Hemispheric specialization also known as cerebral lateralization refers to the functional asymmetry between the left and teh right hemispheres of the brain. This phenomenon allows each hemisphere to process certain cognitive functions differently.

1. Hemispheric
Specialization
Sonam Manoj
II M.Phil. Clinical Psychology
Chairperson: Dr. Dhanya
Chandran

2. We would cover
 Lateralization , Localization and contra laterality
 Structural asymmetries
 Commissural fibres
 Diaschisis
 Hemispherectomy and Split Brain
 Tests used to understand specialization
 Preferred handedness
 Dissociation and Double Dissociation

3. Introduction
 Cortical hemispheres as ‘’mirror image duplicates’ of one another, with the same mental
faculties located homotopically (at the same relative location). However closer
inspection reveals many differences in structure, and behavioural studies suggest
intriguing differences in function too.
 The reason for these so-called asymmetries is unclear although, as the blueprint for
basic structures ( and functioning) of our nervous system is under genetic control,
asymmetries too are assumed to depend on the action of genes .

5. Structural Differences
 Despite their superficial similarity, the two hemispheres of the human brain consistently
differ in a number of characteristic ways .
- Viewed from the top of the head, the right frontal lobe extends several millimeters further
forward, and the left occipital lobe further back ( known as cortical torque).
- The sylvian fissure, which is the dividing line between the frontal and temporal lobe, is
less sloped on the left side than on the right.
- A region of the temporal lobe known as the planum temporale, which is adjacent to the
Sylvian fissure and encompasses Wernicke’s area, is significantly larger on the left than
on the right.

6. Structural Differences
- Cells in the region of the left frontal lobe that we now call Broca’s area have more
synapses (contacts with other neurons) than the equivalent region on the right side.
- The angular gyrus (located in the posterior parietal lobe), which may be important in
reading and semantic aspects of language, is larger on the left than on the right side.
- The parietal area on the right side (just behind the angular gyrus on the left) is larger
and has more synaptic contacts. The region is linked with visual perception and spatial
processing.

7. Structural
differences

8. Commissural Fibres
 The commissural fibres interconnect the identical cortical areas of the two hemispheres
(Interhemispheric fibres). The bundle of such fibres are termed as commissures.
 The important commissures of the brain are:
 Corpus Callosum
 Anterior commissure
 Posterior commissure
 Hippocampal commissure
 Habenular Commissure

13. Corpus
Callosum
 The corpus callosum is the primary
commissural region of the brain consisting of
white matter tracts that connect the left and
right cerebral hemispheres.
 The main role of the corpus callosum is
to serve as a conduit allowing information to
transmit from one side of the brain to the
other (e.g., from the left to right frontal
lobes). It is also hypothesized to play a
major role in movement control, cognitive
functions (such as memory and learning),
and vision.

14. What is Lateralization?
 Lateralization is the idea that two halves of the brain are functionally different, and each
hemisphere has functional specializations, e.g the left is dominant for language and the
right excels at visual motor tasks.
 In other words, the lateralization of brain function is the tendency for some neural
function or cognitive processes to be specialized to one side of the brain or the other.

15. Functional
Asymmetries of
a human brain

16. What is localization?
 Brain localization refers to the idea that brain is made up of specialized modules (to put
it simply, different parts), and that each module has a certain function. For instance, one
part of the brain might be involved in storing memories, another in recognizing faces,
another in producing language.
 For example, damage to the Broca's area, or that part of the brain that is involved in
language expression, will render a person unable to communicate even when he is
perfectly capable of understanding language.

17. Lets understand Localization
Temporal Lobe
Occiptal lobe
Parietal lobe
Temporal lobe

18. Frontal lobe

22. Occipital lobe

25. Parietal lobe

29. Temporal lobe

34. What is contra laterality?
 Contralateral control is the term that describes how each hemisphere of the brain
controls the contralateral side of the body.
 This means that the left hemisphere controls the right side of the body, and the right
hemisphere controls the left side of the body.
 Hippocrates (460–380BC) was aware that injury to one side of the human brain results
in convulsion or paralysis on the opposite side. This anatomical design characteristic is
of fundamental significance to the science of neuropsychology, as witnessed by the vast
literature on laterality in animals and man.

35. Theories of Contra laterality
Santiago Ramon Y cajal theory of decussation

38. Sarnat & Nesky
 Decussation of motor control was an
adaptation in support of the defensive coiling
reflex as observed in amphioxus, providing a
direct pathway for animals to withdraw
the threatened side.

39. Methods use to
understand the
function of the
hemispheres

40. 1. Split brain

41. 2. Chimeric
faces
Levy, Trevarthen, and Sperry (1972) presented
data consistent with the view that face
processing may also be dealt with
preferentially in the right hemisphere. Split-
brain patients were shown images of pairs of
half-faces via a tachistoscope.
These “chimeric” images might, for example,
comprise half the face of a girl on the right
side, and half the face of a man on the left.
The fixation point was exactly on the joint at
the bridge of the nose.

42. Chimeric faces
Levy, Trevarthen, and Sperry (1972) presented
data consistent with the view that face
processing may also be dealt with
preferentially in the right hemisphere. Split-
brain patients were shown images of pairs of
half-faces via a tachistoscope.
These “chimeric” images might, for example,
comprise half the face of a girl on the right
side, and half the face of a man on the left.
The fixation point was exactly on the joint at
the bridge of the nose. When a participant was
asked to say what they had seen, they usually
reported to seeing an intact (i.e., complete)
picture of a woman
However if it was told to recognize from the
cards they would pick the picture of a man.
We might have predicted this, because this
half image went to the right

43. 3. Intracarotid Amobarbital Testing (IAT)
 Popularly called as the ‘WADA test’, reflecting the role of the neurosurgeon Juhn Wada.
 IAT is commonly used to assess the laterality of language in individuals who will be
undergoing brain surgery.
 In this test sodium barbital (a short term anaesthetic) is injected into either the right or
the left internal carotid artery. The injection anesthesizes the hemisphere ipsilateral to
the injection.

44. 4, Dichotic
Listening
 Dichotic listening (DL) is a
noninvasive technique for the
study of brain lateralization, or
hemispheric asymmetry. DL is
the most frequently used
method to reveal a left-
hemisphere dominance for
language processing,
particularly extraction of the
phonetic code from the speech
signal.

45. Concept of Dissociation
In neuropsychology, dissociation involves identifying the neural substrate of a particular
brain function through identification of case studies, neuroimaging, or neuropsychological
testing.
Single dissociation
 When dissecting complex mental tasks into their subcomponents, a researcher can
establish a "single dissociation" between functions. This is done by demonstrating that a
lesion to brain structure A disrupts function X.

46. Concept of Double Dissociation
Double Dissociation:
 To strengthen a single dissociation, a researcher can establish a "double dissociation", a
term that was introduced by Hans-Lukas Teuber in 1955.
 This is the demonstration that two experimental manipulations each have different
effects on two dependent variables; if one manipulation affects the first variable and not
the second, the other manipulation affects the second variable and not the first.

47. Double Dissociation with example
 Paul Broca and Carl Wernicke were two physicians of the 1800s whose patients were
evidence of the double dissociation between generating language (speech) and
understanding language.
 Broca's patients could no longer speak but could understand language (non-fluent
aphasia) while Wernicke's patients could no longer understand language but could
produce jumbled speech (fluent aphasia).

48. Conclusion
By establishing Double Dissociation, scientists are able to determine which mental processes are
specialized to certain areas of the brain.

49. Hemisperectomy and what can we
expect ?
 The operation of hemispherectomy might more correctly be termed hemidecortication,
since not all the hemisphere is removed in most instances.
 Usually parts of the deep nuclear masses such as the thalamus and striate complex
remain untouched.
 Two major conditions have appeared to date as an indication to carry out the surgery
1. Infantile hemiplegia
2. Extensive invasion of the hemisphere by neoplastic disease.

50. Infantile hemiplegia
 The earliest report of removal of a very considerable proportion of one hemisphere for the
treatment of uncontrollable epilepsy associated with hemiparesis of early onset appears to
be that of Mc Kenzie (1938).
 The most striking feature of early reports, apart from clinical improvement, was the absence
of mental deterioration that might be expected on a priori grounds from such massive
removal of cortical tissue.
 Kyrnauw’s first report (1950) described 12 cases with ‘improvement of mentality’ as
adjudged by clinical evidence, though no psychometric evidence was presented in support.
 Cairns and Davidson (1951) reported three cases with no evidence of intellectual loss, but
rather an improvement in scores on the test such as Weshcler Bellevue and Stanford Binet
scales.

51. Case of hemispherectomy – Long term
follow up study
 Hemispherectomy was performed for epilepsy at the age of 5.5 years of age and was
followed up until 21 years of age of the patient. (Smith and Sugar 1975). This case provides
impressive evidence of the brain’s ability to utilize residual tissue in the remaining
hemisphere as the basis for high level ability in both the verbal and nonverbal spheres.
 Before operation (5.5 years) – marked speech defect and mental age as 4 years.
 4 months after the operation – mental age was close to the chronological age, had become
normal
 At 8 years 8 months, his mental age was 7 years 10 months and speech both verbal and
nonverbal were well above the normal range
 At 21 his full-scale IQ was 125

52. Understanding
 Such improvement strongly supported the frequent claim that the deficits seen after
operations on the brain might often be due to the effect of residual pathological tissue
rather than simply due to loss of brain substance.
 Also the brain has its miraculous way of using the residual tissues and function to
almost the same functionality or better than before.

53. Adult hemispherectomy
 The first reports of removal of most of one hemisphere for tumour were made by Dandy
(1928, 1933).
 A case reported Zollinger (1935) showed that not all language was lost with dominant
hemispherectomy . The patient retained an elementary vocabulary that was partially
increased by speech training.
 A second case of hemispherectomy reported by Crockett and Estridge (1951), survived
4 months and although severely impaired, also showed improving capacity for speech
as well as verbal comprehension.

54. Adult hemispherectomy Case followed
up for 7 months
 Immediately after the operation of a tumor in the left hemisphere, the patient showed
the anticipated signs of right hemiplegia, right hemianopia and severe aphasia.
 On later examination the patient showed continuing recovery of language functions, The
patient preserved the ability to sing old songs, suggesting that the right hemisphere
plays an important role in this area.

55. Understanding
 Since these functions are not abolished, and since speaking, reading, writing and
understanding language show improvement even after left hemispherectomy, the right
hemisphere apparently contributes to all these functions, although in varying
proportions.

56. Individual
differences in
brain
organisation

57. Preferred Handedness
 Preferred handedness has been defined by the relative efficiency or speed of the hands.
The latter definition shifts the emphasis from “preference” to “proficiency” of hand use
 Broca (1861) proposed that a person’s preferred handedness was opposite from the
hemisphere specialized for language.
 Researchers continue to debate that whether it is genetic in origin as Annett (1985) or
related to intrauterine factors of the foetus in the womb (previc, 1991)

58. Preferred handedness
 For many years it was more or less assumed by psychologists that the organisation of the
left-hander’s brain was the mirror image of that of the righthander. However, data from the
Wada test put paid to this idea (Rasmussen & Milner, 1977).
 As expected, results indicated a pattern of left lateralised language for almost all right-
handed individuals. But for left-handers a different result emerged. About two-thirds have the
same arrangement as right-handers. Of the remainder, about half show the opposite pattern
(reversed asymmetry) and half show language and non-language skills both distributed in
each hemisphere (bilateral distribution).
 Nevertheless, combining these figures, about 96.5% of the population has left hemisphere
specialisation for language generation. These data have recently been broadly confirmed by
Knecht et al. (2000)

59. Handedness and Cognitive function
 Hardyck and Petrinovich (1977) found that, on average, left-handers with damage to the
right hemisphere were more likely to experience language problems than righthanders
with similar damage (14% versus 7%).
 The incidence of aphasia following left-sided damage was more or less the same for
right- and left-handers.
 Similarly, spatial skills were more likely to be affected after right hemisphere damage in
right-handers than in left-handers.
 Taken together, these findings suggest that left-handers as a group may be less
“lateralised” than right-handers.

60. Sex differences
 Structurally, female brains are slightly lighter, but contain proportionately more grey
matter (cell bodies and dendrites). Male brains have more white matter and larger
ventricles.
 Females have a larger anterior commissure and a larger splenium (the most posterior
part of the corpus callosum). It has been estimated that at birth the general level of
tissue development in boys is between 4 and 6 weeks behind that of girls, and they are
known to be about twice as likely to be born with a range of neurodevelopmental
disorders as girls. It is also well documented that cognitive developmental disorders
including autism, hyperactivity, stutter, aphasia, and dyslexia are all four to six times
more common in boys

61. Sex differences
 As with the earlier debate about the functions of the left and right hemispheres, the
rather simplistic conclusions drawn by early researchers (that boys are better at
visuospatial skills and girls are better at linguistic skills) have required revision in light of
more thorough research.
 Although a male visuospatial advantage is most apparent on tests of mental rotation
and targeting, females outperform males on other non-verbal measures such as manual
dexterity and “spot the difference” tests where subtle differences between similar figures
must be found (Kimura, 2002). Most measures of language function clearly favour
females, but males are better at generating verbal analogies (Halpern, 2005

62. Diaschisis
 Diaschisis, a Greek term meaning “split throughout,” was introduced to neurology in
1914 by Monakow. This concept suggests that damage in one focal area of the brain
can affect distant brain regions.
 Diaschisis was originally described in patients with ipsilateral paralysis following focal
brain lesions. With the advent of brain imaging techniques, we now know that focal
damage can result in large connectivity abnormalities in the brain

63. Summary
 Hemispheric differences exist, but they are not consistent with the simple notion that people are either
“right-brained” or “left-brained” in their behavior. Generally, the brain functions as a cohesive whole with
interconnected pathways and regions performing both distinct and overlapping functions. However,
certain functions, such as speech, tend to be lateralized to one hemisphere or the other.
 Lateralization does not imply that the other hemisphere is not providing a complementary function. For
example, the left hemisphere is generally specialized for verbal speech, whereas the right hemisphere
plays an equally important role in providing the prosodic aspects to speech.
 Sex differences in the performance of neuropsychological tasks are evident, although the sexes
overlap in their performance and the actual differences are small. Morphologic and functional brain
differences have been identified; however, there is a lack of empirical consensus. Because of the
organizing and activating influence of sex hormones, the

64. References
 Darby, D., & Walsh, K. W. (2005). Walsh’s Neuropsychology: A Clinical Approach. Churchill Livingstone.
 Lezak, M. D., et al. (2012). Neuropsychological assessment (5th ed.). Oxford University Press.
 Kolb, B., & Whishaw, I. Q. (1995). Fundamentals of Human Neuropsychology.
http://ci.nii.ac.jp/ncid/BB19299943
 Stirling, J. D., & Elliott, R. (2008). Introducing neuropsychology.

65. Thank you