Chapter 11 - Cerebral Asymmetry Flashcards
What does the term laterality indicate?
- The idea that the hemispheres have distinct functions (and structure)
What’s considered the general laterality between the left and right hemispheres in the human brain?
- Left: producing and understanding language
- Right: Perceiving and synthesizing nonverbal information such as music and facial information, and spatial stuff
What are some variables to consider when discussing laterality?
- Laterality is relative, not absolute (ex. right hemisphere can still have language capabilities)
- Cerebral site is at least as important in understanding brain function as cerebral side
- Environmental and genetic factors affect laterality
- A range of animals exhibit laterality
- We act as though there is a single side
T/F: The right thalamus is larger.
- FALSE
- The left thalamus is larger
Wernicke’s area is larger in which side?
- The left hemisphere
- It specializes in language
Heschl’s gyrus is larger in which side?
- The right hemisphere
- It’s another name for the primary auditory cortex
The lateral fissure has a steeper slope on which side?
- The right side
What are the 8 major anatomical differences between hemispheres?
- Right side is slightly larger and heavier, but left has higher grey: white matter ratio
- Structural asymmetry in temporal lobes likely corresponds to functional difference
- Thalamic asymmetry
- Steeper lateral fissure on right side
- The frontal operculum (part of broca’s area in left hemi) is a larger area buried in sulci (grammar production) while it’s a larger area visible on the surface of the right side (tone of voice?)
- Asymmetric distribution of neurotransmitters in both cortical and subcortical regions
- Right side extends farther anteriorly, left extends farther posteriorly (cerebral torque/Yakolevian torque)
- Thicker cortex in left side, larger surface area in right side, regionally specific asymmetries in 31/34 regions
What’s considered the strongest evidence for studying lateralization?
- Double dissociation methodologies
What’s double dissociation?
- An experimental technique by which 2 neocortical areas are functionally dissociated by two behavioural tests
- Performance on each is affected by a lesion in one area, but not the other
- Demonstrates lateralization and localization
What’s an example of a double dissociation?
- Patient 1: Lesion to A affects function X, but not function Y; Patient 2: Lesion to B affects function Y, but not X
Generally, what functions do the left and right hemispheres specialize in?
- Left: Langauge and speech comprehension
- Right: Spatial navigation and music.
Commissurotomy vs. Callostomy?
- Commissurotomy - When all commissures in the brain have been surgically severed
- Callostomy - Only the corpus callosum is severed
What are the major effects of a commissurotomy on interhemishperic connections?
- Sensory info can be presented to one hemisphere in isolation
- Functions can then be studied without the other hemisphere having access to that information
What’s a general overview of the visual pathway from the eyes to the brain?
1) Light hits the retina
2) Signals travel down the optic nerve to the optic chiasm
3) 50% of signals transfer at the chiasm, info continues down the optic tract
4) Info reaches the LGN in thalamus, then follows the optic radiation
5) Info reaches V1 and is processed subsequently
T/F: Visual input from the left hemifield is processed in the left hemisphere, and info from the right hemifield is processed in the right hemisphere.
- FALSE
- Visual systems are crossed, therefore visual info from the left hemifield is processed in the right hemisphere and so forth.
Nasal retina vs. temporal retina?
- Temporal retina - visual info remains ipsilateral
- Nasal retina - visual info is contralateral
What would happen if a split-brain patient was asked to read a word presented only in their left visual field?
- The patient would claim that they see no word, since the right hemisphere is not responsible for language comprehension, and since the corpus callosum is snipped, info containing the word presented could not be transferred to the left hemisphere.
What would happen if a spoon was presented in the right visual field, and then the left visual field in a split-brain patient?
- When in right field - Patient would be able to verbally state it’s a spoon since the left hemisphere is responsible for speech
- When in left field - Patient would verbally reply that they see nothing (the right hemisphere cannot speak). Can select object that they’re seeing with their left hand though
Where is Broca’s area located and what does it do?
- Located in the left hemisphere
- Responsible for speech production
-Evidence for lateralization
Which hemisphere is considered the “interpreter”?
- The left hemisphere, able to justify image selections in experiments with split-brain patients
- The right hemisphere can’t make non-verbal interpretations or connections of different images, instead chooses at random
What’s the chimeric figures test?
- It’s a form of facial recognition test where one face is made up of two halves of two different faces
- Used to test facial recognition in the two hemispheres
Which hemisphere is better at facial recongition?
- The right hemisphere
- This was demonstrated by the chimeric figures test
What’s a carotid sodium amobarbital injection?
- An injection that produces a brief period of anesthesia in the ipsilateral hemisphere (i.e., injection in the left carotid artery would numb the left hemisphere)
- Initially done in the 1960s
- Can be performed on individuals who aren’t split-brain
- Today injections are made through a catheter in the femoral artery
- Can arrest a hemisphere for up to several minutes
What are the advantages of a carotis sodium amobarbital injection?
- Reversible
- Each hemisphere can be studied separately in the functional absence of the anesthetized one
- Can be used to study a variety of functions, including, memory and movement
What are the advantages of laterality studies?
- Can be done on intact brains
- Takes advantage of the crossed nature of our sensory systems to study the functions of one hemisphere over the other.
What does a tachistoscope do?
- A machine used in laterality studies in the visual system
- An image is flashed in one visual field for 50ms (helps control for confounds)
- Can investigate which hemisphere is best suited for processing specific info types
What was the dichotic listening task?
- Two different sounds were presented to each ear
- Individuals with a functioning corpus callosum were able to hear both
- Split brain patients could only distinguish the sound being played on the right side, so the info being processed on the left
What’s the general auditory pathway?
- Ears - medulla - inferior colliculus - MGN (thalamus) - A1
- Some auditory connections remain ipsilateral due to the inferior colliculus, but these are suppressed during the dichotic listening task
What advantages does the right ear have (i.e., left hemisphere)?
- Digits
- Words
- Nonsense syllables
- Backward speech
- Morse code
- Difficult rhythms
- Ordering temporal information
What advantages does the left ear have (i.e., the right hemisphere)?
- Melodies
- Musical chords
- Environmental sounds
- Emotional sounds and hummed melodies
- Tones processed independently of linguistic content
- Complex pitch perception
What sounds appear to have no ear advantage?
- Vowels
- Isolated fricatives (consonant sound that requires consistent airflow)
- Rhythms
- Nonmelodic hums
In laterality studies on the somatosensory system, which hand performed better?
- The left hand performed better since somatosensory info is processed on the right hemisphere, which is dominant in spatial navigation
T/F: Both blind and sighted participants read braille more rapidly with left hand.
- TRUE
- Right hemisphere is better with braille because it relies on the spatial coordination of the dots
What’s a dichaptic task?
- When there’s a simultaneous presentation of a different tactile stimulus to each of the two hands
- Participants then look at objects and select those they previously touched
What were the respective right and left hand advantages in the dichaptic task?
- Right-hand advantage - letters
- Left-hand advantage - shapes
What’s apraxia?
- Severe deficits in making or copying voluntary movement sequences that are goal-oriented
- Can be observed in those with left hemisphere lesions
What is the major confound when it comes to studying laterality in the motor system?
- Sensory/perceptual asymmetries exist, so the two sides do not start off equal
- Ex. telling someone what to do would already engage the left hemisphere, providing it with an advantage
What’s a major confound with direct observation when studying laterality in the motor system?
- Difficult to tell whether there’s just hand preference or actual functional asymmetry in motor control
What’s one discovery that was made during direct observation during a laterality study in the motor system?
- The right side of the mouth opens slightly more quickly than the left side when making verbal and non-verbal sounds
- Quicker engagement of the left hemisphere due to broca’s area in speech production
Where does the right hemisphere show dominance in direct observation of laterality studies in the motor system?
- The left side of the face shows emotion more strongly, and the onset of facial expressions occurs sooner on the left side as well
- This movement for a purpose is asymmetrical in its function
Which types of memory do the respective hemispheres specialize in?
- Left: verbal memory
- Right: non-verbal memory
What types of spatial properties does the right hemisphere specialize in?
- Geometry, sense of direction, mental rotation of shapes
