sciences Review Theoretical Models of Consciousness: A Scoping Review Davide Sattin 1,2, *, Francesca Giulia Magnani 1 , Laura Bartesaghi 1 , Milena Caputo 1 , Andrea Veronica Fittipaldo 3 , Martina Cacciatore 1 , Mario Picozzi 4 and Matilde Leonardi 1 1 2 3 4 * Citation: Sattin, D.; Magnani, F.G.; Bartesaghi, L.; Caputo, M.; Fittipaldo, A.V.; Cacciatore, M.; Picozzi, M.; Leonardi, M. Theoretical Models of Consciousness: A Scoping Review. Brain Sci. 2021, 11, 535. https:// doi.org/10.3390/brainsci11050535 Neurology, Public Health, Disability Unit—Scientific Department, Fondazione IRCCS Istituto Neurologico Carlo Besta, 20133 Milan, Italy; francesca.magnani@istituto-besta.it (F.G.M.); bartesaghilaura@gmail.com (L.B.); milena.caputo@outlook.com (M.C.); marty9484@live.it (M.C.); matilde.leonardi@istituto-besta.it (M.L.) Experimental Medicine and Medical Humanities-PhD Program, Biotechnology and Life Sciences Department and Center for Clinical Ethics, Insubria University, 21100 Varese, Italy Oncology Department, Mario Negri Institute for Pharmacological Research IRCCS, 20156 Milan, Italy; veronicaandrea.fittipaldo@marionegri.it Center for Clinical Ethics, Biotechnology and Life Sciences Department, Insubria University, 21100 Varese, Italy; mario.picozzi@uninsubria.it Correspondence: davide.sattin@istituto-besta.it; Tel.: +39-02-2394-2709 Abstract: The amount of knowledge on human consciousness has created a multitude of viewpoints and it is difficult to compare and synthesize all the recent scientific perspectives. Indeed, there are many definitions of consciousness and multiple approaches to study the neural correlates of consciousness (NCC). Therefore, the main aim of this article is to collect data on the various theories of consciousness published between 2007–2017 and to synthesize them to provide a general overview of this topic. To describe each theory, we developed a thematic grid called the dimensional model, which qualitatively and quantitatively analyzes how each article, related to one specific theory, debates/analyzes a specific issue. Among the 1130 articles assessed, 85 full texts were included in the prefinal step. Finally, this scoping review analyzed 68 articles that described 29 theories of consciousness. We found heterogeneous perspectives in the theories analyzed. Those with the highest grade of variability are as follows: subjectivity, NCC, and the consciousness/cognitive function. Among sub-cortical structures, thalamus, basal ganglia, and the hippocampus were the most indicated, whereas the cingulate, prefrontal, and temporal areas were the most reported for cortical ones also including the thalamo-cortical system. Moreover, we found several definitions of consciousness and 21 new sub-classifications. Academic Editor: Gerry Leisman Received: 12 April 2021 Keywords: consciousness; neural correlates; cognition; consciousness theory; consciousness definition; subjectivity Accepted: 21 April 2021 Published: 24 April 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). 1. Introduction The scientific literature debating themes on consciousness is huge. Contributing scientists come from different disciplines such as medicine, philosophy, physics, psychology, neurobiology, mathematics, and computer science. Authors from these disciplines have led to an increase in knowledge about consciousness, giving new and interesting inputs for neuroscience. However, all this information has created a multitude of viewpoints and it is difficult to compare (and synthesize) some of the various issues that have emerged in the last few decades. For example, one of the main problems related to consciousness is its definition. Indeed, as remarked by Sommerhoff [1], “A precise definition of the word [‘consciousness’], of course, can only be the endpoint of a theory of consciousness, just as the concepts of work and energy found a precise definition only as part of a theory of mechanics” [1]. Therefore, the probability of having different definitions of the same object correlates directly to the number of theories that try to explain the nature of consciousness itself. In this sense, the concept of ‘consciousness’ can be found in historical documents Brain Sci. 2021, 11, 535. https://doi.org/10.3390/brainsci11050535 https://www.mdpi.com/journal/brainsci
2 of 58 since ancient times, and it can be proven in various traditions and cultures with some perspectives that, although the expression language has become different, seem to be concepts that had evolved throughout history [2]. Fritjof Capra [3] describes how some eastern ancient concepts on a universal consciousness are consistent with modern physics, considering that some physics perspectives try to invalidate the concept of atomic particles as consisting of smaller independent building blocks made of different substances. Capra believes that one substance exists that comes from a ‘cosmic consciousness’ (this was the definition of consciousness used) out of which all matter is made of. This is a position commonly used also by those who argue that consciousness supervenes on the whole embodied animal in dynamic interaction with the environment [4]. Another example proposed in the last decade derived by some eastern reductionist perspective highlighted that consciousness could be defined as “the ability to maintain an alert state, attention, and awareness of self and environment” [5] or as the “the subjective character of our mental states” [6], according to the clinical and representational perspective used to describe it. This provides particular attention to the role of introspection and metarepresentation, two themes cited by Aristotle that contribute to current debates over the nature of consciousness [7]. Therefore, the analysis of the general framework and the perspectives proposed by each model of consciousness is important for improving the science of consciousness and, consequently, its definitions. Another issue is represented by the definition of the neural correlates of consciousness (NCCs). If we consider consciousness as a human phenomenon deriving from matter, one of the main neurobiological questions is the definition of the minimum neuronal mechanisms jointly sufficient for any one specific conscious percept [8,9]. The definition of what we mean by consciousness and the brain neurons/areas/processes linked to the appearance of this phenomenon is a crucial issue for all theories of consciousness. The evolution of imaging and neurophysiological techniques, as well as the possibility to change brain activities online using tailored magnetic and electrical stimulations during tasks requiring wakefulness and awareness of external stimuli, has led to increases in the possibility of verifying/falsifying different hypotheses. Moreover, it has led to study the of consciousness modulation through the stimulation of different brain areas, overpassing the limits derived from the anatomo-clinical model based on the lesional approach. However, every theoretical approach has some assumptions according to which consciousness could be generated by a multitude of processes. This is grounded in the activity of a wide range of structures that vary from the single neuron to the whole brain. Consequently, the number of NCCs is presently very large. There are other main issues debated in the scientific literature related to consciousness. The problem of how we can “quantify” the conscious level, for example, is one of the most important questions that many scientists are still studying. This question is crucial for clinicians during the clinical diagnosis of patients with disorders of consciousness after a severe brain injury (to assess the consciousness level beyond behavioral abilities of patients), as well as for computer science/artificial intelligence researchers involved in machine and computer development. All the examples reported above makeup only a minimal part of the literature on consciousness, which has increased over the last few years. Therefore, the general aim of this article is to offer an exhaustive framework on the theoretical models of consciousness that currently exist, without committing to a specific perspective or viewpoint. To achieve this goal, the specific aims of this research are twofold: Firstly, we sought to systematically review the scientific literature on theoretical models of consciousness published from 2007 to 2017 in major scientific databases and, secondly, to describe each theory collected using a common thematic grid called the dimensional model (DM) in order to qualitatively and quantitatively analyze how each article, related to a specific theory, debates/analyzes specific issues.
3 of 58 2. Materials and Methods We included all the published studies that analyzed theoretical aspects of consciousness. 2.1. Search Strategies and Information Sources The authors performed a systematic search in the Medline (PubMed), Embase (EMBASE.com), Scopus, Web of Science (WoS), and PsycINFO databases. A tailored search strategy was developed for each database according to their thesaurus characteristics. The search method for the identification of studies is reported in Supplemental Materials (Table S1). Results obtained from each database were exported in a web-based bibliographic management software (RefWorks; https://www.refworks.com/it/, accessed on May–December 2017) and duplicate deletion was done by two independent raters, which were then matched. Moreover, all records were imported in a tailored Excel spreadsheet including title, abstract, and record information for each article. 2.2. General Selection Criteria Studies were excluded if they met any of the following criteria: (1) studies published before 2007; (2) article type: book, generic, magazine article, or monograph; (3) studies not published in the English or Italian languages. This review was performed following the preferred reporting items for systematic reviews and meta-analyses criteria [10]. 2.3. Screening and Eligibility Criteria for Each Step Search results were independently screened by different reviewers using a three-step procedure due to the high number of articles selected. Duplicate articles were removed before step 1. 2.3.1. Step 1: Abstract Selection In the first step, one author (Rater #1) screened the articles by title, abstract, and keywords using the following assessment scale: 0 = the article does not debate consciousness itself (the term consciousness appears in the title or the abstract but with general meaning (e.g., people are conscious of their life, etc.); 1 = the rater doubts this article; 2 = the article debates some aspects related to consciousness. In total, 20% (4100 studies) of the total number of articles were selected with random data extraction (using SPSS software; SPSS Inc., Chicago, IL, USA) for an independent rating made by Rater #2. The agreement between the two raters (Cohen K analysis) was predetermined; it had to be over 0.90 to proceed with the second step of the screening process. If the agreement value was below 0.90, a third rater was involved in the screening of abstracts, and then a second analysis was performed. The third author (Rater #3), who had senior experience in systematic reviews and consciousness issues, checked the quality of data collection, helping to solve the discrepancy between the evaluations of the first- and second-rater. 2.3.2. Step 2: Full-Text Selection In the second step, two raters (#2 and #3) analyzed all full-text articles extracted during step 1 to assess them for final eligibility. The assessment scale used in this step was as follows: 0 = the article does not report theoretical aspects related to consciousness; 1 = the rater doubts this article; 2 = the article discusses theoretical aspects related to consciousness. The term “theoretical aspects” was used in referring to papers that accomplished three things: (a) describe a theory of consciousness; (b) describe one dimension of the DM (see below for its description) concerning consciousness as a topic; (c) describe a specific issue concerning different models of consciousness. In the case of a disagreement between raters regarding a selected study, consensus was achieved by means of discussion involving Rater #4, who read the article/s and participated in the discussion.
4 of 58 2.3.3. Step 3: Data Extraction and Management In the final step, full-text analysis of the selected records classified as “included” in the previous step was performed. Raters #2, #4, and #5 independently analyzed the selected articles. Each of them completed the DM (see below for the full description) and then summarized each theory of consciousness. Additional relevant papers were retrieved through a cross-reference check. The bibliography of each article was read to collect information on other studies not included in the final list. 2.4. Outcome Measures Before beginning the review process, we developed a DM. The DM is a thematic grid used to quantify how each article, belonging to one specific theory, debates/analyzes specific issues. Dimensions are selected according to (i) important questions that emerged from previous consciousness studies [11–15] and (ii) questions on which authors wanted to point out. 2.5. Main Dimensions Analyzed 2.5.1. Neural Correlates of Consciousness (NCCs) NCCs are defined as the minimum neuronal mechanisms jointly sufficient for any one specific conscious percept [8,9,16]. In each article, the rater analyzed how much NCC was debated (quantifying this dimension using the Likert scale described below). Further, the rater reported the DM in the main brain structures. Moreover, the raters indicated whether the NCCs were described at the neuronal or brain area levels and whether the NCCs were correlated to a process involving different neural circuits. In this review, we did not consider the distinction between content-related consciousness NCC (i.e., the local state) and awakening state NCC (i.e., the global state or the activity which determines a particular phenomenal distinction within an experience) as eligibility criteria. Rather, raters reported more specific information when a clear description was found in the analyzed articles. 2.5.2. Association between Consciousness and Other Cognitive Functions In this dimension, raters were asked to analyze how much each article explained the relationships between consciousness and other cognitive functions such as attention, memory, mental imagery, and so on. In each study, raters were asked to analyze how much the theoretical process linked conscious information to cognitive functions (specifically about information processing). For instance, raters were required to analyze how stimuli conscious elaboration is related to memory or how consciousness generation mechanisms work in relation to attentional functions [17]. 2.5.3. Translation from Theory to Clinical Practice In this area, we spotlighted information useful for the clinical management of patients with neurological/psychiatric disorders related to consciousness manifestations. Raters evaluated if each article provided information about clinical implications for either the diagnosis or the treatments of patients (e.g., with disorders of consciousness or with loss of consciousness due to epilepsy, etc.). Moreover, raters evaluated if the article reported any implications that would determine some prognostic markers based on the theory described. Specifically, the translation from theoretical aspects to clinical practice was examined, looking for words such as “marker”, “recovery of consciousness”, “diagnosis”, “clinical impact”, and “clinical implications”, as is reported in the supplementary materials. 2.5.4. Quantitative Measures of Consciousness Raters observed if an article reported information about the quantification of consciousness levels. We analyzed if instruments/tools to measure consciousness were described and if tailored algorithms were reported. Special attention was paid to the “index” definition directly related to the consciousness definition reported in the theory.
5 of 58 2.5.5. Consciousness, Sensory Processes, and the Autonomic Nervous System This area was chosen to explore if the selected theories of consciousness debate on the relationships between peripheral information derived from the autonomic nervous system (ANS) (also including sensory processes) and consciousness. In detail, raters were asked to quantify how much the elaboration of information coming from ANS was described in each article and how this information was linked to a consciousness percept (e.g., pathways that provide visceral sensation to conscious perception). 2.5.6. Subjectivity The final dimension analyzed by raters refers to how each article/theory of consciousness explains where subjectivity comes from and how it is related to conscious experience. A specific definition of subjectivity was not provided to raters to avoid the exclusion of new concepts and alternative viewpoints about subjectivity. In this sense, raters were instructed to search within for text-specific words such as “subjective experience”, “subjectivity”, “first-person experience”, and other terms linked to the discussion about how subjective experience could emerge from neural activity, and then to analyze how much this topic is analyzed in the text of each article. 2.6. Outcomes 2.6.1. Quantitative Outcomes For each study collected, we analyzed the full text, giving a score on a Likert scale ranging from 0 to 5. The score for each dimension was assigned according to the following criteria: 0 = the issue was not debated in the article. 1 = the article gave minimal attention to the issue. The theme was cited in the text (introduction or discussion) as a marginal topic that used fewer words. 2 = the issue was not reported in the abstract. The theme was debated in the text using some sentences as collateral/secondary issues in refers to other main topics. 3 = the issue was not reported in the abstract but there was a short paragraph that discussed the issue reporting specific references. 4 = the issue was cited in the abstract. In specific sections of the paper (introduction, results, or discussion) there was at least a long paragraph on the issue reporting specific references. 5 = the issue was cited in the title and/or in the abstract and/or as a keyword. The issue was the main topic of the article, which was debated in different parts of the text. 2.6.2. Qualitative Outcomes A series of information, although not evaluable in a quantitative way, was also collected from full texts. 1. 2. Main definitions of consciousness. Raters were asked to report if there was a main definition of consciousness (e.g., consciousness is/represents/serves/refers to/consists of/results in/defined as/has to do with) linked to the main theory described in the articles they read. The definition of consciousness expressed by other authors and only cited in the text was not reported unless strictly related to the theory presented in the article. Definitions of what is not consciousness were also included. Definitions of consciousness’ components/parts/sub-elements. Raters noted any definitions concerning parts and elements of consciousness and their definitions (if available). Related terms/features. Raters reported specific terms and/or features concerning the nature of consciousness as described by the single article/theory that is complementary to the main definition.
6 of 58 Brain Sci. 2021, 11, 535 3. Results 3.1. Review Results The search identified 21,661 records after duplicates deletion performed in th The search identified 21,661 records after duplicates deletion performed in the step.were ana In total, 11130 full-text articles were analyzed in step 2. Further, 68 articles In total, 11130 full-text articles were analyzed in step 2. Further, 68 articles were analyzed using the DM (see Figure 1). using the DM (see Figure 1). Figure 1. Prisma flow diagram. Figure 1. Prisma flow diagram. The criteria used to exclude records before the final analysis consisted of using the The criteria used to exclude records before the final analysis consisted term “consciousness” with a generic meaning (e.g., “conscious about their condition”) or of usi term “consciousness” with a genericconsequences of his actions”). with a moral viewpoint (e.g., “conscious about the meaning (e.g., “conscious about their conditio with a moral viewpoint (e.g., “conscious about the consequences of his actions”).
Brain Sci. 2021, 11, 535 77 of 58 of 60 During the analysis of the final 68 articles, we identified 29 different theories of conDuring the analysis of the final 68 articles, we identified 29 different theories of sciousness (Figure 2). Each of them was synthetized following alphabetical order. Figure consciousness (Figure 2). Each of them was synthetized following alphabetical order. 2 graphically represents the number of record distribution over the years. Figure 2 graphically represents the number of record distribution over the years. Figure 2. Graphic representation of the number of articles found for each theory in the period analyzed. ADT: apical Figure 2. Graphic representation of the number of articles found for each theory in the period analyzed. ADT: apical dendrite theory; AIM: activation/information/mode-synthesis hypothesis; ART: adaptive resonance theory; AST: attention dendrite theory; AIM: activation/information/mode-synthesis hypothesis; ART: adaptive resonance theory; AST: attention schema theory; COI: cross-order integration theory of consciousness; CP: centrencephalic proposal; CCS: consciousness schema theory; COI: cross-order integration theory of consciousness; CP: centrencephalic proposal; CCS: consciousness state space model; DCT: dynamic core theory; EM/CEMI: electromagnetic field hypothesis/consciousness electromagnetic state space model; DCT: dynamic core theory; EM/CEMI: electromagnetic field hypothesis/consciousness electromagnetic Information field theory; GWT: global workspace theory; GNW: global neural workspace; HOT/FOR: higher order theories Information field theory; GWT: global workspace theory; GNW: global neural workspace; HOT/FOR: higher order theoof consciousness/first orderorder representational theory; IIT: integrated information LRMB: layered reference reference ries of consciousness/first representational theory; IIT: integrated information theory; theory; LRMB: layered model of model of MCTT: memory memory consciousness and temporality theory; NIH: network hypothesis; PFT: passive frame the brain; the brain; MCTT:consciousness and temporality theory; NIH: network inhibitioninhibition hypothesis; PFT: passive frame theory; PToC: psychological theory of consciousness. Q theories—Orch OR: orchestrated objective reduction theory; PToC: psychological theory of consciousness. Q theories—Orch OR: orchestrated objective reduction theory; single theory; consciousness hypothesis; quantum no-go theorems; the three layer three layer model; Koehler’s mathematical particle single particle consciousness hypothesis; quantum no-go theorems; themodel; Koehler’s mathematical approach; approach; timeless and the four-dimensional Einstein; RPT: radical plasticity thesis; SPC: semantic pointer competition timeless and spaceless; spaceless; the four-dimensional Einstein; RPT: radical plasticity thesis; SPC: semantic pointer competition theory of consciousness; Agnati et al.; Bieberich’s; Damasio; Gelepithis’s; Gurwitsch’s; Min’s; O’Doherty’s; Retheory of consciousness; Agnati et al.; Bieberich’s; Damasio; Gelepithis’s; Gurwitsch’s; Min’s; O’Doherty’s; Rejikumar’s. jikumar’s. The Mesocircuit hypothesis is not abbreviated. The Mesocircuit hypothesis is not abbreviated. Figure 3 reports the maximum score obtained by the articles analyzed for each theFigure 3 reports the maximum score obtained by the articles analyzed for each theory. ory. dimension number 1 (NCC), the the maximum score was observed in 11 theories; In inIn in dimension number 1 (NCC), maximum score was observed in 11 theories; in in D2 (Association between consciousness and other cognitive functions) in three theories; D2 (Association between consciousness and other cognitive functions) in three theories; in D3 (translation from theory to clinical practice) in two theories; in D4 (quantitative in D3 (translation from theory to clinical practice) in two theories; in D4 (quantitative measures of consciousness) in four theories; in D5 (consciousness, sensory processes, and measures of consciousness) in four theories; in D5 (consciousness, sensory processes, and the autonomic nervous system) in six theories; and in D6 (subjectivity) in two theories. the autonomic nervous system) in six theories; and in D6 (subjectivity) in two theories. Regarding the total number dimensions debated by by each theory, we found that the Regarding the total number ofof dimensions debated each theory, we found that the range range was comprised of a minimum of two dimensions debated in the selected articles was comprised of a minimum of two dimensions debated in the selected articles (NIH and (NIH and O’Doherty’s theory) to a maximum of all as reported as reported in theories O’Doherty’s theory) to a maximum of all dimensions,dimensions, in Table S2. TheTable S2. The the highest number of analyzed articles were the group of the quantum theories of withtheories with the highest number of analyzed articles were the group of the quantum theories of consciousness (n = (n the and = GWT/GNW (n = 9), whereas whereas 18 consciousness (n = 12), the IIT12),= 8),IIT (nthe8), and the GWT/GNW (n = 9), 18 theories theories were described by a single were described by a single article. article. For each analyzed article, we reported the definition of consciousness (Table 1). Regarding the various definitions found, we decided to report the exact sentences found in each article to avoid misinterpretation. Frequencies of the main terms used in the main definitions of consciousness by each theory were also reported in Table 2.
Brain Sci. 2021, 11, 535 8 of 58 8 of 59 Figure Graphic representation of the maximum scores obtained in the six dimensions Figure 3. 3. Graphic representationof the maximum scores obtained in the six dimensions analyzed by each theory. The each The figures depicted the scores achieved each theory (displayed in in alphabetical order) in each dimension of the dimenfigures depicted the scores achieved by by each theory (displayedalphabetical order) in each dimension of the dimensional sional model. We considered the maximum each for each theory among the articles belonging to it. The y-axis represents model. We considered the maximum score for scoretheory among the articles belonging to it. The y-axis represents the scores the scores ranging from 0 to 5. D1 refers to the neural correlates of consciousness dimension. D2 refers to the association ranging from 0 to 5. D1 refers to the neural correlates of consciousness dimension. D2 refers to the association between between consciousness and other cognitive functions dimension. D3 refers to translation from theory to clinical practice consciousness and other cognitive functions dimension. D3 refers to translation from refers to consciousness, sensory prodimension. D4 refers to the quantitative measures of consciousness dimension. D5 theory to clinical practice dimension. D4 refersandthe quantitativenervous system dimension. D6 refers to D5 refers to consciousness, sensory processes, and the cesses, to the autonomic measures of consciousness dimension. the subjectivity dimension. autonomic nervous system dimension. D6 refers to the subjectivity dimension.
9 of 58 Table 1. Consciousness definitions found in the collected articles. Theory Authors (year) Main Definition Agnati et al.’s proposal LaBerge and Kasevich (2007) [18] Consciousness is an activity that is extended in time and typically continues from the time we awake to the time we fall asleep. Cook (2008) [19] ADT nf Consciousness may be thought as the global result of integrative processes taking place at different levels of miniaturization in plastic mosaics Agnati et al. (2012) [20] AIM Hobson (2009) [21] ART Grossberg (2007) [22] Grossberg (2017) [23] AST Graziano and Kastner (2011) [24] Bieberich’s theory COI Other Terms/Subcategorization Related to Consciousness (Definitions Reported If Available) - Background consciousness: some processes in the brain create “cognitive event that may be called ‘having an impression’ of something”. - Elevated consciousness for selected aspects of background consciousness is assumed to arise when sustained activity of a primary sensory area is sent to higher sensory areas, and where a selected part of the sensory scene is amplified by attentional activity controlled from the frontal lobes. - Foreground consciousness: the elevated attentional activity of a part of the sensory scene in higher sensory areas. - Content of consciousness: the simultaneous impressions from both foreground and background consciousness together; Bieberich (2012) [25] Kriegel (2007) [26] Waking consciousness can be defined as the awareness of the external world, our bodies, and ourselves (including the awareness of our awareness) that humans experience when awake. nf Consciousness is not just a whir of information-processing. Consciousness is not an emergent property or a metaphysical emanation but is itself information computed by an expert system. [ . . . ] Consciousness=awareness (i.e., a perceptual model of attention). Consciousness determines what is perceived as reality. When a subject has a higher-order representation that is unified with the first-order representation it represents, their representational unity constitutes a conscious state. Consciousness determines what is perceived as reality. - Primary consciousness can be defined as simple awareness that includes perception and emotion. - Secondary consciousness depends on language and includes such features as self-reflective awareness, abstract thinking, volition, and metacognition. - Core consciousness (see Damasio’s theory)
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