Prefrontal cortex - week 9 (Chris) Flashcards
(26 cards)
Frontal lobe in different animals
The human one is by far the largest. It looks quite similar to the chimpanzee but there’s a lot more complexity in the folds – in the sulci and gyri in the human brain.
Thus, it seems that the frontal lobes must be doing something important that distinguishes us from other animals
Frontal lobe development
This is a graph showing the developmental trajectory of brain development between age 5 and 20 in humans. The scale shows the density of gray matter across the brain.
As you can see, gray matter thins out over the whole brain. However, some of the biggest changes appear to happen in this red region – the frontal lobe.
Thus, the frontal lobe must be doing something important related to the behavioural changes we see from childhood to adulthood.
Connections in the frontal cortex
It is connected to virtually every other region of the brain, which also suggests it plays a key role in human behaviour.
Frontostriatal loops
Some of the strongest connections occur between regions in the PFC and the basal ganglia (or striatum) which is a collection of old, subcortical structures including the caudate, putamen, globus pallidus and ventral striatum.
These loops seem to occur in parallel, with different loops connecting different PFC regions and hypothesised to play different roles – for example, a reward processing loop that connects the ventral striatum to the OFC and an executive control loop that connects the DLPFC to the dorsal striatum.
Anatomy of the frontal lobe
Lateral surface
Medial surface
Orbital surface
Anterior cingulate cortex (ACC)
Orbitofrontal cortex
Phineas Gage (1823-1860)
One of the first indications of the function of the PFC was provided by the unfortunate case of Phineas Gage, a railway worker who was carrying out some explosives work on the lines when an explosion went off unexpectedly and a metal bar shot up into his face, entered his brain and exited the top of his head.
The amazing this is a) he was alive b) he could speak, interact, go about his business, drink a cup of tea, do all the things healthy people can do.
However, Phineas did suffer some quite serious problems in everyday life. It was something more abstract that appeared to have been broken, something relating to his personality, to the control and organisation of his behaviour. The description by the doctor who dealt with his case explains this quite nicely.
Ryland (1939)
Ryland characterised problems faced by patients with frontal lobe injuries as a dysexecutive syndrome involving problems with…
Attention (easily distracted)
Abstraction (difficulties grasping the whole of a complicated set of affairs)
Novelty (ok with routine, but difficulties in novel situations)
Shallice – Frontal lobe = Supervisory Attention System (SAS)
a system in charge of the control of action and of coping with novelty (Norman & Shallice, 1980).
Required in situations where the routine selection of actions is unsatisfactory and cognitive control or executive function is required
Wisconsin Card Sorting Task
Patients are given a single card (here the one at the bottom) and must choose which of 4 decks to place the card on. They have to learn the rule governing which deck it should be placed on and continue to place different cards on the same deck according to the correct rule. The rule could be based on colour, shape, number of shapes etc. The patient must use trial and error to find the correct rule. Then, after 10 consecutive correct responses, the rule is changed and the patient must discover the new rule.
This process is often referred to as task-set switching (or shifting). The idea is that the patient must acquire a ‘set’ for task performance, which basically means a rule or set of rules. And this set can switch repeatedly during the task.
Problems with the Normal and Shallice supervisory attention system (SAS) theory of the frontal lobe
‘Homunculus’ criticism
- Who controls the controller?
Explains what is controlled but not how control is exercised
Problems with the supervisory attention system theory of the frontal lobe
Patients with frontal lesions tend to perform poorly on complex tasks
Complex tasks tend to require lots of different cognitive processes
E.g. WCST
Planning
Set-shifting
Inhibition
Selective attention
Working memory
Following complex rules
Miyake et al (2000): Factor analysis of executive function
They gave healthy subjects a variety of tasks:
For example, task switching – subjects have to perform two tasks – odd/even or vowel/consonant – depending on the location of the letters/numbers
Subjects are slower in switch trials than they are in repeat trials on this task.
Letter Memory Task – Working memory
Miyake et al (2000)
A letter memory task that requires subjects to remember letters but also to update the letters in memory.
Stop Signal Reaction Time Task
Miyake et al (2000)
And a stop signal task requiring subjects to withhold prepotent responses. So the task is to respond as quickly as you can to the direction of the arrow but to withhold responding if you hear a loud beep after the arrow is presented.
The researchers gave subjects 9 tasks in total measuring a variety of different executive functions like these.
Factor Analysis of Executive Function
They found that there were three very clearly distinct, latent variables that accounted for performance differences on the 9 tasks. These variables (or ‘factors’) are shown in the central part of the figure.
– shifting, which means shifting between task sets.
updating, which means updating the contents of WM.
And inhibition, which means inhibiting prepotent responses.
The idea is that any complex executive task can be accomplished by drawing on (some mixture of) these three functions.
‘Cold’ Cognition
Broadly, functions that do not involve emotional or value-based judgements
Response inhibition
Task switching
Error monitoring
Attention
Working memory
‘Hot’ Cognition
Broadly, functions that do involve emotional or value-based judgements
Value-based decision making
Emotion-guided decision making
Counterfactual thinking
Gambling
Stuss & Alexander (2007)
Tested frontal lobe patients (n = ~40) on a range of neuropsychological tasks including classic frontal tasks (WCST, Stroop), language and memory tests requiring executive functions, and attentional tests.
Brain lesions were mapped out and location of brain damage defined by registration to a standard anatomical template
Focus on parts of the frontal lobe involved in ‘cold’ cognition – dorsolateral and ventrolateral PFC, anterior cingulate cortex (ACC)
Right Lateral PFC
Monitoring
The process of checking the task over time for ‘quality control’ and the adjustment of behaviour.
Miyake’s ‘updating’?
Left Lateral PFC
Task setting
the ability to set a stimulus–response relationship.
Miyake’s ‘shifting’?
Left Medial PFC
Energising
the process of initiation and sustaining of any response
Evidence that the right inferior frontal cortex is a response inhibition ‘module’
Aron et al. (2003) demonstrated the particular importance of the right inferior frontal cortex for response inhibition
They gave patients with different brain lesions a Stop Signal Reaction Time task.
They found that performance on this task was strongly related to the size of the lesion in the right inferior frontal gyrus – there was a positive correlation between the Stop Signal Reaction Time (a measure of how good the subject is at inhibiting their response) and lesion size in this region – the bigger the lesion, the worse the patient was at inhibiting their response.
Evidence that the right inferior frontal cortex is a response inhibition ‘module’ - imaging studies
Imaging studies have largely supported these findings, showing increased activation in the right inferior frontal cortex during response inhibition
Many studies have put healthy people in the scanner and given them go/no-go tasks in which the subject simply has to press a key when they see certain letters (e.g. O) and withhold responding when they see for example an X.
Activation in right inferior frontal cortex is consistently higher for no-go trials than it is for go trials, suggesting a specific role for this region in inhibiting a prepotent response.
An alternative perspective from neuroimaging
- Frontoparietal cortex as a ‘multiple demand’ network
John Duncan and Adrian Owen in 2000 performed a meta-analysis of neuroimaging studies of executive function, in which they plotted the activations associated with multiple different processes, e.g. response conflict, task novelty, on a single brain.
What they found was that rather than there being separate regions of the PFC dedicated to different processes, the different processes all activated remarkably similar regions. There was no clear separation between the different processes – it was more like there was a network of regions, encompassing regions in the lateral PFC, the anterior insula, the medial PFC, and also the inferior parietal cortex, that all seemed to show increased activation when subjects did something cognitively difficult.
Duncan called this network the frontoparietal ‘multiple demand’ network, reflecting the idea that this is a ‘multi-purpose’ network of brain regions that underlies cognitive performance in multiple different types of demanding tasks.
