Origin and evolution of human consciousness

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Origin and evolution of human consciousness

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Franco Fabbroa,b,*, Damiano Cantonec, Susanna Feruglioa, Cristiano Crescentinia a

Cognitve Neuroscience Laboratory, DILL, University of Udine, Udine, Italy PERCRO Perceptual Robotics Laboratory, Scuola Superiore Sant’Anna, Pisa, Italy c Industrial Technical Institute “A. Malignani”, Udine, Italy *Corresponding author: Tel.: +39-3480-171338, e-mail address: [email protected] b

Abstract The study of the origin and evolution of consciousness presents several problems. The first problem concerns terminology. The word consciousness comes from the Latin term conscı˘entı˘a that means “knowledge shared with others.” However, the term consciousness also refers to several other aspects involving both its levels (sleep, coma, dreams and waking state) and contents (subjective, phenomenal and objective). A second issue is the problem of other minds, namely, the possibility to establish whether others have minds very like our own. Moreover, human consciousness has been linked to three different forms of memory: procedural/implicit, semantic and episodic. All these different aspects of consciousness will be discussed in the first part of the chapter. In the second part, we discuss different neuroscientific theories on consciousness and examine how research from developmental psychology, clinical neurology (epilepsy, coma, vegetative state and minimal state of consciousness), neuropsychology (blindsight, agnosia, neglect, split-brain and ocular rivalry), and comparative neuropsychophysiology contribute to the study of consciousness. Finally, in the last part of the chapter we discuss the distinctive features of human consciousness and in particular the ability to travel mentally through time, the phenomenon of joint intentionality, theory of mind and language.

Keywords Problem of other minds, Phenomenic consciousness, Auto-noetic consciousness, Neuropsychology, Theory of mind, Mental time travel, Language and culture

Progress in Brain Research, Volume 250, ISSN 0079-6123, https://doi.org/10.1016/bs.pbr.2019.03.031 © 2019 Elsevier B.V. All rights reserved.

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1 Introduction The word consciousness comes from the Latin term conscı˘entı˘a that means “knowledge shared with others” (Charlton and Short, 1879). In fact, the Latin term conscı˘entı˘a stems from the combination of two words: scio (I know) and cum (with) (Zeman, 2002). The etymology of the term thus indicates that consciousness refers to a form of shared, relational or social knowledge (Dunbar, 2014; Graziano, 2015). Although much has been written about consciousness in recent decades, what consciousness is still largely remains an enigma (Baars, 1997; Damasio, 2010; Dehaene, 2014; Dennett, 2017; Edelman, 2006; Gazzaniga, 1987, 2011; Humphrey, 2011; Kock, 2012; Metzinger, 2009; Nagel, 2012; Revonsuo, 2009; Searle, 2004; Tononi, 2012; Velmans, 2009). This is partly due to the difficulty of defining what is meant by the term “consciousness.” As Adam Zeman pointed out, in neuroscience the term “consciousness” refers to different aspects, such as: the waking state (“conscious” as “awake”); awareness of sensory experiences and psychological processes such as thoughts, emotions, memories, imagination and language (“conscious” as “aware of”); or consciousness as a synonym of “mind,” which implies the ability to know, think, understand, remember, believe, etc. (Zeman, 2002). Moreover, in both common language and neuroscience, “consciousness” has been linked to self-awareness. According to Zeman, there are at least five different ways of understanding self-awareness (Zeman, 2002). The first corresponds to the awareness of being at the center of the attention of another human being (“selfconscious” as “embarrassment-prone”); the second to the awareness of one’s own sensations, perceptions and actions (“self-conscious” as “self-detecting”); the third to the ability to recognize one’s own image in the mirror (“self-conscious” as “selfrecognizing”) (see also Gallup, 1970; Rochat and Zahavi, 2011); the fourth to the awareness of one’s own mental states and those of others (“self-conscious” as “aware of awareness”), an aspect called Theory of Mind (Baron-Cohen, 2003, 2011; Dunbar, 2014); finally, self-awareness is also intended as narrative selfawareness (“self-consciousness” as “self-portraiture and autobiography”) (see also Ananthaswamy, 2015; Feinberg, 2009; Gottschall, 2012).

2 Epistemological aspects More than a century ago, the psychologist William James (1842–1910) argued that consciousness is not a static thing but a process (James, 1904). This position appears to be in agreement with ideas supported by theoretical physics that everything that exists appears to be a process (Rovelli, 2018; Smolin, 2013). For human beings, consciousness corresponds mainly to subjective experience, that is, to the ability to have sensations and perceptions, which have been defined qualia within the field of philosophy of mind (Jackson, 1986; Nagel, 1974). Human beings, as well as all living

2 Epistemological aspects

organisms, are characterized by a physical space, corresponding to the body (self ), which is separated from the rest of the world (not self ) (Humphrey, 1992). Characterizing the differences between knowing ourselves and knowing the world around us, namely, the first- and third-person perspectives, is crucial in the study of consciousness and has generated the problem of “epistemic asymmetry” (Velmans, 2009). A similar issue seems to also involve other scientific disciplines, which do not seem to be completely reducible to one another (for example, biology does not seem to be reducible to physics, nor psychology to biology) (Anderson, 1972; Polany, 1968). In the latter case one can speak of “epistemological pluralism” (Fabbro et al., 2018). Specific psychological characteristics typical of human beings, such as Theory of Mind, cognitive empathy and verbal communication, corroborate the idea that all humans are conscious. This universally widespread belief has been challenged by solipsism, which maintains the impossibility of ascertaining the existence of other conscious minds besides our own (the problem of other minds) (Allen and Trestman, 2017; Russell, 1921). Erwin Schr€ odinger (1887–1961) believed all scientific research, and physics in particular, is based on two undisputed assumptions: first, the so-called “Hypothesis P,” connected to the problem of solipsism, which examines the following questions: “What are the perceptions of another man? How can I even know that he has perceptions? How can I know that he is capable of feeling and thinking something?” (Schr€odinger, 1935) and second, the use of the terms “sensations” and “perceptions” by physicists without knowing what they really are (Schr€odinger, 1967). In fact, the natural attitude of ordinary scientists is to implicitly assume that reality is independent of the mind; this attitude, called naive realism, has been strongly criticized by phenomenology (Gallagher and Zahavi, 2008). odinger, Hypothesis P is not a trivial question; it simply goes According to Schr€ beyond the limits of science. For example, one may wonder if all human beings see the color red in the same way (Humphrey, 2006). The case of individuals with color ontgen (1845–1923), indicates the significance blindness, such as Wilhelm Conrad R€ of the question: in fact before diagnosis, people with color blindness believe that they see colors like everyone else. A central question in the study of consciousness is whether this is a specific mental activity of human beings, as claimed by the French philosopher Rene Descartes (1596–1650), or if it is also present in other living creatures with complex nervous systems (Blackmore, 2005; Panksepp and Biven, 2012; Panksepp et al., 2017). From a philosophical point of view, this is a problem similar to solipsism. Consequently it may be very difficult, perhaps impossible, to establish with certainty whether other living species have phenomenal consciousness (subjective experience) or not. One possibility is to address the question of consciousness in humans and other animal species in a pragmatic way by: (1) rejecting both the positions of those who consider the study of animal consciousness an unsolvable mystery (e.g., Thomas Nagel, Colin McGinn, Steven Pinker), and of those who, such as Nikolaas Tinbergen

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(1907–1988), consider this research useless, because “subjective phenomena cannot be observed objectively in animals” (Tinbergen, 1951, p. 4); (2) assuming, until proven otherwise, that all vertebrates, and some species belonging to other phyla, can experience variable levels of sensations, perceptions and subjective experiences (Fabbro et al., 2015; Feinberg and Mallatt, 2016; Godfrey-Smith, 2016).

3 On the nature of consciousness According to the English psychologist Kenneth Craik (1914–1945), one of the fundamental properties of the mind consists in the ability to represent the world in order to simulate the possible effects of one’s behavior (Craik, 1943). This ability consists of three steps: the transduction of sensory stimuli into neural patterns; their re-elaboration in different symbols, which allow a representation of the world and to perform simulations of it; and the conversion of mental plans into muscular activities. This perspective was endorsed by the paleoneurologist Harry Jerison, according to whom the main function of the nervous system concerns the ability to construct mental models of reality. For Jerison, the ability to represent the world and elaborate simulations constitutes biological intelligence (Jerison, 1973, 1991). This is the ability to represent, in a more or less detailed way, the real world (von Uexk€ull and Kriszat, 1967). According to this perspective, the world, as we know it, and the self, are the most complex and elaborate “objects” of biological intelligence. The idea that the brain generates models is derived from mathematical theories of systems, which have highlighted the need for each regulator to create a model of what is regulated (Conant and Ashby, 1970). To manage survival and reproduction, the brain of a complex organism must create a mental model of the world and the self. Cognitive neuroscience data indicate that both the world and the self are a construction of our brain (Frith, 2007; Libet, 2006), which creates the illusion that they are real. Put in philosophical terms, the models of the world and self are “transparent” (Metzinger, 2009; Revonsuo, 2006). For this reason, the German philosopher Thomas Metzinger has proposed a comparison of consciousness to virtual reality, which likens the brain to a system similar to the most complex flight simulators, with both the operation of the aircraft and that of the pilot being completely simulated, referred to as total flight simulator (Metzinger, 2009, 2018). The images or representations of the self and world generated by the brain are generally effective, allowing the individual to maintain homeostasis, orient him or herself in the world, and pursue personal self-realization. However, the representations of the world and self-generated by the mind can sometimes be incongruous, as happens in visual and somatic illusions, for instance in the so-called rubber-hand illusion, in the illusion of being in another person’s body, or in out-of-body experiences (Botvinick and Cohen, 1998; Lenggenhanger et al., 2007; Metzinger, 2009). Moreover, in dreams, the deafferentated brain is able to generate a truthful image of reality (Hobson, 1988; Hobson et al., 2014).

4 Characteristics of human consciousness

Table 1 Philosophical theories on the ontological nature of consciousness. Philosophers/ scientists Substance dualism: two independent substances exist: the mental and the physical Idealistic monism: the mind is the only existing substance; the external world is either mental itself, or an illusion created by the mind Property dualism: the world is composed of one kind of physical substance, but there are two distinct kinds of properties: physical properties and mental properties Material monism: the variety of existing things can be explained in terms of a single reality or substance

Neutral monism: the mental and the physical are two ways of describing the same elements, which are themselves “neutral,” that is, neither physical nor mental Ontological agnosticism: the relationship between consciousness and brain cannot find an exclusively physical and/ or biological explanation

Neuroscientists

 Pitagoras, Plato, Rene Descartes, Roger Penrose George Berkeley, Johann G. Fichte, Georg W. F. Hegel, Giovanni Gentile, Edmund Husserl Aristoteles, Hilary Putnam, Max Velmans, Karl Popper

Charles Sherrington, John Eccles

Julien O. de la Mettire, Paul Churchland, Thomas Metzinger, Gilbert Ryle, John Searle

Antonio Damasio, Stansilav Dehaene, Atti Revonsuo, Salvatore M. Aglioti

Roger Sperry, Gerald Edelman, Giovanni Berlucchi

Betrand Russell, William James, David Chalmers

Henry Bergson, Thomas Nagel

Franco Fabbro

The nature of illusions and dreams is believed to be of fundamental importance in understanding the problem of mind and consciousness (Metzinger, 2009; Revonsuo, 2009). The ontological nature of consciousness has been the subject of several philosophical and neuroscientific debates. In this regard, several philosophical theories have been put forward, among the most widespread are: the dualism of substance, idealistic monism, property dualism, monist materialism and neutral monism (Kriegel, 2007; Savoldi et al., 2013; Seager, 2007; Searle, 1997) (Table 1).

4 Characteristics of human consciousness According to the Austrian psychologist Franz Brentano (1838–1917), human consciousness always involves a content concerning the world, the body or the mind itself. Thus, consciousness is always awareness of something (Bodei, 2015). This

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characteristic is called intentionality (Brentano, 1874; Gallagher and Zahavi, 2008). The content of consciousness, for example the cherry tree I see out the window, can be an object that belongs to the world I am observing (third-person perspective), or it can be a personal experience, such as the smell of a flower or the experience of pain (first-person perspective) (Revonsuo, 2006, 2009; Velmans, 2009). Thus, in regard to consciousness, one can recognize primarily cognitive aspects, defined as noetic— from the ancient Greek word νο~ υς (nouˆs), meaning intellect or intelligence (Lesher, 1973)—in addition to aspects concerning the ability to feel (sentience) (GodfreySmith, 2016), and connected with sensory, enteroceptive and affective experiences (qualia) (Jackson, 1986; Nagel, 1974; Ramachandran and Hirstein, 1997). The scientific explanation of how phenomenal consciousness and qualia are generated is the “hard problem of consciousness” (Chalmers, 1995, 2010; Grossberg, 2017). The Canadian psychologist Edving Tulving has proposed distinguishing three states of consciousness: anoetic, noetic and autonoetic (Tulving, 1985, 2002, 2005). These three states have been linked to three different forms of memory: procedural/implicit (anoetic state), semantic (noetic) and episodic (autonoetic). Most of the mental activity involved in linguistic and cognitive processing seems to be carried out at a non-conscious (a-noetic) level (Dehaene, 2014). The ability to recognize a person, or to distinguish concepts (trees with respect to cars), refers to cognitive tasks concerning noetic consciousness and semantic memory. Instead, the ability to remember a personal event in one’s life, reconstructing the here and now of a multisensory scene from the past, is a typical cognitive activity involving autonoetic consciousness. This form of consciousness involves the ability to imagine a mental timeline, through which it is possible to remember and reconstruct, in the present moment, the memories of the past. The ability to be simultaneously aware of the present moment and memories of the past is a specific characteristic of autonoetic consciousness, hence a form of higher-order consciousness (Rosenthal, 2005).

5 Neuroscientific theories of human consciousness According to Gerald Edelman (1929–2014), the construction of mental scenes is related to the activity of reentrant circuits, which connect the cerebral cortex with the thalamus. According to Edelman, perceptual experience depends on memory, since primary consciousness (noetic consciousness) is a sort of “remembered present” (Edelman, 1990, 1992). He uses the metaphor of the “remembered present” because the scene, which I now have in my mind’s eye, is constructed on the basis of the elaboration of the stimuli that have occurred in the immediate past. The scene is therefore a mental image, characterized by a background (the space) in which an object or an event is placed in front of a subject who is not yet aware of himself (Edelman, 2004; Edelman and Tononi, 2000; Llina´s, 2001). In humans, there is a higher order consciousness (autonoetic consciousness), which affords personal identity and self-awareness and depends on one’s own language, culture, and capacity to generate a model of the self that mentally extends through time, from the past to

5 Neuroscientific theories of human consciousness

the future. Edelman believed that human self-awareness, with its phenomenal dimension, is an emerging process that depends on the enormous amount of re-entrant brain circuits’ interactions between numerous neuronal maps (Edelman, 2004, 2006). For Giulio Tononi phenomenal consciousness depends on the wealth of integrated information. The wealth of information refers to the fact that each conscious experience is distinguished by an incalculable number of other possible experiences. Integration makes it possible to explain a peculiar aspect of consciousness: its uniqueness and indivisibility. Tononi has developed a mathematical model to measure the integrated information of a system, indicated by the Greek letter Φ, where the vertical line stands for “information” and the circle for “integration” (Tononi, 2012). Tononi and Massimini have developed an experimental technique for measuring the level of integrated information (Φ) in the brain in different states of consciousness. The procedure involves the delivery of magnetic pulses (via Transcranial Magnetic Stimulation, TMS) and the simultaneous recording of evoked brain electrical activity (via Electroencephalography, EEG) during waking, sleep and dream states. The idea is simple: if the system is integrated, the magnetic pulse can spread to many cortical areas; if instead the system is segregated there will be no diffusion. The results confirmed the idea that the integration of electrical activity at the level of different cortical areas is an essential element of conscious experience. In fact, magnetic pulses spread to many cortical areas during waking and dream states, two states of phenomenal consciousness, while they remain segregated in the areas of stimulation in slow-wave sleep and general anesthesia, two conditions in which there is no phenomenal consciousness (Massimini and Tononi, 2018; Tononi et al., 2016). In addition to self-consciousness (autonoetic consciousness) and primary consciousness (noetic consciousness), human beings are probably endowed with a minimal form of consciousness organized in subcortical structures and in particular in the upper portions of the brain stem. Based on an investigation of epilepsy, the Canadian neurosurgeon Wilder Penfield (1891–1976) was one of the first to indicate the critical role subcortical structures (midbrain and thalamus nuclei) played in the genesis of consciousness (Penfield, 1975; Penfield and Roberts, 1959). Subsequently, the North American psychobiologist Jaak Panksepp (1943–2017) correlated the functions of a large series of subcortical structures, responsible for the release of the most important neurotransmitters dopamine, acetylcholine, noradrenaline, serotonin and endogenous opioids, with the processing of emotional-affective consciousness (Denton, 2006; Panksepp, 1998; Panksepp and Biven, 2012). More recently, from a neurological and a neurophysiological perspective, Antonio Damasio (Damasio, 2010; Parvizi and Damasio, 2001) and Bjorn Merker (Merker, 2007, 2013), have supported the idea that minimal states of consciousness are organized in some structures of the midbrain and brain stem, such as the superior colliculus and the periaqueductal gray, and of the thalamus, such as the dorsal pulvinar. The study of the origin and evolution of human consciousness has used data and knowledge coming from: (i) studies on the development of consciousness in children; (ii) neurophysiological research on different states of consciousness

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(sleep, dream, hypnosis, sleepwalking); (iii) neurological and neuropsychological analyses of patients with impaired consciousness; (iv) investigations of the effects of anesthetic and psychotropic drugs; (v) studies on consciousness in animals; and (vi) paleoanthropology, neuroscience, psychology and linguistics studies concerning the evolution of hominids and in particular of Homo sapiens.

6 The development of consciousness in children Research on the development of consciousness in children suggests the existence of different levels of consciousness in relation to the age of children (Rochat, 2001, 2010; Zelazo et al., 2007). From birth to the first year of age, children present a minimal level of consciousness (minC) characterized by the implicit ability to distinguish oneself from others and by the development of an intermodal body schema that allows children to explore the environment and build an implicit sense of self. The minimum level of consciousness seems to emerge in the fetus between the 24th and 30th week of gestation. Between 9 and 13 months of age, when the children begin to indicate the objects and produce the first words, a new level of consciousness emerges, referred to as recursive consciousness (recC). Through this level of consciousness, children are able to connect the contents of two distinct moments of minimal consciousness. For example, when a child sees a cat and utters the word “cat,” the child is able to link the perceptual experience (the recognition of the cat) with the word that labels the experience that is retrieved from semantic memory. At the age of two, children reach the first forms of self-consciousness (SelfC). During this period, they begin to recognize themselves in the mirror, use personal pronouns and manifest conscious emotions, such as embarrassment and shame. They are also able to describe past and future events. For example, a 2-year-old child may be able to remember that yesterday he went to visit the zoo, while being aware of what is happening now in the present. As mentioned before, in order to remember past events and imagine future events, the child must have the capacity to imagine a timeline while maintaining some stable sense of self; in other words, the child has to have the capacity to mentally travel through time, called mental time travel (MTT). Toward the third year of age, children develop the first stage of reflective consciousness (refC1). Through this form of consciousness, children are able to compare two types of arbitrary rules, through their executive and working memory functions, in order to choose one of them instead of persevering in using only one rule. In addition, many 3-year-old children are able to recognize that right now in a small box there are pens while previously there was some candy (Gopnik and Astington, 1988). From 4 to 5 years of age, children develop the second stage of reflective consciousness (refC2). During this period, children’s executive functions and theory of mind develop further. For this reason, children are able to distinguish and coordinate two series of temporal lines: one that relates to their own life and the other that

7 Contributions of neurology to the study of consciousness

relates to the world itself. For this reason, 5-year-olds observed in the original Theory of Mind test are able to realize there were pencils in a Smarties tube (the past of the world) and that continue to be there (present of the world), while recognizing that at a certain moment in their past (personal history) they had believed Smarties were in the tube despite the fact there are now pencils (personal present). Thanks to reflective consciousness, children are able to appreciate the existence of multiple temporal perspectives that refer to the same reality. Because consciousness is a dynamic process, throughout life adults can manifest one of the different levels described in children. In fact, there is a continuous oscillation within the different levels of consciousness present in adults, from the minimum level to the levels of maximum reflective consciousness (Rochat, 2001). For this reason, specific training programs have been developed in many cultural and religious traditions that allow individuals to increase their own states of consciousness and self-awareness, such as mindful awareness (Fabbro, 2019; Tomasino and Fabbro, 2016; Tomasino et al., 2014).

7 Contributions of neurology to the study of consciousness In neurology there are two main aspects of consciousness: contents and levels (Bayne et al., 2016; Posner et al., 2007). The contents of consciousness concern a series of systems operating to elaborate sensory, motor, cognitive and affective information. Some neurological lesions can cause deficits that affect the contents of consciousness, such as aphasias (language), amnesia (memory), agnosia (recognition of objects), neglect (awareness of space), and dementia (deficit of memory and awareness of the self ). On the other hand, the levels of consciousness depend on the activity of a set of brain systems, located mainly in the structures of the brainstem and diencephalon, operating to maintain alertness, attention and awareness of oneself and the world. The level of consciousness can vary physiologically (in waking and in sleep), be modified by drugs (general anesthesia), or change as a result of neurological diseases, such as in coma, vegetative state, minimal state of consciousness and in some forms of epilepsy. A neurologist can objectively observe a patient’s state of consciousness (thirdperson perspective) by evaluating his/her EEG activity, the level of vigilance (wakefulness), as well as the somatic expression of emotions in the patient and his/her ability to organize behavioral plans to achieve complex objectives. Nonetheless, the level of personal experience (first-person perspective), which concerns the ability to develop mental images related to real or imagined objects or events of the world, together with the ability to create a sense of self able to act and experience, is a dimension that can be explored only through verbal communication (Damasio and Meyer, 2009). Within the physiological variation of consciousness levels, many studies have examined sleep, and REM sleep in particular. REM sleep is a condition generally associated with dreams containing visual images, emotions and a preserved

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representation of the self. REM dreams are often associated with spatial and temporal disorientation, bizarreness and unawareness of living a dream experience (Hobson, 1988, 2009). Much attention has been focused on the lucid dream, a condition in which the subject is aware of being immersed in a dream (Baird et al., 2018). General anesthesia is a pharmacologically induced condition that resembles slow-wave sleep and coma. The most common used anesthetics (propofol, barbiturates and benzodiazepines) work through their binding to GABA type A receptors. Their action determines a functional deactivation of the brainstem and some structures of the central thalamus, with consequent loss of consciousness, oculomotor and corneal reflexes, apnea and atony. During general surgery, an individual under general anesthesia reaches a level of consciousness similar to that present in stage 2 of non-REM sleep. In other therapeutic conditions the administration of anesthetics (propofol or barbiturates) allows reaching a level of consciousness in which the EEG becomes isoelectric or flat (Brown et al., 2010). Through brain imaging techniques, it was possible to show that in individuals regaining consciousness after a general anesthesia, the first forms of minimal consciousness to emerge are related to the activity of some structures of the brainstem (locus coeruleus), hypothalamus, thalamus and anterior cingulate cortex (Mashour and Alkire, 2013). Preservation or loss of consciousness is also of fundamental importance in epilepsy. Indeed, forms of epilepsy that affect only focal regions of the brain, partial epilepsies, can be distinguished from the forms that affect extensive and bilateral regions of the brain, generalized epilepsies (Blumenfeld and Meador, 2014). Sudden loss of consciousness may be present in both generalized epilepsies (absence epilepsy, tonic-clonic epilepsy) and in some focal epilepsies (temporal lobe epilepsy). Loss of consciousness in these forms of epilepsy is correlated with an abnormal increase in activity in some structures of the midbrain and medial thalamus, and a decrease in activity in the cingulate gyrus, medial frontal cortex, and medial parietal lobe, or precuneus (Blumenfeld, 2009). The most serious neurological disorders of consciousness include coma, vegetative state and minimally conscious state. These disorders are due to brain injuries of traumatic origin, or to hemorrhagic stroke. Coma is characterized by a complete loss of consciousness; the sleep-wake cycle is lost and the patient’s eyes are kept closed. It is a temporary condition that can last from a few hours to weeks. Thereafter, patients die or emerge from the coma and pass into the vegetative or minimally conscious state. In the vegetative state the sleep-wake cycle is present, the eyes can be opened but patients do not seem to be aware since they only show non-purposeful movements. Instead, in the minimally conscious state, patients sometime show signs of awareness, for example by executing a command or reacting appropriately to an environmental stimulus (Laureys and Tononi, 2009). Brain imaging techniques have led to the discovery that approximately 20% of people in the vegetative state are in fact conscious. These individuals are able, on verbal request, to imagine they are playing a game of tennis (activation of the premotor cortex), or to mentally explore their home (activation of the parietal lobe and the parahippocampal gyrus). Through fMRI or EEG it has been possible to establish

7 Contributions of neurology to the study of consciousness

a communication code with these patients: in order to say “yes,” they have to imagine playing tennis, to say “no” they have to imagine walking in their homes. In this way patients in the vegetative state were able to correctly answer a series of questions such as “Do you have any siblings?” or “Do you have any sisters?” and experimentally show that they were conscious (Monti et al., 2010; Owen, 2014). Particular attention has also been given to assessing the level of consciousness in children with anencephaly who, due to a severe cerebral malformation, do not develop cerebral hemispheres. In the past it was believed that these children, who can live up to even a few decades, were in a persistent vegetative state. Because they cannot verbally express themselves it was difficult to understand their state of consciousness (Hypothesis P). However, several investigations on small groups and more recently, a study on a group of >100 anencephalic children have revealed that they produce a sleep-wake cycle and may express emotions and moods congruent with ongoing social situations (Aleman and Merker, 2014; Shewmon et al., 1999). Moreover, after brief lapses of consciousness, which may be present due to episodes of absence epilepsy, contact with parents is resumed. In these children, the minimal state of consciousness is maintained through activity by some midbrain structures (superior colliculus) connected with the hypothalamus and the periaqueductal gray (Merker, 2007). In a recent literature review, Christof Koch et al. analyzed neural correlates of consciousness (Koch et al., 2016). Different from proposals put forward by Wilder Penfield, Bjorn Merker and Jaak Panksepp, these authors argue that midbrain and diencephalon structures are not sufficient to sustain consciousness. Moreover, based on reports of some exemplar clinical cases (e.g., agenesis of the cerebellum, bilateral frontal lobectomy), they argued that consciousness and its contents are not dramatically affected by lesions of the cerebellum and frontal lobes. They instead recognize a temporal-parietal-occipital area of the posterior cerebral cortex as the best candidate for the neural correlates of consciousness. In our view, however, it is possible that the authors’ general conclusions may be affected by the lack of adequate neuropsychological tools necessary for reliable measures of patients’ consciousness (different from the neuropsychological tests currently available for many other cognitive and affective domains such as language, memory, attention, etc.). For example, a few years ago one of us (Fabbro) had the opportunity to study one of the few reported cases of a patient with an acquired lesion of the periaqueductal gray. She was believed to be in coma by clinicians because she could not open her eyes (eyelid paralysis) and nor speak. After some time, the clinicians realized that the patient was suffering from a very serious condition known as acinetic mutism. The patient appeared to be frozen, but if sufficiently stimulated she was able to understand complex sentences (Esposito et al., 1999). How this could be associated with the patient’s level of consciousness is hard to say, since only the usual, rather coarse, clinical measures for disorders of consciousness had been used. The same reflections can be made for one of the first reported cases of complete congenital agenesis of the cerebellum, which was also studied by one of us (Fabbro) (Tavano et al., 2007a,b). The patient had mild intellectual impairment but was autonomous, with

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no significant motor, cognitive, or linguistic problems. His consciousness was defined as normal, but due to the limitations of tools available it could not be measured accurately. The same way of proceeding, once defined as anecdotal, is generally used to define the state of consciousness of patients with brain tumors affecting, for example, the thalamus nuclei, basal ganglia, or cerebral areas such as the frontal lobes (Fabbro and Paradis, 1995; Fabbro et al., 1997; Peru and Fabbro, 1997; Urgesi et al., 2010).

8 Contributions of neuropsychology to the study of consciousness The neuropsychological investigation of patients with specific visual impairments has made a significant contribution to the study of consciousness (Naccache, 2009). In particular, the analysis of a series of patients presenting the phenomenon of “blindsight” highlighted a dissociation between the unconscious recognition of stimuli and the visual phenomenal consciousness. In the first half of the 1970s, the English neuropsychologist Lawrence Weiskrantz (1926–2018) studied the visual skills of a patient (D.B.) who had undergone the surgical removal of a malformation in the right occipital lobe (Weiskrantz, 1986). Analyzing the lesion-related visual deficits (hemianopia of the right upper quadrant), short light stimuli were sent into D.B.’s blind quadrant: the patient reported he saw nothing, although he was able to guess well above chance level (i.e., above 50%), the effective presence of stimuli in this blind quadrant (phenomenal consciousness). Studies in monkeys (macaques) with lesions to the primary visual areas showed the presence of the same dissociation (Stoerig and Cowey, 1997). David Milner and Melvyn Goodale described a second type of dissociation in a patient (D.F.) with a bilateral lesion to specific areas of the occipito-temporal cortex (Goodale and Milner, 1992). Following the lesion, the patient was no longer able to recognize common objects or discriminate the orientation of simple lines and geometric shapes. Brain imaging revealed a lesion to occipito-temporal areas (ventral visual pathways) but not to occipito-parietal areas (dorsal visual pathways). Though unable to describe the orientation of the slot in a postbox that could be rotated, D.F. was able to post the letter, orienting her hand appropriately, without making mistakes. The study of this patient led to the hypothesis of the existence of the two visual pathways: the ventral visual pathway would be involved in the phenomenal visual consciousness or the “what” pathway, while the dorsal visual pathway would mainly subserve unconscious visuo-motor transformation, representing the “how” pathway (Milner and Goodale, 2006). Recently, a similar organization has also been hypothesized for language: ventral pathways of language processing could be related to semantic awareness while dorsal (unconscious) pathways could be more involved in the phonological and syntactic aspects of language processing (Friederici, 2015; Skeide and Friederici, 2016).

8 Contributions of neuropsychology to the study of consciousness

Unilateral spatial neglect is another important area of consciousness studies. It is characterized by the inability to explore, perceive and respond to stimuli presented in the contralateral space of a lesion, independent of significant sensory or motor deficits (Vallar and Calzolari, 2018). The neglect syndrome generally depends on lesions located in the right parietal lobe. Many patients with neglect lose the phenomenal awareness of objects placed in the left part of a space. Spatial neglect affects not only the ability to explore and recognize objects of the external world, but also the contents of one’s imagination or spatial memory (Bisiach and Luzzatti, 1978). The neglect syndrome has been associated with a dysfunction of spatial attention systems; this means that in order to have phenomenal consciousness, both the ventral visual pathways and neural structures supporting spatial attention must be preserved (Naccache, 2009). A very important neuropsychological condition for the study of consciousness is the split-brain syndrome, also called callosal disconnection syndrome. This condition is characterized by a number of neurological abnormalities arising from the partial or complete lesioning of the corpus callosum; for example, a surgical procedure performed for the purposes of treating intractable forms of epilepsy. After the complete section of the corpus callosum the patients appear normal, without evident cognitive disturbances; however, they are no longer able to verbally recognize objects placed in their left hand, nor write with their left hand. By using the tachistoscopic technique, Roger Sperry (1913–1994) and Michael Gazzaniga showed that split-brain patients could consciously comply with verbal requests sent to their left hemisphere, but unconsciously respond to stimuli presented to the right hemisphere (single words). According to Gazzaniga, human beings have multiple mental systems, each one capable of perceiving, memorizing and coordinating effective actions; nevertheless, only one of these systems is endowed with awareness: the verbal interpreter, which is located in the left cerebral hemisphere. When faced with behaviors organized by non-verbal mental systems (located in the right cerebral hemisphere), the verbal interpreter continually constructs explanation for actions, possibly trying to make sense of what it sees happening (Gazzaniga, 1987, 2011). The neural correlates of consciousness have also been studied from a neurophysiological perspective, analyzing the phenomenon of ocular rivalry: a condition consisting in the simultaneous presentation of two different images to both eyes. Instead of experiencing a superimposed view of the two figures, the subject is aware of seeing only one of the two images alternately and with random duration. The Greek neurophysiologist Nikos Logothetis has used this paradigm to study the neural correlates of visual consciousness in primates (Logothetis, 1998). He trained macaques to pull a lever when they saw a human face, a second lever when they saw a flag and to release both levers when they saw a quilt. In this way he managed to associate the motor response with the conscious perception of a visual stimulus. While animals performed the task, electrical activity was recorded at various points in the visual pathways. It was shown that the activity of primary visual areas was not correlated with the awareness of visual stimuli. Neurons in these areas discharged

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both when the image was perceived and when it was not. The situation was different in higher order visual areas, such as in the inferior temporal cortex, where neurons responded only to the image that the monkey was aware of seeing. Furthermore, the prefrontal cortex also appeared to mediate conscious visual perception. Similar results have been obtained studying ocular rivalry in humans using brain imaging techniques (Blake and Logothetis, 2002).

9 Consciousness in vertebrates Most of the animal taxonomic groups (phyla) first appeared during a short period of a few million years, called the “Cambrian explosion” (approximately 541 million years ago). The animals of previous periods were small, soft-bodied and slow. After the Cambrian explosion, animals with complex active bodies appeared on Earth (Trestman, 2013). In particular the animals belonging to three phyla (arthropods, chordates and mollusks) developed complex active bodies equipped with: (i) appendages (claws, hands, tentacles, mouths); (ii) bodies capable of producing movements with many degrees of freedom; (iii) sensory organs suitable for receiving distal information (eyes); and (iv) information processing systems capable of developing autonomous mobility and active manipulation of objects. The factors that caused the Cambrian explosion have not been completely clarified. One of the most significant factors seems to have been the increase of oxygen concentration in the atmosphere and water. Subsequently, organisms with sensory organs (vision) developed and started to engage in active predatory behaviors. The predatory behaviors, initially manifested in particular by species belonging to the arthropoda phylum, promoted the differentiation of taxonomic groups; in fact, some groups developed defensive armor, while others evolved defensive behaviors based on an increase of mobility (chordates). The British zoologist Andrew Parker claimed that a crucial factor in the Cambrian explosion was the development of vision in some phyla (Parker, 2003). In his opinion, the appearance of eyes led to an escalation in predatory skills in some species, which in turn led to the development of appropriate defensive strategies in others. By contrast, Michael Trestman, a philosopher of mind, pointed out that one of the critical aspects in the Cambrian explosion was not primarily due to the appearance of eyes, but to the development of neural and cognitive systems connected to vision, defined as basic cognitive embodiment (BCE). In his opinion, these cognitive skills have developed to integrate sensory information and link them to active movement. This allowed the creation of a representation of the “space,” which comprised a “scene” composed of a center (the “self”) placed in front of the “object” (Jerison, 1973, 1991). Trestman argues that all animals with BCE capacities, that is at least those belonging to the taxonomic groups of arthropods, chordates and mollusks, have acquired an agentive or action-oriented bodily self-awareness, since they are able to move autonomously in space, interacting with the objects contained in it (Colin and Trestman, 2017; Trestman, 2013).

9 Consciousness in vertebrates

However, as mentioned before, the problem of animal consciousness remains an area of active debate among neuroscientists, psychologists, veterinarians, breeders, philosophers and politicians. This is not just a philosophical or cognitive argument as it has important practical implications on how to breed and care for animals. In general, studies on consciousness in non-human animals are based on research integrating neuroanatomical, neurophysiological and behavioral data (Edelman and Seth, 2009). Based on these studies, a substantial part of neuroscientists and cognitive psychologists believes that primary consciousness (noetic consciousness) is present in mammals and birds, and this should be related to the presence of corticothalamic reentrant and reverberating circuits (Edelman, 1992; Edelman and Seth, 2009; Edelman and Tononi, 2000). Other neuroscientists believe that primary consciousness is organized in some structures of the midbrain and the diencephalon (optic tectum, periaqueductal gray, hypothalamus, central thalamic nuclei). These structures are involved in sensorimotor integration (including eye movement), the regulation of communicative and affective basic functions, and motor initiation (Damasio, 2010; Fabbro et al., 2015; Feinberg and Mallatt, 2016; Merker, 2007; Panksepp, 1998; Panksepp and Biven, 2012). According to this perspective, primary consciousness would already be present in fish showing eye movement (Trestman, 2013; Trevarthen, 1968) (Table 2). As already mentioned, Hypothesis P (the problem of other minds) deals with the problem of ascertaining the existence of other minds besides our own through our phenomenal consciousness. The assumption that other human beings are conscious is essentially based on the ability to communicate our inner states through language. In the case of non-human animals this capacity is not available (Farah, 2008; Sober, 2000). Nevertheless, neuroscience and cognitive psychology studies have shown the presence of significant imitative, cognitive and social skills in numerous species of vertebrates, including many species of fishes (Fabbro et al., 2015; Feinberg and Mallatt, 2016; Watanabe et al., 2017). For example, recently a species of fishes (Labroides dimidiatus) appeared to pass the “mirror test,” which is often used as a benchmark for self-consciousness (Kohda et al., 2018). Given the complexity Table 2 Levels of the self and consciousness in the various vertebrate classes. Proto- Core- SelfNarrative- Possible level of self self consciousness self consciousness Fish (Teleostei) Amphibians Reptiles Birds Mammals Homo Sapiens (current)

+ + + + + +

+ ? + + + +

? ? ? + + +

+

Notes: Based on Fabbro et al. (2015) and Feinberg and Mallatt (2016).

Noetic consciousness ? Noetic consciousness Noetic consciousness Noetic consciousness Autonoetic consciousness

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of the issue concerning consciousness in non-human animals, it seems appropriate to cautiously hypothesize that some form of primary consciousness is likely present in all vertebrates, i.e., that all vertebrates are sentient beings (Low et al., 2012). This assumption should be taken into account in the breeding and care of all vertebrates.

10 Origin and characteristics of consciousness in Homo sapiens The social and cultural dimensions exhibited by Homo sapiens distinguish the species from other primates. As regards the social dimension, human beings are born, grow up and live in social contexts nested inside each other. The nucleus of human sociality is the mother-child dyad and the extended “family” in which little humans develop (about 10–15 individuals). Some families meet periodically and constitute so-called “bands” (about 30–40 individuals). Five or six bands form a village or “clan” of about 150 individuals, while about 10 villages constitute a community (about 1500 individuals). The British evolutionary psychologist Robin Dunbar has shown that human group size (families, bands, clans and communities) is a universal characteristic, which is present in every social structure of hunter-gatherers in any part of the world (Dunbar, 1993, 2010). Furthermore, neuroanthropological research by Dunbar indicates that the size of the fundamental human group, namely, that of the village or clan (150 individuals), was one of the main factors connected with the progressive growth of the cerebral cortex in the Homo species versus other species of human primates (Dunbar, 1992, 1997, 2014). The second main feature of Homo sapiens is its high propensity for cultural learning. By culture we mean the ability to develop specific types of learning concerning: (i) social behavior (ambulation, sphincter control, singing and dancing), (ii) the construction of tools, (iii) the learning of a language, and (iv) the appropriation of typical habits and customs of a society (clothing, culinary traditions, religious traditions, narratives and literature, etc.). In addition to showing a great propensity to learn, humans have also developed the inclination to teach, which is absent in other primates. In fact, humans dedicate a large part of the interaction between mother and child to teaching, a behavior that is very sporadic in chimpanzees (Mithen, 1998). For this reason, the Russian neuropsychologist Alexander R. Luria (1902–1977) claimed that humans’ higher cognitive functions, in particular memory, voluntary attention, language, mathematical skills, reasoning, reading and writing, depend on both specific neurological substrates (especially the cerebral cortex) and the historical-cultural environment in which such functions have developed (Luria, 1962, 1973). Indeed, if a child does not grow up in a human culture, he will not be able to learn to walk, speak, control sphincters, or develop fine motor skills (Blakmore, 1977; Rymer, 1994). Moreover, all these different types of learning have critical windows of sensitivity whose closure makes learning more difficult (Fabbro, 1999; Fabbro and Cargnelutti, 2018; Kuhl, 2010).

10 Origin and characteristics of consciousness in Homo sapiens

If we consider the behavioral and cognitive abilities of Homo sapiens, we find a series of distinctive features that we do not share with the species closest to us, such as Pan troglodytes (common chimpanzee) and Pan paniscus (bonobo). These characteristics concern the bipedal ambulation, fabrication of lithic tools, cooking of food, shared attention, language, culture and Theory of Mind (Table 3). The first of these, bipedal ambulation, is believed to have developed more than 4 million years ago (Hay and Leakey, 1982; Tattersall, 2012). This brought about radical physiological and cognitive changes, modifying breathing, freeing upper limbs and hands for new functions, thus transforming the way one could have “access” or affordance to the world (Gibson, 1979; Provine, 2000). Moreover, the cooking of foods occurred between 1.8 and 1 million years ago and changed the digestive system, which increased the percentage of nutrients absorbed from food, thus favoring an increase in brain volume (Tattersall, 2012; Wrangham, 2009). The fabrication of stone tools began about 3 million years ago and represented a crucial feature differentiating the ancestors of the Homo species from all other animals (Shumaker et al., 2011). The ancestors of the Homo species could make ever more complex lithic instruments, which were conserved for a long time after their use. In addition, they were made of materials coming from areas far from the place the tools were actually manufactured (Dunbar, 2014; Mithen, 1998; Tattersall, 2012). From a cognitive point of view, this means ancestors of the Homo species, probably since the australopithecines, already showed the ability to imagine the future use of a stone, which was collected and conserved for this reason (Fabbro et al., 2015). Thus, technological skills in humans are related to the temporal dimension of existence (Heidegger, 1927). The ability to travel mentally through time (Mental Time Travel), which is associated with episodic memory and autonoetic consciousness, is a cognitive dimension that seems to be present only in Homo sapiens and in some of its ancestors (Corballis, 2011, 2018; Suddendorf and Corballis, 2007; Tulving, 2005). Another cognitive aspect that appears to be typical of the human species and some of its ancestors (Homo heidelbergensis) is the ability to cooperate, which is associated with the emergence of Theory of Mind, i.e., the ability to attribute beliefs, intentions, thoughts and moods to oneself and to others (Tomasello, 2014, 2018). In fact, it is necessary to imagine the intentions of others (joint intentionality) in order to effectively collaborate with them, a task that even chimpanzees seem unable to accomplish. The Theory of Mind ability does not only include the possibility to imagine what others believe or are thinking about or second-order intentionality, but also what others think that I am thinking or third-order intentionality (Baron-Cohen, 2003, 2011). The Theory of Mind is a cognitive device that plays a fundamental role in human social relationships. For example, the narratives present in novels, or in the most diverse religious traditions, may underlie complex cognitive abilities that imply up to a fifth-order intentionality: I want [1] you to believe [2] that there is a supernatural agent that understands [3] that I wish [4] that he intends to intervene [5] if you do not conform to the established rules (Dunbar, 2010; Dunbar et al., 2005). These are scenarios that deal with mental/virtual worlds that have been created through social interactions and underlie many human institutions.

333

Table 3 Cognitive and communicative skills in the different species of hominids.

Pan troglodytes (common chimpanzee) Australopithecus Homo habilis Homo erectus (ergaster) Homo heidelbergensis Homo neanderthalensis Homo sapiens (>100,000) Homo sapiens (<100,000)

Bipedal locomotion

Manufacture of lithic tools

Habitual mental time travel

Cooking of food

+ + +

+ + +

+ + +

+

+ +

+ +

+ +

+ +

+ +

+ +

+

+

+

+

+

+

+

+

+

+

+

+

Notes: Based on Tattersall (2012) and Tomasello, (2014).

Theory of mind

Pantomime gestures

Vocal language

? +

+

10 Origin and characteristics of consciousness in Homo sapiens

Finally, language is what most radically distinguishes human cognition from that of other animals. Language does not only represent a doubly articulated communication system (Jakobson and Waugh, 1979; Sapir, 1921), but also a cognitive faculty that allows humans to expand their creativity and knowledge in an almost infinite way (Chomsky, 1975; Jerison, 1973). Convergent studies of genetics, linguistics and paleo-archeology indicate that language was probably invented shortly before the origin of symbolic culture (100,000–80,000 years ago) by a small group (several thousand individuals) of Homo sapiens in Africa (cf. Tattersall, 2012, 2017). It is possible to conceive that after a rather short period of time (a few thousand years), numerous languages then developed. This probably favored the increase of cultural diversity, the development and diffusion of new hunting technologies (use of arches and poisoned arrows), and the progressive migration of Homo sapiens in all continents correlated with the extinction of all other Homo species (Homo neanderthalensis, Homo erectus, Homo floresiensis) (Fabbro, 1999, 2018). According to this perspective, the invention of language is comparable to the invention of writing that took place about 5400 years ago. In both cases some brain structures, which had evolved for other functions, were subsequently recycled or exploited for subserving language, reading and writing, known as exaptation phenomena (Dehaene and Cohen, 2007; Lieberman, 2013). In fact, the acquisition of the mother tongue (L1) is a complex sensory-motor and cognitive task that takes many years to reach an adequate level (Locke and Bogin, 2006). Furthermore, some children with no evident neurological or intellectual disorders present significant difficulties in the acquisition of language and/or in reading and writing (Dehaene, 2009; Krishnan et al., 2016). Human language is subserved by a complex system of brain structures linked with different types of memory (nondeclarative memories, semantic and episodic memories) (Paradis, 2009; Ullman, 2001). According to the notion of exaptation in human evolution, the development of phono-articulatory structures must have preceded the invention of language by hundreds of thousands of years. It is likely that human vocal organs evolved for singing, a behavior that probably played a considerable role in sexual selection, as Charles Darwin emphasized (Darwin, 1871). Singing also played an important role in the social interaction of human groups, allowing a significant reduction of social and individual stress (Dunbar et al., 2012; Giustison and Bergman, 2017; Masataka, 2008; Mithen, 2006). In this sense, musical experience anticipated and favored the development of language (Brown, 2017; Merker et al., 2015). The possibility of using a language to exchange knowledge, reason, reflect, and tell stories, including one’s own biography, has radically changed the mind and human consciousness. An example of this transformation was described in the book A Man without Words by Susan Schaller, which documented the story of a young Mexican deaf-mute man. Through the learning of American Sign Language, the man enters the world of language, a dimension that appeared shocking, yet allowed him to get out of the isolation and “cognitive prison” in which he had lived up to that moment (Schaller, 1991). It is thus likely that the structural characteristics of different languages, the ways of life of diverse cultures, as well as the new

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media of communication, can influence human consciousness in ways that are not yet clear (Deutscher, 2010; Jaynes, 1990). As highlighted by many spiritual and meditative traditions, the autonoetic state of consciousness is also probably characterized by different levels. For example, the practice of mindfulness meditation shows how easy it is in everyday life to slip from phenomenal states of consciousness to noetic or anoetic levels of consciousness (Fabbro, 2019; Malinowski, 2013).

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Further reading

Ullman, M.T., 2001. A neurocognitive perspective on language: the declarative/procedural model. Nat. Rev. Neurosci. 2, 717–726. Urgesi, C., Aglioti, S.M., Skrap, M., Fabbro, F., 2010. The spiritual brain: selective cortical lesions modulate human self-transcendence. Neuron 65, 309–319. Vallar, G., Calzolari, E., 2018. Unilateral spatial neglect after posterior parietal damage. Handb. Clin. Neurol. 151, 287–312. Velmans, M., 2009. Understanding Consciousness. Routledge, London. Von Uexk€ull, J.V., Kriszat, G., 1967. Ambiente e comportamento (di P. Manfredi, trad.). Il Saggiatore, Milano. Watanabe, S., Hofman, M.A., Shimizu, T. (Eds.), 2017. Evolution of the Brain, Cognition, and Emotion in Vertebrates. Springer, Tokyo, Japan. Weiskrantz, L., 1986. Blindsight: A Case Study and Implications. Oxford University Press, Oxford. Wrangham, R., 2009. Catching Fire: How Cooking Made Us Human. Basic Books, New York. Zelazo, P.D., Gao Hong, H., Todd, R., 2007. The development of consciousness. In: Zelazo, P.D., Moscovitch, M., Thompson, E. (Eds.), The Cambridge Handbook of Consciousness. Cambridge University Press, Cambridge, pp. 405–432. Zeman, A., 2002. Consciousness. A User’s Guide. Yale University Press, New Haven.

Further reading LeDoux, J.E., 2015. Anxious: Using the Brain to Understand and Treat Fear and Anxiety. Penguin Books, New York.

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