Prefrontal cortex

frontal lobes, anatomy & history. Modulaity of the forntal lobe, behavioural & neural evidence, is there a response inhibition module in the frontal lobe, frontoparietal multiple demand network

PFC & other animals

• human PFC is largest

• similar to chimpanzees but there is more complex in the folds (sulci & gyri)

Developmental trajectory of brain development

• from ages 5-20yrs density of grey matter decreases

• large change to the frontal lobe indicating important behavioural changes we see form childhood to adulthood

Connections to the PFC

• frontal lobe is connected to virtually every other region of the brain

• key role in human behaviour

• positioned to perform control/modualte

Frontostriatal loops

• strongest connections occur between regions in PFC & basal ganglia (striatum)

  • which is a collection of old, subcortical structures including the caudate, putamen, globus pallidus and ventral striatum

• loops occur parallel w/diff loops connecting diff PFC regions

• hypothesised to play diff roles

  • reward processing loop that connects ventral striatum to OFC

  • executive control loop that connects DLPFC to dorsal striatum

anatomy of the frontal lobe

• lateral surface

• medial surface

• orbital surface

• prefrontal refers to the front of the frontacortex

Lateral surface

• Lateral frontopolar cortex

• dorsolateral prefrontal cortex

• Anterior prefrontal cortex

• premotor cortex

• primary motor cortex

Medial surface

• Anterior cingulate cortex

orbital surface

• orbital frontal cortex

History of frontal lobes

• Phineas Gage

• Ryland (1939)

• Shallice

Phineas Gage

• one of 1st indications of function of PFC

• railroad foreman, metal pole through his head

• personality began to change

  • swearing, inpatient and impulsive

• no longer gage

Ryland (1939) - Frontal lobe or dysexcutive syndrome

• characterised problems faced by patients w/frontal lobe injuries as dysexecutive syndrome

  • attention (easily distracted)

  • abstraction (difficulties grasping whole of a complicated set of affairs)

  • novelty (ok w/routine, difficulties in novel situations)

Shallice - model of the frontal lobe = supervisory attention system

• frontal lobe acts like a system in charge of control of action & coping w/novelty

• it is required in situations where routine selection of actions is unsatisfactory & cognitive control or executive function is required

classic executive function task

• wisconsin card sorting task

• P given single card & must choose which of 4 decks to place the card on

• they have to learn rule governing which deck should be placed where

• set shifting task = patient must acquire a ‘set’ for task performance, which can switch repeatedly during the task

  • have to adapt to the problem

Problems w/Norman & shallice SAS theory

• homunculus criticism (lil man)

  • who controls the controller

  • if we suggest the frontal lobe controls us pushes back the control

    • frontal lobe controlling every other area lacks explanation

  • explains what is controlled but not how this control is exercise

• P w/frontal lesions tend to perform poorly on complex tasks

  • complex tasks tend to require lots of different cognitive processes e.g planning, inhibition, WM

  • cluster of problems → cannot test specific

Can we fractionate executive function into component processes?Can we map different executive functions onto separate brain systems?

Miyake et al (2000): Factor analysis of executive function

• attempted to fractionate executive function in component variables using behavioural tasks & factoral analysis

• Healthy p did several tasks → task switching, response inhibition task etc

• they slower in switch trials than repeated trials

• study identified 3key components of executive function:

  • updating (updating working memory)

  • shifting (changing approach when required)

  • inhibition

    • any complex task requires these 3 different sources

Letter memory task

• letter memory task that requires P to remember letters

Stop signal reaction time task

• requiring subjects to withhold prepotent responses

• 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.

Factory analysis of executive

• found three clearly distinct, latent variables 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

• has been quite an influential model of executive function, and it tends to be used as a kind of template for understanding how executive functions can be fractionated. 

• The idea is that any complex executive task can be accomplished by drawing on (some mixture of) these three functions.

• But perhaps the real test of this attempt to fractionate executive function is whether we can map these different functions onto different brain regions.  

can we map different executive functions onto anatomically separate regions of the PFC

• General consensus that regions in lateral/medial PFC (DLPFC, VLPFC, ACC) are doing something different from the orbitofrontal cortex (OFC)

  • clearly something between them

• hot vs cold cognition

hot cognition

• Broadly, functions that do involve emotional or value-based judgements

• Value-based decision making

• Emotion-guided decision making

• Counterfactual thinking

• Gambling

cold cognition

• Broadly, functions that do not involve emotional or value-based judgements

• dorsolateral & ventrolateral PFC, anterior cingulate cortex (ACC)

  • Response inhibition

  • Task switching

  • Error monitoring

  • Attention

  • Working memory

Is there a dysexecutive syndrome? (Stuss et al, 2007)

• Tested frontal lobe P on a range of neuropsychological tasks

  • classic frontal tasks (WCST, Stroop), language, memory tests requiring executive functions & attentional tests

• Brain lesions were mapped out & location of brain damage defined by registration to a standard anatomical template

• Focus on parts of the frontal lobe involved in ‘cold’ cognition

  • dorsolateral & ventrolateral PFC, anterior cingulate cortex (ACC)

• found some correspondence between the Miyake model & patients

• Right lateral → monitoring

  • Miyake’s “updating” variable included monitoring tasks

• Left lateral → task setting (separating stimuli)

  • necessary for shifting as in Miyake’s model

• Convergence in medial PFC → energising

  • No space for inhibition → Stuss says that inhibition may not exist at the psychological level

• Some agreement between behavioural and neuropsychological evidence – e.g. task setting < left lateral PFC

• But also some disagreement, even on fundamentals, e.g. the existence of specific components such as response inhibition

Is there a response inhibition module in the PFC?

Evidence that the right inferior frontal cortex is a response inhibition ‘module’

• Aron et al (2003)

• imaging studies

Evidence that the right inferior frontal cortex is a response inhibition ‘module’ (Aron et al, 2003)

• Stuss suggested this region was involved in task checking, quality control & the adjustment of behaviour (‘monitoring’)

• Aron et al (2003) suggested that a part of the right lateral PFC plays an important role in response inhibition (the process of inhibiting a prepotent or inappropriate response)

• gave P w/different brain lesions a Stop Signal Reaction Time task in which they had to respond whether an arrow was pointing to the left or right but on occasional trials withhold their response when they heard a loud beep

• found that performance on this task was strongly related to size of the lesion in the right inferior frontal gyrus

  • there was a positive correlation between the Stop Signal Reaction Time and lesion size in this region

  • the bigger the lesion, the worse the patient was at inhibiting their response.

• demonstrated the particular importance of the right inferior frontal cortex for response inhibition

Imaging studies

• fMRI studies have supported inhibition module in right inferior cortex, showing increased activation in the right inferior frontal cortex during response inhibition.

• Many studies have put healthy people in the scanner & given them go/no-go tasks in which the subject simply has to press a key when they see certain letters and withhold responding

• 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.

limitations to right inferior lateral PFC having a module response inhibition module

• may be only due to role in attention

• Hamisphire found fmri evidence for other areas

• Dodds found that it may be that it involved with attention or that task always does involve attention

a module for response inhibition

• Right inferior frontal cortex may play an important role in response inhibition

• However, this may in fact be due to a role in attention

• may be a network instead → is it then reasonable to call it a module

alternative perspective from neuroimaging - Frontoparietal cortex as a ‘multiple demand’ network

• Not everyone believes that it is possible to fractionate the PFC into separate executive processes.

• Duncan & Owen (2000) performed a meta-analysis of neuroimaging studies of executive function → plotted the activations associated w/multiple different processes, e.g. response conflict, task novelty, on a single brain

• found was that rather than there being separate regions of PFC dedicated to different processes, the different processes all activated remarkably similar regions

• No clear separation between diff processes → there was a network of regions, encompassing regions in the lateral PFC, anterior insula, medial PFC, & inferior parietal cortex, that all seemed to show increased activation when subjects did something cognitively difficult.

• Duncan called the frontoparietal ‘multiple demand’ network

  • reflecting that this is a ‘multi-purpose’ network of brain regions that underlies cognitive performance in multiple types of demanding tasks

  • when we need it → its for harder cognitive functions

• there is no clear pattern

What does the ‘multiple demand’ network do?

• Construction of ‘attentional episodes’

• Neurons have highly dynamic response properties, adapting to code the specific information and events within the current attentional focus”

• With the transition between one episode and the next, neural coalitions for one kind of information processing dissolve and coalitions for the next episode form, producing a system in constant flux.”

• Adaptive coding – PFC neurons adapt their responses depending on task demands

• frontal lobes are a unified system

fMRI evidence for coding of information about task rules in frontoparietal cortex (Woolgar et al, 2011)

• Woolgar suggested PFC may perform a slightly more complex role

• P performed a task in which they had to learn different rules mapping locations to responses

  • when they saw a blue screen had to remember a particular mapping between locations & button presses

• used multivoxel pattern analysis to see which brain areas encoded different information about the task such as rules, colours & responses.

• Found that although frontalparietal regions did encode information about the position of the stimuli, the colours and the responses, these regions showed strongest coding of rules.

• Results demonstrate that primary role of the PFC in this kind of task is encoding the information about quite abstract aspects of task performance such as the rules governing stimulus-response mappings

• It could be claimed, therefore, that these regions are not simply directing attention to specific stimuli in WM but are, in fact, performing a slightly more complex function

• 

Key points

• PFC plays a key role in organised, goal-directed behaviour. Its structure, connections and functional architecture make it well suited to playing a key role in intelligent human behaviour.

• Some evidence that PFC can be fractionated into different functions but also disagreement, even about what the fundamental executive functions are.

• Multiple demand network hypothesis offers an alternative viewpoint – integrated network involved in performing cognitively demanding tasks