lecture 23 - decision making II: executive function

prefrontal cortex

• not a motor area

• involved in aspects of sensation, attention, covert decision making

• influences motor circuits to output movements that are related to particular goals

• interacts with BG circuits

hierarchy of action representations

bottom → top

1. primary motor cortex: controlling limb movements, not individual muscles, but individual directions of limb movement

2. supplementary motor cortex: sequences, action motifs, planning

3. prefrontal cortex: sensori-motor integration, processing/establishing higher order goals + complex decisions

4. sensory context plays a role, what you’re experiencing in your environment feeds into systems to influence which actions are generated

• these goals/predicted outcomes that we motivate actions based upon exist along different time scales (motor program = immediate action-outcome; conceptual “action program” = long term action-outcome)

• BG interacts with all of these systems, serving as a gate, determining which of these programs at diff levels of the hierarchy are enacted/suppressed (whether its worth it)

elaboration of PFC in humans

• one of the regions of the brain that we think makes us uniquely human

• prefrontal cortex in humans is much larger + more developed

• using animal models to study brain processes is hard here b/c the PFC of humans is different than PFCs in other species

damage to PFC results in loss of behavioral inhibition

• phineas gage case

• damage to areas of the frontal lobe resulting from accident working on railroad

• significant (but transient) changes in aggression and behavioral inhibition

• personality changed significantly overtime, started to exhibit increasing aggression, social behavior changed, developed deficits in decision making

• but still could function in society to some extent

frontal lobe lobotomies

• antonio egas moniz

• destruction of the frontal lobe was a surgical procedure in the 40s & 50s to treat a variety of “unidentified” mental conditions like schizo

• surgeries resulted in a wide range of highly variable effects:

  • increased passivity (but sometimes increased aggression)

  • loss of goal-directed motivation

  • loss of higher reasoning

  • increased impulsitivity

  • social impairments

executive functions

1. flexibility

2. behavioral inhibition (suppressing impulsivity)

3. abstract thought (complex rules) 

4. working memory

5. attention

6. goal-directed behavior (costs vs. benefits)

7. social behavior

behavioral flexibility

PFC is involved in creating mental models that allow for flexible adjustments in behavior

• Model based/flexible: mental map of possible actions, plan best route (may be trajectory never actually experienced); allows for adjustments based on changes in context (ex: traffic)

• Model free/inflexible: learn value of simple actions based on experience (go right to avoid freeway)

behavioral inhibition

suppressing an inappropriate/automatic response

• ex: social norms (cake); habit suppression

lack of this: myelination + synaptogenesis/pruning stabilize last in the PFC, which develops last

multiple systems compete for behavior control

prefrontal cortex: context-dependent, complex/multi-dimensional, long-term goals

hypothalamus: context-independent, simple low dimensional, short term needs

multiple paths to action!

wisconsin card sorting task

task to assess behavioral flexibility and rule learning

• subjects have to learn to apply a rule to match the sample card with one of the 4 others

• after learning is achieved, the rule is switched and subjects have to learn to flexibly alter their choices

• patients with frontal lobe damage/disorders like schizophrenia are impaired in flexibly adjusting to new rule (perseverate!)

stroop task

task to assess flexibility and behavioral inhibition

• conflict between rapid automatic response to read word and rule to read color

• requires active suppression of rapid response in favor of slower response

• performance impaired in schizophrenia, ADHD, depression, addictive disorders

abstract thought

• humans can apply complex rules to solve problems

• exhibit creativity

PFC neurons can selectively represent complex rules

working memory

ability to hold something in mind over a period of time until an appropriate response can be made; holding info “online” in the face of distractors

working memory in PFC

• fixated in the center

• stimulus is presented at a certain location on the screen

• stimulus is then removed, monkey has to remember where the stimulus was presented (working memory of the location)

• after some delay, the monkey can make a response

task is to remember the position of a stimulus to guide a later response

• raster plot shows that neuron responds most when stimulus is shown at bottom middle; single neuron recorded in PFC

• delay period response: activated when stimulus comes on, neuron continues to fire after stimulus is turned off

• burst of firing shows working memory of that stimulus

dopamine agonists can enhance working memory encoding in PFC neurons

• same task where an animal has to observe where the stimulus is presented at different locations on the screen, hold that location in working memory, and create a response

• response of the same neuron when a preferred stimulus is presented (left) and nonpreferred (right); preferred = receptive field

• increases firing during delay period in the preferred stimulus and less so for nonpreferred

• what happens if you inject a drug that enhances signaling of the d1 receptor (agonist) into the PFC? 

  • neuron increases firing when the stimulus is in its preferred location + decreases in nonpreferred location

  • dopamine, the d1 binding to the receptors, is enhancing the signal (working memory response when stimulus is in preferred location) to noise (response in nonpreferred location)

• drugs that enhance dopamine = enhance working memory function through G-Protein coupled receptors

• too much DA can cause working memory effect to break down

inverted U effects of dopamine on cognitive performance

• drugs that enhance DA function have inverted U relationship with cog performance enhancement

• effect depends on basal levels of DA and is task dependent

• subjects/tasks with high basal DA will experience cognitive/working memory deficits with DA drugs, vice versa for low DA

• implications for pharmacological treatment of ADHD and other disorders

selective attention

• PFC has been implicated in the ability to attend to things (attention)

• working memory is connected to selective attention!

• attention is drawn towards the goal, you’re able to filter out and not react to all the other stuff around you

attentional modulation

• task is to attend to stimulus in one part of visual space + ignore the other one (make response based on that stimulus based on color, shape, etc.)

• when the rule is to attend to bottom stimulus, there is an increase in responding of that neuron for the stimulus presented in receptive field

• enhancement of the response of that neuron based on the rule = attentional effect

top down modulation of attention by PFC

• other parts of the brain are processing the visual stimuli (v1 receiving info about the basic properties of stimuli, where they are in space)

• PFC is providing this signal that’s saying which of those locations we need to pay attention to (attentional modulation)

• sends connections down the hierarchy/low levels of the sensory + motor systems & can influence responses at those levels/regulate context dependent attention

• PFC is master switch saying to all of these low-level systems that theres bars here, colors there, shapes, etc and to hone in on that, pay attention to those!