Attention and consciousness

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Naotsugu Tsuchiya and Christof Koch (2008), Scholarpedia, 3(5):4173. doi:10.4249/scholarpedia.4173 revision #129964 [link to/cite this article]
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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 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).
  • 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 considerations

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.

Table 1. A four-fold classification of percepts and behaviors depending on whether or not top-down attention is necessary and whether or not these percepts and behaviors necessarily give rise to phenomenal consciousness. Different percepts and behaviors are grouped together according to these two, psychophysically defined, criteria.
May not give rise to consciousness Gives rise to consciousness
Top-down attention is not required Formation of afterimages

Rapid vision (< 120 msec)

Accommodation reflex

Pupillary reflex

Zombie behaviors


Iconic memory


Animal & gender detection in dual-tasks

Partial reportability

Top-down attention is required Priming


Processing of objects

Visual search


Working memory

Detection and discrimination of unexpected & unfamiliar stimuli

Full reportability

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.

How to manipulate and measure visual consciousness

Visual consciousness can be manipulated using a multitude of illusions, such as backward masking, the standing wave of invisibility, visual crowding, bistable figures, binocular rivalry, flash suppression, continuous flash suppression (Tsuchiya and Koch, 2005; Tsuchiya et al., 2006), motion-induced blindness and attentional blink (for a review see Kim and Blake, 2005). These techniques control the visibility of an object or a part thereof in both space and time. Yet how is visibility assayed? More generally, how can the degree of consciousness be probed?

Subjective measures

The most lenient criterion is to accept what subjects subsequently report verbally; e.g., “I never saw the face.” Though widely used (such as when obtaining reports immediately after an fMRI session), this method is unsatisfactory because unattended items or task-irrelevant (implicit) features of stimuli may be inaccessible in subsequent recognition or recall tasks (Sperling, 1960; Wolfe, 1999; Landman et al., 2003). A somewhat more stringent criterion for non-conscious processing is to ask subjects about their experience directly at the time the stimulus is processed. When subjects deny seeing a stimulus, the implication is that it was processed at a subjectively non-conscious level. Although many studies involving non-conscious states adopt this convention, the definition suffers from the possibility of criterion shifts: for the same subjective experience of visibility, some subjects may deny seeing a stimulus while others may report seeing it, because their criterion of what to count as “seen” differs (Kunimoto et al., 2001).

Objective measures

The strictest procedure is to demonstrate null sensitivity using an appropriate overt behavioral measure, that is, d’ = 0. For example, subjects can be given two alternative temporal intervals (or locations), each of which contains the stimulus equally often. If they are at chance in detecting/discriminating one from the other, they are (objectively) unaware of the stimulus (our use of “subjective” and “objective” here refers to the method used, not to the nature of the conscious experience, which is of course always subjective in terms of its phenomenology). Note that above-chance behavioral discrimination performance does not necessarily demonstrate conscious awareness, since patients with blindsight exhibit precisely such performance.

Objectively measuring subjectivity

However, such an objective definition does not directly reflect phenomenal experience, which is the central issue. By applying the objective measure of signal discriminability to one’s own judgment of whether the stimulus is seen or not, one can objectively measure subjectivity. That is, one can consider the discriminability (d’ or area under the ROC curve (A')) of one’s own experience. For this method, subjects first make a detection/discrimination judgment, then rate the confidence in their decision, where confidence is presumably based on whether their phenomenal experience was strongly, moderately, weakly or perhaps not at all in support of their decision. Defining ‘hit’ as proportion of high confidence ratings given that the decision was correct - p(high confidence | correct) - and ‘false alarm’ as the proportion of high confidence ratings given that the decision was incorrect - p(high confidence | incorrect) - one can calculate the discriminability (d’ or area under the curve) of those instances where phenomenal experience informed the decision from those where it did not. In signal detection theory, this is called Type 2 analysis (Galvin et al., 2003), and it has been applied to evaluation of above chance behavior in non-conscious perception (Kolb and Braun, 1995; Kunimoto et al., 2001).

Post-decision wagering

However, reflecting upon one’s own judgment may require substantial internal focus and such an act itself can modify conscious experience significantly (Maia and McClelland, 2004). With a recently proposed new method, post-decision wagering, this contamination due to introspection can be minimized (Kunimoto et al., 2001; Persaud et al., 2007). Following each response, subjects wager on their performance, betting either high or low. If the subject is confident that she saw the stimulus, reward maximization would presume that she would wager a higher amount than when she is unaware of the stimulus and is guessing.

Here, subjects’ awareness is gauged by the discriminability of their own judgment. This method proves to be easy and intuitive for subjects to use and very effective in reflecting one’s subjective aspects of consciousness while minimizing interference to the quality of the experience. Persaud and colleagues (Persaud et al., 2007) observed above-chance behaviors in blindsight patients, and in implicit learning and Iowa gambling tasks with normal subjects while demonstrating, using post-decision wagering, that there was little or no conscious access to the information that informed those behaviors.

Psychophysical tools to manipulate top-down attention

Top-down attention and consciousness are usually tightly coupled. To dissociate these two, experimental tools that manipulate either one independently in a specific manner with few side effects are called for.

There exist at least two forms of selective attention: stimulus-driven, bottom-up, saliency-mediated attention as well as task- and goal-dependent top-down attention. Previously neutral stimuli (such as text, or abstract images) can be associated with reward or punishment to acquire additional saliency. Biologically relevant stimuli may be preferred or disliked based on individual differences (e.g., snakes, spiders, and nude pictures).

A variety of techniques to manipulate these components of attention has been invented. It is not always easy to compare them, as each method interferes with attention at a different level of processing (Sperling and Dosher, 1986; VanRullen et al., 2004).


In Posner’s cueing paradigm, popular in the study of orienting (Posner et al., 1980), a target is preceded by an informative or a non-informative cue that appears at the target location or at fixation. Attentional effects are inferred in terms of reaction time and/or accuracy of target detection. Variants of the method demonstrated that an invisible cue can direct exogenous attention to a particular spatial location (McCormick, 1997; Kentridge et al., 2004; Rajimehr, 2004; Feng et al., 2006; Jiang et al., 2006; Sumner et al., 2006), clear support for the orienting of exogenous attention without the intervention of consciousness.

Visual search

In visual search, subjects need to find a target among distractors; reaction time is related to the number of distractors. When the search slope is steep, the search process is said to be serial, and when flat, parallel. The former is usually taken as the evidence of serial processing by top-down attention. However, the steep serial search may arise due to completely bottom-up factors (VanRullen et al., 2004). This exemplifies a case where dual-tasks and visual search methods may yield inconsistent results.

Figure 1: Dual-tasks paradigm. How performance of a secondary task in the periphery (empty red circle) is affected when a centrally presented attention-demanding task is performed simultaneously is studied with the aid of the dual-tasks paradigm.

Dual-task paradigm

Figure 2: Deciding whether or not a natural scene includes an animal can be done at the same time as the central task – here a demanding letter discrimination - (single task performance (y-axis) is the same as dual-task performance (x-axis)), while discriminating a red-green disk from a green-red one can’t be done when attention is engaged at the center. Modified from (Li et al., 2002). From (Koch and Tsuchiya, 2007) with permission.

The dual-tasks paradigm (Sperling and Dosher, 1986; Braun and Sagi, 1990; Braun and Julesz, 1998) manipulates top-down, focal attention without affecting bottom-up saliency: a central, attentionally-demanding discrimination task is present at the center of gaze, while a secondary stimulus is projected somewhere into the periphery (Figure 1). Subjects carry out either the central, the peripheral, or both tasks simultaneously while the scene and its layout remain the same.

Surprisingly, seemingly complex peripheral tasks can be done equally well under either single or dual-task condition (Li et al., 2002; Reddy et al., 2004; Reddy et al., 2006) (Figure 2, left), while other, computationally simpler tasks deteriorate when performed simultaneously with the central task (Figure 3, right). The dual-task paradigm quantifies what type of stimulus attributes can be signaled and possibly consciously perceived in the near absence of spatial attention (VanRullen et al., 2004).

Independent manipulation of top-down attention and visibility

Figure 3: (Left) An example of a bistable conscious percept (Rubin vase: two silhouettes versus a vase). (Right) It would be interesting to characterize the effect of withdrawing top-down attention from Rubin’s vase illusion by embedding this bistable percept into a dual-task experiment (Paffen et al., 2006; Pastukhov and Braun, 2007). From (Koch and Tsuchiya, 2007) with permission.

Most importantly, the dual-task paradigm can be combined with a multitude of visual illusions that render stimuli invisible, allowing the independent manipulation of top-down attention and consciousness (Figure 3), although such a full factorial analysis for many popular experiments awaits future work #Opposite Effects.

Caveat of dual-task paradigm

The inference of attentional requirements from dual-task performance demands caution. High proficiency in such tasks is only achieved after extensive training of many hours. Such an extended training phase renders the experience of the task quite different for trained subjects from what naïve subjects experience (Joseph et al., 1997; Braun, 1998).

Neurological disorders of attention & other attention-related visual illusions

Finally, there is a class of neurological conditions as well as visual illusions in normal subjects where stimuli become invisible because of impairments in the mechanisms of top-down or bottom-up attention. Hemineglect and extinction (Driver and Mattingley, 1998), attentional blink (Raymond et al., 1992; Chun and Potter, 1995), inattentional blindness (Mack and Rock, 1998), and change blindness (Simons and Rensink, 2005) are sometimes used as positive evidence for “without attention, no consciousness” (O'Regan and Noe, 2001). Although some attributes of the visual input need attentional amplification to rise to the level of consciousness, other aspects, such as the gist of the scene and its emotional content, are quite resistant to such attentional manipulations (Mack and Rock, 1998; Anderson and Phelps, 2001).

Bottom-up or exogenous components of attention

Exogenous cues are image-immanent features that transiently attract attention or eye gaze, independent of any particular task. Thus, if an object attribute (for example, flicker, motion, color, orientation, depth, or texture) differs significantly from its value in some neighborhood, the object will be salient.

This definition of bottom-up saliency has been implemented into a popular suite of neuromorphic vision algorithms that have at their core a topographic saliency map that encodes the saliency or conspicuity of locations in the visual field independent of the task (Itti and Koch, 2001) (see for a C++ implementation and for a Matlab toolbox implementation). Such algorithms account for a significant fraction of fixation eye movements (Parkhurst et al., 2002; Peters et al., 2005).

Candidates for such a map in the primate brain include the initial responses of neurons in the frontal eye field (FEF) and the lateral intraparietal sulcus (LIP) (Constantinidis and Steinmetz, 2005; Thompson and Bichot, 2005).

Processing without top-down attention and consciousness

Rapid visual categorization

Visual input can be classified very rapidly. As famously demonstrated by Thorpe and colleagues (Thorpe et al., 1996; Kirchner and Thorpe, 2006) around 120 msec following image onset, some brain processes begin to respond differentially to images containing one or more animals from pictures than contain none. At this speed, it is no surprise that subjects often respond without having consciously seen the image (VanRullen et al., 2001; VanRullen and Koch, 2003); consciousness for the image may come later or not at all.

Dual-task and dual-presentation paradigms support the idea that such discriminations can occur in the near-absence of focal, spatial attention (Li et al., 2002 #How to Manipulate Attention; Rousselet et al., 2002), (but see (Einhauser et al., 2007)) implying that purely feed-forward networks can support complex visual decision-making in the absence of both attention and consciousness (VanRullen et al., 2001; VanRullen and Koch, 2003). Indeed, this has now been formally shown in the context of a purely feed-forward computational model of the primate’s ventral visual system (Serre et al., 2007).

Animal experiments could substantiate this assertion. Imagine that all the cortico-cortical pathways from prefrontal cortex back to higher level visual cortex and from there on even further back to primary and secondary visual cortices could be transiently knocked out using a molecular silencing tool (without compromising feed-forward processing). That is, for a couple of hours, the brain of the monkey would only support feed-forward pathways. It is quite likely that such an animal could still carry out a previously learned simple discrimination task with the same level of performance as prior to the intervention (upper left quadrant in Table 1 Attention and Consciousness), without any top-down attention (since prefrontal cortex would have no means to modulate the processes in the visual brain), and possibly without conscious perception (although this latter would be difficult to demonstrate in an animal preparation).

Attention without consciousness

Processing of invisible stimulus requires top-down attention

In lateral masking (visual crowding), the orientation of a peripherally-presented grating is hidden from conscious sight but remains sufficiently potent to induce an orientation-dependent aftereffect (He et al., 1996). In this case, the orientation of the grating is not consciously accessible to the subject yet something is still seen at that location (that is, in this case only some attributes of the object are invisible). Montaser-Kouhsari and Rajimehr (2004) showed that an aftereffect induced by an invisible illusory contour required focal attention to the aftereffect-inducing object, even though the attribute of that object at the center of attention was invisible. Naccache and colleagues (Naccache et al., 2002) elicited priming for invisible words (suppressed by a combination of forward and backward masking) but only if the subject was attending to the invisible prime-target pair at the right timing; without attention, the same word failed to elicit priming. In both cases, (spatial or temporal) attentional selection was necessary to obtain the aftereffect or priming even though the relevant attribute (orientation, letter identity) was invisible.

Invisible stimulus attracts spatial attention

Male/female nudes attracted attention and induced involuntary eye movements when they were rendered completely invisible by continuous flash suppression (Feng et al., 2006; Jiang et al., 2006). Interestingly, in heterosexuals, these effects were only apparent for nudes of the opposite sex (see also (McCormick, 1997; Rajimehr, 2004; Sumner et al., 2006)). Note that by themselves (i.e. without the mask), these stimuli are clearly visible.

Likewise, the blindsight patient GY has the usual reaction-time advantages for the detection of targets in his blind visual field when attentionally cued, even when the cues are located in his blind field and are therefore invisible to him (Kentridge et al., 1999a, 1999b, 2004).

Feature-based attention can spread to invisible target

Feature-based attention can spread to invisible stimuli (Melcher et al., 2005; Kanai et al., 2006; Schmidt & Schmidt, 2010). Indeed, when searching for an object in a cluttered scene (e.g., keys in a messy room), attention is paid to an invisible object and its associated features.

Imaging attention to invisible stimulus

Figure 4: Object images presented to the periphery of the non-dominant eye are rendered invisible by continuous flash suppression (CFS) at the corresponding and two other locations in the dominant eye. Modified from (Bahrami et al., 2007) with permission.
Figure 5: At the center of the display, subjects performed a rapid serial visual presentation (RSVP) task. In the low load condition, a target letter "T" of white or black color must be detected. In the high load condition, either a "white N" or a "black Z" must be detected, embedded within a stream of letters. Modified from (Bahrami et al., 2007) with permission.
Figure 6: fMRI response amplitude in primary visual cortex to invisible objects is modulated by the attention load on the central task. Modified from (Bahrami et al., 2007) with permission.

Spatial attention modulates the processing of invisible stimuli

More direct evidence comes from a recent fMRI study by Bahrami and colleagues (Bahrami et al., 2007), demonstrating that the processing of objects hidden from sight (with d’ = 0) via continuous flash suppression (Tsuchiya and Koch, 2005) depended on the availability of spatial attention (Figure 4,Figure 5,Figure 6). They varied the load of the central task in a dual-task design #How to manipulate attention. The hemodynamic blood-oxygen-level-dependent contrast (BOLD) response to the invisible objects in primary visual cortex, V1, was stronger when the central task was easy, that is, when spatial attention was available for processing the invisible, peripheral stimulus than when the central task was hard and more attentional resources were drawn to it. In other words, available attentional capacity modulated the fMRI response to an invisible stimulus.

Subthreshold motion is more distracting than suprathreshold motion

Figure 7: Subthreshold motion is more distracting than suprathreshold motion. (A) Subjects had to detect two target digits embedded in a rapid stream of letters (RSVP). Surrounding the letter stream, moving random dots of variable coherence were presented. (B) The central RSVP task performance (on the y-axis) dropped when the peripheral motion was below threshold for detecting coherent motion. Note that stronger motion (i.e., higher coherency) did not interfere with the central task. (C) Activation of the cortical region MT+ was highest when peripheral motion was below threshold. (D) Activation of lateral prefrontal cortex (LPFC), which inhibits activity in MT+, was higher when motion coherence was above the threshold. Subthreshold motion does not activate LPFC. Modified from (Tsushima et al., 2006) with permission.

An even more paradoxical effect - that invisible stimuli can be more distracting than visible ones – was discovered by Tsushima and colleagues (Tsushima et al., 2006) (Figure 7A). In this study, subjects had to detect foveally-placed targets in a stream of characters – a rapid serial visual presentation (RSVP) task - surrounded by an annulus of moving dots. The fraction of dots moving coherently in one direction – the motion coherence - was varied from 0% (truly random dot motion) to 50% (half of the dots move in the same direction). When the central task was combined with the task-irrelevant surround motion, the central performance dropped when the coherent motion was perceptually below threshold (say at 5%, where the cloud of dots was not perceived to move coherently) compared to when the motion coherence was 0% or above threshold (e.g., 20%) (Figure 7B). This counterintuitive finding was explained by the parallel fMRI study in which the authors looked at BOLD activity in area MT+, which reflects the degree of distraction by motion stimuli, and in the lateral prefrontal cortex (LPFC), which provides an attentional suppression signal to MT+ (Figure 7CD). Compatible with the behavioral findings, invisible motion did not elicit activity in the LPFC, resulting in higher distractor-related activity in MT+. On the other hand, visible motion evoked a stronger LPFC signal but a weaker MT+ one. The authors hypothesize that invisible motion activates MT+, impairing performance, but not the LPFC, which fails to inhibit MT+; thereby stimuli that are not consciously perceived can escape inhibitory control, a phenomenon more familiar from psychoanalysis than from sensory psychology.

Consciousness in the Absence of Attention

Perception in the near absence of attention in a dual-task paradigm

In a dual-task paradigm, the subject’s attention is drawn to a demanding central task, while at the same time a secondary stimulus is flashed somewhere in the periphery (see #How to Manipulate Attention). Using the identical retinal layout, the subject either performs the central task, or the peripheral task, or both simultaneously (Sperling and Dosher, 1986; Braun and Sagi, 1990; Braun and Julesz, 1998).

Natural scenes, gender and identity of faces can be perceived in the near absence of attention

With focal attention busy at the center, the subject can still distinguish a natural scene containing an animal (or a vehicle) from one that does not include an animal (or a vehicle) while being unable to distinguish a red-green bisected disk from a green-red one (Li et al., 2002).

Likewise, subjects can tell male from female faces or even distinguish a famous from a non-famous face (Reddy et al., 2004; Reddy et al., 2006), but are frustrated by tasks that are computationally much simpler (e.g. discriminating a rotated letter ‘L’ from a rotated ‘ T’). This is quite remarkable. Thus, although we cannot be sure that observers do not deploy some limited amount of top-down attention in these dual-task experiments that require training and concentration (that is, high arousal), it remains true that subjects can perform certain discriminations but not others in the near-absence of top-down attention. And they are not guessing. They can be quite confident of their choices and “see”, albeit often indistinctly, what they can discriminate.

Can we study perception in the complete absence of attention?

Can perception be studied in the complete absence of attention? This seems possible if, in the above-mentioned dual-task paradigm, subjects must perform a very demanding central task without needing to monitor the periphery. Such an experiment has been conducted to investigate the effects of attention on bistable perception (Pastukhov and Braun, 2007). A fundamental question in the perception of ambiguous figures is why they switch spontaneously despite constant retinal input. One influential theory posits that top-down attention triggers perceptual transitions (James, 1890). To test this, Pastukhov and Braun (Pastukhov and Braun, 2007) examined whether unattended and unreported bistable motion stimuli continued to switch. Consistent with other studies (Paffen et al., 2006), they found that drawing attention away from the peripheral ambiguous percept slowed down the dominance periods but their statistical variability remained; even a complete withdrawal of attention failed to abolish transitions. In other words, top-down attention is not necessary for switches in the content of visual consciousness. Similar dual-task experiments can likewise be applied as a strict test for the necessity of top-down attention in learning, memory, adaptation, and other cognitive functions.

Can top-down attention be opposed to consciousness?

Attention enhances neuronal processing, yet, it sometimes impairs performance of the task

Attention and its neuronal correlate can be understood in the context of selection and biased competition (Desimone and Duncan, 1995): attention acts as a winner-take-all, enhancing one coalition of neurons (representing the attended object) at the expenses of others (non-attended stimuli) (Lee et al., 1999). Paradoxically though, reducing attention can enhance awareness (Olivers and Nieuwenhuis, 2005) and certain behaviors (Reber, 1976; Wong and Weisstein, 1983; Yeshurun and Carrasco, 1998; Beilock et al., 2002).

In the Rubin’s ambiguous face/vase figure (see #How to manipulate and measure visual consciousness), the percept switches between two faces seen in profile and a vase. Discrimination of high-frequency stimuli, such as a line, presented on the face area when it is perceived as the figure is better than when it is perceived as the ground. If a blurred, low spatial frequency stimulus is presented in this region, it is better discriminated when the face is perceived as the ground. Something similar occurs when the target stimulus is presented on the vase area. In other words, a low spatial frequency stimulus is better detected on the unattended ground (Wong and Weisstein, 1982, 1983). Likewise, Yeshurun and Carrasco (Yeshurun and Carrasco, 1998) showed that attention impairs the performance of texture segregation when the subject is required to process low spatial frequency information.

Opposing effects of attention and consciousness

Note that a complete orthogonal manipulation of attention and consciousness has not been performed in any of the following examples.

Figure 8: When an adaptor is attended, the associated afterimage becomes weaker and appears later (blue gradation (Lou, 2001; Suzuki and Grabowecky, 2003; Wede and Francis, 2007)). Yet paradoxically, when the adaptor is rendered invisible, its afterimage is substantially weakened (pink gradation (Gilroy and Blake, 2005; Tsuchiya and Koch, 2005)).

Attention weakens afterimages, visibility enhances afteriamges

Consider the formation of afterimages (Figure 8). If an item is attended during adaptation, the intensity of the subsequent afterimage becomes weaker and its duration shorter compared to an unattended item(Lou, 2001; Suzuki and Grabowecky, 2003; Wede and Francis, 2007). If, however, the image is suppressed during adaptation, the afterimage is substantially weakened (Gilroy and Blake, 2005; Tsuchiya and Koch, 2005). Thus, focal attention and consciousness have opposing effects (Tsuchiya, 2006).

Table 1. Opposing effects of attention and consciousness for perception of afterimages.
Adaptor invisible Adoptor visible
Inattention to adaptor Weaker afterimages (?) Strong afterimages (++)
Attention to adaptor Weakest afterimages (--) Weak afterimages (+)

Speed of perceptual switches is modulated by attention and visibility of bistable stimuli

Figure 9: When attention is withdrawn from a visible bistable/rivalry target (here, a Necker cube), the rate of perceptual flips slows down (blue gradation (Paffen et al., 2006; Pastukhov and Braun, 2007)). When the target stimulus is intermittently presented (stabilization), the opposite may occur; withdrawing attention from the target causes less stabilization, that is, perceptual flips speed up (pink gradation (Kanai and Verstraten, 2006)).

Next, consider freezing in bistable perception (Figure 9) (Orbach et al., 1963; Leopold et al., 2002). During continuous viewing of an ambiguous stimulus, the percept flips stochastically. Yet if the bistable figure is briefly removed (leaving the display empty), the dominant percept at the start of the new display is the same as the one when the percept disappeared. This freezing is disrupted if spatial attention is distracted from the empty display(Kanai and Verstraten, 2006), most likely by disrupting memory buildup. This can be thought of as speeding up perceptual switching. Yet distracting focal attention during bistable perception slows down the switching rate (Paffen et al., 2006; Pastukhov and Braun, 2007). In other words, withdrawing focal attention when the stimulus is invisible, not consciously seen, disrupts perceptual freezing, while withdrawing attention when the stimulus is visible slows down switching.

Table 2. Opposing effects of attention and consciousness for switches of bistable figure.
Rivalry invisible Rivalry visible
Inattention to rivalry Faster switches,

less freezing (-)

Slower switches, more freezing (+)
Attention to rivalry Slower switches, more freezing (+) Faster switches, less freezing (-)

Opposing effects of attention and consciousness in decision making

Figure 10: When confronted with a complex decision where many items must be remembered (i.e., the list is invisible), distracting subjects from the decision making process improves performance (pink gradation (Dijksterhuis et al., 2006)). The last figure is modified from (Dijksterhuis et al., 2006). From (Koch and Tsuchiya, 2007) with permission.

Finally, consider complex decision-making (Figure 10). The Dijksterhuis’ (Dijksterhuis et al., 2006) study consisted of three phases: examination of items, deliberation, and decision. One of either 4 or 12 properties for each of 4 cars was shown one at a time during the examination phase. Subjects then deliberated for several minutes without the attributes being visible (that is, subjects had to remember them; this can be thought of as an ‘invisible’ condition) before making a purchasing decision. Dijksterhuis and colleagues manipulated whether or not subjects were cognitively engaged during the deliberation period. They concluded that when faced with working memory overload, an explicit strategy based on deliberate and rational thought leads to poor decision making for a complex decision, while distracting subjects when they decide which car to buy greatly increased the probability of a correct choice. We surmise that if the list of items would have been present throughout the decision-making period – thereby reducing working memory load – an attentional distracting task would degrade purchasing performance. For a related finding in implicit learning, see (Reber, 1976).

Table 3. Opposing effects of attention and consciousness for decision making.
List invisible List visible
Inattention to decision making Better decision (+) Worse decision (?)
Attention to decision making Chance performance (-) Best decision (?)

Other related issues

  • Phenomenal consciousness without cognitive access by Ned Block
  • Iconic memory
  • Stanislas Daheane’s tripartite ontology of consciousness


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Internal references

  • Ernst Niebur (2007) Saliency map. Scholarpedia, 2(8):2675.

External links

See also

Alertness, Arousal, Attention, Attentional Blink, Biased Competition Model of Attention, Bottom-up attention, CODAM Model, Change blindness, Consciousness, Cueing, Endogenous Attention, Exogenous Attention, Inattentional Blindness, Inattentional blindness, Neuronal Correlates of Consciousness, Post-decision wagering, ROC curve, Saliency Map, Self-organization of brain function, Signal Detection Theory, Top-down Attention, Visual Attention, Visual Search

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