Topic 4: Cerebral Asymmetry Flashcards
Laterality
Refers to the side of the brain that controls a given function. Hence, studies of laterality are undertaken to determine which side of the brain controls various functions.
- Laterality is relative, not absolute. Both hemispheres participate in nearly every behavior; thus, for example, although the left hemisphere is especially important for producing language, the right hemisphere also has many language capabilities.
planum temporale (Wernicke’s area)
An area comprising the anterior and posterior superior temporal planes (aSTP and pSTP), together with the auditory cortex (Heschl’s gyrus) within the lateral (Sylvian) fissure.
Major anatomical differences between the two hemispheres:
- The right hemisphere is slightly larger and heavier than the left, but the left contains more gray matter (neurons) relative to white matter (connections).
- The marked structural asymmetry of the left and right temporal lobes = difference in language and in music functions, respectively.
- The anatomical asymmetry in the temporal lobes’ cortex correlates with an asymmetry in the thalamus, shown in Figure 11.1, bottom drawing. This asymmetry complements an apparent functional asymmetry in the thalamus: the left thalamus is dominant for language functions.
- The slope of the lateral fissure is gentler on the left hemisphere than on the right (see Figure 11.1, top). The region of the temporoparietal cortex lying ventral to the lateral fissure, therefore, appears larger on the right.
- The frontal operculum (Broca’s area) is organized differently on the left and right. The area visible on the brain surface is about one-third larger on the right than on the left, whereas the area of cortex buried in the region’s sulci (ridges) is greater on the left than on the right. This anatomical asymmetry probably corresponds to the lateralization of these regions, with the left side affecting grammar production and the right side possibly influencing tone of voice (prosody).
- The distribution of various neurotransmitters is asymmetrical in both the cortical and the subcortical regions. The particular asymmetries in the distribution of ACh, GABA, NE, and DA depend on the structure under consideration.
- The right hemisphere extends farther anteriorly than does the left, the left hemisphere extends farther posteriorly than does the right, and the occipital horns of the lateral ventricles are five times as likely to be longer on the right as on the left (see Figure 11.1, bottom). The left/right asymmetry in frontal and parieto-occipital regions is known as cerebral torque or Yakolevian torque.
- generally thicker cortex in the left hemisphere but larger surface area in the right hemisphere. But there were substantial regionally specific asymmetries in 31 of the 34 regions, as shown in Figure 11.2.
Commissurotomy
Surgical disconnection of the two hemispheres by cutting the corpus callosum.
- Surgical procedure of severing corpus callosum (200-250 million nerve fibers)
Any higher order of communication is no longer shared between the two hemispheres:
- Vision will not be impacted
- Motor pathways will not be impacted
- Motor cortex in the left hemisphere cannot communicate with the right hemisphere
Split Brains
A brain in which the two hemispheres are isolated.
- the effect of commissurotomy on typical brain function. After sectioning, the two hemispheres are independent: each receives sensory input from all sensory systems, and each can control the body’s muscles, but the two hemispheres can no longer communicate with one another.
The Visual Fields / Contralateral Relationship
Input from left visual field is sent only to right brain and input from right visual field is sent only to left brain
- RVF image falls on left hemiretina of each eye
- RVF to LH (contralateral relationship) in place at LGN, then to V1
RVP (Right visual field), LH *Left hemisphere)
Grand Mal seizure
Grand Mal seizure, also known as tonic-clonic seizure, is a type of epilepsy characterized by a loss of consciousness and violent muscle contractions. It occurs as a result of abnormal electrical activity in the brain.
- activation spread across the corpus callosum to both hemispheres
- Split brain PREVENTS the seizure from crossing to the other hemisphere (cutting the corpus callosum)
GABA (gamma-aminobutyric acid)
GABA (gamma-aminobutyric acid) is the primary inhibitory neurotransmitter in the central nervous system and plays a crucial role in regulating brain activity and preventing seizures. In individuals with epilepsy, the balance between excitatory and inhibitory neurotransmitters can be disrupted, leading to excessive electrical activity and seizures.
- GABA primary inhibitory neurotransmitter (helps keep things in check, so activation does not get out of control
- In epilepsy, the transmission/activation is out of control (a belief is this has to do with GABA)
Optic Chiasam
The optic chiasm is the area where the optic nerves from the two eyes cross over in the brain. The optic nerve from each eye carries visual information from the corresponding half of the visual field to the brain. At the optic chiasm, the fibres from the nasal half of each eye cross over to the opposite side of the brain, while the fibres from the temporal half remain on the same side. This allows the brain to process visual information from both eyes to form a single, three-dimensional image.
Split-Brain Phenomenon
Patient N.G.’s behavior clearly demonstrates the different capacities of the two hemispheres when they are not interacting. The speaking left hemisphere could respond to the picture of the cup. The picture of the spoon was presented to the nonspeaking right hemisphere, and with the speaking left hemisphere disconnected from it, N.G. failed to identify the picture. The abilities of the right hemisphere were demonstrated when the left hand, controlled by the right hemisphere, picked out the spoon. But when asked what the still-out-of-sight left hand was holding, the left hemisphere did not know and incorrectly guessed “pencil.”
chimeric-face test
Consists of pictures of faces and other patterns that have been split down the center and recombined in improbable ways. When the recombined faces shown in Figure 11.11 were presented selectively to each hemisphere, split-brain patients appeared to be unaware of the gross discordance between the pictures’ two sides. When asked to pick out the picture they had seen, the patients chose the face seen in the left visual field — by the right hemisphere — demonstrating that the right hemisphere has a special role in recognizing faces.
- Faces that are created by a combination of two other halves of faces
- You are putting two halves of different faces to different brain hemispheres
- Ask: What face did you just see? Split brain patients will record the face that is in the right hemisphere (left visual field)
- Differences may relate to the laterality of face processing
Confabulation in Split Brain Patients: The Interpreter
Split-brain patients are presented with two images, one for each hemisphere, and asked to select a third image that matches the scene. Each hand will choose an option appropriate to the scene visible to the corresponding visual field, but when asked to explain the choice, the patient will describe the image selected by the right hand in terms of the scene on the right, suggesting that only the left hemisphere is engaged in the interpretation of the situation.
- Spontaneous production of false memories events which never occurred actual events displaced in space or time
- May be elaborate, detailed, bizarre or mundane e.g., had eggs for breakfast
- Not lying deliberately or trying to mislead
- low levels of awareness
Example: In the figure, the right hand chose the cow, which would match with the container of milk, whereas the left hand chose the beach ball, which would match with the beach scene. When asked why the left hand was pointing at the ball, a patient would typically fabricate a response, such as “Cows like to play with balls.”
Wada Technique
The Wada technique helps to determine which hemisphere is dominant for a speech by temporarily anesthetizing one hemisphere at a time and evaluating the individual’s language and speech abilities.
To avoid damaging speech zones in patients about to undergo brain surgery, surgeons inject sodium amobarbital into the carotid artery. The drug anesthetizes the hemisphere on the side where it is injected (here, the left hemisphere), allowing the surgeon to determine whether that hemisphere is dominant for speech.
- Surgery planning
- Sodium amobarbital
- Blood supply is unilateral
- One hemisphere is anesthetized
- Contralateral paralysis
- Speech output test
- Some tests conducted: Object naming and spelling, count, days of week forwards and backwards
The contralateral arm falls to the bed with flaccid paralysis, and a firm pinch of the skin of the affected limbs elicits no response whatever. If the injected hemisphere is nondominant for speech, the patient may continue to count and carry out the verbal tasks while the temporary hemiparesis is present. Often the patient appears confused and is silent for as long as 20 to 30 seconds but can typically resume speech with urging. When the injected hemisphere is dominant for speech, the patient typically stops talking and remains completely aphasic until recovery from the hemiparesis is well along, usually in 4 to 10 minutes.
Left Hemisphere
Associated with:
- processing high frequency information (details)
- dominant at visual detail
- Language (but in a small percentage of people, most left-handed, the right hemisphere houses language); i.e., Right ear preference for language (LH)
- Verbal communication and comprehension
Right Hemisphere
Associated with:
- low-frequency information (global)
- better at processing more global aspects, visually
- spatial reasoning, visual-spatial processing, and the processing of nonverbal communication and emotions.
- Facial recognition; there is a dominance in looking and recognizing a face - the right hemisphere has specialization in face processing.
Dichotic Listening Tasks
An auditory procedure for simultaneously presenting different auditory inputs to each ear through stereophonic earphones.
- Right ear dominance; language resides in the left hemisphere (i.e., right ear will send info to language areas of the brain fastest) contralateral relationship
Figure:
(A) Information played to either ear reaches both hemispheres by both ipsilateral and contralateral pathways.
(B) In the dichotic presentation, the contralateral pathways have preferred access to the hemisphere, possibly because the ipsilateral pathways are suppressed. Thus, the syllable “ba” presented to the left ear can gain access to the left hemisphere only through the corpus callosum. If the callosum is cut, the patient can only report hearing “ga.”
Superior Olive
Information becomes bilateral at superior olives.
In the auditory system, the superior olivary complex is a key structure involved in processing auditory information. When information becomes bilateral at the superior olivaries, it means that it is being processed and integrated by both of the superior olivary nuclei, one in each side of the brain.
The superior olivary complex receives auditory information from both ears, and is responsible for processing interaural time and intensity differences, which are important cues for localizing sound in the environment. By integrating information from both ears, the superior olivary complex helps to sharpen the perception of sound and enhance auditory spatial awareness.
Bilateral processing at the superior olivaries is important for normal auditory function, as it allows for the integration of auditory information from both ears, which helps to enhance spatial awareness and improve the perception of sound. Deficits in bilateral processing at the superior olivaries can lead to auditory processing disorders and difficulties with spatial hearing, such as difficulty in localizing sound and hearing in noisy environments.
Spatial Frequency Hypothesis
The hypothesis is that the two hemispheres are different when it comes to processing information in terms of frequencies.
- Low frequency (increased distance between peaks), in vision, refers to looking at things more globally. E.g., in the figure, you can look at the first set of pictures and look at global features to detect that they are not the same person.
- High frequency (decreased distance between peaks), in vision, refers to looking at things in detail.
- Looking at the differences in processing information between the right and left hemispheres.
Dichotic Listening Task: Hemispheric Differences in Auditory Processing of Pitch
Supports the Spatial Frequency Hypothesis: proposes that the right hemisphere performs better for low frequencies(global), while the left hemisphere performs better for high frequencies(tiny details).
This specific task also wanted to test if this phenomenon was relative or absolute. This study revealed that it is not about the specific absolute frequency. It’s about the range of frequencies you are hearing.
- Conditions A and B both show the separation of dominance between the two hemispheres within a given range.
Results:
- the right ear (left hemi) makes fewer mistakes with the higher frequencies; the left hemi is better at processing higher frequencies - left hemi is dominant for high frequencies
- the left ear (right hemi) makes fewer mistakes with the lower frequencies; the right hemi is better at processing lower frequencies - right hemi dominance for low frequencies
Situation A: (192 Hz - 208 Hz)
Situation B: (1860 Hz - 1940 Hz)
- participants are to push a button when they identify the target tone
- different tones enter each ear as they aim to identify the target tone
- researcher tracks the % errors associated
Global versus Local Processing: Interpret the Diagram
Chimeric-figure test: are there differences in assessing left and right hemi when assessing the function of the object vs. the appearance of the object?
The objects in this study can be matched by appearance or by function. Matching by appearance will give a different result than when you match by function. Results:
- Left hemisphere = function decision
- Right hemisphere = appearance decision
Method:
- These objects can be altered to represent chimeric stimuli (taking half of an object, and mixing it with half of another object), then the split-brain patient will attempt at matching by appearance or functionality.
- E.g., looking at chimeric objects’ hands/scissors; the patient will have the hand in the LVF (right hemi) and the scissors in the RVF (left hemi).
- Patients asked to match by function, would (freely w hands) choose a match that was introduced to their left hemisphere. E.g., matching the scissors with the sewing basket.
- Patients asked to match by appearance, would (freely w hands) choose a match that was introduced to their right hemisphere. E.g., matching the hand with the bird.
Objects 1, 2, 3, and 4 = hands, scissors, cake, and bird’s nest.
Objects A, B, C, and D: bird, utensils, hat, sewing basket.
Global versus Local Processing: Interpret the diagram
Similar pattern is seen in both the linguistic and non-linguistic cases.
Looking @ RIGHT-hemisphere damage: we see that there is a loss in global function which is typically associated with the right hemisphere. But local processing is maintained since the left hemisphere is intact.
- Global function = lose sight of the big “M” i.e., global overall M shape is lost
- Local function = maintain the idea that the “M” is made up of smaller “z”’ letters
Looking @ LEFT-hemisphere damage: we see that there is a loss in the local function which is typically associated with the left hemisphere. But the global processing is maintained since the right hemisphere is intact.
- Global function = maintain the idea of the big “M” i.e., global overall M shape is maintained
- Local Function = lose the idea that the “M” is made up of smaller “z” letters
Transfer of Sensory Information Between Hemispheres (Neurologically Intact Function)
Looking at EEG; measuring occipital lobe activity, primary visual cortex.
O1 - Occiputal Left Hemisphere
O2 - Occipital Right Hemisphere
5 - 20 ms = amount of time it takes for the primary visual cortex in one hemisphere to share information with the other side of the brain.
Method: participants will look at the green central spot, and at some point, one of the circles on the LVF or the RVF will shift its box colours from white to black or vice versa, where the other circle stays.
- Shift will be registered in the primary visual cortex.
- Seeing a shift in the LVF, the right hemisphere will register that shift first - in O2.
- Looking at the LVF graph, the peak begins earlier in O2 (we see the processing of information in the right hemisphere first).
- Then we see a peak slightly after in O1 after the information has been transformed.
- We see the same contralateral relationship in the left hemisphere.
Participants had to indicate when they saw a shift using their hands to push a button.
- When comparing the results between the handedness, we do not see any differences because we are measuring activity in the primary visual cortex, not the motor cortex.
- plans to respond with hands have not been made yet, need to place EEG electrodes elsewhere
Inter-hemispheric Interaction (Neurologically intact) - Physical Identity Task
The best performance, in this case, was when the information was on the same side, as it was just an identity task - only matching.
Method:
- Fixation point in the center = LVF and RVF
- When identifying the items, one number will end up in the LVF or the RVF first due to placement (recall that the information will be quickly shared due to the intact corpus callosum).
- When the matching information is in the same hemisphere, we found that people did better and were able to respond more quickly. PROVIDES SUPPORT FOR DIRECT ACCESS THEORY
- When the number that you needed to match is in the opposite hemisphere, we found that people did worse and took longer to respond - requires communication. PROVIDES EVIDENCE FOR CALLOSAL RELAY MODEL
Inter-hemispheric Interaction (Neurologically intact) - Ordinal Task / Computation Task
Prompt: Bottom item + (one of top items) > 10?
- Seems to be better to have visual information presented in opposite visual fields (therefore opposite hemispheres), to force the hemispheres to communicate
- Result: communication between hemispheres makes performance better
