A very brief history of the power struggle between the heart and the brain, and how the brain came out on top.
Cardiocentrism: with great power, comes great responsibility
In ancient Egypt the heart was believed to be responsible for the personality, desires, memory and intellect of an individual, yet its duties did not end there. Upon death, it needed to be lighter than an ostrich feather in order to be admitted to the afterlife, a task that if failed would lead to its destruction at the hands of the goddess Ammit (aka ‘the devourer of the dead)’ [1]. Although their cardiocentric hypothesis (that the heart controls the body’s movement and sensation as well as thought) was erroneous, Egyptian physicians were well respected across the ancient world for their observational and clinical diagnosis capabilities. Even today, the contribution ancient Egyptians had on cardiac medicine should not be overlooked, with the early physicians recognising a number of cardiac abnormalities including aneurysms (ballooning and weakening of a section of the artery) and congestive heart failure (insufficient pumping of the heart), both which are observed by modern doctors today [2].
Figure 1. A hungry Ammit awaits in anticipation as Anubis weighs a heart. Image from [3].
Cold blood: the rise of cephalocentrism
Nevertheless, in the late years of the Ancient Egyptian civilisation and over 1500 kilometres away in ancient Greece, an antithetical hypothesis was emerging. The brave Alcmaeon of Croton is thought to be the first supporter of the cephalocentric theory (that the brain controls the body’s movement and sensations as well as thought). It is believed that the philosopher and physiologist discovered the connection between the brain and sensory organs through his close examination of the optic nerve while conducting animal autopsies. Alcmaeon went on to coin the term ksynienai, which describes the complex ‘mixing’ of information gathered by the senses to be processed by the brain [4].
Hippocrates of Kos offered a celebrity endorsement for the cephalocentric hypothesis. Through his treatise writings on epilepsy, which many believe to cement his position as the father of modern medicine, he was able to identify the brain as the ‘interpreter of consciousness’. He also identified it as the origin of complex neurological diseases and injuries, including hemiplegia where paralysis or weakness in one side of the body is caused by damage to the opposite brain hemisphere [5].
Importantly, both the cardiocentric and cephalocentric hypotheses did coexist. More than 100 years after Alcmaeon and Hippocrates, Aristotle believed that the heart was the hegemonikon (ruling power) and was responsible for both feeling and movement. His cardiosinew theory stated that tension on the heart sinews controlled movement, much like a puppet master controlling a puppet. Through his animal dissections, he observed the brain's superior position, far from the centre of the body, which led him to the conclusion that it could not possibly be the control centre, but rather it was merely a cooling apparatus for the blood [6].
Localisations: growing support for cephalocentrism
It was through cerebral localisation, the process of examining symptoms to deduce the location of an injury, that support for the cephalocentric theory grew. During the Renaissance, English physician Thomas Willis with the help of his Oxford University colleagues, made a habit out of dissecting human bodies in the back rooms of houses and inns [7]. Although not the typical social get-together, these meet-ups resulted in the localisation of a higher order brain function including memory and imagination which were published in his treatise writings Cerebri Anatome [8]. Although retrospectively the localisations were inaccurate, he was one of the first to associate physical parts of the brain with mental faculties, ultimately paving the way for future localisations.
Figure 2. Thomas Willis enjoyed a more ‘hands-on’ approach to learning. Image from [9].
The correlation between structure and function has slowly become understood through gaining a deeper and more widespread understanding of the structural brain, combined with detailed descriptions of patients with neurological diseases/injuries across thousands of studies spanning many years. This understanding has been greatly assisted by examining cases of extreme neurological injury, most notably Phineas Gage’s accident, in which the 19th century American railroad worker’s left frontal lobe was penetrated by an iron rod. His miraculous survival and the subsequent study of his temporary but nonetheless irreverent personality changes contributed to the understanding that specific areas of the cerebral cortex had unique functions [10].
Figure 3. Modelling the path taken by the iron rod which penetrated Phineas Gage’s skull. Image from [11].
Fear modulation: heart and brain working in concert
Although we may believe that we have come a long way since the days when we were thinking with our hearts, we should not be too self-assured. In 2016, a group of neuroscientists discovered that the insular cortex (a subsection of the cerebral cortex) is able to manage fear response through feedback signals from the heart. When freezing in response to fear occurs, heart rate decreases and consequently, the insular cortex responds by decreasing its activity to keep fear within a functional equilibrium [12]. This study highlights the potential of novel therapies for anxiety disorders which target physiological sites outside the brain, since this could be more accessible and less invasive.
Our understanding of the brain is dynamic to say the least. As the neuroscientific landscape continues to evolve, we must remember those scientists that came before.
References:
[1] Carelli F. The book of death: weighing your heart. London Journal of Primary Care (Abingdon). 2011;4(1),86-87.
[2] Willerson J, Teaff R. Egyptian contributions to cardiac medicine. National Centre for Biotechnology Information.1996; 23(3),191–200.
[3] British Museum. Weighing of the Heart. Wikimedia [Internet]. Available from: https://upload.wikimedia.org/wikipedia/commons/5/52/Weighing_of_the_heart.jpg
[4] Zemelka AM. Alcmaeon of Croton - Father of Neuroscience? Brain, Mind and Senses in the Alcmaeon’s Study. Journal of Neurological Neuroscience. 2017, 8(3).
[5] Breitenfeld T, Jurassic M. Hippocrates- The forefather of neurology. Neurological sciences. 2014, 35(9).
[6] Smith CU. Cardiocentric neurophysiology: the persistence of a delusion. J Hist Neurosci. 2013;22(1):6-13
[7] O'Connor JP. Thomas Willis and the background to Cerebri Anatome. J R Soc Med. 2003;96(3):139-143.
[8] Engelhardt E. Cerebral localization of higher functions: The period between Thomas Willis and Paul Broca. Dement Neuropsychol. 2019;13(2),238-243.
[9] Loggan D. Thomas Willis. ODNB. Wikimedia [Internet]. Available from: https://commons.wikimedia.org/wiki/File:Thomas_Willis_ODNB.jpg
[10] Garcia-Molina, A. Phineas Gage and the enigma of the prefrontal cortex. Neurologia. 2012;27(6).
[11] Van Horn JD, Irimia A, Torgerson CM, Chambers MC, Kikinis R. Mapping Connectivity Damage in the Case of Phineas Gage. Wikimedia [Internet]. Available from: https://commons.wikimedia.org/wiki/File:Simulated_Connectivity_Damage_of_Phineas_Gage.png
[12] Klein AS, Dolensek N, Weiand C, Gogolla N. Fear balance is maintained by bodily feedback to the insular cortex in mice. Science.2021; 374(6570).