Measuring consciousness: relating behavioural and neurophysiological approaches
Authors: Anil K. Seth, Zoltán Dienes, Axel Cleeremans, Morten Overgaard, Luiz Pessoa
Journal: Trends in Cognitive Sciences (2008)

The resurgent science of consciousness has been accompanied by a recent emphasis on the problem of measurement. Having dependable measures of consciousness is essential both for mapping experimental evidence to theory and for designing perspicuous experiments. Here, we review a series of behavioural and brain-based measures, assessing their ability to track graded consciousness and clarifying how they relate to each other by showing what theories are presupposed by each. We identify possible and actual conflicts among measures that can stimulate new experiments, and we conclude that measures must prove themselves by iteratively building knowledge in the context of theoretical frameworks. Advances in measuring consciousness have implications for basic cognitive neuroscience, for comparative studies of consciousness and for clinical applications. The resurgent science of consciousness has been accompanied by a recent emphasis on the problem of measurement. Having dependable measures of consciousness is essential both for mapping experimental evidence to theory and for designing perspicuous experiments. Here, we review a series of behavioural and brain-based measures, assessing their ability to track graded consciousness and clarifying how they relate to each other by showing what theories are presupposed by each. We identify possible and actual conflicts among measures that can stimulate new experiments, and we conclude that measures must prove themselves by iteratively building knowledge in the context of theoretical frameworks. Advances in measuring consciousness have implications for basic cognitive neuroscience, for comparative studies of consciousness and for clinical applications. The problem of measurementHow can we measure whether and to what extent a particular sensory, motor or cognitive event is consciously experienced? Such measurements provide the essential data on which the current and future science of consciousness depends, yet there is little consensus on how they should be made. The problem of measuring consciousness differs from the problem of identifying unconscious processing. For instance, in subliminal perception experiments it is desirable to know whether or not a stimulus has been consciously perceived, and in implicit learning paradigms it is interesting to know whether the relationships between different consciously represented stimuli are unconsciously inferred. Measuring consciousness, however, requires saying something about conscious level (Glossary) and conscious content beyond the zero-point of unconsciousness.Here, we review current approaches for measuring consciousness, covering both behavioural measures and measures based on neurophysiological data. We outline a variety of broad theoretical positions before describing a range of measures in the context of these theories, emphasizing recent contributions. We find that potential and actual conflicts among measures suggest new experiments (Table 1); we also assess how different measures can track the graded nature of conscious experience (Table 2). We conclude that it is only by behaving sensibly in a theoretical context that proposed measures pick themselves up by their bootstraps, both validating themselves as measures of what they say they measure and the theories involved.Table 1Conflicts between measuresaRows indicate a measure finds the content conscious and columns indicate the measure finds the content unconscious. Entries reflect a scale according to which a particular conflict is (i) experimentally noted, (ii) not yet shown but entirely possible, (iii) experimentally open, (iv) possible but not tested, (v) unlikely given current evidence and/or theory, or (vi) not possible.Content unconscious according to:ObjectiveStrategic controlSubjectiveWageringContent conscious according to:ObjectiveUnconscious knowledge by Jacoby's process dissociation procedure is ipso facto conscious by objective measures (e.g. Refs 60Destrebecqz A. Cleeremans A. Can sequence learning be implicit? New evidence with the process dissociation procedure.Psychon. Bull. Rev. 2001; 8: 343-350Crossref PubMed Scopus (379) Google Scholar, 72Debner J.A. Jacoby L.L. Unconscious perception: attention, awareness, and control.J. Exp. Psychol. Learn. Mem. Cogn. 1994; 20: 304-317Crossref PubMed Scopus (204) Google Scholar)In both subliminal perception and implicit learning, subjects often pass objective tasks while claiming to have no knowledge or showing no relation between confidence and accuracy (e.g. 22Overgaard M. et al.Is conscious perception gradual or dichotomous? A comparison of report methodologies during a visual task.Conscious. Cogn. 2006; 15: 700-708Crossref PubMed Scopus (208) Google Scholar, 73Cheesman J. Merikle P.M. Priming with and without awareness.Percept. Psychophys. 1984; 36: 387-395Crossref PubMed Scopus (348) Google Scholar, 74Dienes Z. Longuet-Higgins H.C. Can musical transformations be implicitly learned?.Cogn. Sci. 2004; 28: 531-558Crossref Google Scholar, 75Szczepanowski R. Pessoa L. Fear perception: can objective and subjective awareness measures be dissociated?.J. Vis. 2007; Vol. 4 (10): 7Google Scholar)Shown in blindsight and in the Iowa gambling task by 34Persaud N. et al.Post-decision wagering objectively measures awareness.Nat. Neurosci. 2007; 10: 257-261Crossref PubMed Scopus (296) Google ScholarStrategic controlNot possibleSubjects can control which grammar to employ while claiming to be guessing 32Dienes Z. et al.Unconscious knowledge of artificial grammars is applied strategically.J. Exp. Psychol. Learn. Mem. Cogn. 1995; 21: 1322-1338Crossref Scopus (231) Google Scholar and hypnotized subjects can engage in strategic control while reporting no awareness 76Dienes Z. Perner J. The cold control theory of hypnosis.in: Jamieson G. Hypnosis and Conscious States: The Cognitive Neuroscience Perspective. Oxford University Press, 2007: 293-314Google ScholarNot yet shown but entirely possible (see below)SubjectiveNot possibleShown in Stroop effects – a person can report the word's meaning but cannot control its rapid useAs yet only shown in our unpublished work – a person can report awareness but still wager indiscriminatelyWageringNot possibleNot yet shown but entirely possible (see Box 2)Not yet shown but entirely possible (see Box 2)Widespread activationCognitive control system, including prefrontal cortex, activated by objectively invisible stimuli 56Lau H.C. Passingham R.E. Unconscious activation of the cognitive control system in the human prefrontal cortex.J. Neurosci. 2007; 27: 5805-5811Crossref PubMed Scopus (255) Google ScholarLikely for Stroop with clearly shown words 44Dehaene S. et al.Cerebral mechanisms of word masking and unconscious repetition priming.Nat. Neurosci. 2001; 4: 752-758Crossref PubMed Scopus (947) Google ScholarShown in a 'relative blindsight' paradigm 31Lau H.C. Passingham R.E. Relative blindsight in normal observers and the neural correlate of visual consciousness.Proc. Natl. Acad. Sci. U. S. A. 2006; 103: 18763-18768Crossref PubMed Scopus (314) Google ScholarLikely given the results with verbal subjective measures, but not yet testedSynchronyγ synchrony persists during non-REM sleep and under anaesthesia 53Vanderwolf C.H. Are neocortical gamma waves related to consciousness?.Brain Res. 2000; 855: 217-224Crossref PubMed Scopus (76) Google ScholarAs leftAs left, also, similar levels of γ synchrony are observed during non-REM and during (reportable) REM sleep 52Bullock T.H. et al.Temporal fluctuations in coherence of brain waves.Proc. Natl. Acad. Sci. U. S. A. 1995; 92: 11568-11572Crossref PubMed Scopus (125) Google ScholarAs leftComplexity measuresPossible in theory (but see Φ). Not tested in practiceAs leftAs leftAs leftContent unconscious according to:Widespread activationSynchronyComplexity measuresContent conscious according to:ObjectiveLocal neuronal activity can support discriminatory behaviour in many non-conscious organisms (e.g. nematodes and worms). In humans, at least sensory and motor cortices need to be activeUnlikely given current evidenceNot yet tested but entirely possibleStrategic controlUnlikely: strategic control probably requires activation in both perceptual and frontal regionsAs abovePossible but not testedSubjectiveAs aboveAs aboveAs aboveWageringAs aboveAs aboveAs aboveWidespread activationExperimentally open. Some studies show increased long-range synchrony accompanying conscious access 48Melloni L. et al.Synchronization of neural activity across cortical areas correlates with conscious perception.J. Neurosci. 2007; 27: 2858-2865Crossref PubMed Scopus (507) Google ScholarPossible in theory. Not tested in practiceSynchronyGamma synchrony is often localized 47Palva S. et al.Early neural correlates of conscious somatosensory perception.J. Neurosci. 2005; 25: 5248-5258Crossref PubMed Scopus (214) Google ScholarAs aboveComplexity measuresHigh neural complexity (or Φ, or cd) probably requires widespread activity: all else being equal, larger networks will give rise to higher complexity values 17Seth A.K. et al.Theories and measures of consciousness: an extended framework.Proc. Natl. Acad. Sci. U. S. A. 2006; 103: 10799-10804Crossref PubMed Scopus (194) Google ScholarPossible in theory. Not tested in practicea Rows indicate a measure finds the content conscious and columns indicate the measure finds the content unconscious. Entries reflect a scale according to which a particular conflict is (i) experimentally noted, (ii) not yet shown but entirely possible, (iii) experimentally open, (iv) possible but not tested, (v) unlikely given current evidence and/or theory, or (vi) not possible. Open table in a new tab Table 2Sensitivity to graded consciousnessaConscious level can be graded on a scale from coma to full wakefulness, and conscious contents can also be graded (e.g. fringe consciousness and Ganzfeld experiences). This table indicates how different measures are able to track graded consciousness, as well as their primary theoretical affiliation.TypeMeasurePrimary theoretical affiliationSensitivity to graded conscious levelSensitivity to graded conscious contentBehaviouralObjectiveDiscrimination behaviourWDTNone (either an organism is sufficiently conscious to show choice behaviour, or it is not)The d′ value in SDT might index graded consciousness, though typically any d′ > zero is taken to imply full consciousness 1Dulany D.E. Consciousness in the explicit (deliberative) and implicit (evocative).in: Cohen J. Schooler J. Scientific Approaches to Consciousness. Lawrence Erlbaum Associates, 1997: 179-211Google ScholarObjectiveStrategic controlIntegration theoryNone (see above)None so far. Various equations developed assume that a content is either clearly conscious or unconscious (e.g. Ref. 9Jacoby L. A process dissociation framework: separating automatic from intentional uses of memory.J. Mem. Lang. 1991; 30: 513-541Crossref Scopus (2901) Google Scholar)SubjectiveIntrospective reportHOTPoor and indirect; poor verbal coherence might indicate low conscious levelIntrospective reports are explicitly highly sensitive to conscious content and can indicate close mismatches between observed and reported statesSubjectiveConfidence ratingsHOTPoor and indirect; confidence might diminish with conscious levelConfidence can indicate degrees of higher-order beliefSubjectivePDWHOTPoor and indirect though various continuous measures can be used 35Ruffman T. et al.Does eye gaze indicate knowledge of false belief?.J. Exp. Child Psychol. 2001; 80: 201-224Crossref PubMed Scopus (151) Google ScholarGambling measures can indicate degrees of higher order belief (see Box 2)Objective and subjectiveGlasgow coma scaleNoneHighNoneNeuropyhsiologicalEEGBispectral indexNoneHighNoneEEG/MEGEarly ERP ('awareness negativity' 77Wilenius M.E. Revonsuo A.T. Timing of the earliest ERP correlate of visual awareness.Psychophysiology. 2007; 44: 703-710Crossref PubMed Scopus (37) Google Scholar)Localized integration 14Lamme V.A. Towards a true neural stance on consciousness.Trends Cogn. Sci. 2006; 10: 494-501Abstract Full Text Full Text PDF PubMed Scopus (586) Google Scholar, 39Koivisto M. et al.Independence of visual awareness from the scope of attention: an electrophysiological study.Cereb. Cortex. 2006; 16: 415-424Crossref PubMed Scopus (91) Google ScholarMost ERPs are attenuated by sleep and low arousal, but yet not directly tested for awareness negativitySome. Early ERPs are delayed for low-contrast stimuli 77Wilenius M.E. Revonsuo A.T. Timing of the earliest ERP correlate of visual awareness.Psychophysiology. 2007; 44: 703-710Crossref PubMed Scopus (37) Google ScholarEEG/MEGLate ERP (P300)Global Integration 40Del Cul A. et al.Brain dynamics underlying the nonlinear threshold for access to consciousness.PLoS Biol. 2007; 5: e260Crossref PubMed Scopus (504) Google ScholarP300 can be elicited during sleep though with different profile 78Colrain I.M. Campbell K.B. The use of evoked potentials in sleep research.Sleep Med. Rev. 2007; 11: 277-293Abstract Full Text Full Text PDF PubMed Scopus (98) Google ScholarLow. P300 dichotomously characterizes 'seen' versus 'not seen' trials 40Del Cul A. et al.Brain dynamics underlying the nonlinear threshold for access to consciousness.PLoS Biol. 2007; 5: e260Crossref PubMed Scopus (504) Google ScholarGeneral neuroimagingWidespread activationIntegrationImaging of consciousness-impaired patients can distinguish different conscious levels 45Laureys S. The neural correlate of (un)awareness: lessons from the vegetative state.Trends Cogn. Sci. 2005; 9: 556-559Abstract Full Text Full Text PDF PubMed Scopus (491) Google ScholarLow. Access to global workspace is usually considered all-or-none 10Dehaene S. et al.A neuronal network model linking subjective reports and objective physiological data during conscious perception.Proc. Natl. Acad. Sci. U. S. A. 2003; 100: 8520-8525Crossref PubMed Scopus (538) Google ScholarSynchronyInduced γ activityIntegration (local and/or global)Synchrony is present even in non-REM sleep 53Vanderwolf C.H. Are neocortical gamma waves related to consciousness?.Brain Res. 2000; 855: 217-224Crossref PubMed Scopus (76) Google ScholarNot tested (to our knowledge)SynchronySSVEP (frequency 'tag')Global integrationAuditory frequency tag is modulated by arousal level 79Griskova I. et al.The amplitude and phase precision of 40 Hz auditory steady-state response depend on the level of arousal.Exp. Brain Res. 2007; 183: 133-138Crossref PubMed Scopus (43) Google ScholarNot tested (to our knowledge)ComplexityNeural complexityIntegrationHigh (in principle but not yet shown)LowComplexityInformation integration (Φ)IntegrationHigh (in principle but not yet shown)Some (in principle Φ can gauge conscious contents)ComplexityCausal densityIntegrationHigh (in principle; shown only in our own unpublished work)Possibly revealed by causal interaction patterns but not yet showna Conscious level can be graded on a scale from coma to full wakefulness, and conscious contents can also be graded (e.g. fringe consciousness and Ganzfeld experiences). This table indicates how different measures are able to track graded consciousness, as well as their primary theoretical affiliation. Open table in a new tab Theories of consciousnessWorldly discrimination theoryPerhaps the simplest theory that still impacts the experimental literature is that any mental state that can express its content in behaviour is conscious; thus, a person shows they are consciously aware of a feature in the world when they can discriminate it with choice behaviour [1Dulany D.E. Consciousness in the explicit (deliberative) and implicit (evocative).in: Cohen J. Schooler J. Scientific Approaches to Consciousness. Lawrence Erlbaum Associates, 1997: 179-211Google Scholar, 2Eriksen C.W. Discrimination and learning without awareness: a methodological survey and evaluation.Pyschol. Rev. 1960; 67: 279-300Crossref PubMed Scopus (283) Google Scholar]. This theory often makes use of signal-detection theory (SDT), a statistical framework for quantifying the discriminability of a stimulus [3Green D.M. Swets J.A. Signal Detection Theory. Wiley, 1966Google Scholar]. SDT itself is mute on the subject of consciousness and can, thus, be combined with different theories. The combination of SDT with the worldly discrimination theory (WDT) asserts that continuous information available for discriminations is necessarily the content of conscious mental states. This theory captures one property of conscious knowledge, namely that it enables choice behaviour. However, rightly or wrongly, it does not respect other properties that are associated with consciousness. For example, according to this theory blindsight patients see consciously because forced-choice discrimination is the result by which we infer that they can see at all. However, two properties of blindsight indicate intuitively that the seeing is not conscious [4Weiskrantz L. Blindsight: A Case Study and Implications. Oxford University Press, 1998Google Scholar]. First, blindsight patients do not spontaneously attempt to use the information practically or inferentially. Second, blindsight patients themselves think they cannot see.Integration theoriesOther theories attempt to locate a divide between conscious and unconscious processes that respect one or both of the intuitions just mentioned. According to integration theories, conscious contents are widely available to many cognitive and/or neural processes. This core idea has been expressed in various ways. In philosophy, it has been described as inferential promiscuity [5Stich S. Beliefs and subdoxastic states.Philos. Sci. 1978; 45: 499-518Crossref Google Scholar], fame in the brain [6Dennett D. Sweet Dreams: Philosophical Obstacles to a Science of Consciousness. MIT Press, 2005Google Scholar], the unified field theory [7Searle J. Mind: A Brief Introduction. Oxford University Press, 2004Google Scholar] and global access; in cognitive psychology as broadcast within a global workspace [8Baars B.J. A Cognitive Theory of Consciousness. Cambridge University Press, 1988Google Scholar] and in a more constrained way as the process dissociation framework [9Jacoby L. A process dissociation framework: separating automatic from intentional uses of memory.J. Mem. Lang. 1991; 30: 513-541Crossref Scopus (2901) Google Scholar]; and in neuroscience as a neuronal global workspace [10Dehaene S. et al.A neuronal network model linking subjective reports and objective physiological data during conscious perception.Proc. Natl. Acad. Sci. U. S. A. 2003; 100: 8520-8525Crossref PubMed Scopus (538) Google Scholar], a dynamic core [11Tononi G. Edelman G.M. Consciousness and complexity.Science. 1998; 282: 1846-1851Crossref PubMed Scopus (1045) Google Scholar, 12Edelman G.M. Naturalizing consciousness: a theoretical framework.Proc. Natl. Acad. Sci. U. S. A. 2003; 100: 5520-5524Crossref PubMed Scopus (342) Google Scholar], integrated information [13Tononi G. An information integration theory of consciousness.BMC Neurosci. 2004; 5: 42Crossref PubMed Scopus (962) Google Scholar], and, in a more constrained way, as locally recurrent activity [14Lamme V.A. Towards a true neural stance on consciousness.Trends Cogn. Sci. 2006; 10: 494-501Abstract Full Text Full Text PDF PubMed Scopus (586) Google Scholar] or neuronal synchrony [15Engel A.K. et al.Temporal binding, binocular rivalry, and consciousness.Conscious. Cogn. 1999; 8: 128-151Crossref PubMed Scopus (271) Google Scholar, 16Crick F. Koch C. Towards a neurobiological theory of consciousness.Semin. Neurosci. 1990; 2: 263-275Crossref Scopus (15) Google Scholar]. The neuroscience theories in particular have given rise to several putative measures that have been used to quantify simultaneous integration and differentiation in neural dynamics on the basis that conscious experience is also simultaneously integrated and differentiated [17Seth A.K. et al.Theories and measures of consciousness: an extended framework.Proc. Natl. Acad. Sci. U. S. A. 2006; 103: 10799-10804Crossref PubMed Scopus (194) Google Scholar]. According to these theories a mental state is conscious if it provides a sufficiently informative discrimination among a large repertoire of possible states, in which successful discrimination requires both differentiation and integration [11Tononi G. Edelman G.M. Consciousness and complexity.Science. 1998; 282: 1846-1851Crossref PubMed Scopus (1045) Google Scholar, 12Edelman G.M. Naturalizing consciousness: a theoretical framework.Proc. Natl. Acad. Sci. U. S. A. 2003; 100: 5520-5524Crossref PubMed Scopus (342) Google Scholar].Higher-order thought theoriesAccording to higher-order thought (HOT) theories, a mental state is conscious when a person is actually aware [18Rosenthal D.M. Consciousness and Mind. Clarendon, 2005Google Scholar] or disposed toward being aware [19Carruthers P. Language, Thought, and Consciousness. Cambridge University Press, 1996Crossref Google Scholar] of being in that state. Theories differ according to whether awareness of the mental state is achieved by perceiving it [20Lycan W.G. The superiority of HOP over HOT.in: Gennaro R.J. Higher-Order Theories of Consciousness: An Anthology. John-Benjamins, 2004: 93-113Google Scholar] or thinking about it [18Rosenthal D.M. Consciousness and Mind. Clarendon, 2005Google Scholar]. HOT theories differ from WDTs in that it is the ability of a person to discern their mental state, rather than the state the world is in, which determines whether a mental state is conscious. In the context of the SDT, HOT theories is associated either with the criterion of standard SDT or with the second level of discrimination – discriminating not the world (as in the WDT) but the accuracy of our responses [21Dienes Z. Subjective measures of unconscious knowledge.in: Banerjee R. Chakrabarti C. Models of Brain an Mind: Physical, Computational and Psychological Approaches. Elsevier, 2008: 49-64Google Scholar].Because of their differing theoretical affiliations, measures of consciousness can, and do, conflict with each other, as detailed in Table 1. Also, measures of consciousness not only should distinguish between conscious and unconscious processing but also indicate the degree to which an organism or a mental state is conscious [22Overgaard M. et al.Is conscious perception gradual or dichotomous? A comparison of report methodologies during a visual task.Conscious. Cogn. 2006; 15: 700-708Crossref PubMed Scopus (208) Google Scholar, 23Cleeremans A. Conscious and unconscious cognition: a graded, dynamic, perspective.in: Progress in Psychological Science Around the World. Vol. 1. Psychology Press, 2006: 401-418Google Scholar]. Sensitivity to graded consciousness is described in Table 2. All theories described so far, with the exception of some neural integration theories [11Tononi G. Edelman G.M. Consciousness and complexity.Science. 1998; 282: 1846-1851Crossref PubMed Scopus (1045) Google Scholar, 13Tononi G. An information integration theory of consciousness.BMC Neurosci. 2004; 5: 42Crossref PubMed Scopus (962) Google Scholar], describe conditions for asserting whether a particular mental state is conscious (conscious content). They do not generally pertain to whether an organism is conscious or unconscious at a particular time (conscious level). As we will see, measures of consciousness can, and do, address both of these issues.Behavioural measures'Objective measures' assume the ability to choose accurately under forced choice conditions as indicating a conscious mental state [24Pessoa L. et al.Target visibility and visual awareness modulate amygdala responses to fearful faces.Cereb. Cortex. 2006; 16: 366-375Crossref PubMed Scopus (210) Google Scholar, 25Smyth A. Shanks D.R. Awareness in contextual cueing with extended and concurrent explicit tests.Mem. Cognit. 2008; 36: 403-415Crossref PubMed Scopus (109) Google Scholar]. For example being able to pick which item might come next indicates conscious knowledge of regularities in sequences. Conversely, knowledge is unconscious if a distinction in the world expresses itself only in non-intentional characteristics of behaviour (such as its speed) or in galvanic skin response, functional magnetic resonance imaging (fMRI) or other physiological characteristics not expressed in behaviour at all [26Naccache L. et al.A direct intracranial record of emotions evoked by subliminal words.Proc. Natl. Acad. Sci. U. S. A. 2005; 102: 7713-7717Crossref PubMed Scopus (149) Google Scholar]. That is, knowledge is unconscious if it expresses itself in an indirect – but not a direct – test [27Reingold E.M. Merikle P.M. On the inter-relatedness of theory and measurement in the study of unconscious processes.Mind Lang. 1990; 5: 9-28Crossref Scopus (164) Google Scholar, 28Greenwald A.G. et al.Long-term semantic memory versus contextual memory in unconscious number processing.J. Exp. Psychol. Learn. Mem. Cogn. 2003; 29: 235-247Crossref PubMed Scopus (87) Google Scholar]. Unqualified trust in objective measures presupposes WDT and conflicts with most other measures (Table 1).'Strategic control' determines the conscious status of knowledge by the person's ability to deliberately use or not use the knowledge according to instructions. In Jacoby's process dissociation procedure [9Jacoby L. A process dissociation framework: separating automatic from intentional uses of memory.J. Mem. Lang. 1991; 30: 513-541Crossref Scopus (2901) Google Scholar], a person either tries to avoid using the information (exclusion task) or makes sure they do use it (inclusion task); any difference in influence of the stimulus between these conditions indicates conscious knowledge, and any use of it despite intentions in the exclusion condition indicates unconscious knowledge (e.g. Refs [29Norman E. et al.Gradations of awareness in a modified sequence learning task.Conscious. Cogn. 2007; 16: 809-837Crossref PubMed Scopus (60) Google Scholar, 30Destrebecqz A. Cleeremans A. Temporal effects in sequence learning.in: Jimenez J.C. Attention and Implicit Learning. John Benjamins, 2003: 181-213Google Scholar]). Unqualified trust in this measure presupposes integration theory.'Subjective measures' require subjects to report their mental states. Most simply, subjective measures have been used to ascertain whether a person knows that they know. The WDT indicates knowledge but not the awareness of knowing. To test for awareness of knowing, confidence ratings can be given. If for all the trials when the person says 'guess', the discrimination performance is still above baseline, then there is evidence that the person has knowledge that they do not know they have: unconscious knowledge by the 'guessing criterion' [31Lau H.C. Passingham R.E. Relative blindsight in normal observers and the neural correlate of visual consciousness.Proc. Natl. Acad. Sci. U. S. A. 2006; 103: 18763-18768Crossref PubMed Scopus (314) Google Scholar]. If a person's knowledge states are conscious, they will know when they know and when they are just guessing. In this case, there should be a relationship between confidence and accuracy, indicating conscious knowledge with no relationship indicating unconscious knowledge by the 'zero-correlation criterion' [32Dienes Z. et al.Unconscious knowledge of artificial grammars is applied strategically.J. Exp. Psychol. Learn. Mem. Cogn. 1995; 21: 1322-1338Crossref Scopus (231) Google Scholar, 33Kolb F.C. Braun J. Blindsight in normal observers.Nature. 1995; 377: 336-338Crossref PubMed Scopus (150) Google Scholar]. Unqualified trust in subjective measures presupposes one or other of the higher-order theories.An advantage of subjective measures is that the conscious status of a range of mental states can be assessed, including both knowledge content and phenomenal content (Box 1). For example a blindsight patient can consciously know without consciously seeing – if they think they know but they do not think they see. Graded degrees of conscious seeing were assessed by Overgaard et al.[22Overgaard M. et al.Is conscious perception gradual or dichotomous? A comparison of report methodologies during a visual task.Conscious. Cogn. 2006; 15: 700-708Crossref PubMed Scopus (208) Google Scholar]: normal subjects consistently reported glimpses or impressions of content they were not willing to say they actually saw (Table 2). Subjective specification of conscious content is often associated with introspection, but not all subjective reports are introspective given that introspection requires being consciously aware of being in a mental state (rather than merely being consciously aware of states in the world) [18Rosenthal D.M. Consciousness and Mind. Clarendon, 2005Google Scholar, 22Overgaard M. et al.Is conscious perception gradual or dichotomous? A comparison of report methodologies during a visual task.Conscious. Cogn. 2006; 15: 700-708Crossref PubMed Scopus (208) Google Scholar].Box 1Structural knowledge and judgment knowledgeTasks can involve a range of knowledge states, the conscious status of each can be assessed subjectively. For example, when a person is exposed to a structured domain (e.g. strings from an artificial grammar), they learn about the structure (structural knowledge). Artificial grammar learning might consist of the knowledge that an 'M' can start a string, about whole strings that were presented and so on. In the test phase, the structural knowledge is brought to bear on a test item to form a new piece of knowledge: the judgment, for example, that this string is grammatical (judgment knowledge) [58Dienes Z. Scott R. Measuring unconscious knowledge: distinguishing