The brain is made up of about 86 billion neurons, and the connectome, the structure of connections between these neurons, determines our thoughts and emotions, according to the theory of connectionism, which suggests that analyzing the connectome could potentially allow us to read the human mind.
Our bodies are made of cells. There are about 37.2 trillion cells in the body, and about 86 billion of them make up the brain, which is called a neuron. A neuron is divided into three main parts: the dendrite, the cell body, and the axon. The dendrite is the part that receives electrical signals from other neurons, the cell body is literally the center of the cell, and the axon is the part that carries electrical signals from the dendrite to other neurons. Where two neurons meet is called a synapse. For example, when the axon of neuron A and the dendrite of neuron B meet, there is a gap between them called a synapse. An electrical signal is converted into a neurotransmitter at the end of the axon of neuron A, spreads across the synaptic gap to the other side, reaches the dendrites of neuron B, and is converted back into an electrical signal. Each neuron is connected to many other neurons. The dendrites of neuron A are connected to millions or tens of millions of axons, and the axons of neuron A are connected to just as many dendrites. In order for a particular neuron to fire, the voltage must cross a certain threshold, and when the sum of the electrical signals it receives from all the other neurons exceeds that threshold, the neuron fires. Conversely, if the voltage is even slightly insufficient, the neuron cannot transmit any electrical signals to the next neuron.
Connectionism or Connectome Theory is a theory in cognitive science that states that all human mental activity (thoughts, emotions, etc.) can be explained through the connectome. So what is the connectome? You’ve probably heard the word “genome” before. It refers to the entire set of genes of an organism, which determines how it looks, how it metabolizes, and many other things. For example, analyzing a person’s genome can tell you how likely they are to develop Alzheimer’s disease. If the genome is the totality of genes, the connectome is the totality of connections between neurons. In the not-too-distant future, when technology is advanced enough, it may be possible to reveal how all 86 billion neurons in the human brain are connected to each other. According to the theory of connectionism, we will then be able to read a person’s mind through his or her connectome.
The way we study the connectome with current technology is very limited. To put it simply, we first take a section of the brain and cut it into very thin layers. Then, one by one, those many layers are put under a microscope, analyzed, and the images are fed into a computer. The computer then analyzes and synthesizes the many two-dimensional images to create a three-dimensional image. For now, only deceased brains can be used to study the connectome. Non-invasive scanning techniques like fMRI aren’t sophisticated enough to distinguish neuron by neuron, and can only tell us roughly which parts of the brain respond to which stimuli.
So what are the implications of analyzing a dead brain? To understand this, we need to recognize two aspects of the brain and mind. The first aspect of the brain and mind is what we see in our daily lives. We go through the day thinking about different things and then realizing that we need to wash the dishes. We also go from enjoying a beautiful view to being entertained by a TV show, from being angry to being happy. The pathways and patterns of electrical signals traveling along neurons in our brains are constantly changing. It’s like a flowing river. But for a river to flow, it needs a riverbed. This is the Connectome. Compared to the speed at which the river flows, the shape of the riverbed is fairly constant. Similarly, the connectome is relatively constant compared to electrical signals, which can change in an instant. Despite the fickleness of our emotions, each of us has a unique personality, which is shaped by our connectome. Furthermore, our timeless traits (such as our childhood memories) are all housed in the same connectome as our riverbed. This doesn’t mean that the connectome stays the same forever. Just as the river bed erodes and changes shape as the river flows, so does our connectome. As we study our majors, watch our favorite shows, and get angry, the electrical signals traveling through our neurons gradually reshape our connectome. In other words, the flow of electrical signals and the connectome interact. This is where the connectome is very different from the genome. The genome is unchanged from the moment of fertilization until the end of the individual’s life. However, a connectome is not as deterministic as a genome because it changes with experience. Because the connectome encompasses both nature and nurture, it has an advantage over the genome in describing humans. Hence the central hypothesis of connectionism: “You are your connectome”.
