9 unanswered questions about the human brain
Much has been revealed about the inner workings of the human brain, but many mysteries remain unsolved. Dr Rebecca Tan looks at some neuro puzzlers
The average human brain has about 90 billion neurons that make 100 trillion connections or synapses. Scientists believe this staggering number of neurons is responsible for the traits that make us uniquely human: our thoughts, memories and emotions.
Recent technological advances in functional magnetic resonance imaging, optogenetics and whole-brain imaging have made the brain accessible in ways that previous generations of neuroscientists could only dream of. Yet, many pieces are still missing from the puzzle. Here are nine unsolved questions in neuroscience.
1. How is information encoded by neurons?
The brain has often been likened to a computer. In essence, both follow the same basic principles: electrical inputs arrive in the brain, which are then processed before leaving as electrical outputs. While computers encode information in a binary fashion, much less is known about how neurons encode information. What we do know is that neurons conduct electrical impulses down specialised extensions known as axons, before releasing chemical signals to neighbouring neurons at junctions called synapses. These electrical and chemical signals carry information about everything we see, hear, taste and touch.
2. How do we perceive pain?
Have you ever instinctively withdrawn your hand after touching a boiling pot by accident? Those pain sensations started with the activation of pain receptors – or nociceptors – in your hand. The information is conducted to the spinal cord, before entering higher brain areas for the perception of pain. However, perception of pain is a subjective experience, and some people experience pain more or less severely than others. Why? The gate control theory of pain, as proposed by Canadian researcher Ronald Melzack, suggests that pain signals that enter the spinal cord can be modified – amplified, diminished or completely blocked – before they enter the brain. Supporting this theory are the many accounts of how people injured on battlefields or while playing sports do not experience pain until much later.
3. Why do we sleep and dream?
We spend about a third of our lives sleeping. Why do we invest so much time in sleep? The most straightforward answer is that sleep is restorative, and that it replenishes the body’s energy stores. However, intense neural activity during rapid eye movement (REM) sleep, the stage in which most dreams occur, suggests there may be more to the story. One theory, which by far has the largest body of evidence, is that sleep plays a critical role in learning and consolidating memories. It is probably why infants and toddlers need up to 14 hours of sleep a day, with half of it spent in REM sleep. In adults, dreams may also play a role in brain plasticity and learning, which is why sleep-deprived adults perform worse in memory tests and tasks.
4. How are memories stored and retrieved?
Memories make each of us unique and give us continuity in life. Further insight into how memories are stored and retrieved came from a 1953 study on an epileptic patient Henry Molaison. In an attempt to treat his severe epileptic seizures, neurosurgeons removed his medial temporal lobe, a section of the brain that includes the hippocampus. The surgery worked and the seizures ceased, but it also rendered Molaison unable to make memories of new events. This finding revealed that the hippocampus is crucial for making new memories. Scientists now believe that the hippocampus makes new memories via long-term, sustained signal transmission between neurons, a phenomenon known as long-term potentiation.
5. How do we make decisions?
Each day we are faced with hundreds of decisions which can range from trivial to life-changing. According to a 2012 study published in the Proceedings of the National Academy of Science, our decisions are the outcome of two separate brain systems: cognitive control and the valuation network. While the valuation network supplies the brain with information about the value of each choice, it is cognitive control which keeps the overall goal in focus, preventing the brain from being overwhelmed with information.
6. How can emotions be regulated?
The 2015 Pixar movie Inside Out has popularised the work of psychologist Paul Eckman, who identified six basic emotions that seem to be universally recognised: anger, fear, sadness, disgust, joy and surprise. These emotions play a role in ensuring our survival; fear, for example, helps us to prioritise our actions in the face of danger. Even seemingly negative emotions such as sadness could help us by prompting us to change for the better. But emotional disorders such as post-traumatic stress disorder and depression also take their toll. Despite the advent of blockbuster drugs like selective serotonin reuptake inhibitors, we still barely understand how emotions arise and in which specific areas of the brain, much less how to treat complex emotional disorders.
7. What is intelligence?
Despite the ubiquity and extensive use of the intelligence quotient (IQ) test, the very definition of intelligence is itself an unanswered question. Although scientists agree that intelligence is not the straightforward outcome of having a larger brain, they are unsure if intelligence is the result of a higher number or particular pattern of neural connections, an unusually efficient wiring pathway, or something else. In fact, intelligence may not even be a single trait but a whole range of skills spanning emotional astuteness, analytical ability and linguistic flair. Are these abilities related or are they mutually exclusive? To what degree do non-human animals have intelligence, and even more controversially, are there genetic differences that account for varying intelligence between human beings? These are just some of the most pressing questions in the minefield that is intelligence.
8. How do specialised parts of the brain integrate with one another?
To the untrained eye, the brain looks like a wrinkly, pink blob. Flip open any medical textbook, however, and you will probably find a colour-coded map of the different regions of the brain, with each area responsible for functions such as speech (pre-frontal cortex), sight (occipital lobe), long-term memory (temporal lobe) and so on. This sub-division of labour within the brain is a rough guide at best, and many functions actually require rapid communication between multiple brain regions. However, no one part of the brain acts as a control centre that integrates signals from various regions. Instead, multiple parallel connections seem to form a dense and overlapping network between the different regions. Teasing out the pathways linked to specific processes is likely to occupy neuroscientists for decades to come.
9. What is consciousness?
Consciousness is hard to define, but can be thought of as the difference between a digital photograph of a person (a bunch of pixels) and the memory of a person, which is recognised as something which has meaning and evokes a subjective experience. The problem with such a definition of consciousness is that it is impossible to objectively quantify and measure. Instead, scientists have used what they call neural correlates of consciousness, which refers to the minimal brain activity that is required for conscious thought. Even though correlates have been found, such as a region known as the claustrum, they bring us no closer to explaining what consciousness is.