Learning objectives
- Difference between strength and direction of lateralisation
- Differences between individual and population biases
- Pro’s and cons of lateralisation
- Define lateralisation
- Negative frequency dependent selection of lateralisation
- Chimera’s
Lecture 6 – Riedstra 1,2
Evolution is the change in the inherited characteristics of biological populations over successive
generations, there are three conditions for evolution:
1. Heredity
2. Variability
3. Selection
Tinbergen’s 4 questions
The Tinbergen’s four questions are:
- Proximate (how)
o Development (ontogeny)
o Causation (mechanism)
- Ultimate (why)
o Function (adaptation)
o Evolution (phylogeny)
Unhemispheric sleep
Humans sleep with both hemispheric at the same time. However, some animals, birds and dolphins
for example sleep with one hemispheric awake, this is called unhemispheric sleep.
Handedness
There are not many left handlers (sinistrals) about 1 in 10. Even less are ambidextrous (no
preference).
Hemispheric functions
Lateralisation of brain and behaviour
- Lateralisation is about which functions are dominant in one hemisphere
- Hemispheres contain compatible functions and one of the two may be dominant, this causes
behavioural output —> side preferences
- Morphological differences
- Difference/asymmetry in the left-right axis and its differences sensation, perception in the
different domains such as physiology, anatomy, behaviour, etc.
, Strength and direction of lateralisation
- Birds have no corpus callosum
- Birds with laterally placed eyes have little overlap in the frontal visual field
- Almost complete decussation (crossing) of the visual projection
- First processing in contra lateral hemisphere
Advantages of lateralisation
There are multiple advantages of lateralisation:
- Conflict avoidance by dividing incompatible functions over hemispheres
- Higher capacity through specialisation
- Increased processing speed by avoiding slow interhemispheric communcation (corpus
callosum)
- Parallel processing —> multi-tasking
!!! NOTE that the corpus callosum is a mammalian structure !!!
Literature indicates that strongly lateralised animals do better than weakly lateralised animals in both
single tasks as well as dual tasks.
It seems that an increased strength (a certain direction as well) of lateralisation is beneficial for an
individual, but if this works via dual tasking is unclear.
Disadvantages are:
- Damage to a hemisphere may cause loss of a function
- Behaviour becomes more predictable
- Social components (for example during kissing; one turns right, and one turns left)
Ontogeny and gyanodromoph
Ontogeny is the origination and development of an organism usually from the time of fertilisation of
the egg to adult.
A gynandromorph are organisms with both male and female characteristics: most pronounced in
animals with large sexual asymmetry. It occurs in both mosaic forms as well in bilateral asymmetrical
forms.
Hypotheses are historically based on male-female differences in lateralisation and testosterone
exposure.
- T decreases the number of axones in the CC (Witelson)
o dextral males have a smaller CC than sinitral males
- T suppresses growth of LH (Geschwind & Galaburda)
o More sinistral males, men are better in visiospatial tasks, whereas women are better
in language tasks
- Lateralisation is part (side effect) of sexual differentiation that is under the influence of T in
mammals (Grimshaw)
o T masculinises the brain: looks like geschwind en galaburda
- T enhances individual specific axonal pruning (Lauter)
Children
It occurs that only after 4 years stability in handedness occurs, and only after 8 years the preference
is crystallised.
There are different determining factors for handedness:
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