Prefrontal Cortex

425 Section 2, Lecture 1

Phrenology

“Mapping” the contours of the skull in specific regions to specific mental functions.

Measuring “bumps” on a person’s skull to predict mental traits and character.

What is the frontal cortex broken down into?

Prefrontal cortex and the motor cortex.

Function of the motor cortex

Responsible for movement.

What is the prefrontal cortex broken down into?

Lateral prefrontal cortex and the medial prefrontal cortex.

Lateral prefrontal cortex

Involved in enabling complicated thoughts, e.g. listening to others speaking, solving equations.

Medial prefrontal cortex

Involved with consuming your own thoughts and subjective experiences, e.g. inner voice.

What is the prefrontal cortex not involved in?

Sensory processing/perception systems, like visual, tactile, auditory, gustatory and olfactory processing.

It is more involved in taking that sensory information and processing it into something more.

What is the main function of the prefrontal cortex?

Planning, making decisions and controlling how we act.

How big is the prefrontal cortex compared to other animals, e.g. chimpanzees or macaques.

The grey matter volume in the human prefrontal cortex is 1.2x greater than in chimpanzees and 1.9x greater than macaques.

White matter tracts are also 1.7x denser subcortical connection in humans compared to chimpanzees.

Motor and visual areas are also larger in humans.

Why has there been an increase in the size of the prefrontal cortex across time?

Evolution.

Synaptogenesis/synaptic pruning: forming new synaptic connections.

Dendritic arborization: more tree-like structures in the dendrites.

What is the first cortex to develop (and what age?).

Sensorimotor cortex.

Age 4.

What is the last cortex to develop (and what age)?

Prefrontal cortex.

20’s (myelination even continues into the 20s).

The case of Phineas Gage

1848.

An iron bar went right through his frontal cortex through his eye.

Gage survived the accident. His walking and talking was completely fine. However, his personality completely changed.

He found himself to be irritable and couldn’t plan, despite being considered a “model employee” before the accident.

What did the case of Phineas Gage show?

The frontal cortex was crucial in the role of planning and personality.

Pros of using single case studies, e.g. Phineas Gage.

No one is intentionally harmed to examine the effects of frontal lobe lesions, making them ethical.

Cons of using single case studies, e.g. Phineas Gage.

Problems with causality: how do we know the deficits were due to the frontal lobe lesions? Was it due to the trauma of the accident?

Pros of using ANIMAL WORK to study the prefrontal cortex.

Full experimental control: you can control their development and genetic factors to increase causality.

You can enable manipulation of precise neurobiology component.

Cons of using ANIMAL WORK to study the prefrontal cortex.

Not humans: animal brains are different to human brains, therefore can we generalise these findings to human brains?

Ethics: some work could put animals at risk of death or severe pain.

Pros of using SELF-REPORT METHODS to study the prefrontal cortex.

Patients with frontal cortex damage can give us rich, qualitative evidence of what it is really like from someone living with it.

Gaining qualitative data is often cheap and does not usually produce a burden on the patient.

Cons of using SLEF-REPORT METHODS to study the prefrontal cortex.

Lack of control: we do not have control of what exact region has lesions.

Some patients do not even know they have lesions and may be unaware of some of the difficulties they may face, therefore leading to missing information.

It is also very subjective - are they telling the truth?

Patient (case) / control studies.

Gathering a group of patients and compare them to control subjects.

In order to measure how good someone’s planning is, we can reduce planning to: getting A to B while overcoming an obstacle in the way.

Tower of London task

6 coloured balls.

Participants need to arrange the a bottom display to match the top display. The magenta ball is an obstacle in the way of obtaining this.

IV: patient group

DV: accuracy and reaction time.

Owen et al (patients / control study).

Tested patients with frontal lobe damage on the Tower of London task. Healthy controls were also tested.

Frontal lobe patients required more moves to solve the problem compared to the controls, meaning they have suggested impaired planning.

What did Owen et al’s patient / control study show?

The frontal lobe and the prefrontal cortex has involvement in planning.

Pros of using PATIENT / CONTROL STUDIES to study the prefrontal cortex.

We can increase our sample and see if we can get consistent results across lots of patients.

Cons of using PATIENT / CONTROL STUDIES to study the prefrontal cortex.

Lack of control: we did not experimentally induce the lesion, so therefore is the lesion definitely causing this behaviour.

Functional Magnetic Resonance Imaging (fMRI)

Having a patient perform a planning task and see what brain regions are active during the task by measuring blood flow (BOLD).

Blood oxygenation-level dependant (BOLD) = changes in the ratio of oxygenated and deoxygenated haemoglobin.

Cognitive subtraction (fMRI)

Patients complete task A compared to task B (a control task), subtracting the two images to reveal which parts of the brain have higher blood flow.

E.g. task A may be a task involving planning, but task B may be a task that is completely different.

What part of the prefrontal cortex is involved in planning?

Dorsolateral prefrontal cortex.

What part of the prefrontal cortex is involved in “mind wandering”/default mode?

Medial prefrontal cortex.

When a set of brain regions decrease their activity in task performance and is consumed by their own thoughts.

Spreng and Schacter (fMRI)

Participants were younger adults and older adults, participating a Tower of London task in an fMRI.

Older adults showed impaired performance on the task with barely no deactivation of the medial prefrontal cortex. Whereas, younger adults could deactivate their medial prefrontal cortex and do better on the task.

This shows that life requires a balance between the dorsolateral prefrontal cortex and the default mode medial prefrontal cortex and the older you get, the harder it is to deactivate the medial prefrontal cortex and find a balance.

Pros of fMRI to study prefrontal cortex.

We can examine whether there are increases and decreases in different parts of the brain with spatial precision.

Cons of using fMRI to study the prefrontal cortex.

fMRI scans are very expensive (~£800 per person).

Problems with causality: just because a brain region is active, does not mean it is essential for the task.

Transcranial magnetic stimulation (TMS)

Involves using a magnetic field to excite and inhibit certain regions of the brain.

Van den Heuvel et al (non-invasive brain stimulation).

Used inhibitory (decreasing neuronal activity) TMS on the left dorsolateral prefrontal cortex prior to planning. They also did another “sham session” where a participant receives a placebo.

When the participants had their left dorsolateral prefrontal cortex inhibited, they performed worse at the task, compared to the sham session.

It was also found that after TMS inhibition, decreased BOLD signal was found in other regions of the brain, suggesting that the prefrontal cortex is sending signals to other regions of the brain via white matter tracts,

Pros of using NON-INVASIVE BRAIN STIM. METHODS to study the prefrontal cortex.

Causality: we can directly change activity and control the certain areas to see their affect on planning.

Focal: we can apply stimulation to specific regions of the brain.

Cons of using NON-INVASIVE BRAIN STIM. METHODS to study prefrontal cortex.

We are limited to the number of sessions of stimulation as we do not want to cause any long-lasting brain damage or injuries.

Having a deactivated prefrontal cortex for a few minutes is likely to be quite different to having it deactivated for a few years.