This blog post explores how the connectome—the network of connections among 86 billion neurons—shapes our thoughts and emotions.
Our bodies are made of cells. We have approximately 37.2 trillion cells, and about 86 billion of these make up the brain. These brain cells are called neurons. A single neuron is broadly divided into three parts: the dendrite, the cell body, and the axon. Dendrites receive electrical signals from other neurons. The cell body is, as the name suggests, the central part of the cell. The axon transmits the electrical signals received by the dendrites to other neurons. Where two neurons meet, there is a structure called a synapse. For example, when the axon of neuron A meets the dendrite of neuron B, a gap called a synapse exists between them. The electrical signal is converted into a neurotransmitter at the end of neuron A’s axon, diffuses across the synaptic gap, and is reconverted into an electrical signal upon reaching the dendrite of neuron B. A single neuron is connected to countless other neurons. The dendrites of neuron A connect to millions or tens of millions of axons, and the axon of neuron A connects to just as many dendrites. For a specific neuron to activate, it must exceed a specific voltage threshold. If the sum of the electrical signals received from multiple other neurons exceeds that threshold, the neuron activates. Conversely, if the voltage is even slightly insufficient, that neuron cannot transmit any electrical signal to the next neurons.
Connectionism or Connectome Theory is a theory in cognitive science that posits all human mental activities (thoughts, emotions, etc.) can be explained through the connectome. So, what exactly is the connectome? You’ve probably heard the word ‘genome’. The genome refers to the complete set of genes in an organism, determining its appearance, metabolic activities, and countless other traits. For example, analyzing a person’s genome can reveal their likelihood of developing Alzheimer’s disease. If the genome represents the entirety of genes, the connectome represents the entirety of connections between neurons. In the distant future, when technology advances sufficiently, we may be able to fully map how all 86 billion neurons in the human brain are interconnected. According to connectionism theory, at that point, we might be able to read a person’s mind through their connectome.
Current methods for studying the connectome are severely limited. To explain simply: first, a portion of the brain is removed and sliced into extremely thin layers. Then, each of these countless layers is placed under a microscope for analysis, and the resulting images are input into a computer. The computer then analyzes and synthesizes these numerous two-dimensional images to create a three-dimensional image. Therefore, for connectome research, only brains from deceased individuals can currently be used. Non-invasive scanning technologies like fMRI are not sophisticated enough to distinguish individual neurons; they can only roughly indicate which brain regions respond to specific stimuli.
So what is the significance of analyzing dead brains? To understand this, we must recognize two aspects of the brain and mind. The first aspect manifests in our daily lives. We might ponder various concerns one day, then suddenly think we need to do the dishes. We might admire a beautiful landscape, then find joy watching a TV variety show; we might get angry, then quickly become happy. Each time, the pathways and patterns of electrical signals flowing along neurons in our brain constantly change. It can be likened to flowing river water. However, for river water to flow, there must be a riverbed. This riverbed is the connectome. Compared to the speed of the flowing water, the shape of the riverbed is quite constant. Similarly, the connectome is relatively constant compared to the rapidly changing electrical signals. Even if our emotions are fickle, everyone possesses their own unique personality. That personality is shaped by each individual’s connectome. Furthermore, characteristics that remain largely unchanged over time (such as childhood memories) are all anchored in the connectome, which is like the riverbed. That doesn’t mean the connectome stays exactly the same forever. Just as a riverbed erodes and changes shape as the river flows, our connectome also changes. As we study our respective majors, watch variety shows, or get angry, the electrical signals flowing through our neurons gradually alter the shape of our connectome. In other words, the flow of electrical signals and the connectome interact. This is precisely where the connectome differs significantly from the genome. The genome remains absolutely unchanged from the moment of fertilization until the end of an individual’s life. However, the connectome changes due to experience and is therefore not as deterministic as the genome. The connectome encompasses both nature and nurture, giving it an advantage over the genome in explaining human beings. This is why the core hypothesis of connectionism emerged: “You are your connectome.”
