Biological and Physiological Markers of Consciousness

What is consciousness?  Is it unique to humankind? What about animal consciousness? There is little that is more tantalising than the question of how and why consciousness came to exist in science. In the following paragraphs, I will try to give a short glimpse into the scientific definition and markers of consciousness based on the most recent theories on the topic.

According to researchers Bering and Borklund, consciousness is a “higher-order cognitive system enabling access to intentional state.” Cognitive neuroscientist Stanislas Dehaene argues in his own words, for a “functionalist” view of consciousness, which posits that consciousness evolved in order to fulfil a particular operational function: that of, “transforming incoming information into an internal code that allows it to be processed in unique ways”. Dehaene argues that consciousness’ specific evolutionary role is “learning over time,” and that it is “a social sharing device”. Similar to Dehaene’s view, according to Tononi and his integrated information theory (IIT), consciousness is integrated information and its quality is given by the informational relationships generated by a conglomerate of components.

Definition of Consciousness

The main reason why it is very difficult to pinpoint the neural markers, and hence the evolution of consciousness, is that there isn’t a widely-agreed-upon description of what consciousness actually is. According to the Oxford English dictionary, consciousness is “the state of being aware of and responsive to one’s surroundings.” Dehaene described consciousness as “the mind’s virtual-reality simulator”, and quoting cognitive scientist Marvin Minsky, “the mind is what the brain does.”

Consciousness is usually separated into primary consciousness and higher-order consciousness. Primary consciousness refers to the existence of a multimodal scene composed of perceptual and motor events that is reportable. This means having mental images in the present, without a sense of a person with a past and future. Higher-order consciousness includes referral of the contents of primary consciousness to interpretative semantics, and includes a sense of self and the ability to explicitly formulate past and future events, and an awareness of being aware.

The current framework we have for defining and explaining consciousness is mainly anthropocentric. This creates a drawback for grasping the totality of the question of the emergence of consciousness. The standard behavioural indicator for consciousness in humans is ‘‘Accurate report (AR)”. This is chiefly due to the fact that descriptive reports are not limited to, but mainly available to us verbally, in language.

However, the fact that consciousness is not limited to human beings is by now a widely accepted idea. While on the one hand, some researchers have posited that consciousness is a uniquely human trait, there have been some to suggest that there is consciousness in insects, and some scientists who have even hypothesised that consciousness exists in protozoans. Thus, we look for certain markers of consciousness that can be generalisable to all beings that possess the trait of consciousness, but since each species has a particular version of anatomical and behavioural traits,  defining consciousness becomes problematic. Words are typically made up to describe human features, but when these words are used to describe other species, the question of appropriate use of consciousness depends on a somewhat arbitrary cut-off as to degree of resemblance with the relevant human feature. Therefore whether an animal has an analogous or homologous human attribute which would justify a shared terminology is subjective and depends on semantics. For this reason, a very rigid description of consciousness is not practical. However in order to evaluate consciousness in non-human animals, it is crucial to identify certain relevant correlates.

Markers of Consciousness

There are a number of physiological-anatomical, and cognitive-behavioural criteria within the anthropocentric framework that have been associated with conscious and unconscious states. Since our point of origin is that the healthy human brain is conscious, human beings are our main reference for defining consciousness. Currently, attempts at establishing such criteria are more effective along the physiological-anatomical line than along the cognitive-behavioural line. These attempts have been made with EEG, MEG, fMRI, and PET scan technologies and have resulted in several hypothetical criteria. I will begin with summarising what is known about the general markers of consciousness within this anthropocentric framework.

Behavioural-cognitive signs of consciousness

Unlike in humans, establishing behavioural measures for consciousness in animals is a risky endeavour, since accurate report cannot be applied to other species. The most homologous version of the accurate report is doing matching tasks with primates. The ability to distinguish between and categorise stimuli representative of cognition, is very varied across all species, and has been shown in mammals, birds, reptiles, fish, invertebrates, insects and might even exist in single-celled organisms. Some of the cognitive-behavioural characteristics of consciousness that are generalisable across species are, but not limited to, facilitation of learning, conscious knowing and decision making, intentionality, allocentricity, and subjectivity.  Higher-order consciousness can be thought of as to what makes the accurate report possible. Therefore, for the purposes of this essay I will focus on the markers of primary consciousness, since the species-specific accurate report of most non-human animals is unachievable up to date.

Physiological-anatomical signs of consciousness

We know that low-amplitude, high-frequency (gamma band) and irregular EEG patterns ranging from 12 to 70 Hz play critical roles in conscious states. Contrastingly, on the EEG level, high-amplitude, synchronous, low-frequency slow voltages at less than 4 Hz, and local cortical activity define unconscious states.  Even though in EEG, conscious activity is generally reflected by large-scale asynchronous activity, it has also been associated on a finer scale with the synchronous activity of selected populations of neurones (rhythmic oscillations).

Wide-spread MEG activity over cortex has also been linked to consciousness. While unconscious stimulation induces only local cortical activity, conscious content is associated with widespread brain activity. This is due to the fact that conscious content involves a much wider range of effects than simply the current conscious content, such as implicit learning, episodic memory, biofeedback and training of autonomic and motor functions.

 

 

These hypothetical criteria have been further tested in studies of epilepsy and anaesthesia. EEG patterns of anaesthetised and epileptic patients show the aforementioned high-amplitude, synchronous and low-frequency activity.  These preliminary physiological criteria are quite basic but can be further investigated to determine anatomical criteria. When the brain structures that are responsible for these low-amplitude, unsynchronised and high-frequency oscillations for conscious states, and the synchronised, low-frequency oscillations of the unconscious brain states are investigated, the thalamo-cortical circuit has been found to have a very important role in creating consciousness. There are a number of bottom-up and top-down approaches, and theories that focus on the sensory or the motor system to further specifize the anatomical markers of consciousness.

Furthermore with regards to anatomy, we know by now that the cerebral cortex is a pivotal element of consciousness. The destruction of the cerebral cortex renders humans in a vegetative state, permanently unconscious. However, removing the entire cerebellum, which is even richer in neurones does not affect consciousness at all. Studies have also shown that damage to different parts of the cerebral cortex have entirely different consequences. While damage to certain parts of the cortex changes our experience of color, damage to other areas hinders our perception of shapes. Within the cerebral cortex, there are a number of different theories which distinguish between regions of the cortex. While some theories rule out the primary visual cortex as contributing to the conscious experience, some specifically include this cortical area as being a major contributor.

All in all, it has been found that the cortico-thalamic system plays an important role in all theories and there has been no proposal to date regarding the neural correlates of consciousness which has excluded the cerebral cortex.  The temporal and parietal association areas, and frontal lobe regions such as the anterior cingulate, premotor, and prefrontal cortical areas have also been emphasised in the literature.