Attention and consciousness
|Naotsugu Tsuchiya and Christof Koch (2008), Scholarpedia, 3(5):4173.||doi:10.4249/scholarpedia.4173||revision #126988 [link to/cite this article]|
Attention and consciousness are two closely related psychological concepts that are often conflated, even among scholars. However, modern psychological and neurophysiological researchers can now independently manipulate top-down selective attention and perceptual consciousness. This allows them to untangle the distinct contributions these two make to processing in the mind and their underlying neuronal mechanisms.
Current controversies revolve around three questions
- Can we become conscious of some object without attending to this object?
- Can we attend to objects that are consciously suppressed, that is, invisible?
- Can top-down selective attention and perceptual consciousness have opposing effects?
Although by no means conclusive, current evidence suggests that top-down attention and perceptual consciousness are two distinct but often allied processes with distinct neurobiological processes. As a consequence, it will be important to carefully distinguish the neuronal correlates of consciousness from the neuronal correlates of selective attention (Tse et al., 2005).
The relationship between attention and consciousness
Few would dispute that the relationship between selective attention and perceptual consciousness is an intimate one. When we pay attention to an object, we become conscious of its various 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 (Posner, 1994; Jackendoff, 1996; Velmans, 1996; Merikle and Joordens, 1997; Chun and Wolfe, 2000; O'Regan and Noe, 2001; Prinz, 2004). Others, going back to the 19-th century (Wundt, 1874), however, have argued that attention and consciousness are distinct phenomena, with distinct functions and distinct neuronal mechanisms (Iwasaki, 1993; Hardcastle, 1997; Naccache et al., 2002; Lamme, 2003; Woodman and Luck, 2003; Kentridge et al., 2004; Koch, 2004; Baars, 1997 & 2005; Block, 2005; Bachmann, 2006; Dehaene et al., 2006).
Even if the latter proposition is true, what is 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 and attention are unitary concepts, which is not the case. Indeed, consciousness has been dissected on conceptual (access vs. phenomenal consciousness; Block, 2005), ontological (Hard vs. Easy problem; Chalmers , 1996), and psychological (explicit vs. implicit processes; Tulving 1993) grounds, and attention has similarly been dissected into orienting, filtering, and searching functions, anterior and posterior brain circuits, exogenous (bottom-up) and endogenous (top-down) trigger mechanisms, and so forth (Posner & Peterson, 1990). Some recent psychophysical and neurophysiological evidence favors a dissociation between selective attention and consciousness and provides functional justifications for this position. The supportive evidence is summarized in the following three categories:
- Events or objects can be attended to without being consciously perceived, that is, attention can select invisible objects. In particular, ongoing neuroimaging studies are measuring attentional modulation of fMRI responses to invisible stimuli (Tsushima et al., 2006; Bahrami et al., 2007).
- An event or object can be consciously perceived in the near absence of top-down attentional processing. For empirical methods to manipulate the visibility of stimuli independently of top-down attention, see How to manipulate and measure visual consciousness and How to Manipulate Attention.
- Top-down attention and consciousness can have opposite effects.
Note that the usage of “attention” in this entry always implies selective attention, rather than the processes that control the overall level of arousal and alertness of the organism. Furthermore, this entry focuses on visual attention and visual consciousness, as visual perception and the neurophysiology of vision are much easier to manipulate and also much better understood than are similar phenomena and their supporting brain mechanisms in other modalities.
Functional role of attention
Complex organisms, in particular those with brains, suffer from information overload. In primates, about one million fibers leave each eye and carry on the order of one megabyte per second of raw information. One way to deal with this deluge of data is to select a small fraction of it and to process this reduced input in real-time, while the non-selected portion of the input is processed at a reduced bandwidth. In this view, attention is a mechanism that selects information of current relevance to the organism while leaving the non-selected, and thus non-attended, data to suffer from benign neglect.
Attentional selection is known to be based on either ‘’bottom-up/exogenous Bottom-up Attention’’ or ‘’top-down/endogenous’’ factors (James, 1890; Braun and Julesz, 1998; Duncan, 1998). However, under many conditions, subjects can disregard salient, bottom-up cues when searching for particular objects in a scene by exercising top-down, task-dependent control of their attention (Henderson et al., 2006). Bringing top-down, sustained attention to bear on an object or event in a scene takes time, however. Top-down attention selects input defined by a circumscribed region in space (focal attention), by a particular feature (feature-based attention), or by an object (object-based attention). It is the relationship between these volitionally-controlled forms of selective, endogenous attention and consciousness that is the topic of this entry.
Functional role of consciousness
Consciousness is surmised to have quite different functions from those of attention. These range from summarizing all relevant information pertaining to the current state of the organism and its environment and making this compact summary accessible to the planning stages of the brain, to detecting anomalies and errors, decision making, language, inferring the internal state of other animals, setting long-term goals, making recursive models, and rational thought.
To the extent that one accepts that attention and consciousness have different functions, one has to accept that they cannot be the same process.
Consider the four different ways in which a particular percept or behavior can be classified depending on whether or not it requires top-down attention and whether or not it necessarily gives rise to consciousness.
The four-fold way of processing visual events and behaviors
While many scholars agree that attention and consciousness are distinct, they insist that the former is necessary for the latter, and that non-attended events remain "sub rosa" from the point of view of consciousness. For example, Dehaene and colleagues (Dehaene et al., 2006) argue that without top-down attention, an event cannot be consciously perceived but will remain preconscious.
|May not give rise to consciousness||Gives rise to consciousness|
|Top-down attention is not required|| Formation of afterimages
Rapid vision (< 120 msec)
Animal & gender detection in dual-tasks
|Top-down attention is required||Priming|| Working memory
Detection and discrimination of unexpected & unfamiliar stimuli
Processing that requires top-down attention and results in consciousness
More than a century of research efforts have quantified the ample benefits that accrue to attended and consciously perceived events. For example Mack and Rock (Mack and Rock, 1998) compellingly demonstrated that subjects must attend to become conscious of novel or unexpected stimuli. These occupy the lower right quadrant of the attention x consciousness design matrix (Table 1).
Processing that does not require top-down attention and that can remain non-conscious
On the other end of the spectrum are objects or events that are neither sufficiently salient to attract bottom-up attention nor are the target of top-down attentional bias. The net-wave of spiking activity associated with these non-attended objects or events, moving from the retina into primary visual cortex and beyond may not trigger a conscious percept (but see further below). Nonetheless, this activity can still be causally effective and leave traces in the brain that can be picked up with sensitive behavioral techniques, such as priming.
It is known that invisible stimuli can cause negative afterimages and that withdrawing attention from the afterimage inducer strengthens afterimages (For details, see Opposite Effects). Therefore, afterimages can be induced without top-down attention and without seeing the inducer; these occupy the upper left quadrant of Table 1.
Other likely examples include visuo-motor reflexes such the accommodation and the pupillary reflexes, as well as so-called zombie behaviors (Koch & Crick, Nature, 2001). These highly trained, rapid, automatic and stereotyped sensory-motor actions - examples include rapid eye movements, reaching and grasping, posture adjustment, running and action sequences such as tying shoe-laces, playing piano, tennis or soccer, driving, climbing, trail running and so on - are likely to run off independent of attention. This hypothesis could be tested using a combination of masking and dual-task paradigms. It is known that paying attention to elements of the trained action sequences interferes with their rapid execution (Beilock 2002), whereas executing an automatic behavior sequence interferes very little or not at all with execution of a non-automatic behavior in dual task paradigms as long as input and output modalities do not interfere (e.g., Schneider et al, 1984).
Rapid visual categorization is yet another candidate for a process that may require neither top-down attention nor consciousness Processing Without Attention And Consciousness.
What about the two remaining quadrants, covering events that require top-down attention but that do not give rise to conscious perception and events that give rise to consciousness yet without requiring top-down attention? These can be studied with techniques that independently manipulate top-down attention and visual consciousness How to Manipulate Attention and How to manipulate and measure visual consciousness.
Attention without consciousness
Consider that subjects can attend to a location for many seconds and yet fail to see one or more attributes of an object at that location (lower left quadrant in Table 1).
Psychological experiments as well as fMRI experiments have demonstrated
- Processing of invisible stimuli that requires top-down attention
- Invisible stimuli that attract spatial attention
- Feature-based attention spreading to an invisible target
For details, see Attention Without Consciousness. These experiments compellingly demonstrate that in some cases, subjects can attend to something without consciously experiencing any attribute of that very thing. This evidence is consistent with the view that attention is a selection process whose output may or may not give rise to phenomenal sensation.
Consciousness in the absence of attention
The converse can also occur and may be quite common (upper right quadrant in Table 1). When focusing intensely on one event, the world is not reduced to a tunnel, with everything outside the focus of attention gone. Subjects are always aware of some aspects of the world surrounding them, such as its gist. Indeed, gist is immune from inattentional blindness (Mack and Rock, 1998): when a photograph covering the entire background was briefly flashed completely unexpectedly onto a screen, subjects could accurately report a summary of what it contained. In the 30 msec necessary to apprehend the gist of a scene (Biederman, 1972; Fei-Fei et al., 2007), top-down attention cannot play much of a role (because gist is a property associated with the entire image, any process that locally enhances features is going to be only of limited use).
Take perception of a single object (say a bar) in an otherwise empty display, a non-ecological but common arrangement in many experiments. Here, what function would top-down, selective attention need to perform without any competing item in or around fixation? Indeed, the most popular neuronal model of attention, biased competition (Desimone and Duncan, 1995), predicts that in the absence of competition, little or no attentional enhancement occurs.
Further support for consciousness without top-down attention comes from a series of experiments using a dual-task paradigm Consciousness Without Attention.
Given our current inability to selectively, deliberately, transiently, reversibly and safely intervene in the brains of subjects, it is very difficult to be sure that all attentional resources or components have been removed from a particular location or object. What seems clear is that conscious perception can occur in the near-absence of, or at least without the necessity of, top-down attention.
Attention and consciousness can oppose each other
Most remarkably, withdrawing top-down attention from a stimulus and cloaking it from consciousness can cause opposing effects. When observers try to find two embedded targets within a rapidly flashed stream of stimuli, they often fail to see the second target, a phenomenon known as the attentional blink (Raymond et al., 1992; Chun and Potter, 1995). Counter-intuitively, Olivers and Nieuwenhuis (Olivers and Nieuwenhuis, 2005) reported that observers can see both the first and the second targets better when they are distracted by a simultaneous auditory secondary task or encouraged to think about task-irrelevant events.
Paying attention usually improves processing speed, lowers detection threshold or increases response accuracy. However, under certain conditions, low-spatial-frequency stimuli can be better discriminated without spatial attention than with it (Wong and Weisstein, 1982, 1983; Yeshurun and Carrasco, 1998). During implicit learning, attentively trying to discover the underlying complex rule delays learning and impairs subsequent recognition (Reber, 1976). Recent work on afterimages, stabilization of bistable figures, and complex decision-making hint at striking dissociations between top-down attention and consciousness Opposite Effects. Such findings are difficult to understand within a framework that aligns top-down attention closely with consciousness.
List of subpages in this article
- How to manipulate and measure visual consciousness
- How to Manipulate Attention
- Bottom-up Attention
- Processing Without Attention And Consciousness
- Attention Without Consciousness
- Consciousness Without Attention
- Opposite Effects
- Phenomenal consciousness without cognitive access by Ned Block
- Iconic memory
- Stanislas Daheane’s tripartite ontology of consciousness
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The following emphasize the difference between attention and consciousness
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- Bachmann T (2006) A single metatheoretical framework for a number of conscious-vision phenomena. In: Psychological Science Around the World (Jing Q, ed), pp 229-242. Sussex: Psychology Press.
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- Koch C (2004) The Quest for Consciousness: A neurobiological Approach. CO: Roberts and Publishers.
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The following emphasize the close relationship between attention and consciousness
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- Posner, M.I. (1994). Attention: the mechanisms of consciousness. Proc Natl Acad Sci U S A 91, 7398-7403.
- Velmans, M. (1996). The science of consciousness, (London: Routledge).
- Merikle, P.M., and Joordens, S. (1997). Parallels between perception without attention and perception without awareness. Conscious Cogn 6, 219-236.
- Jackendoff, R. (1996). How Language Helps Us Think. Pragmatics and Cognition 4, 1-34.
- Prinz, J. (2004). Gut Reactions, (New York: Oxford University Press).
- Chun, M.M., and Wolfe, J.M. (2000). Visual attention. In Blackwell's Handbook of Perception, E.B. Goldstein, ed. (Bla), pp. 272-310.
Neuronal Correlates of Consciousness, Consciousness, Attention, Arousal, Alertness, Visual Attention, Bottom-up attention, Exogenous Attention, Top-down Attention, Endogenous Attention, Biased Competition Model of Attention, Attentional Blink, Inattentional Blindness, Saliency Map, CODAM Model, Self-organization of brain function