Brain development (8.10, 8.11, 8.13, 8.12, 8.19)
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Vision requires neuronal connections from the left and right retina to the thalamus and then from the thalamus to the visual cortex. The ordering of these connections ensures that what is seen with the right eye can be mapped onto what is seen with the left. These ocular dominance columns are present at birth.
The critical period is a window of time after birth during which the neuronal pathways between the retina and the visual cortex exhibit sufficient neuroplasticity to be adjusted and strengthened.
During the critical period, the number and strength of neuronal connections between the ocular dominance columns and the visual cortex increase.
Strengthening and maintaining neuronal connections with the visual cortex requires neuronal activity and, therefore, sufficient exposure to light and pattern stimuli during the critical period.
If one eye receives more stimulation than the other during the critical period, that eye will become better connected and dominant in the brain.
Animal models have been extensively used to study biological processes, including how the brain functions.
Scientists use animals in experiments in the hope that their findings on processes in the animal model may be translatable to equivalent processes within humans.
Animal models are selected through consideration of various factors:
- Ease of obtaining the animal.
- Rate of breeding / length of life cycle (e.g., for genetic studies).
- Similarity to humans (e.g., for system functions and drug testing).
Monkeys and kittens were used in experiments into the critical period by Hubel and Wiesel.
Monkeys and kittens were selected for their similarity to humans in brain connectivity and visual field.
Several studies were conducted to adjust the visual stimulation provided to animal models at various stages of their visual development. The impact on their vision was recorded.
Hubel and Wiesel subjected their kitten animal models to monocular deprivation during various stages of their development. This involved preventing stimuli from reaching one eye whilst maintaining stimulation in the other eye.
In kittens, the critical window occurs between three weeks and three months, with a peak at 4 weeks.
Hubel and Wiesel’s monocular deprivation in monkeys during the critical window demonstrated that the lack of stimulation prevented the necessary development of the optic nerve connections, which connect the retina to the visual cortex via the thalamus.
The retina was still able to respond correctly to stimuli, but the signal was not received by the brain. This supports the idea of a period of neuroplasticity.
Some people believe that animals have rights, linking the use of animal models in research to the historic practice of using humans as slaves. Is using animals in a laboratory setting a violation of their rights?
European law has developed to protect animal welfare but does not reference rights. The legislation is designed to ensure that animals are treated ‘well’ as much as is possible.
In Europe:
“Vertibrates should not be used in medical research if there are non-animal alternatives. Where no alternative exists, strict guidelines are in place, and the research must be sufficient to warrant the use of vertebrates.”
It is debated, scientifically and philosophically, whether animals are capable of experiencing suffering and pleasure. If animals cannot “suffer”, then does their experience matter?
The arguments in favour of animal research on these grounds are most valid when discussing the use of invertebrates and fish, for which studies support the absence of these emotional responses.
Utilitarianism is the most commonly used argument for the use of animal models in scientific research. In basic terms, utilitarians strive for the “greatest good for the greatest number”. Utilitarianism states that it can be correct to choose animal modelling in research, where the overall expected benefit outweighs the overall expected harm.
Memories can be created by altering:
- the pattern of neuronal connections within the brain
- the strength of the connections in different parts of the brain.
Both the pattern and strength of connections are altered through nerve stimulation.
Habituation develops when exposure to, and sometimes reaction to, a stimulus is repeated.
Habituation uses nerve memory to allow the body to ignore unimportant repetitive stimuli.
These two mechanisms enable the most effective use of other mental resources when processing more threatening or rewarding stimuli.
During habituation:
- The presynaptic membrane becomes less permeable to calcium.
- Intracellular calcium levels drop.
- Fewer vesicles fuse with the presynaptic membrane.
- Fewer neurotransmitter molecules bind to receptors in the postsynaptic membrane.
- Lack of depolarisation means sodium channels do not open.
As a result, the signal of this stimulus to the brain becomes weaker.
Habituation can be observed across humans and other animals.
Examples of habituation include:
- Mentally blocking out background noise.
- Not having awareness of the sensation of your clothing.
- An animal’s lack of fear of their feeder at the zoo.
CBT, Cognitive Behavioural Therapy, uses habituation to manage phobias. Repeated safe exposure to a perceived threat reduces and potentially eliminates the nervous system’s reaction to the threat.
When differences in the nature of adult brains are investigated, there is a need to differentiate between “nature” and “nurture”.
Studies are conducted to determine whether behaviours and conditions are purely genetic (nature), purely environmental (nurture), or a combination of both.
To determine whether elements of behaviour are innate or learned, some studies are conducted as soon after birth as possible. This limits the test subject’s ability to be influenced by their environment.
Studies quickly confirmed a human baby’s innate ability to recognise human faces, cry and breastfeed.
Studies on depth perception were more complex, as they required gross motor development; therefore, test subjects had to reach a certain age before they could be tested, allowing for greater exposure to environmental influences.
Identical twins are used in studies of nature vs nurture for factors which become visible at later developmental stages. When twins are genetically identical but exposed to different environments, the observable differences are likely at least partially due to nurture.
Animal studies of critical developmental regions have highlighted that some neurological functions are innate (nature), while others are learned (nurture)
Findings on innate vs learned behaviour can vary from species to species.
Findings from animals do not always relate to humans.
Studying individuals with injuries to different parts of the brain has increased scientific understanding of brain function.
Where there is significant damage to an area of the brain but no impact on a given function, this brain area cannot be critically linked to the function.
Where a function is inhibited by a brain injury, that part of the brain must be connected with that function.
The impact on memory (amnesia) varies depending on the area damaged, implying that memories are formed and stored across the brain.