Human Neuropsychology, Bryan Kolb & Ian Q. Whishaw’s Fundamentals of Human Neuropsychology, Sixth Edition Chapter 11 Lecture PPT Prepared by Gina Mollet, Adams State College

Cerebral Asymmetry

Portrait: Words and Music M.S. Lifetime history of epilepsy Had a cyst removed from her left temporal lobe Developed an infection that damaged the left hemisphere Diagnosed as a global aphasic, but could still sing songs she had known before

Anatomical Asymmetry in the Human Brain Laterality Left and right hemispheres have different functions Left hemisphere Language Controlling movement on the right side of the body Right hemisphere Perceiving and synthesizing nonverbal information Controlling movement on the left side of the body

Anatomical Asymmetry in the Human Brain Four variables that complicate laterality research Laterality is not absolute Cerebral site is just as important as cerebral side Laterality is affected by environmental and genetic factors Laterality is exhibited by a range of animals

Anatomical Asymmetries Right hemisphere is larger and heavier than the left; it contains more gray matter Temporal lobes have a marked asymmetry Planum Temporale Larger in the left hemisphere Heschl’s gyrus Larger in the right hemisphere Asymmetry in the temporal lobes correlated with asymmetry in the thalamus

Anatomical Asymmetries Slope of the lateral fissure is gentler on the left hemisphere The frontal operculum is organized differently on the left and right sides Distribution of neurotransmitters is asymmetrical Right hemisphere extends further anteriorly; the left hemisphere extends further posteriorly Anatomical asymmetries affected by sex and handedness

Neuronal Asymmetry Neurons in the left frontal operculum and in the left precentral cortex have more dendritic branches than those in the right frontal operculum and right precentral cortex

Genetic Asymmetry Tao Sun and colleagues Genes are expressed differently in the right and left hemispheres May regulate the production of growth factor May account for functional properties (i.e. handedness)‏

Asymmetry in Neurological Patients Double Dissociation Two areas of the cortex are functionally dissociated by two behavioral tests Each test is affected by a lesion in one zone but not in the other Example: left hemisphere lesions produce language deficits that do not appear after right hemisphere lesions

Asymmetry in Neurological Patients P.G.: Left temporal lobe tumor S.K.: Right temporal lobe tumor

Patients with Commissurotomy Surgical procedure of severing the corpus callosum Two hemispheres can no longer communicate Split-brain Sensory information is sent to only one hemisphere Example vision: Input from the left visual field is sent only to the right brain and input from the right visual field is sent only to the left brain

The Split-Brain Phenomenon

The Split-Brain Phenomenon Illustrates the right hemisphere’s role in facial recognition Jere Levy Split brain patients identify the face that is seen by the right hemisphere

Brain Stimulation Effects of applying electrical current to the cortex Localized movements, localized dysthesias, light flashes, or buzzing sensations “Interpretive” and “Experiential” responses Production of déjà vu, fear, dreaming states, visual or auditory memories Acceleration of speech production Stimulation can block function if the subject is engaged in an ongoing task

Carotid Sodium Amobarbital Injection The Wada Test Injection of sodium amobarbital to produce a period of anesthesia in one hemisphere Allows for the study of each hemisphere Useful for determining lateralization of speech

Behavioral Asymmetry in the Intact Brain Asymmetry in the Visual System Tachistoscope can be used to present information to each hemisphere independently Infer which hemisphere is better at processing certain types of information by looking at accuracy of identification in each visual field

Behavioral Asymmetry in the Intact Brain Asymmetry in the Auditory System Dichotic listening Simultaneous presentation of two different stimuli to the left and right ears Subjects are asked to report which stimulus they heard most clearly Studies find a: left ear advantage for music and stimuli with a tonal quality right ear advantage for verbal stimuli

Dichotic Listening

Behavioral Asymmetry in the Intact Brain Asymmetry in the Somatosensory System Subjects are blindfolded and then asked to perform various tasks with their hands Differences in efficiency are evidence of lateralization of function Braille read more rapidly with the right hand Dichaptic Test Subjects feel objects and then identify those that they touched Right hemisphere advantage for shapes; left hemisphere advantage for letters

Behavioral Asymmetry in the Intact Brain Asymmetry in the Motor System Apraxia - severe deficits in copying sequences of movements Appear after left-hemisphere lesions Direct Observation Watch subjects performing different tasks to see which hand is more active Wolf and Goodale Right side of the mouth opens wider and more quickly for verbal and nonverbal tasks Onset of facial expressions sooner on the left side of the face

Behavioral Asymmetry in the Intact Brain Asymmetry in the Motor System Interference tasks Simultaneous performance of two tasks Example: playing the piano and speaking

What Do Laterality Studies Tell Us about Brain Function? Behavioral studies may be measuring more than just laterality Laterality studies provide correlations between anatomy and behavior Measures of laterality are not concordant Robert Efron Laterality can be explained by the way the brain “scans” sensory input

Neuroimaging and Asymmetry Reveal activation in the left hemisphere in Broca’s area and Wernicke’s area when speaking and listening to speech

Theoretical Arguments: What is Lateralized?

Specialization Models Kimura Left hemisphere may be specialized for some kind of motor control, rather than verbal functioning Two hemispheres process information in different ways Left: Logical, analytical, computer-like fashion, analyzing stimuli sequentially Right: Overall stimulus configuration, organizations and processes as gestalts

Interaction Models Two hemispheres function simultaneously, but work on different aspects of processing Two hemispheres have the capacity to perform a given function and they inhibit each other’s activity Information processing models

Snapshot: Imaging the Brain’s Plasticity Holloway and colleagues Investigated sensorimotor function in hemispherectomy patients Found that the passive movements produced activation in areas of the brain that normally respond to the opposite side of the body

Preferred Cognitive Mode Preference of one thought process to another Left hemisphere people analytical, logical, verbal, and meticulous Right hemisphere people concerned with organizing concepts and visualizing meaningful wholes Cognitive set Tendency to approach a problem with a bias Can affect tests of lateralization

Measuring Behavior in Neuropsychology Results can be difficult to replicate Numerous ways to measure the same thing Sometimes multiple measurements may be needed Example: Foot measurement studies