BP - Hemispheric Lateralisation - The Brain

Question 1

What is hemispheric lateralisation?

Answer 1

• The idea that the two hemispheres of the brain have functionally different and that certain mental processes and behaviours are mainly controlled, or specialised to, one hemisphere rather than the other
• They have functional specialisations
• Hemispheric lateralisation is similar and different to localisation

Question 2

Lateralisation

Answer 2

• About one half of the brain

Question 3

Localisation

Answer 3

• About one particular part of a hemisphere
• Some functions, such as language, are localised and lateralised
  e. g. lateralised to the left hemisphere and localised to Wernicke and Broca’s Areas

Question 4

The Corpus Callosum

Answer 4

• The two hemispheres do not work in isolation
• This brain area is a thick bundle of nerves which connects the two hemispheres
• It allows the hemispheres to communicate with each other so the brain can work as a complete organ (information received by one can be passed to the other)
• As a result, the two hemispheres retain their own roles while working together to control behaviour in the whole body

Question 5

Sperry and Gazzaniga’s Research

Answer 5

• Studied 11 individuals who had undergone surgery in which the corpus callosum was cut in order to control frequent and severe epileptic seizures
• This is a procedure called commissurotomy
• Sperry presented stimuli to one visual field using a tachistoscope - participants fixate on a dot or cross in the centre of the screen whilst stimuli are presented to one half of the visual field so briefly (1/10 of a second) that the participant does not have time to look at the stimuli with both eyes
• This means the information is only received in one hemisphere and since the corpus callosum is cut it cannot be communicated to the other hemisphere
• They were then compared to a control group with no history of epilepsy

Question 6

Anything in the left visual field

Answer 6

Goes to the right hemisphere (RH)

Question 7

Anything in the right visual field

Answer 7

Goes to the left hemisphere (LH)

Question 8

Split brain scenarios 1 - Describing what you see

Answer 8

Scenario - A picture of an object was shown to either the left or right visual field and the participant was asked to name the object.

Result -

• If shown to the RVF, they will be able to as the LH controls language processing
• If shown to the LVF, they will not be able to as the RH does not control language; typically they will also report seeing nothing

Question 9

Split brain scenario 2 - Recognition by touch

Answer 9

Scenario - Objects were shown to the participants’ left visual field and they were asked to select a matching object from a grab-bag of different objects using their left hand. The objects were placed behind a screen so as to not be seen. Participants were sometimes asked to select a related object rather than a matching one.

Result -

• The participant would be able to select a matching object and a related object, but they wouldn’t be able to describe the object as the LH controls language and has not received the information
• They would ‘understand’ the object and physically respond accordingly but not be able to name it

Question 10

Split brain scenario 3 - Composite Words

Answer 10

Scenario - Two words were presented simultaneously, one on either side of the visual field (e.g. ‘key’ on the left and ‘ring’ on the right). Participants were asked to say what they saw and draw it if they couldn’t say it.

Result -

• They would be able to name the object shown to their RVF, as language is lateralised to the LH, but they would only be able to draw or select the object in the LVF as drawing is lateralised to the RH
• They cannot communicate, so each half of the body can do one or the other

Question 11

Split brain scenario 4 - Matching faces

Answer 11

Scenario - Participants were shown an image of a face (different faces to the left and right visual field) and then were asked to match it to one from a series of other faces. They were also given composite pictures made up of two faces (one half presented to each visual field).

Result -

• They would be able to match the faces shown in the LVF as the RH would be able to identify the faces as it is responsible for facial recognition - the face shown in the RVF could be described but not re-identified as the LH does not control this and the two hemispheres cannot communicate
• In the second task, the half of the face in the RVF could be described but not identified before description (split-brain patients can draw / identify an object after verbally describing it to themselves) and the half of the face in the LVF could be identified but not described
• This suggests that facial recognition is lateralised to the right hemisphere, and language is lateralised to the left hemisphere

Question 12

Conclusion

Answer 12

• The LH is responsible for speech and language
• The RH is responsible for visual-spatial processing and facial recognition
• However, split-brain research has not shown that the brain is organised into discrete regions with specific sections responsible for specific tasks (localisation)
• Instead, it suggests that the connectivity between different regions is more important that the operation of different areas

Question 13

The functions of the left hemisphere

Answer 13

• Controls the right hand
• Right visual field
• Speech
• Logical thinking
• Analytical tasks
• Understanding written and spoken language

Question 14

The functions of the right hemisphere

Answer 14

• Control of the left hand
• Emotional content of language
• Creativity
• Left visual field
• Recognising faces
• Musical ability
• Spatial awareness
• Drawing

Question 15

Strengths of hemispheric lateralisation and split-brain research

Answer 15

1. Sperry and Gazzaniga’s work into the split-brain phenomenon has produced an impressive and sizeable body of research findings, the main conclusion of which appears to be that the left hemisphere is more geared towards analytic and verbal tasks, whilst the right is more adept at performing spatial tasks and music; the RH can only produce rudimentary words and phrases but contributes emotional and holistic content to language.
   - This is a key contribution to our understanding of brain processes
2. Split brain research made use of highly sophisticated and standardised procedures with the method of presenting visual information to one hemispheric field at the time was ingenious (describe research method of S&G) and the prevention of allowing the patient enough time to use both eyes and therefore both visual fields to observe the stimulus restricted the receiving of information to one hemisphere only, which they couldn’t communicate to the other without a corpus callosum. This allowed researchers to vary aspects of the basic procedure and ensured that only one hemisphere was receiving information at a time.
   - Therefore, the procedure developed was very useful and well-controlled, and consequently we can determine cause and effect from the research and so support the internal validity of the theory of hemispheric lateralisation

Question 16

Weaknesses of hemispheric lateralisation and split-brain research

Answer 16

1. The researchers of split brain research have urged caution in the widespread acceptance of their theories and results as split-brain people are an unusual sample of people - small sample size of 11 who took part in procedure variations, and all had a history of epileptic seizures, which can be argued to the source of unique changes to the brain that could have influenced the findings - it is also the case that some participants may have more disconnection of the two hemi. as part of the surgical procedure than others and some patients had experienced drug therapy for much longer than others. The control group may have also been inappropriate as they had no history of epileptic seizures and the data was artificially produced as in real life, a severed corpus callosum can be compensated for by the use of two unrestricted eyes.
   - This suggests the research cannot be considered to be completely valid either internally or externally and so we cannot confidently support the theory of hemispheric lateralisation from the split-brain research
2. Pop-psychological literature overemphasises and oversimplifies the functional distinction between the hemispheres on the basis of split-brain research - modern neuroscientists would contend the actual distinction as less clear cut and much more messy than the basic ‘verbal and nonverbal’ distinction; in the normal brain, the two hemispheres are in constant communication when performing everyday tasks, and many of the behaviours typically associated with one hemisphere can effectively be performed by the other when the situation requires it
   - Therefore, hemispheric lateralisation may be more complex than originally stated and the brain may work more holistically