BRAIN HEMISPHERES AND LATERISATION
Our brain is constructed of two hemispheres, which are responsible for different facets of human personality and behaviour. Despite some essential differences in some functions, these hemispheres communicate through inter-hemispheric neural connections, most through the corpus callosum. This communication allows the brain to integrate different elements to produce coordinated, apparently seamless action and a unified personality.
Laterality refers to the degree to which one hemisphere may be dominant for a specific function such as language. If a function is the responsibility of one hemisphere, that function is said to be lateralised. The most well know lateralised behaviour is hand preference. Generally, research has focussed on language and spatial ability as neurologists have focussed on these areas to localise. As mentioned above, the brain consists of two hemispheres, which are similar, but are not totally alike, known as cerebral hemispheres.
Brain lateralisation refers to the fact that the two halves of the human brain are not identical, as each hemisphere has their own specific functional specialisations. The terms “right brain” and “left brain” function refer to this difference. For example, in humans, speech and language ability is the most obvious functional specialisation. Research conducted in the mid 1800s by Paul Broca identified an area in the left hemisphere that plays a primary role in speech production, while shortly afterwards, Carl Wernicke found that the left hemisphere was primarily concerned with language comprehension.
Most humans have a left hemisphere specialisation for language abilities, though this is not black and white. The direct tests to determine this are too invasive for healthy people, so most of the information we have is from clinical reports of people with brain diseases or brain injuries. Based on the data we have collected and other indirect measures, it is possible to estimate that 70 – 95% of humans have a left hemisphere language specialisation, while the rest have a different pattern. This pattern might include right hemisphere language specialisation or not having/having a minor lateralised specialisation. As more research is done in relation to lateralisation, the more complicated it appears to become. For example, prosody (the emotive content of speech) is specialised in the right hemisphere of the brain of people with left hemisphere language specialisation. Therefore, even though we may talk about right brain and left brain functions, there is still a lot about brain lateralisation that we simply do not yet know.
General hemisphere function
Popular psychology has been criticised in the past for being too specific on the functions of the two hemispheres. As we mentioned above, it is hard to determine which areas of the brain are responsible for what. However, the following functions are thought to relate to the different sides of the brain:
Left hemisphere function
- Linear reasoning
- Language functions (grammar, vocabulary)
- Sequential
- Verbal
- Analytical
- Linear algorithmic processing
- Mathematics (counting and measurement)
- Present and past
- Literal
Right hemisphere function
- Holistic language functions (accentuation, intonation)
- Spatial manipulation
- Facial perception
- Artistic ability
- Holistic
- Simultaneous
- Imagistic
- Intuitive
- Holistic algorithmic processing
- Mathematics (perception of shape and motion)
- Present and future
- Contextual
Bilateral Function
- Intuitive and heuristic arithmetic
- Emotions
- Binaural sound localisation
This is hard to determine, but research with people with brain damage can show us how each of the hemispheres function. For example, dyscalculia is a neurological syndrome which is associated with damage to the left tempero-parietal junction. The syndrome is associated with poor mental arithmetic skills and an inability to understand and apply mathematic concepts and poor numeric manipulation.
Left Handedness
Handedness is a vague term. Many will define handedness as the hand you use to write. However, researchers have considered this too vague and define handedness using five different theoretical assumptions, including:
- The hand we prefer to use, regardless of performance
- The hand that performs faster in manual tests.
- Two types of handedness: Left or right, or right or non-right.
- Three types of handedness, including ambidexterity.
- Different types of ambidexterity.
Other researchers think that handedness should not be lumped into five categories, but measured along a continuum scale. Paul Broca also suggested that a person’s handedness was the opposite of the specialised hemisphere, so a right handed person probably had a left hemispheric language specialisation. However, the majority of lefthanders also seem to have a left-hemispheric brain specialisation for language abilities, so it is not that simple.
