L7 - Cortical Communication and Functional Organization

Question 1

What are the 3 main types of cortical white matter pathways?

Answer 1

1. Ascending / descending pathways (into and out of cortex). Consists of ascending fibres from lower brain centres to neocortex and descending fibres from neocortex to lower brain areas.
2. Intrahemispheric (association) pathways. Consists of long fibre bundles that connect distal ipsilateral regions and short, U-shaped fibres that connect proximal areas.
3. Interhemispheric (commissural) pathways. Connect homologous structures in both hemispheres.

Question 2

What is the relationship between cortical gray matter and adjacent white matter volume?

Answer 2

• Cubic relation: gray matter volume is proportional to the cube of the average white matter fiber length.
• Each unit piece of cortex sends and receives the same total cross-sectional area of long distance fibres to and from other cortical regions
• Cajal’s principle of conservation of space, conduction time, cellular materials / principle of minimum axon length – Brain is organized so that evolution can allow for sparse coding structure

Question 3

What are the 3 main techniques that are used to provide information about the location and distribution of white matter?

Answer 3

• Blunt dissection: Dissection with instrument that doesn’t cut much
• Anterograde / retrograde staining: Introduce substance (e.g. GFP into soma – allows you to trace projections because axons turn bright green. However, tedious because there are so many slices to look through.
• Cerebral axonal tracing techniques: In DTI, it is assumed that water molecules are anisotropic. However, when they are confined in an axon, the water molecules will become isotropic (they will diffuse at right angles to the cell membrane).

Question 4

Explain autoradiography.

Answer 4

• Inject animal with isotope that binds to glucose (dense in brain and taken up by active neurons)
• Sacrifice animal and remove brain
• Make slices of brain and have slices make contact with x-ray film
• wrap it up in x-ray film to protect it from radiation and place it in freezer for 3 months
• When taken out of freezer, isotope will radiate out of neurons that took up glucose and expose x-ray film
• Film density will be darker where radiation was stronger

Question 5

What is the superior longitudinal fasciculus?

Answer 5

• A long, bidirectional bundle of axons connecting distal regions of ipsilateral cortex (frontal, occipital, temporal, and parietal lobes)
• Anterior extreme: White matter radiations in frontal lobe
• Posterior extreme: White matter radiations in posterior parietal, occipital, and temporal lobes
• Courses through operculum to posterior end of lateral sulcus

Question 6

What are the 4 main divisions of SLF?

Answer 6

1. Dorsal component: Originates in superior and medial parietal cortex and terminates in dorsal and medial frontal lobes and supplementary motor areas. Aids in regulating motor behaviour, including selecting amongst competing tasks.
2. Middle component: Originates in caudal-inferior parietal cortex and terminates in dorsolateral PFC. Aids in selection and retrieval of spatial information.
3. Ventral component: Originates in supra marginal gyrus of the inferior parietal lobes and terminates in ventral premotor and prefrontal cortex. Aids in transfer of somatosensory information (e.g. speech articulation)
4. Arcuate fasciculus: Originates in caudal superior temporal gyrus & sulcus and passes around sylvian fissure to terminate in dorsal PFC. Involved in transmitting auditory information.

Question 7

What is the inferior longitudinal fasciculus (occipitotemporal projections)?

Answer 7

• Originates in visual association areas of occipital pole and projects to lateral and medial anterior temporal regions
• Transfers visual information to temporal regions, so may be involved in object recognition

Question 8

What is the inferior fronts-occipital fasciculus? What are its two components?

Answer 8

• Interconnects frontal and occipital lobes
  1) Superficial / dorsal: Connects frontal lobe to superior parietal lobe and posterior portion of superior and middle occipital gyri
  2) Deep / ventral: Connects frontal lobe to posterior portion of inferior occipital gyrus and posterior temper-basal area

Question 9

The corpus callosum is larger in what 4 individuals?

Answer 9

1. Left handers
2. Musicians
3. Typists
4. Einstein

Question 10

What are the 3 general types of CC connections?

Answer 10

1. Midline homotopic connections
2. Homotopic connections with contralateral hemisphere
3. Diffuse terminal distribution (to alert appropriate zones in one hemisphere that the other is active)

Question 11

What are the 9 regions of the CC from anterior to posterior?

Answer 11

1. Rostrum: small diameter unmyelinated that connect frontal lobes
2. Inferior genu: smaller diameter axons that have PFC fibres that course anteriorly and contralaterally
3. Superior genu
4. Posterior genu: Premotor cortex projections
5. Anterior midbody: Precise connections between premotor, motor, somatosensory, posterior parietal cortices
6. Middle midbody
7. Posterior midbody
8. Isthmus: posterior parietal cortex, superior temporal gyrus, inferior temporal gyrus
9. Splenium: Inferior temporal gyrus and occipital visual cortex. Consists of midline connections that forms a functional ‘zipper’. No fibres from peripheral visual fields.

Question 12

What regions do the anterior forceps, tapetum, and posterior forceps connect in each hemisphere?

Answer 12

• Anterior forceps: Connects frontal lobes through genu
• Tapetum: Connects hemispheres through midbody
• Posterior forceps: Connects occipital lobes through selenium

Question 13

Explain how the evolution of the corpus callosum improved function for humans.

Answer 13

• CC arose in eutherian brain as more direct and effective system for inter hemispheric integration of topographically organized sensory cortices (moreso than the anterior and hippocampal commissures already present in nonplacental animals
• Callosal regions connecting primary and secondary sensory areas tend to have higher proportions of coarse-diameter, highly myelinated fibres than callosal regions connecting higher order areas. This suggests that in primary / secondary areas there are strong timing constraints for inter hemispheric communication that may be related to the process of midline fusion of the two sensory hemifields across the hemispheres.

Question 14

What are some symptoms of congenital agenesis of the CC?

Answer 14

• Vision impairments
• Low muscle tone
• Hypotonia
• Delays in motor milestones (sitting and walking)
• Low perception of pain
• Delayed toilet training
• Chewing and swallowing difficulties
• “Everyday” cognition remains intact (in normal range_ and symptoms are typically only revealed with neuropsychological tests

Question 15

What types of disorders can result in callosal disconnection syndromes?

Answer 15

• Callosal resection for intractable seizures

- Partial callosal resection for tumor removal

Question 16

Explain how callosal deficits differentially affect the right and left sides of the body.

Answer 16

• Objects presented to left visual field or stimuli held in the left hand cannot be named – sensory / perceptual information and tactile systems project to the contralateral cortex. Language centres located in LH, so information initially presented to the RH cannot cross over.
• Single words presented to right or left ear are reported normally, but if different words are presented to both ears simultaneously, there is a right ear advantage. This is because 90% of fibres cross over at the brainstem rather than the corpus callosum. The signal from the ipsilateral ear is inhibited by the stronger signal from the contralateral ear.
• Difficulty following verbal commands affecting the left hand (RH has poor language comprehension). However, later recovery can occur where the patient develops control of the left hand.
• Specific postures of right hand cannot be copied by the left. Objects held in one hand cannot be transferred to the other.
• Patients fail to recognize their left hand as their own. They can only control their hands through actions of the ipsilateral hemisphere.
• Verbal anosmia: inability to name odours presented only to the right nostril even though the olfactory receptors are intact
• Hemianopsia: visual information presented to the left of fixation point projects to the right occipital lobe, so patient cannot speak about it. Visual information presented to the right of fixation reaches the left occipital lobe, so patient can speak about it. For either stimulus, the patient cannot raise his or her contralateral hand to indicate detection.
• Unilateral apraxia of left hand: Tasks easily performed by right hand to verbal command cannot be performed with the left hand. Lost ability to write with left hand.

Question 17

Explain how Hebb’s theory of reverbatory circuits might help explain cortical neural networks.

Answer 17

• According to Hebb, this is a circuit that continues to respond after the original stimulus that excited it has ceased.
• Networks might reflect scaled versions of these circuits
• Lowest level = circuitry within cortical columns / layers
• Input layers combines sensory input with a modulatory location input to form sparse representations that correspond to features at specific locations on the object
• Output layer receives feedforward inputs from input layer and converges to a stable pattern representing the object
• Complex behaviour in humans is enabled by the networked interaction of distributed brain regions each specialized for specific behavioural functions
• Some areas are temporally correlated, co-active during certain tasks, or are active during complex cognitive tasks

Question 18

What are the 4 major MRI modalities that provide views of the structure, function, and connectivity of the human brain?

Answer 18

1. Structural MRI: Volume-based MRI analysis provides estimates of grey matter regions; surface-based MRI analysis provides estimates of cortical thickness and folding patterns
2. Task functional MRI: Identifies regions of increased or decreased fMRI blood oxygen level that is correlated with brain activity (synaptic currents and neuronal spiking) via neuromuscular coupling
3. Diffusion imaging and tractography: Enables characterization of ‘structural connectivity’ using preferential diffusion of water molecules along the length of axons to estimate the dominant fiber orientations and to infer long-distance connectivity from probabilistic tractographic algorithms
4. resting state fMRI: Relies of correlated fluctuations in the BOLD signal to infer “functional connectivity” that typically reflects brain regions sharing a history of co-activation.

Question 19

List 7 examples of large-scale functional networks.

Answer 19

1. Task positive network
2. Task negative network
3. Default mode network
4. Dorsal attention
5. Ventral attention
6. Salience network
7. Fronto-parietal

Question 20

What are the 4 major DMN nodes?

Answer 20

1. Posterior cingulate cortex
2. Ventromedial PFC
   3/4. Lateral parietal cortex

Question 21

Explain how networks develop and differentiate with age.

Answer 21

• At 8.5 years old, network is simplistic (1 network / areas in close proximity)
• At 13, networks differentiate into 2 different areas
• At 25.5, networks differentiate into 3 areas
• In children, regions are local and organized by physical location. In adults, networks become highly correlated despite physical distance (they are now connected by myelinated tracts).

Question 22

Explain the 4 functional hubs of the DMN.

Answer 22

• Posterior cingulate cortex and precuneus: Combines bottom-up attention with information from STM, information regarding others
• Medial PFC: Decisions about self-processing and goals, information regarding others
• Angular gyrus: Connects perception, attention, spatial cognition, and episodic recall

Question 23

Explain how white matter can exhibit neuroplasticity.

Answer 23

• DTI measures: Detected a localized increase in fractional anisotropy in white matter underneath the right posterior intraparietal sulcus following training on juggling. White matter changes were accompanied by changes in grey matter.
• Other studies have shown that myelin increases in task dependent brain regions as a result of experience. A slower rate of motor learning resulted in a greater increase in myelin.

Question 24

What are connector nodes?

Answer 24

• Nodes that connect various brain networks
• Activity at local nodes within the modules does not increase in tasks that require more cognitive functions, suggesting that the brain contains autonomous information processing modules linked together at connector nodes
• TMS can be used to interfere with the communication process that links them together
• Connector nodes exhibit increased activity when more cognitive functions are engaged in a task: Play a role in between-module communication that maintains the modular function of the brain
• Located in association cortex: Dense connections between motor and sensory processes