘consciousness’ have resisted clear and compelling definition. As argued elsewhere 1, 2, this unfortunate state of affairs will remain until the mechanistic basis of these phenomena has been enunciated thoroughly at the neuronal and molecular levels.

Few would dispute that the relationship between selective attention and consciousness is an intimate one. When we attend to an object, we become conscious of its attributes; when we shift attention away, the object fades from consciousness. This has prompted many to posit that these two processes are inextricably interwoven, if not identical 3, 4, 5, 6. Others, however, going back to the 19th century [7], have argued that attention and consciousness are distinct phenomena that have distinct functions and neuronal mechanisms 2, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17. If this proposition were true, what would be the nature of their causal interaction? Is paying attention necessary and sufficient for consciousness? Or can conscious perception occur outside the spotlight of attention? Of course, this presupposes that consciousness is a unitary concept, which is not the case; consciousness has been dissected on conceptual grounds (access versus phenomenal consciousness), ontological grounds (hard problem versus easy problem) and psychological grounds (explicit versus implicit processes).

Here, we summarize recent psychophysical evidence in support of a dissociation between attention and consciousness, and provide functional justifications for this viewpoint. We argue that events or objects can be attended to without being consciously perceived. Furthermore, an event or object can be consciously perceived in the near absence of top-down attentional processing. Note that our usage of ‘attention’ always implies selective attention, rather than the processes that control the overall level of arousal and alertness.

Section snippets

Functional roles of attention and consciousness

Complex organisms and brains can suffer from informational overload. In primates, about one million fibers leave each eye, carrying on the order of one megabyte per second of raw information. One way to deal with this deluge is to select a small fraction and process this reduced input in real time, while the non-attended portion of the input is processed at a reduced bandwidth. In this view, attention ‘selects’ information of current relevance to the organism, while the non-attended data are

The four ways of processing visual events and behaviors

Although many scholars agree that attention and consciousness are distinct, they insist that attention is necessary for consciousness, and that non-attended events remain hidden. For example, Dehaene et al. [16] state that considerable evidence indicates that, without attention, conscious perception cannot occur. We now review evidence that argues otherwise.

Processing without top-down attention and consciousness

Visual input can be classified rapidly. As demonstrated by Kirchner and Thorpe [41], at ∼120 ms, the brain can begin to determine whether a briefly flashed image contains animals or not. At this speed, it is no surprise that subjects often respond without having consciously seen the image; consciousness of the image might come later or not at all. Dual-tasks and dual-presentation paradigms support the idea that such discriminations can occur in the near absence of focal, spatial attention 38, 42

Attention and consciousness can oppose each other

Withdrawing top-down attention from a stimulus and cloaking it from consciousness can have opposing effects. When observers try to find two embedded targets within a rapidly flashed stream of images, they often fail to see the second target, the attentional blink [46]. Counterintuitively, Olivers and Nieuwenhuis [47] found that observers can see both the first and the second targets better when they are distracted by a simultaneous auditory dual task or when they are encouraged to think about

Relationship to other conceptual distinctions

Dehaene and colleagues [16] propose a tripartite ontology, based on the global workspace hypothesis of Baars (for an updated view, see Ref. [14]) and Dehaene et al. [23], whereby any physical stimulus triggers subliminal, preconscious or conscious processing. What decides the fate of a stimulus is its strength and whether or not top-down attention is deployed. This threefold distinction maps onto our fourfold one if subliminal processing is equated with the two left quadrants and preconscious

Do these conclusions hold for real life?

It could be contested that top-down attention without consciousness and consciousness with little or no top-down attention are arcane laboratory curiosities that have little relevance to the real world. We believe otherwise. A lasting insight into human behavior – eloquently articulated by Friedrich Nietzsche – is that much action bypasses conscious perception and introspection. In particular, Goodale and Milner [48] isolated highly trained, automatic, stereotyped and fluid visuomotor behaviors

Acknowledgements

Owing to space limitations, we could not discuss all the relevant literature and apologize to those whose work was not cited here. We thank R. Blake, N. Block, A. Cleeremans, S. He, Y. Jiang, R. Kanai, V. Lamme, C. Paffen, and M. Snodgrass for discussion. We thank the participants of our tutorial at ASSC10 Oxford for their feedback. This research was supported by the NIMH, the NSF, the Keck Foundation, the Moore Foundation, and the Tom Slick Research Awards from the Mind Science Foundation.

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