Chapter 29. Material theories of the Mind

29.1 Psychology

Studies of the mind branched from philosophy to psychology in the latter half of the 19th century. By 1874, the year of Irish natural philosopher John Tyndall’s Belfast address (discussed in Chapter 28), French physician Pierre Paul Broca[1,2] and German neurologist Carl Wernicke had identified areas of the brain associated with different aspects of language.[3]

In 1879, German physician Wilhelm Wundt founded the first laboratory dedicated exclusively to psychology,[4] and German psychologist Hermann Ebbinghaus became the first person to describe the ‘learning curve’ during his studies on memory in 1885.[5] Ebbinghaus is also known for discovering the Ebbinghaus illusion, an optical illusion that illustrates our relative perception of size.

Illustration of the Ebbinghaus illusion.

Figure 29.1
Image credit

The Ebbinghaus illusion.

29.1.1 Psychoanalysis and behaviourism

In 1913, American psychologist John Watson suggested that psychology should be a science of behaviour, not of the mind.[6] Behaviourism rejects the idea that internal mental states like qualia (discussed in Chapter 26) exist, and arouse in the early 20th century as a response to Austrian psychologist Sigmund Freud’s psychoanalysis.[7]

Watson was influenced by the idea of classical conditioning, discovered by Russian physiologist Ivan Pavlov in 1897.[8] In 1920, Watson and his assistant Rosalie Rayner used classical conditioning to make an eleven-month-old baby, Albert, fear stimulus that would not normally be feared, like a white rat. Albert soon generalised the response so that he was afraid of anything furry.[9,10]

Further criticisms of psychoanalysis arose in 1948, when American psychologist Burrhus Frederic “B. F.” Skinner argued that thoughts and feelings are merely examples of behaviour.[11]

Three years later, German psychologist Hans Eysenck showed that some personality traits are genetic[12] (discussed in Book I) and, in 1963, Austrian-British philosopher Karl Popper claimed that psychoanalysis is not a science because it cannot be falsified[13] (discussed in Chapter 32). Popper argued that science can be distinguished by the fact that it makes unique predictions that can be proven false.

29.1.2 Identity theory and functionalism

In 1959, American linguist Noam Chomsky showed that behaviourism cannot explain how people can produce an infinite variety of sentences, unique in both structure and meaning.[14] He suggested that there must be internal mental structures, and behaviourism was soon replaced with identity theory.

Identity theory suggests that particular states of the mind are identical to particular states of the brain. Identity theory was first suggested by British psychologist Ullin Place in 1956,[15] and was extended by Australian philosopher Jack Smart three years later.[16] This approach was criticised by American philosopher Hilary Putnam in 1960.[17]

Putnam showed that the same sensations can arise from different brain states, an animal may see the same thing as a human, for example, but this image will correspond to different parts of their brain. Putnam suggested that the mind is more like a computer.

In 1965, American philosopher Jerry Fodor suggested that the mind can be explained in functional terms, this means anything that performs the function of a mind, is a mind, and must therefore be conscious.[18] Functionalism has been criticised by the Chinese room argument, presented by American philosopher John Searle in 1980.[19]

29.2 Neuroscience

29.2.1 The neuron doctrine

In the 1870s-1890s, Italian neuroscientist Camillo Golgi and Spanish neuroscientist Santiago Ramón y Cajal developed tissue-staining techniques that allowed them to map the path of nerve cells - known as neurons - in the brain.[20,21]

Neurons are cells that can gather and transmit information along a neural pathway. We now know that there are three types of neurons: sensory neurons, motor neurons, and interneurons. Sensory neurons carry signals from the outer parts of the body to the central nervous system in the brain and spinal cord, and motor neurons do the opposite. Interneurons carry information between motor and sensory neurons.

The simplest type of neural pathway does not involve the brain. In a knee-jerk reaction, for example, when a person’s knee is taped in the right place a sensory neuron sends a signal to a motor neuron, which sends a signal to make a muscle in their leg contract. The brain only becomes involved in more complex situations.[22]

The discovery of neurons led to the development of the neuron doctrine, first proposed by German anatomist Heinrich Wilhelm Gottfried von Waldeyer-Hartz in 1891.[23] The neuron doctrine states that neurons are the fundamental units of the nervous system and are responsible for all brain activity.

Illustration of neurons in the cerebellum of a baby chicken.

Figure 29.2
Image credit

Drawing of the cells found in the brains of chickens by Santiago Ramón y Cajal, c. 1905.

Photograph of neurons.

Figure 29.3
Image credit

Neurons in the brain of a macaque.

29.2.2 Cognitive science

British chemist Christopher Longuet-Higgins coined the term ‘cognitive science’ in 1973.[24] Cognitive science accepts the neuron doctrine, which shows that signals in the brain are mostly sent through networks composed of neurons. Neurons receive stimulus from branches known as dendrites and then communicate this information across a synapse, either electrically or chemically.

  • At electrical synapses, a nerve impulse is fired along an axon, this crosses a gap junction to connect to the dendrite of another neuron.
  • At chemical synapses, electrical impulses are converted into chemical signals. This process is called exocytosis and it is slower than electrical transmission.
Diagram of a nerve cell.

Figure 29.4
Image credit

Diagram of a neuron.

There are about 100 billion neurons in the human brain, and the amount of electronic signals they emit depends on how much stimulus they are receiving.[25] Neurons emit more electronic signals per second, and therefore have a higher electronic frequency, when they receive more stimuli. This frequency can be measured by placing electrodes on the top of a person’s head, in what is known as ‘electroencephalography’ (EEG). Our brain waves have the lowest frequency when we are asleep.

29.3 Sleep

29.3.1 What is sleep?

Almost all animals are thought to sleep. In complex animals, sleep can be defined as a state of reversible unconsciousness, illustrated by a change in brain wave patterns and eye movements.[26] In simpler animals, like invertebrates, sleep can be defined as a state where the animal periodically stops responding to external stimuli. The simpler the life form, the more difficult it is for researchers to determine whether or not they have fallen asleep.[27]

The most primitive creature thought to sleep is the Caenorhabditis elegans, a type of roundworm that becomes unresponsive before it moults, indicating that sleep may be connected to physical development.[28]

More complex invertebrates are thought to sleep, although this is difficult to prove. Reptiles sleep, and this can be shown from their brain wave patterns, which differ from those of sleeping birds and mammals.[29] Birds and mammals also periodically undergo a different stage of sleep to other animals, known as rapid eye movement (REM).[30] This is the stage of sleep in which we’re most likely to dream.[31]

Different species of mammals sleep for different durations, and members of the same species have been shown to sleep longer when in captivity.[32] Carnivores tend to sleep for longer than omnivores, and herbivores tend to sleep the least, with smaller herbivores often sleeping for longer than larger ones.[30] The state that an animal enters when hibernating is different from sleep, and so hibernating animals also sleep periodically.[33]

Some birds and aquatic mammals like eared seals, whales, and dolphins can exhibit unihemispheric sleep.[34] This means that one side of their brain can sleep while the other is awake. When the right side is asleep, the left eye is shut, and vice versa. This allows birds to continue to fly while sleeping, and allows aquatic mammals to swim to the surface to breathe. Evidence of unihemispheric sleep has also been found in cats that have had the connection between their left and right hemispheres severed.[35]

REM sleep does not occur during unihemispheric sleep, but many animals that exhibit unihemispheric sleep will start to sleep with both hemispheres when possible - such as when they move to land or enter captivity - and will then exhibit REM sleep.[36]

29.3.2 What happens to our brain when we sleep?

We can determine some of what happens to us during sleep by looking at brain wave patterns. Brain wave patterns can be divided into five types:[26]

  • Gamma brain waves have the highest frequency, and are associated with states when we are highly alert, such as when we are afraid, or when we are concentrating on a complex task.
  • Beta brain waves have a slightly lower frequency than gamma brain waves, and are associated with less complex tasks.
  • Alpha brain waves have an even lower frequency, and occur when we are relaxed, and during REM sleep.
  • Theta brain waves have a lower frequency than alpha brain waves, and also occur during REM sleep.
  • Delta brain waves have the lowest frequency, occurring during deep sleep or unconsciousness, although young children and babies can exhibit delta waves while they are awake.[37]

Humans undergo five stages of sleep, which repeat every 90 minutes or so. These are categorised by different brain wave patterns, different muscle movements, which are measured with an electromyogram (EMG) machine, and different eye movements, which are measured with an electrooculogram (EOG). Heart rate, breathing rate, and blood pressure can also be measured during sleep.[26]

The first stage of sleep consists of very light sleep, as brain waves slow from alpha to theta waves, during this stage you can experience sudden muscle spasms, known as hypnic jerks, which can make you feel as if you’re falling, you may also experience dream-like images or sounds.[26]

In the second stage of sleep, theta brain waves can exhibit spindles and K-complexes.[26] Spindles are clusters of rapid waves that are often followed by muscle movement. K-complexes are single large waves, which may occur when the brain decides that it’s safe to ignore external stimulus. They may also be associated with processing memories.[38]

The third stage of sleep is deeper than the second stage, and brain waves slow to become delta waves, while stage four sleep is deeper still. Sleepwalking, sleep talking, and night terrors typically occur during stages three and four.[39]

The fifth stage of sleep consists of REM sleep, which was discovered by physiologists Eugene Aserinsky and Nathaniel Kleitman in 1953.[40] When a person undergoes REM sleep, many of their muscles are paralysed and their eyes move rapidly under their closed eyelids, their heart rate and breathing rate increase, their blood pressure rises, and their brain activity can increase to the same level as when they’re awake.[41]

Dreams mostly occur during periods of REM sleep, but have also been found to occur at any stage. As we repeat the five stages of sleep, we experience longer periods of REM sleep and shorter periods of deep sleep.[26]

No one knows why we experience REM sleep. One possibility is that it’s important for brain development, providing the neural stimulation needed to develop the nervous system. This is known as the ontogenetic hypothesis. Evidence for this comes from the fact that we experience less REM sleep as we get older. If the ontogenetic hypothesis is true, then it would mean that REM sleep has no function in adults.[42]

29.3.3 Why do we sleep?

Sleep appears to be vital for survival. This has been illustrated in laboratory experiments on rats,[43] and in people suffering from fatal familial insomnia (FFI).[44] FFI is a very rare condition that causes increasingly worsening insomnia, leading to hallucinations, and eventually organ failure. We still don’t know exactly why we need to sleep, but there’s evidence that sleep can aid in physical development, in memory recall,[45] and in improving the immune system.[46]

29.4 Dreams

29.4.1 What are dreams?

While dreaming, we see images without external stimulus, in a similar way to how we can imagine visual images that do not really exist. Dreams differ from these images, however, because when we are dreaming we are not actively in control of what we experience, and we tend to be unaware that the images we see in dreams are not real. This may be because we are unable to imagine any other type of reality than the one presented to us while we are dreaming.[47]

The fact that we do not often realise when we are dreaming, even though there are many clues that the dream is not real, led philosophers such as Zhuang Zhou[48] and Rene Descartes[49] to claim that we cannot trust our ability to distinguish reality from illusion (discussed in Chapter 26).

People have recorded their dreams for at least 5000 years, with the earliest evidence coming from the Sumerians in Mesopotamia.[50] In the late 1800s and early 1900s, Freud began one of the first efforts to systematically study dreams.[7] Freud believed that dreams occur so that we can fulfil wishes that we may not want to admit we have in our waking life. Psychiatrist Carl Jung expanded on this by stating that dreams have messages the dreamer should act upon.[51] Neither of these theories are currently accepted.

Scientific studies were made in the 1900s, when dreams became the subject of neurology. We now think that everyone dreams several times a night for about 2 hours in total. Individual dreams last about 5 to 20 minutes, with the period spent dreaming, and the vividness of dreams, increasing the longer we sleep.[52] We still don’t know exactly why animals and people dream.

One reason why it’s so difficult to study dreams using the scientific method is that dreams are inherently subjective experiences. The scientific study of dreams is also complicated by the fact that people find it much more difficult to remember their dreams than their waking life.

Painting of ‘The Dream’ by Henri Rousseau.

Figure 29.5
Image credit

‘The Dream’ by Henri Rousseau, 1910.

Painting of ‘The Nightmare’ by John Henry Fuseli.

Figure 29.6
Image credit

‘The Nightmare’ by John Henry Fuseli, 1781.

Dreams can have continuously changing visuals that blend together and do not necessarily make sense, and people have the tendency to create narrative in random images.[53] There’s also a bias where more intense or unusual dreams are easier to remember.[54]

The memories of dreams can be triggered if similar events occur when we’re awake, particularly if they’re unusual, and this can lead people to think they’ve had a dream that predicted the future. Although there’s no evidence that this is more than a coincidence.[55]

The fact that we find it difficult to remember our dreams suggests that there’s no evolutionary advantage to remembering them. There are people who claim to never remember dreaming, and so the ability to remember our dreams is apparently not vital for survival.

The most commonly reported dreams include sexual dreams and nightmares. Negative emotions are more commonly reported than positive emotions, with anxiety being the most common emotion.[56] Some people only recall dreams in black and white, and this has been linked to watching black and white television and films as children.[57]

Some people experience lucid dreams of varying complexity. Lucid dreams occur when a person obtains some kind of self-awareness while they are dreaming. Some people can then control certain aspects of their dreams, or force themselves awake. Although the brain will often find a way to adapt to this, preventing the dreamer from awakening or providing a false awakening, where the dreamer merely dreams they have awoke.[58]

Lucid dreaming allows dreamers to communicate with researchers, if they can, for example, deliberately move their eyes while they are asleep. In 1985, psychophysiologist Stephen LaBerge showed that our experience of time in a lucid dream is the same as when we’re awake.[59] He did this by having dreamers count for ten seconds while dreaming, signalling at the beginning and end. This was confirmed in 2004, when it was also shown that muscle movements took longer to perform during lucid dreams.[60]

29.4.2 Why do we dream?

While it’s still not known why we dream, there are a number of theories. In 1983, biologists Francis Crick and Graeme Mitchison proposed the reverse learning theory, which states that dreams are needed for the brain to remove unnecessary information that it’s received during the day.[61]

There’s also evidence of the opposite idea, that the brain needs dreams to remember information.[62] Studies have shown that you can improve your chances of recalling specific things if a memory of them if triggered while you’re asleep.[63] Psychiatrist Jie Zhang suggested that dreams provide something analogous to a temporary storage space, which we can use to transfer short-term memories into long-term memories.[64] This is known as the continual-activation theory.

Another idea is that dreams have the purpose of providing us with simulations that we can learn from. Neuroscientists Katja Valli and Antti Revonsuo argue that dreams evolved to prepare life forms for threatening events.[65] This is known as the threat simulation theory. Others argue that dreams provide more general simulations, which can also help with problem solving[66] and emotional control.[67]

Research has already shown that people can learn new information in their sleep. In a 2012 study, volunteers were taught to associate different scents with different sounds while they were asleep, where the different scents led them to involuntarily produce different breathing patterns.[68] While the volunteers had no memory of this when waking, when they were played the different sounds, their breathing pattern changed to match the smell they were taught to associate with it.

It’s also possible that dreams are related to the defence mechanism of animals that pretend to be dead in order to escape threats, since both states are related to temporary paralysis.[69]

Finally, dreams may not have an intrinsic purpose at all and may occur as the by-product of something else that is happening to our brains when we sleep. In the 1970s, psychiatrists J. Allan Hobson and Robert McCarley proposed the activation-synthesis theory.[70] This suggests that dreams are a by-product of REM sleep, caused by random signals from the brain stem being interpreted by the forebrain. The random nature of the signals causes dreams to appear somewhat nonsensical.

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