PSYC327: Large-Scale Brain Networks

Large-Scale Brain Networks

networks of brains structures that extent across large areas of the brain that work together to perform certain tasks

The Brain as a Network: Language

Cortical regions interact to form interconnected processing "streams" (dorsal & ventral stream).

The arcuate fasciculus is a key language tract and has white matter fibers that supports the communication between the language network.

The Brain as a Network: Visual Attention

Two spatial attention networks

- Dorsal attention network

- Ventral attention network

White Matter Tracts

Analysing the white matter tracts allows us understand how different regions of the cortex interact

Locating white matter tracts helps us understand the anatomical interconnections between regions.

Identifying white matter tracts = structural connectivity (structures that support interaction between brain areas)

an MRI technique to identify white matter tracts

Diffusion Tensor Imaging (DTI)

Diffusion Tensor Imaging (DTI)

- Tracks the direction of movement of water molecules

- Able to register what direction the water molecules move in

- Able to detect white matter tracts because we can detect areas in the brain where water molecules more along one line/linear dimension

- Systematic flow in one direction indicates a large white matter tract

- Software can identify primary direction of flow and can be colour coded to tell you the primary direction flow

DTI: Clinical Applications

- Diseases that affect long axons, can be identified via DTI

- Diffuse axonal injury in head injury and concussion can lead to tearing and damage to white matter tracts and DTI is important for identifying whether communication is damaged

Structural Connectivity

the anatomical white matter tracts connections between regions

- White matter tracts are like the "roadways" of the brain

Functional Connectivity

patterns of activity in white matter tracts during task

- The actual traffic during some activity

- Not just where are the fibres but what is going on and how much interactions are there during tasks

Functional Connectivity Between Regions/Dynamic Networks

- Areas of the brain area have a degree of "cross-talk" between regions varies depending on task

- To measure "cross-talk", measure the degree of synchrony between activity in different regions (statistical modelling of fMRI, BOLD signal, EEG data).

- What we experience as "thought" involves synchronised activity across the entire cerebral cortex

Functional Connectivity Example: Resting State fMRI

Found synchrony between posterior cingulate cortex (PCC) & MPF structures, thus seems to work in a network

- BOLD goes down in one, it goes down in the other.

- BOLD goes up in one, it goes up in the other

Found that the IPS (location in parietal lobe) is activating almost in total opposition to PCC & MPF.

- Activation to PCC & MPF, there's deactivation in IPS and vice versa

Statistical Analysis of Networks in the Brain: Time Series Analysis

- apply statistics to determine correlation between activation in different brain areas

- Correlation between signal in one region and another region to determine functional connectivity

- Activity in Regions A & B correlates: as region A BOLD increases, so does region B

The Default Mode Network

Network of structures with tightly synchronized activity while a person is under an fMRI and is at rest

- Active at rest, eyes closed

- Active when focused on internal thoughts

- Deactivated when focus shifts to external stimuli

- The "daydreaming" network

Structures = temporo-parietal cortex, hippocampus, vmPFC

Regions in the DMN become More Active (cf. to rest) When a Person is:

- Retrieving memories for past experiences

- making social judgements about themselves relative to others e.g., rating how perfectionistic they are

- Mind wandering

Structures Involvement in DMN:

- vmPFC: retrieving past memories, social cognition, and making judgements about the possible thoughts/feelings of others

- Hippocampus: retrieving past memories

Christoff et al. (2009): Mind Wandering

- Participants had to do a boring button press tasks

- At regular intervals, researchers interrupted the flow of the task to ask whether pp attention was focused on the task or whether it was "off task" (they were day-dreaming)

- Found greater activation and synchronisation in parts of the default mode network during day dreaming.

Advantages of DMN:

- Innovative way of thinking about how we think (networks rather than structures cause each function)

- Can use fMRI to measure neural activity when pp aren't required to speak and internally-focused tasks

Networks:

- constantly in compe between other networks

- Constant battle between networks

The Frontoparietal Control Network

- Activated when performing a demanding task

- Highly externally focused cognitive control

- Involved in orienting to outside world

- Structures = Lateral PFC & Parietal lobe & Dorsomedial PFC

Lateral PFC

cold cognitive control, effortful tasks involving on regulating behaviour and focusing attention

Frontoparietal Control & Default Mode Networks

- We actively "shift" between these networks depending on what's required

- There is alteration between these two networks as we shift between one to another

- IPS = part of the frontoparietal control network

- PCC & MPF = part of the default mode network

The Salience Network:

Switching between these "modes" of thinking; this network signals when something is aversive or emotionally salient (not talking about fear-response) and can respond to a thought or intention

Regions of Salience Network

The dorsal anterior cingulate and the anterior insula form the two main nodes of the salience network

Salience Network: Dorsal Anterior Cingulate

- signals when we need to exert effort/more resources on a task

- Plays a role in helping initiate new activities and upregulate autonomic nervous system to do a task

- Making an effort

Salience Network: Anterior Insula

- evaluates emotional and bodily states (semantic states)

- Tucked right under the hanging lobes of the frontal lobes

- Works closely with dorsal anterior cingulate due to importance in identifying and signally emotion and bodily states

Salience Network: How it Functions (Menon, 2011)

- DMN & FP control network = negatively correlated/actively inhibit each other/alternating activation

- The shift between the two networks can be initiated by the salience network

- The salience network rapidly switches from the DMN to FPC network and go into thinking mode and allocating resources

- These networks influence each other and are controlled by the salience network

Example: Complex Problem Solving after Anterior PFC Damage (Burgess, 2007)

- Real life problems & multitasking involve dynamic switching between modes

- Anterior PFC crucial for switching between modes of thinking

- Suggested that damage to anterior PFC prevents an individual from switching between modes of thinking/impacts complex tasks (e.g. multiple errands) and multitasking

Applications: Depression (Williams, 2016) & (Mulders et al., 2015)

- Difficult to measure due to the heterogeneity of depression and MDD

- Finding: people with MDD spend more time in the DMN under the scanner

- Finding: people with MDD had increased connectivity/synchroneity (and overall activation) within the default mode network

Limitation of Applications for Depression

is this cause or effect?

- fMRI activity is just another observation, doesn't necessarily reveal the root cause or the valence of the thought

- Recent study found these effects are not as prominent

- Reverse inference problem

Example: Creativity in Healthy People (Beaty et al., 2018)

- Alternate uses task: list as many uses as possible for a given object

- Control condition: list all the characteristics of a given object (cylindrical, opening at top, often has a handle, usually made of heatproof material)

- Suggests that creative thought involves flexible interplay between: default mode network & Frontoparietal control network