HEMISPHERIC BRAIN LATERALISATION
SPECIFICATION: Hemispheric lateralisation: visual, auditory and language centres; Broca’s and Wernicke’s areas
Cerebral Hemispheres:
A deep furrow divides the cerebrum into two halves, the left and right hemispheres. Although the two hemispheres look mostly symmetrical, each side functions slightly differently. Sometimes, the right hemisphere is associated with creativity, and the left hemisphere is associated with logic abilities. The corpus callosum is a bundle of axons that connects these two hemispheres.
One Brain...or Two? How many brains do you have - one or two? This is quite easy to answer...you have only one brain. However, the cerebral hemispheres are divided right down the middle into the right and left hemispheres. Each hemisphere appears to be specialised for some behaviours. The hemispheres communicate through a thick band of 200-250 million nerve fibres called the corpus callosum. (A smaller band of nerve fibres called the anterior commissure also connects parts of the cerebral hemispheres.)
Handedness
Are you right-handed or left-handed? As you probably know, most people (about 90% of the population) are right-handed - they prefer to use their right hand to write, eat and throw a ball. Another way to refer to people who use their right hand is to say that they are dominant. It follows that most of the other 10% of the population is left-handed or "left-hand dominant." Few people use each hand equally; they are "ambidextrous." (Most people also have a dominant eye and dominant ear.
Exactly why people are right-handed or left-handed is somewhat of a mystery. Dr William Calvin has developed a fascinating theory about the origin of handedness and has written an essay called The Throwing Madonna to explain it.
In 95% of right-handers, the left side of the brain is dominant for language. Even in 60-70% of left-handers, the left side of the brain is used for language.
Lateralisation of function: Each brain hemisphere is dominant for other behaviors. For example, the right brain appears dominant in spatial abilities, facial recognition, visual imagery, and music. The left brain may be more dominant in calculations, mathematics, and logical abilities.
The two hemispheres work together, are connected, and share information through the corpus callosum. Much of what we know about the right and left hemispheres comes from studies in people who have had the corpus callosum split - this surgical operation isolates most of the right hemisphere from the left hemisphere. This type of surgery is performed in patients suffering from epilepsy. The corpus callosum is cut to prevent the spread of the "epileptic seizure" from one hemisphere to the other.
Dominant Functions -Left Hemisphere: Language, Mathematics and Logic. Right Hemisphere: Spatial abilities, Face recognition, Visual imagery, object recognition and Music
The lateral sulcus is generally longer in the left hemisphere than in the right hemisphere, and functionally, Broca's area and Wernicke's area are located in the left cerebral hemisphere for about 95% of right-handers but about 70% of left-handers.
• Language functions such as grammar, vocabulary and literal meaning[are typically lateralised to the left hemisphere, especially in right-handed individuals. While language production is left-lateralised in up to 90% of right-handed subjects, it is more bilateral or even right-lateralised in approximately 50% of left-handers. In contrast, prosodic language functions, such as intonation and accentuation, often are lateralised to the right hemisphere of the brain
• .. Dyscalculia is a neurological syndrome associated with damage to the left temporoparietal junction. It is associated with poor numeric manipulation, poor mental arithmetic skills, and the inability to understand or apply mathematical concepts.
• Depression is linked with a hyperactive right hemisphere, with evidence of selective involvement in "processing negative emotions, pessimistic thoughts and unconstructive thinking styles", as well as vigilance, arousal and self-reflection.
Brain terminology related to lateralisation:
Lateral (from Latin lateralis, meaning "to the side") refers to the sides of an animal, as in "left lateral" and "right lateral". The term medial (from Latin medius, meaning "middle") is used to refer to structures close to the centre of an organism, called the "med” [2] For example, in a fish, the gills are medial to the operculum but lateral to the heart.
Contralateral: (from Latin contra, meaning "against") on the side opposite to another structure. For example, the left arm is contralateral to the right arm or leg.
Ipsilateral: (from Latin ipse, meaning "same"): on the same side as another structure. For example, the left arm is ipsilateral to the left leg.
Bilateral: (from Latin bi, meaning "two"): on both sides of the body. For example, bilateral orchiectomy(removal of testes on both sides of the body's axis) is surgical castration.
Unilateral (from Latin uni, meaning "one"): on one side of the body. For example, unilateral paresis is hemiparesis.
Corpus Callosum is a huge bundle of nerve fibres (axons) that connects the two cerebral hemispheres. There are * over 200 million axons! (as many as 10x more than the spinal cord).
Commissurotomy - surgically cutting nerve fibre tracts which connect the two hemispheres
Split Brains prove lateralization to an extent……
Sperry first became interested in "split-brain" research when he was working on the topic of interocular transfer, which occurs when "one learns with one eye how to solve a problem then, with that eye covered and using the other eye, one already knows how to solve the problem".Sperry asked the question: "How can learning with one eye appear with the use of the other?" Sperry cut nerves in the eyes of cats so the left eye was connected to the left hemisphere and the right eye was connected to the right hemisphere; he also cut the corpus callosum. The cats were then taught to distinguish a triangle from a square with the right eye covered. The cats were presented with the same problem with the left eye covered; the cats had no idea what they had just learned with the right eye, and because of this, they could be taught to distinguish a square from a triangle. Depending on which eye was covered, the cats would either distinguish a square from a triangle or a square, demonstrating that the left and right hemispheres learned and remembered two different events. This led Sperry to believe that the left and right hemispheres function separately when not connected by the corpus callosum.
Sperry’s research with "split-brain" cats helped lead to the discovery that cutting the corpus callosum is a very effective treatment for patients who suffer from epilepsy. Initially, after the patients recovered from surgery, there were no signs that the surgery caused any changes to their behaviour or functioning. This observation made the question: if the surgery had no effect on any part of the patient's normal functioning, then what is the purpose of the corpus callosum? As Karl Lashley jokingly put it, was it simply there to keep the two sides of the brain from collapsing? Sperry was asked to develop a series of tests to perform on the "split-brain" patients to determine if the surgery caused changes in the patient's functioning.
Working with his graduate student Michael Gazzaniga, Sperry invited several "split-brain" patients to volunteer to participate in his study to determine if the surgery affected their functioning. These tests were designed to test the patients' language, vision, and motor skills. When a person views something in the left visual field (on the left side of their body), the information travels to the brain's right hemisphere and vice versa. In the first series of tests, Sperry would present a word to either the left or right visual field for a short period. If the word were shown to the right visual field, meaning the left hemisphere would process it, then the patient could report seeing the word. If the word were shown to the left visual field, meaning the right hemisphere would process it, then the patient could not report seeing the word. This led Sperry to believe that only the left side of the brain could articulate speech. However, in a follow-up experiment, Sperry discovered that the right hemisphere does have some language abilities. In this experiment, he had the patients place their left hands in a tray full of objects under a partition so the patient would not be able to see them. Then, a word was shown to the patient's left visual field, which was processed by the right side of the brain. This word described one of the objects in the tray, so the patient's left hand picked up the object corresponding to the word. When participants were asked about the word and the object in their hand, they claimed they had not seen the word and had no idea why they were holding it. The right side of the brain recognized the word and told the left hand to pick it up, but because the right side of the brain could not speak and the left side had not seen the word, the patient could not articulate what they had seen.
OUTLINE AND EVALUATE SPERRY'S SPLIT-BRAIN RESEARCH
APFC AIMS:
Sperry realised that ‘split-brain’ patients allowed researchers to study the functions of the two hemispheres in ways that would not be possible by true experimental methods.
To study the psychological effects of hemispheric disconnection in split-brain patients and to use the results to understand how the right and left hemispheres work in “normal” people. In other words, to demonstrate that hemispheres have different functions/abilities and that each hemisphere may have its conscious awareness & memory.
METHOD:
DESIGN: A natural experiment.
PARTICIPANTS:
• 11 male patients who had undergone surgery because of epilepsy.
• All had a history of severe epilepsy, which had not responded to drug treatment.
• Two had been operated on to sever the corpus callosum long before the experiment.
• nine had undergone surgery recently.
PROCEDURE:
Sperry designed an apparatus that allowed information to be sent to just one hemisphere to see what it was capable of, known as the divided field technique.
The subject has one eye covered and gazes at a fixation point on an upright, translucent screen (tachistoscope).
VISUAL STIMULI TESTS:
Slides have been projected on either side (or both) of the fixation point at one picture per 1/10 seconds.
PP’S have to say or write what they have seen.
RESULTS: VISUAL STIMULI:
When an object is displayed in the right visual field (thus processed in the left hemisphere), Ps can describe it in speech and writing
When an object is displayed in the left visual field (thus processed in the right hemisphere), Ps can only draw it OR
if asked to use the left hand to point to a matching object on the table, they can do so while still insisting nothing was seen. Sperry: * Results: * Tests imply that two hemispheres have different abilities & functions. Sperry: Results. Images & objects are only recognised when presented to the same eye or hand. When an object is displayed on one half of the screen (i.e., the left) and then in the other, the P has no recollection of seeing it before
Is this evidence for two separate memories?
TACTILE STIMULI TESTS:
Objects are presented to the left or right hand (or both) behind the screen. PP’S must point, feel, or draw objects (with left hand).
Tests to Right hemisphere: Range of tests/puzzles.
SPERRY: RESULTS TACTILE STIMULI:
Objects placed in right hand: PP’s described the object in speech and writing.
Objects placed in the left hand: Ps made wild guesses - seemed unaware of the object in their hand, but could draw, point, or select an object by touch • When objects were placed in one hand, subjects could point to the object with the same hand.
RESULTS WHEN TWO DIFFERENT OBJECTS DISPLAYED SIMULTANEOUSLY: • E.G. KEY & RING
Ps asked to draw what they saw with their left hand: Drew what was on the left half of the screen (KEY). But said they had drawn what was on the right half of the screen (RING) imply that one side of the brain does This implies that one side of the brain did not know what the other side has.
SPERRY: RESULTS • EXTRA TESTS TO RIGHT HEMISPHERE:
Can carry out simple maths problems.
Can sort objects by size, shape, & texture.
Picture of nude presented amongst geometric shapes produces giggling & blushing, but PP’S have no awareness of seeing pictures.
CONCLUSIONS:
Hemispheres have different functions: only the left can produce language.
The right hemisphere can recall & identify stimuli but cannot verbalise this.
Hemispheres have independent perception, awareness & memory.
SUMMARY The LEFT HEMISPHERE (LH)
In right-handed people, the LH specialises in speech and writing and in the organisation of language.
It can communicate the visual experiences of the RIGHT VISUAL FIELD and the experiences of the RIGHT half of the body.
SUMMARY The RIGHT HEMISPHERE (RH)
The RH is MUTE and cannot speak or write (aphasic and graphic)
But it can show non-verbally that mental processes, centred around the left visual field and the left half of the body, are present.
SUGGESTED CHARACTERISTICS OF THE TWO CEREBRAL HEMISPHERES
LEFT-HEMISPHERE
Verbal/ language-based: speech, reading and writing.
Visual-spatial, face recognition (Fusiform facial area), pattern and object recognition
Sequential processing
Simultaneous, parallel processing
Analytical
RIGHT-HEMISPHERE
Gestalt and Holistic
Rational
Emotional
Divergent
Intuitive, creative
Convergent Thought
Divergent thought.
Scientific
Artistic
SPERRY EVALUATION:
LIMITATIONS
SMALL SAMPLE
Because it is such drastic treatment, very few commissurotomies have ever been carried out – only eighty in total- and they are extremely rare nowadays. Only about 20 of these individuals have been given Psychological testing, and most of the data has been from less than ten of them. Therefore, the sample is too small to generalise to the whole population.
NORMALITY OF PARTICIPANTS?
Some form of brain abnormality causes epilepsy and patients must live with it many years before the surgery takes place. Moreover, the split-brain patients were also very likely to have been receiving drug therapy. It is hard to argue, therefore, that these were normal patients before surgery as both epilepsy and the drug therapy could have changed their brains – or indeed the split-brain patients may have had unusual brains to begin with.
Indeed, some of the patients showed evidence of right hemisphere language by writing with their left hand. Other split-brain patients showed evidence of bilateral organisation of language (language on both sides of the hemispheres.
It is, therefore, difficult to generalise these findings to neurotypical people, e.g., people with left lateralised language, which accounts for over 95% of the population.
UNCONTROLLABLE VARIABLES
There are too many uncontrollable variables for the split-brain group to be considered a uniform group. They varied on just about every point: age, gender, age of onset of epilepsy, causes of epilepsy, age at which surgery was performed and age of testing.
It has been claimed that the human corpus callosum shows sex differences and that the splenium (the posterior portion) is larger in women than in men. Data collected before 1910 from cadavers indicate that, on average, males have larger brains than females and that the average size of their corpus callosum is larger.
EVALUATION OF RESEARCH
For the key reasons listed above, it is unjustified to use the experimental findings from split-brain patients to build models of the neurotypical brain.
However, in parallel with split-brain work, thousands of studies were carried out on normal participants. Many of these relied on two basic techniques: Sperry’s divided field and dichotic listening.
Aims and procedures) As normal people have an intact corpus callosum the divided field cannot be used unmodified because a single stimulus presented to one hemisphere is rapidly communicated to the other. The simple modification is to present two different stimuli simultaneously, one in each visual field (or ear with dichotic listening). With brief exposure (less than 100 milliseconds, the participant usually reports one stimulus only
Furthermore, since Sperry’s studies, much more advanced technology such as fMRI and PET scans have shown that language is in the LH and visual abilities are in the right hemisphere in over 95% of people.
Other methods that confirm Sperry’s findings are case studies of people who have damaged their brains: tumours, strokes, falls, crashes, disease etc.
However, not all participants show the expected lateralisation. Some participants are left-handed with opposite organisations, and others are bilaterally organised. This may affect their self-awareness.
STRENGTHS
DISCOVERIES ABOUT COGNITIVE FUNCTIONS OF THE RIGHT HEMISPHERE
From Sperry’s work, it seems likely that the right hemisphere, previously thought to be unimportant compared to the verbal left hemisphere, did contain important cognitive functions related to language and the processing of visual-spatial stimuli, including faces. Before this, the classic view of the brain was that the left brain dominated thinking and was primarily the seat of language, analysis, and high-level learned motor skills. The right, or “minor,” hemisphere was considered less highly evolved and unable to understand reading or speech. True, the right side was a whiz at recognizing faces, reading maps, and dealing with other spatial relationships. Still, some scientists considered the right brain so mentally retarded that it was not even conscious.
ROLE OF LANGUAGE AND CONSCIOUS AWARENESS
Studies on split-brain patients show the central role of language and self-awareness or consciousness. When asked to comment on their situation, the participants always refer to the experiences of the left hemisphere. Only under special circumstances can the right hemisphere express itself clearly.
What does this mean for congenitally deaf and mute people, by the way? How do they process self-awareness without language?
ROLE OF THE CORPUS CALLOSUM
Sperry was intrigued by the role of the corpus Callosum in brain function. The disconnection syndrome seen in split-brain patients suggests that the corpus callosum enables each hemisphere to be aware of activities in the other. Each hemisphere seems to have its own specialised functions, and in the intact brain, the corpus callosum allows these to be coordinated and integrated, producing an integrated personality. Thus, we can reflect (using the left hemisphere) on our visual-spatial abilities in the right hemisphere.
COMMENTARY
The inability to verbalise a thought or feeling that is represented elsewhere in the brain of these participants seemingly indicates that consciousness is not required to perform a task.
Valid criticism is that this interrupts signals to the language centres, which are not necessarily conscious. However, there is an implicit assumption that our sense of consciousness is perceived internally through language. This raises the difficult question of what constitutes consciousness. Can there be consciousness without language?
We are used to interpreting consciousness with language, but can we say communicating through a drawn image indicates consciousness? Alternatively, is the ability to draw a picture based on a visual cue something that can happen unconsciously, without the need for consciousness? Perhaps even language perception and vocalization happen primarily at an unconscious level despite our sense that it is intimately tied to consciousness.
MIND OVER MATTER
The implications of split-brain research have been widely debated. Scientists and philosophers have long argued over what is known as the mind-body quandary, the relationship between our mind and the physical brain. Some scientists saw the work of Sperry and others as supporting the notion that the brain operates almost entirely mechanically, and that consciousness, reasoning and free will have almost no effect. But Sperry strongly felt otherwise.
SPERRY’S DIVIDED FIELD TECHNIQUE
One underestimated achievement is the contribution of Sperry’s work on the divided field technique. It has become one of the most popular experimental techniques and has produced much research on lateralisation.
