Vadim Bolshakov

Neurobiology of Uniquely Human Traits

This contribution is about recent neurobiological studies of uniquely human behavioral traits. At the beginning, however, I would like to give a very short general overview of biological mechanisms underlying different types of behavioral responses. Behaviors could be roughly divided into two major categories: they could be innate (inborn), or they could be learned (acquired). Learned behaviors, which are determined by specific experiences, were investigated extensively by behaviorists (for instance, B. F. Skinner). Another major type of behaviors, which do not require any learning, is inborn. We are just born this way. The studies of Konrad Lorenz and Nikolaas Tinbergen provided clear evidence that innate or inborn behaviors differ mechanistically from those behaviors that are acquired. Innate behaviors are fully formed the first time this type of behavior is performed, and, normally, they are triggered by a simple cue. Inborn reflexes provide an example of behaviors that neither require previous experience nor implicate new learning. They are inherited because of their adaptive value, and can be observed in every animal species. In this case, the link between an external trigger (cue) and behavioral response is normally not modifiable. However, the relationship between innate and learned behavioral processes is far from being completely understood.

The behaviors, both learned and innate, are species-specific and they could be changed in response to modifications of the genetic material. We know from our own experience that certain types of behaviors can run in human families. In fact, there is significant experimental evidence that genetic factors contribute to behavioral traits in humans. The most direct support for this notion came from studies of human twins (which are genetically identical). A question about the relationship between inborn and acquired behaviors is, apparently, a part of the broader question about an interaction between genetic factors and environment in defining the behavioral processes. Presently, a commonly held view is that behaviors in humans and animals may result from a combination of both acquired and genetic traits. Moreover, while most behaviors are genetically determined, with certain limits they could be modified by the environment.

Many researchers in the field of neuroscience presently share a view that it might be possible to gain an insight into the human mind by studying the human brain. However, the complexity of the human brain, consisting of approximately 100 billion neurons, makes it, perhaps, the most complex scientific object imaginable. Behavior is not a function of the brain as a whole, but different types of behavior could be controlled in a region-specific or even neurocircuitry-specific fashion. Thus, it has been possible to identify the brain areas responsible for specific functions: speech, sense of smell, for intellectual and emotional functions, and many other functions which humans possess. The mechanisms of inter-cellular communication in the brain are reasonably well characterized. When information about either external or internal stimuli arrives to a certain region of the brain, it modifies the firing rate of activated brain cells (neurons). The generated electrical signals could be transmitted along neuronal axons to other brain cells, and this is how the latter could be connected to each other. They could fire in large groups in such a way that this firing could be modulated both spatially and temporally. Changes in firing patterns may underlie cognitive functions that the brain mediates. Importantly, there is no difference in respect to physical mechanisms underlying neuronal firing in human and animal brains. Nevertheless, it remains to be explained how the firing patterns could be translated into the subtleties of perception: for instance, distinguishing between slightly different colors or sorts of coffee. From a physical point of view, neuronal spikes are identical, but the brain somehow manages to perform these extremely complex evaluative processes. The questions related to understanding the mechanisms of perception have been fascinating neuroscientists for decades. As mentioned above, neurons have the capability to process the incoming information and modulate it, before transmitting signals to other brain cells or regions of the brain. The brain constantly receives external signals of different sensory modalities (visual, auditory, olfactory). A fundamental question here is how the brain distinguishes activity that is generated internally from the external signals.

How could these questions be addressed experimentally? Over the last decade, many different techniques were developed, which, in fact, allowed one to explore directly how the human brain functions. The techniques, which are presently used to study both the human and animal brain, could be divided into several different categories. Some of these techniques, such as encephalography or magneto-encephalography, could provide higher temporal resolution, allowing for faster measurements of neuronal activity associated with distinct processes in the brain. Thus, using encephalography, the fast brain oscillations, termed gamma oscillations, were detected. These fast oscillations were linked to the exchange of information between cortical and sub-cortical areas. Magneto-encephalography allows the functional imaging of different brain regions, because brain tissue has magnetic properties, which differ between the distinct brain structures. Other techniques, while being slower, could provide better spatial resolution, thus permitting the study of brain structures in smaller details. The latter techniques include functional magnetic resonance imaging (fMRI) or positron emission tomography (PET). One of the newly developed techniques, termed “diffusion tensor imaging,” permits picturing of the distribution of neuronal axons in white matter of the brain. Moreover, the brain could be stimulated using the trans-cranial magnetic stimulation approach. This technique allows the selective activation of groups of cells in cortical areas. With this experimental method, it becomes feasible to test the hypotheses about the contribution of specific brain regions to the complex cognitive processes. The problem with some of the mentioned techniques (for instance, fMRI) is that they are dependent on a subtraction of the background activity from the activities induced by certain types of behavior. It might be difficult to control, however, the background brain activity in humans, who are constantly thinking. These approaches were still very beneficial for the studies aiming at understanding the human brain functions.

How could these experimental approaches be used for studies of the human consciousness, as well as unconscious mental processes, and the interactions between them? Some of these issues were addressed in the elegant experiments using fMRI techniques. In the course of this work, the human subjects were given sequences of words that were hidden in a text, so that they could be seen but could not immediately consciously processed. Functional magnetic resonance imaging allowed the brain regions that were activated during the conscious processing of words to be identified, as opposed to the masked word processing. These experiments revealed significant differences between conscious versus unconscious processing of words in respect to the brain structures involved, indicating that these two types of mental activity might be structurally segregated. But what is consciousness? According to a leading figure in the field of neuroscience, Antonio Damasio, it is dependent on self-awareness and is greatly influenced by emotions and memory. A major postulate of his theories is that there is a cross interaction between conscious and unconscious cognitive processes. Formally, consciousness is a cognitive state in which we are aware of our current situation and ourselves. Thus, a conscious person is someone who is aware of his or her immediate environment, including inner awareness (thoughts, feelings, and memories). I should also say that it, probably, includes the ability to reflect on our own thoughts. Importantly, humans possess a unique ability to think about the process of thinking, and realize that they do not know something. This ability could be found in a rudimentary form in animals, but humans have this ability to a much greater extent.

Presently, there are several theories of consciousness, which differ significantly in certain aspects, but there is, however, one commonality between them: all these theories postulate that consciousness might be determined by firing patterns of brain cells. To explore the processes underlying consciousness, the investigators applied the above-mentioned techniques, exploring the processes mediating the ability of the human brain to discriminate and react to environmental stimuli. We can now look at the brain functioning during processing the information, as well as evaluate the difference between wakefulness and sleep. A serious problem with these experiments is that there is a significant subjective component in any type of perception (for instance, sound of music, perceived quality of color, taste, emotion, etc.). For example, we know what the brain is; we can study it, and look at the brain regions which light up when the subject experiences pain. However, why does it hurt, and what is this feeling made of? There is no direct experimental way to answer these questions.

It was established that during evolution the volume of the human brain significantly increased. It led to a popular hypothesis that our mental uniqueness is largely due to our larger brains. Neuroanatomist Thomas Henry Huxley (1825–1895), for example, did not see any difference between humans and animals besides the brain size. This theory, however, failed to explain many experimental observations. If we look at different animals and normalize the brain size by the size of the body, humans are not on the very top of this hierarchical ladder, as there are animal species whose brain-to-body mass ratio is much larger than that of humans. For example, the pocket mouse’s brain size is ten percent of the whole body weight, while the human brain is only two percent of the body weight. While the human brain size significantly increases during early postnatal development, it is not only the size of the brain, but also the specificity of its organization is likely to be responsible for human behavioral traits.

Thus, personality, for example, was linked to the function of the prefrontal cortex. This was based on the famous case of Phineas P. Gage (1823–1860), who was a railroad engineer. On September 13, 1848, his prefrontal cortex was destroyed in an accident, where a long iron rod was blasted through his skull. Surprisingly, he did not lose the ability to live, and did not even become much less intelligent than he was, but his personality was completely changed. He became an extremely unpleasant person, who was unable to communicate with his peers or with his family members. Due to these personality changes, he had serious difficulties in keeping any job. People, who knew him well, would say that he was no longer Gage. This is a classical anecdotal case which, in fact, found some scientific confirmation decades later in the experiments utilizing functional imaging techniques, and in other observations of brain-damaged patients. It was established that the prefrontal cortex is responsible for the self-identity. Cortical areas, which are responsible for self-awareness and sense of identity, are extremely complex, containing many sub regions responsible for processing of different sensory inputs.

Human emotions could also be studied using modern imaging techniques. In one of the recently completed studies, researches were performing functional imaging of the brains of teenagers and adult persons, when the experimental subjects were responding to certain sentences. It was found that the brain of a teenager and the brain of an adult person responded quite differently to the words associated with emotional responses, such as disgust. This demonstrates that the interactions between the ability of frontal cortical areas to process external stimuli and innate behavioral responses might change during development.

Another interesting example of how fMRI imaging was applied to explore the mechanisms of human emotions comes from the experiments where the imaged brains of people in love were compared to the imaged brains of mothers looking at their newborn children. It was found that similar areas were activated both during expression of maternal and romantic love.

Instructing a person to respond to external stimuli in a certain way also provides an example of a uniquely human behavioral trait. During fear conditioning experiments, human subjects were asked to look at a computer screen, where the person would be presented with different geometrical figures in a random manner. The presentation of the certain types of geometrical shapes was paired with the mild electric shocks to the person’s wrist. The next time the subject saw the same shape on a screen, it was associated with stereotypic fear responses, indicating that this person was conditioned to fear certain visual external stimuli. It was demonstrated that another person, who was observing the subject during conditioning, was also conditioned to fear the same visual stimuli. The observational fear learning provides evidence for a social component in emotional responding in humans. This type of learning could, however, be seen in animals as well: animals may learn to fear something from observing other animals. Unlike the animals, human subjects could also be instructed to fear something without conditioning. When the subject is told that he or she might receive an electric shock to the wrist when a certain shape type appears on a computer monitor, this instruction would be sufficient to have this person fear-conditioned. The subject would exhibit a fear response when the particular shape is presented. This shows the role of language in human behavioral processes, which, obviously, is uniquely human, and could not be seen in animals.

Arguably, humans are more intelligent than animals. The present theories of the mechanisms of intelligence and how it could be mediated are based on the functional studies of the brain’s electric activity. A current hypothesis is that intelligence reflects the interconnectivity of different regions of the brain. Specifically, the frontal cortex in humans is essential for connecting the distinct mental processes mediated by different brain regions.

Studies of creativity, which is one of the sides of human intelligence that is not observed in animals, indicate that creativity is not homogeneous, involving many processes, such as arousal, motivation, and memory retrieval. There were some attempts to study creativity using modern experimental techniques. On a descriptive level, it appears that some people exhibit increased brain electric activity and a very high arousal level when they are searching for a solution of a difficult problem. It has been demonstrated that it could be associated with activation of the dopaminergic system of the brain. It was suggested that such increases in the brain’s dopamine level could lead to the enhanced insightfulness and excitement during the first phases of creativity. This could be followed by depression, enhanced criticality, perfectionism, and an intense re-evaluation of what was created during the initial creative phase.

In summary, there are two major mental systems, conscious processes, which are opposed by subconscious, automatic, intuitive or reflexive systems. Animals normally have mostly automatic minds. It appears, however, that humans may use both levels of mental processing. We still have automatic, reflexive systems, but additionally, we have the ability of conscious processing of our experiences and our thoughts.