OvergaardpersName (2017date). “The Status and Future of Consciousness Research.” Frontiers in Psychologytitle·j.
Object of study — The two entangled obstacles of consciousnessterm·dom research - how to measure consciousnessterm·dom, and what counts as a neural correlate of it.
Research question — What are the central unresolved obstacles in empirical consciousnessterm·dom research, and what challenges will the field face next?
Contribution — Argues that the field's two current obstacles (no agreed measure of consciousnessterm·dom, and contested neural correlates) are entangled because any objective or 'no-report' measure must be calibrated against introspectionterm·mth, and projects two future challenges: measuring consciousnessterm·dom 'from the outside' in non-reporting patients and artificial systemsterm·dom, and reconnecting empirical work to the metaphysical mind-brainterm·dom question.
phenomenon · claim-type · cross-domain-transfer · mechanism · claim-relation · replication-status · limitation — grounded in the full text| phenomenon | measurement of consciousness and its neural correlates — “Historically, the attempt to “measure” consciousness has unfolded as a debate between direct and indirect approaches.” | |
| claim-type | critique — “One major obstacle for consciousness research is the lacking consensus of how to optimally measure consciousness empirically.” | |
| method — evidence | — “As it will be argued below, consciousness research may face problems in the future that are currently less debated but which are logical extensions of the challenges above.” | |
| object of study — evidence | — “One major obstacle for consciousness research is the lacking consensus of how to optimally measure consciousness empirically.” | |
| mechanism | Any objective measure must be calibrated against something already known to track consciousness, and that anchor is a subjective report - so the weaknesses of introspection are re-imported into every objective method — “Furthermore, in order to arrive at any one particular objective method, one must have “calibrated it” to something else in order to know that this particular behavior can be considered a measure of consciousness—and not something else.” | mechanisms[0] |
| mechanism | Objective measures work only under an unstated assumption that the threshold for a correct response coincides with the threshold for having the experience, which is what makes them fallible as measures of awareness — “For one thing, objective measures must assume that the “threshold” of giving a correct response is the same as the “threshold” of having a subjective experience of the same content” | mechanisms[1] |
| mechanism | Empirical consciousness science arose less from new neuroscience technology than from an openness that let philosophical, cognitive and neuroscientific questions, methods and arguments be integrated — “This development was of course influenced by the technological advancements in neuroscience during those decades, but more important and fundamental was a new openness to interdisciplinary integration of research questions, methods and arguments.” | mechanisms[2] |
| mechanism | The two obstacles are one problem: you cannot look for the correlates of something without first fixing how you will measure it — “This challenge clearly relates to the first as one needs to apply a measure of consciousness in order to identify its correlates.” | mechanisms[3] |
| mechanism | An objective measure has to be calibrated against something, and the only available something is a subjective report - so the weaknesses it was designed to avoid come back in through the calibration — “This would typically involve associating a subjective report with a particular behavior—a process by which one would “import” all the weaknesses related to subjective reports that one tried to avoid in the first place (Overgaard, 2010 ).” | mechanisms[4] |
| mechanism | No-report paradigms do not escape this, because associating a behavioural marker with experience in the first place requires introspection — “However, and as mentioned above, the only way one may associate a phenomenon as nystagmus with conscious experience is by the direct use of introspection (to establish the “correlation”) (Overgaard and Fazekas, 2016 ).” | mechanisms[5] |
| mechanism | Objective measures also assume the threshold for a correct response coincides with the threshold for experiencing the content, which is an assumption and not a finding — “For one thing, objective measures must assume that the “threshold” of giving a correct response is the same as the “threshold” of having a subjective experience of the same content (Fu et al., 2008 ; Timmermans and Cleeremans, 2015 ).” | mechanisms[6] |
| mechanism | One proposed resolution of the prefrontal-versus-occipital dispute is that both are right about different things, because there are two kinds of consciousness — “Block ( 2005 ) argues that there may be two neural correlates of consciousness: One relating to phenomenal consciousness (the “early” activations), and one relating to access consciousness (the “late” activations).” | mechanisms[7] |
| mechanism | Existing evidence cannot adjudicate the metaphysical question, because it fits every position equally — “Existing data, however, seems to fit easily into every theoretical understanding of this problem.” | mechanisms[8] |
| builds-on | Overgaard and Fazekas (2016) — “However, and as mentioned above, the only way one may associate a phenomenon as nystagmus with conscious experience is by the direct use of introspection (to establish the “correlation”) (Overgaard and Fazekas, 2016 ).” | claim_relations[0] |
| critiques | no-report paradigms as a solution to the measurement problem — “However, and as mentioned above, the only way one may associate a phenomenon as nystagmus with conscious experience is by the direct use of introspection (to establish the “correlation”) (Overgaard and Fazekas, 2016 ).” | claim_relations[1] |
| critiques | the position that objective measures alone can settle questions about consciousness — “For one thing, objective measures must assume that the “threshold” of giving a correct response is the same as the “threshold” of having a subjective experience of the same content (Fu et al., 2008 ; Timmermans and Cleeremans, 2015 ).” | claim_relations[2] |
| supports | Block's two-correlates proposal as a reading of the prefrontal/occipital dispute — “According to other perspectives, this debate is partially misunderstood.” | claim_relations[3] |
| extends | the REF-CON model, on which there need be no universal correlate at all — “According to the REF-CON model of consciousness, subjective experience is intrinsically related to a particular kind of “strategy” that makes information available for action (Overgaard and Mogensen, 2014 ; Mogensen and Overgaard, 2017 ).” | claim_relations[4] |
| sense | neural correlate of consciousness - of CONTENT: the minimal activations sufficient for one specific conscious content — “Although it is rarely debated what we mean with a “neural correlate of consciousness,” most experiments aim to identify the minimal neural activations that are sufficient for a specific content of consciousness (Chalmers, 2000 ).” | senses[0] |
| sense | neural correlate of consciousness - AS SUCH: what distinguishes being awake from asleep or dead, not tied to any content — “Contrary to this, other scientists are preoccupied with finding neural correlates of consciousness “as such”—i.e., neural correlates that mark the difference between being dead, asleep, awake, etc., and which are not content-specific in the sense above.” | senses[1] |
| sense | measuring consciousness - DIRECT: the subject reports their own experience, with the known limits of self-report — “Direct approaches, at least intuitively, are the most informative as participating experimental subjects here simply report about their own experiences.” | senses[2] |
| sense | measuring consciousness - INDIRECT: performance on a task stands in for experience, which avoids self-report and imports a threshold assumption instead — “Experiments on consciousness that are based on objective measures—the “indirect” approach—typically involve asking subjects to choose between alternatives, e.g., in forced-choice tasks.” | senses[3] |
| sense | early activations - as the correlate of phenomenal consciousness, which is the currently favoured reading — “According to most recent reviews and articles, evidence is lending support toward the latter view (Andersen et al., 2016 ; Koch et al., 2016 ; Hurme et al., 2017 ).” | senses[4] |
| sense | early activations - as PRECONSCIOUS: information not yet conscious, which is the opposite reading of the same data — “Nevertheless, proponents of the opposite view—that consciousness is associated with prefrontal cortex activity—argue that “early” activations in fact represent preconscious states—i.e., information that is not yet conscious (e.g., Lau and Rosenthal, 2011 ).” | senses[5] |
| edge | subjective report —contrasted-with→ objective measures — “As subjective reports however have demonstrable limits (e.g., lack of insights into personal bias, memory problems etc.), many scientists have refrained from their use and insisted on the use of objective measures only (e.g., Nisbett and Wilson, 1977 ; Johansson et al., 2006 ).” | concept_edges[0] |
| audit | no-report paradigms —same-as→ objective measures — Audit 2026-09-28: DOUBTFUL - read against its span and kept, but the span says 'in a certain sense' no-report paradigms are objective ones; same-as is hedged. — “Essentially, all paradigms using objective measures only are without report, so in a certain sense, paradigms labeled “no-report paradigms” have not introduced anything new.” | concept_edges[2] |
| weight | no-report paradigms —same-as→ objective measures — 0.9 — “Essentially, all paradigms using objective measures only are without report, so in a certain sense, paradigms labeled “no-report paradigms” have not introduced anything new.” | concept_edges[2] |
| audit | phenomenal consciousness —distinct-from→ access consciousness — Audit 2026-09-28: DOUBTFUL - read against its full span and kept, but Block 'ARGUES there MAY be' two correlates, for phenomenal and access consciousness; distinct-from is attributed and hedged. — “Block ( 2005 ) argues that there may be two neural correlates of consciousness: One relating to phenomenal consciousness (the “early” activations), and one relating to access consciousness (the “late” activations).” | concept_edges[5] |
| weight | preconscious —distinct-from→ consciousness — 0.9 — “Nevertheless, proponents of the opposite view—that consciousness is associated with prefrontal cortex activity—argue that “early” activations in fact represent preconscious states—i.e., information that is not yet conscious (e.g., Lau and Rosenthal, 2011 ).” | concept_edges[8] |
| audit | contrastive analysis —operationalised-as→ vegetative state — Audit 2026-09-28: DOUBTFUL - read against its span and kept, but vegetative patients are one contrast group; operationalised-as is the wrong type. — “Some studies have attempted to contrast healthy subjects with patients in vegetative state or minimally conscious state (Boly et al., 2013 ), although there are a number of problems: Some experiments indicate that not all such patients are unconscious (Owen et al., 2006 ), and—at the same time—most brain injured patients have many different lesions and, consequently, massive reorganization, which makes comparisons very difficult.” | concept_edges[9] |
| weight | contrastive analysis —operationalised-as→ vegetative state — 0.9 — “Some studies have attempted to contrast healthy subjects with patients in vegetative state or minimally conscious state (Boly et al., 2013 ), although there are a number of problems: Some experiments indicate that not all such patients are unconscious (Owen et al., 2006 ), and—at the same time—most brain injured patients have many different lesions and, consequently, massive reorganization, which makes comparisons very difficult.” | concept_edges[9] |
| audit | neural correlate of consciousness —distinct-from→ mind-brain — Audit 2026-09-28: DOUBTFUL - read against its full span and kept, but the span says evidence of a correlate does not settle whether consciousness is identical to brain activity; distinct-from is loose. — “In and of itself, it seems not to be the case that evidence that perceptual experience is associated with—say—activity in primary visual cortex also provides evidence to determine whether consciousness should be seen as—say—identical to or metaphysically different from brain activity.” | concept_edges[11] |
| note | neural correlate of consciousness —distinct-from→ mind-brain — DOUBTFUL, found by an audit that read the edge against its span: hedged metaphysical distinction. The edge is kept live; a reader should weigh it with this in mind. — “In and of itself, it seems not to be the case that evidence that perceptual experience is associated with—say—activity in primary visual cortex also provides evidence to determine whether consciousness should be seen as—say—identical to or metaphysically different from brain activity.” | concept_edges[11] |
| for which use | limitation — the list of challenges itself: THEY OBVIOUSLY ONLY REPRESENT A FEW OF THE MANY SCIENTIFIC AND THEORETICAL ISSUES THAT SCIENTISTS IN THIS AREA FACE — “The challenges highlighted above obviously only represent a few of the many scientific and theoretical issues that scientists in this area face.” | limitations[0] |
| for which use | limitation — every NCC result in the field: IT IS RARELY DEBATED WHAT WE MEAN WITH A NEURAL CORRELATE OF CONSCIOUSNESS, while MOST EXPERIMENTS AIM TO IDENTIFY THE MINIMAL NEURAL ACTIVATIONS THAT ARE SUFFICIENT FOR A SPECIFIC CONTENT OF CONSCIOUSNESS — “Although it is rarely debated what we mean with a “neural correlate of consciousness,” most experiments aim to identify the minimal neural activations that are sufficient for a specific content of consciousness (Chalmers, 2000 ).” | limitations[1] |
| for which use | limitation — any quantitative claim about consciousness: WE STILL HAVE NO UNIVERSALLY ACCEPTED MEASURES OF CONSCIOUSNESS — “Whereas we still have no universally accepted measures of consciousness, much progress has been made with regards to how one may grasp the content of an experience in the context of an experiment.” | limitations[2] |
| for which use | limitation — the patient comparisons: MOST BRAIN INJURED PATIENTS HAVE MANY DIFFERENT LESIONS AND, CONSEQUENTLY, MASSIVE REORGANIZATION, WHICH MAKES COMPARISONS VERY DIFFICULT — “most brain injured patients have many different lesions and, consequently, massive reorganization, which makes comparisons very difficult” | limitations[3] |
| for which use | limitation — the unconscious contrast group: SOME EXPERIMENTS INDICATE THAT NOT ALL SUCH PATIENTS ARE UNCONSCIOUS — “Some experiments indicate that not all such patients are unconscious (Owen et al., 2006 ), and—at the same time—most brain injured patients have many different lesions and, consequently, massive reorganization, which makes comparisons very difficult.” | limitations[4] |
| for which use | limitation — the no-report label: ALL PARADIGMS USING OBJECTIVE MEASURES ONLY ARE WITHOUT REPORT, SO IN A CERTAIN SENSE, PARADIGMS LABELED NO-REPORT PARADIGMS HAVE NOT INTRODUCED ANYTHING NEW — “Essentially, all paradigms using objective measures only are without report, so in a certain sense, paradigms labeled “no-report paradigms” have not introduced anything new.” | limitations[5] |
| replication | untested | |
| limitation | The paper explicitly presents its list of challenges as a selective sample rather than a complete survey of the field's problems. — “The challenges highlighted above obviously only represent a few of the many scientific and theoretical issues that scientists in this area face.” | limitations[0] |
| limitation | The field's central term is used without its ambiguity being examined — “Although it is rarely debated what we mean with a “neural correlate of consciousness,” most experiments aim to identify the minimal neural activations that are sufficient for a specific content of consciousness (Chalmers, 2000 ).” | limitations[1] |
| limitation | There is still no universally accepted measure of the thing being studied — “Whereas we still have no universally accepted measures of consciousness, much progress has been made with regards to how one may grasp the content of an experience in the context of an experiment.” | limitations[2] |
| limitation | The patient comparisons that would settle levels of consciousness are confounded by lesion variability and reorganisation — “most brain injured patients have many different lesions and, consequently, massive reorganization, which makes comparisons very difficult” | limitations[3] |
| limitation | Some patients classified as unconscious appear not to be, so the contrast group is unreliable — “Some experiments indicate that not all such patients are unconscious (Owen et al., 2006 ), and—at the same time—most brain injured patients have many different lesions and, consequently, massive reorganization, which makes comparisons very difficult.” | limitations[4] |
| limitation | The label 'no-report paradigm' names nothing new, since every purely objective paradigm is already without report — “Essentially, all paradigms using objective measures only are without report, so in a certain sense, paradigms labeled “no-report paradigms” have not introduced anything new.” | limitations[5] |
finding · quantity · variable · definition · key-term — grounded in the full text| finding | The field's two dominant problems are not independent: neural correlates cannot be identified without first committing to a measure of consciousness, so the measurement problem is prior to the correlate problem. — “This challenge clearly relates to the first as one needs to apply a measure of consciousness in order to identify its correlates.” | content_tags.findings[0] |
| finding | Objective (indirect) measures cannot escape the weaknesses of introspective report, because calibrating any objective measure requires pairing a behaviour with a subjective report in the first place. — “This would typically involve associating a subjective report with a particular behavior—a process by which one would “import” all the weaknesses related to subjective reports that one tried to avoid in the first place” | content_tags.findings[1] |
| finding | So-called no-report paradigms introduce nothing that escapes introspection either, since the link between the objective marker and conscious experience can only be established by direct introspective report. — “the only way one may associate a phenomenon as nystagmus with conscious experience is by the direct use of introspection” | content_tags.findings[2] |
| finding | Different measurement methods seem to generate different answers about the neural correlates of consciousness: some yield prefrontal correlates of visual consciousness, others occipital/parietal correlates without prefrontal involvement. — “Whereas this is most likely what is necessary, it is important to notice that different methods seem to generate different results, so that some methods are associated with the finding that the neural correlates of visual consciousness involve prefrontal activity, whereas other methods are associated with the finding that visual consciousness mainly involve occipital/parietal activity but not prefrontal.” | content_tags.findings[3] |
| finding | Most recent reviews lend support to the view that consciousness is associated with occipital/parietal rather than prefrontal activity. — “According to most recent reviews and articles, evidence is lending support toward the latter view” | content_tags.findings[4] |
| finding | Existing consciousness data seem to fit easily into every theory of the mind-brain relation. — “Existing data, however, seems to fit easily into every theoretical understanding of this problem.” | content_tags.findings[5] |
| finding | The author names measuring consciousness "from the outside" as one major future challenge. — “One major future challenge will be how to measure consciousness “from the outside.”” | content_tags.findings[6] |
| finding | Despite these unsolved foundations, consciousness research already works as a discipline producing meaningful and reproducible results. — “Consciousness research has proven to be an actual and functioning discipline able to provide meaningful and reproducible results.” | content_tags.findings[7] |
| finding | different methods of measuring consciousness yield different conclusions about which brain regions are its correlates — “Whereas this is most likely what is necessary, it is important to notice that different methods seem to generate different results, so that some methods are associated with the finding that the neural correlates of visual consciousness involve prefrontal activity, whereas other methods are associated with the finding that visual consciousness mainly involve occipital/parietal activity but not prefrontal.” | content_tags.findings[8] |
| finding | recent reviews favour the occipital/parietal reading over the prefrontal one — “According to most recent reviews and articles, evidence is lending support toward the latter view (Andersen et al., 2016 ; Koch et al., 2016 ; Hurme et al., 2017 ).” | content_tags.findings[9] |
| finding | the same early activations are read as the correlate of consciousness by one camp and as preconscious by the other — “Nevertheless, proponents of the opposite view—that consciousness is associated with prefrontal cortex activity—argue that “early” activations in fact represent preconscious states—i.e., information that is not yet conscious (e.g., Lau and Rosenthal, 2011 ).” | content_tags.findings[10] |
| finding | the existing evidence seems to fit every metaphysical position — “Existing data, however, seems to fit easily into every theoretical understanding of this problem.” | content_tags.findings[11] |
| finding | the field has proven able to produce meaningful and reproducible results — “Consciousness research has proven to be an actual and functioning discipline able to provide meaningful and reproducible results.” | content_tags.findings[12] |
| definition | neural correlates of consciousness — “most experiments aim to identify the minimal neural activations that are sufficient for a specific content of consciousness” | content_tags.definitions[0] |
| definition | neural correlates of consciousness — “Contrary to this, other scientists are preoccupied with finding neural correlates of consciousness “as such”—i.e., neural correlates that mark the difference between being dead, asleep, awake, etc., and which are not content-specific in the sense above.” | content_tags.definitions[1] |
| definition | direct approach — “Direct approaches, at least intuitively, are the most informative as participating experimental subjects here simply report about their own experiences.” | content_tags.definitions[2] |
| definition | indirect approach — “Experiments on consciousness that are based on objective measures—the “indirect” approach—typically involve asking subjects to choose between alternatives, e.g., in forced-choice tasks.” | content_tags.definitions[3] |
| definition | no-report paradigm — “Nevertheless, experiments of this kind attempt first to associate a particular objective measure (e.g., a behavior or a brain activation) with conscious experience, and then to apply this measure as a measure of consciousness so that no direct report is needed” | content_tags.definitions[4] |
| definition | REF-CON model of consciousness — “According to the REF-CON model of consciousness, subjective experience is intrinsically related to a particular kind of “strategy” that makes information available for action” | content_tags.definitions[5] |
| definition | neural correlate of consciousness — “Although it is rarely debated what we mean with a “neural correlate of consciousness,” most experiments aim to identify the minimal neural activations that are sufficient for a specific content of consciousness (Chalmers, 2000 )” | content_tags.definitions[7] |
| definition | consciousness as such — “neural correlates that mark the difference between being dead, asleep, awake, etc., and which are not content-specific in the sense above” | content_tags.definitions[8] |
| definition | no-report paradigms — “experiments of this kind attempt first to associate a particular objective measure (e.g., a behavior or a brain activation) with conscious experience, and then to apply this measure as a measure of consciousness so that no direct report is needed” | content_tags.definitions[9] |
| definition | REF-CON model — “According to the REF-CON model of consciousness, subjective experience is intrinsically related to a particular kind of “strategy” that makes information available for action” | content_tags.definitions[10] |
| key-terms | consciousness, neural correlate of consciousness, subjective report, objective measures, no-report paradigms, forced-choice tasks, Perceptual Awareness Scale, phenomenal consciousness, access consciousness, prefrontal cortex, preconscious, metacognition, contrastive analysis, vegetative state, introspection, mind-brain |
| claim type | critique — a second reading reaching the same value; recorded: critique — “This challenge clearly relates to the first as one needs to apply a measure of consciousness in order to identify its correlates.” | alternatives.claim_type |
| phenomenon | measurement of consciousness and its neural correlates — a second reading reaching the same value; recorded: measurement of consciousness and its neural correlates — “Scientists across disciplines generally agree that consciousness is subjective, characterized by a kind of privileged first-person access.” | alternatives.phenomenon |
Full text — tagged
The reference list is not part of the text we hold for this paper. Our copy was extracted from the publisher's page and the bibliography did not survive that extraction, so the article below ends before its references. Nothing has been removed by us.
The Status and Future of Consciousnessterm·dom Research Morten OvergaardpersName Morten OvergaardpersName 1 Cognitive Neuroscience Research Unit (CNRU), CFIN, Institute of Clinical Medicine, Aarhus University, Aarhus, Denmark 1, * 1 Cognitive Neuroscience Research Unit (CNRU), CFIN, Institute of Clinical Medicine, Aarhus University, Aarhus, Denmark Edited and reviewed by: Axel Cleeremans, Free University of Brussels, Belgium ✉ *Correspondence: Morten OvergaardpersName mortover@rm.dk This article was submitted to Consciousnessterm·dom Research, a section of the journal Frontiers in Psychology Received 2017 May 23; Accepted 2017 Sep 19; Collection date 2017.
Keywords: consciousnessterm·dom, consciousnessterm·dom disorders, measures of consciousnessterm·dom, neural correlates of consciousnessterm·def, artificial consciousnessterm·dom Copyright © 2017 OvergaardpersName.
This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) or licensor are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.
PMCID: PMC5641373 PMID: 29066988 As papers about consciousnessterm·dom are so often introduced, consciousnessterm·dom was until few decades ago considered a philosophical problem only, and the current interest in empirical consciousnessterm·dom research was unforeseen. This development was of course influenced by the technological advancements in neuroscience during those decades, but more important and fundamental was a new openness to interdisciplinary integration of research questions, methods and arguments. Cognitive scientists and neuroscientists agreed that the philosophical problems of why and how there is consciousnessterm·dom are also their problems. Philosophers agreed that empirical evidence may resolve or at least influence this debate. Scientists across disciplines generally agree that consciousnessterm·dom is subjective, characterized by a kind of privileged first-person access.
Consciousnessterm·dom research has proven to be an actual and functioning discipline able to provide meaningful and reproducible results. Nevertheless, it has yet only scratched the surface in the attempt to solve some its bigger challenges, e.g., its many underlying questions of metaphysics (i.e., why does consciousnessterm·dom exist?) and questions of mechanisms (how does consciousnessterm·dom exist?).
One major obstacle for consciousnessterm·dom research is the lacking consensus of how to optimally measure consciousnessterm·dom empirically. Another major challenge is how to identify neural correlates of consciousnessterm·def. This challenge clearly relates to the first as one needs to apply a measure of consciousnessterm·dom in order to identify its correlates. Current consciousnessterm·dom research is already occupied with these questions that may even be said to dominate the scientific debate.
As it will be argued below, consciousnessterm·dom research may face problems in the future that are currently less debated but which are logical extensions of the challenges above. It is a natural ambition when developing a measure of consciousnessterm·dom to be able to determine whether non-reporting subjects or even machines are conscious and of what. And it is a natural ambition when finding neural correlates of consciousnessterm·def to understand how these correlates relate to a deeper metaphysical understanding of the relation between subjective experience and the physical substrate of the brain.
How do we measure consciousnessterm·dom?
Historically, the attempt to “measure” consciousnessterm·dom has unfolded as a debate between direct and indirect approachesterm·def. Direct approachesterm·def, at least intuitively, are the most informative as participating experimental subjects here simply report about their own experiences. As subjective reportsterm·mth however have demonstrable limits (e.g., lack of insights into personal bias, memory problems etc.), many scientists have refrained from their use and insisted on the use of objective measuresterm·mth only (e.g., Nisbett and Wilson, 1977 ; Johansson et al., 2006 ).
Experiments on consciousnessterm·dom that are based on objective measuresterm·mth—the “indirect” approach—typically involve asking subjects to choose between alternatives, e.g., in forced-choice tasksterm·mth. Although such methods may stay clear of classical limitations of subjective methods, they are confronted with other problems, which, according to some scientists, are greater. For one thing, objective measuresterm·mth must assume that the “threshold” of giving a correct response is the same as the “threshold” of having a subjective experience of the same content (Fu et al., 2008 ; Timmermans and Cleeremans, 2015 ). Furthermore, in order to arrive at any one particular objective method, one must have “calibrated it” to something else in order to know that this particular behavior can be considered a measure of consciousnessterm·dom—and not something else. This would typically involve associating a subjective reportterm·mth with a particular behavior—a process by which one would “import” all the weaknesses related to subjective reportsterm·mth that one tried to avoid in the first place (OvergaardpersName, 2010 ).
Proponents of the “direct” approach have attempted to develop precise and sensitive scales to capture minor variations in subjective experience, e.g., the Perceptual Awareness Scaleterm·mth and gradual confidence ratingsterm·mth (Ramsøy and OvergaardpersName, 2004 ; Sandberg and OvergaardpersName, 2015 ). Although different approaches to this idea disagree about what consistutes the optimal measure (Dienes and Seth, 2010 ; Timmermans et al., 2010 ; Szczepanowski et al., 2013 ), they share the view that a detailed subjective reportterm·mth may be imprecise yet better than an indirect measure.
In recent years, the arsenal of indirect measures have been supplied with what is named “no-report paradigmsterm·mth.” Essentially, all paradigms using objective measuresterm·mth only are without report, so in a certain sense, paradigms labeled “no-report paradigmsterm·mth” have not introduced anything new. Nevertheless, experiments of this kind attempt first to associate a particular objective measureterm·mth (e.g., a behavior or a brain activation) with conscious experience, and then to apply this measure as a measure of consciousnessterm·dom so that no direct report is needed (e.g., Frässle et al., 2014 ; Pitts et al., 2014 ). Such methods intuitively seem to circumvent some of the criticism mentioned above. However, and as mentioned above, the only way one may associate a phenomenon as nystagmus with conscious experience is by the direct use of introspectionterm·mth (to establish the “correlation”) (OvergaardpersName and Fazekas, 2016 ).
It has been proposed that the best and most practical way forward is to combine methods and learn what we can from the results we get (Tsuchiya et al., 2016 ). Whereas this is most likely what is necessary, it is important to notice that different methods seem to generate different results, so that some methods are associated with the finding that the neural correlates of visual consciousnessterm·dom involve prefrontal activity, whereas other methods are associated with the finding that visual consciousnessterm·dom mainly involve occipital/parietal activity but not prefrontal.
Neural correlates of consciousnessterm·def Most neuroscientific research on consciousnessterm·dom has had the explicit aim to identify the neural correlates of consciousnessterm·def. Although it is rarely debated what we mean with a “neural correlate of consciousnessterm·dom,” most experiments aim to identify the minimal neural activations that are sufficient for a specific content of consciousnessterm·dom (Chalmers, 2000 ). Contrary to this, other scientists are preoccupied with finding neural correlates of consciousnessterm·def “as such”—i.e., neural correlates that mark the difference between being dead, asleep, awake, etc., and which are not content-specific in the sense above.
With regards to the attempt to isolate neural correlates of conscious content, one central debate in recent years has been whether neural correlates of consciousnessterm·def should primarily be associated with prefrontal cortexterm·dom (“late” activations) or whether (visual) consciousnessterm·dom should be associated with occipital/parietal activations (“early” activations). According to most recent reviewsterm·mth and articles, evidence is lending support toward the latter view (Andersen et al., 2016 ; Koch et al., 2016 ; Hurme et al., 2017 ). According to this view, “late” activations are not actual correlates of consciousnessterm·dom, but are confounds associated with metacognitionterm·dom and report (e.g., Aru et al., 2012 ). Nevertheless, proponents of the opposite view—that consciousnessterm·dom is associated with prefrontal cortexterm·dom activity—argue that “early” activations in fact represent preconsciousterm·dom states—i.e., information that is not yet conscious (e.g., Lau and Rosenthal, 2011 ).
According to other perspectives, this debate is partially misunderstood. Block ( 2005 ) argues that there may be two neural correlates of consciousnessterm·def: One relating to phenomenal consciousnessterm·dom (the “early” activations), and one relating to access consciousnessterm·dom (the “late” activations). Others suggests that there is an identity between subjective experience and certain causal properties of physical systems rather than an identity between experience and particular brain parts (Tononi et al., 2016 ). According to the REF-CON model of consciousnessterm·def, subjective experience is intrinsically related to a particular kind of “strategy” that makes information available for action (OvergaardpersName and Mogensen, 2014 ; Mogensen and OvergaardpersName, 2017 ). From this perspective, there need not be any “universal” correlate of consciousnessterm·dom at all. But even in such theoretical models according to which finding neural correlates of consciousnessterm·def is very different from explaining consciousnessterm·dom, neural correlates of consciousnessterm·def are essential as evidence to show how and if they work in practice.
There has been relatively more research into the neural correlates of the contents of consciousnessterm·dom than into “consciousness as suchterm·def.” Research attempting to identify particular “levels” of consciousnessterm·dom obviously also face many methodological challenges, not least relating to contrastive analysisterm·mth. Some studies have attempted to contrast healthy subjects with patients in vegetative stateterm·dom or minimally conscious state (Boly et al., 2013 ), although there are a number of problems: Some experiments indicate that not all such patients are unconscious (Owen et al., 2006 ), and—at the same time—most brain injured patients have many different lesions and, consequently, massive reorganization, which makes comparisons very difficult.
The future challenges The “upsurge” of interest in a science of consciousnessterm·dom did not begin but certainly took off with the publications of Chalmers ( 1995 , 1996 ) and the Tucson-based conference series “Toward a Science of Consciousnessterm·dom”—soon to be further strengthened by the annual conferences organized by the ASSC (Association for the Scientific Study of Consciousnessterm·dom). Since then, much has happened in the attempt to discover neural and cognitive correlates of consciousnessterm·dom. It is however as uncertain today as it was then how exactly to apply these findings. In principle, there are many potential applications of consciousnessterm·dom research, but whereas some are extensions of the more fundamental questions (e.g., ethics and law), others are close to the heart of what consciousnessterm·dom research is (arguably) about, i.e., the mind-brainterm·dom or mind-body problem.
One such fundamental problem relates to the fact that consciousnessterm·dom is subjective and in this way accessible from the first person only. Whereas we still have no universally accepted measures of consciousnessterm·dom, much progress has been made with regards to how one may grasp the content of an experience in the context of an experiment. One major future challenge will be how to measure consciousnessterm·dom “from the outside.” This problem is currently being faced in coma and vegetative stateterm·dom patients who either do not respond or respond in a minimal or strange fashion. It will very likely be an even greater challenge for a future science of consciousnessterm·dom to consider how to evaluate whether artificial systemsterm·dom (e.g., computers or robots) can be conscious or whether experience is a privilege for biological creatures. Essentially, these questions force us to try to make scientifically based decisions about how to measure conscious experience in highly different situations: In coma/vegetative stateterm·dom patients, there are little or no responses, yet a neural (however altered) system. In artificial systemsterm·dom, there may be high responsiveness (even, in principle, explicit expressions of being conscious) but no neural (biological) system.
One possibly even greater challenge will be to reintegrate the philosophical metaphysical debate into the scientific work. It will be a challenge to the future science of consciousnessterm·dom to demonstrate that empirical work on consciousnessterm·dom directly aids an understanding of the fundamental questions about consciousnessterm·dom. This challenge may seem unavoidable as the current preoccupation with cognitive functions and neural activations associated with subjective experience in most cases seems so directly linked to and motivated by the mind-brainterm·dom problem. Existing data, however, seems to fit easily into every theoretical understanding of this problem. In and of itself, it seems not to be the case that evidence that perceptual experience is associated with—say—activity in primary visual cortex also provides evidence to determine whether consciousnessterm·dom should be seen as—say—identical to or metaphysically different from brain activity. Accordingly, it will require something “extra” to answer this challenge. Either, if possible, experimental investigations must be designed in order to “test” theoretical positions that currently are stated within the framework of philosophy of mind. Alternatively, experimental consciousnessterm·dom research must work even closer with theoretical consciousnessterm·dom research in order to make empirical data available as arguments.
Future directions The challenges highlighted above obviously only represent a few of the many scientific and theoretical issues that scientists in this area face. Consciousnessterm·dom remains one of the biggest scientific challenges among all disciplines as the most fundamental questions are not simply unanswered—it is still highly unclear how one should even begin to answer them.
Currently, consciousnessterm·dom research is often considered a “topic”—or even “niche”—under the umbrella of cognitive neuroscience. Nevertheless, consciousnessterm·dom researchers often point out that subjective experience is the underlying and fundamental reason for many questions in neuroscience. Scientists interested in the brain are often seeking answers to questions such as why we become addicted, how we remember, perceive, or solve problems. Such questions arguably presume conscious experience and make little sense without. Terms such as “memory” or “perception” do not solely refer to behavior, but also to particular kinds of conscious content which we know about from introspectionterm·mth. For this reason, one future ambition for consciousnessterm·dom research could be to become a more integral part of the overall ambition to understand the brain, and as such become part of the basic curriculum for any neuroscientist.
Author contributions The author confirms being the sole contributor of this work and approved it for publication.
Conflict of interest statement The author declares that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.