OFC, striatum - week 10 (Chris) Flashcards
(18 cards)
IOWA Gambling Task
In this task, participants are presented with 4 virtual decks of cards on a computer screen. They are told that each time they choose a card they will win some game money. Every so often, however, choosing a card causes them to lose some money. The goal of the game is to win as much money as possible. Every card drawn will earn the participant a reward. Occasionally, a card will also have a penalty. The decks differ from each other in the number of trials over which the losses are distributed. Thus, some decks are “bad decks”, and other decks are “good decks”, because some will lead to losses over the long run, and others will lead to gains.
Bechara et al. (1994)
What they found was that normal controls generally learned to avoid the risky decks and choose the decks that would give them an overall profit. Patients with lesions to occipital, temporal and even dorsolateral prefrontal cortical regions showed equivalent performance, generally preferring the advantageous decks.
However, patients with OFC lesions showed an overall preference for the risky decks, resulting in an overall loss of money – they were unable to learn about the response-reward contingencies.
In their 1996 paper, Bechara et al. went on to test the skin conductance response (SCR) of OFC lesion patients performing this task. SCR is essentially a measure of sweating, and is taken as a measure of how anxious a subject is.
What they found was that SCRs during reward and punishment were roughly equivalent in patients and controls. However, anticipatory SCRs (measured between the choice of deck and the receipt of the reward/punishment) were greatly reduced in patients with OFC lesions.
Somatic Marker Hypothesis (Damasio)
Emotional information, in the form of physiological arousal, is needed to guide decision making
Somatic markers are the bodily reactions to emotional stimuli
OFC supports learning of associations between somatic markers and complex situations
OFC can then use this information to assess likely outcomes of behavioural choices
Problems with the Iowa Gambing Task data
Cards are presented in a fixed order that induces a preference for the risky decks (wins experienced early, losses experienced later)
In order to perform well on the task, subjects must at some point overcome this preference and switch to choosing the ‘good’ decks
This process of inhibition and switching is known as reversal learning
Fellows & Farrah (2005)
VMF patients are impaired relative to controls on the standard version of the IGT, in which risky losses are experienced only later (and therefore reversal learning is required).
This impairment disappears in the variant of the IGT in which risky losses are experienced earlier, and therefor reversal learning is not required.
Suggests deficit is in reversal learning rather than value-based decision making more broadly.
Role of OFC in reversal learning – Neuropsychological Evidence
Studies have further investigated the precise role of the OFC in value based decision making.
In a reversal learning task subjects have to learn which card predicts a reward and then the contingencies are shifted.
This study included patients with DLPFC and ventromedial (OFC) lesions.
DLPFC were unimpaired. OFC had specific deficit at the reversal stage. They could learn the initial response contingencies but had problems learning about the new contingency.
The researchers argued that OFC is necessary not so much for learning about stimulus-reward contingencies but for unlearning of established associations.
Problems with reversal learning hypothesis of OFC function
Monkeys with OFC lesions do not always show reversal learning deficit
In one study, monkeys had to select either a large or small reward (e.g. 4 peanuts or 1 peanut)
If monkey selected the small reward, they were given the large one
If monkey selected the large reward, they were given the small one.
Why do monkeys with OFC lesions not show deficits on this type of reversal learning?
The task in this study requires overcoming an innate preference for larger food rewards – quite different to a standard reversal learning task, which involves learning (and suppression) of arbitrary stimulus-outcome relationships
Rate of learning: monkeys took a long time to learn this task (50 sessions) whereas reversal learning takes place over much shorter periods of time
- OFC may only be required for reversal learning when stimulus-outcome computations are made ‘on the fly’ and suppression involves arbitrary stimulus-outcome relationships
OFC and counterfactual thinking - neuropsychological evidence
Camille et al. (2004)
Also evidence that this function plays a role in more complex emotions such as regret.
In this task subjects chose one of two wheels with different chances of winning.
Then they either got partial feedback, where they found out what they won and they also know what the possibilities were if they chose the other wheel, but they don’t actually know what they would have won if they chose the other wheel. Or they got full feedback where they found out what they won and also what they would have won if they had chosen the other option.
They asked patients to rate their level of satisfaction with the outcome. What you find in normal healthy subjects is that they show higher ratings of satisfaction when they win compared to lose, but they also show much higher ratings of satisfaction when they find out they would have won less or lost more if they had chosen the alternative wheel. And they show much lower ratings when they find out they would have won more or lost less if they had chosen the other wheel.
OFC patients did not show this pattern of counterfactual thinking – they didn’t modulate their ratings according to whether the alternative wheel would have been a win or lose.
Suggests a key role for the OFC in being able to experience the consequences of winning and losing, but more importantly to integrate information from different sources of outcome
Dopamine
Dopamine is a neurotransmitter (chemical released by neurons to transmit an electrical signal between one neuron and another)
DA is produced by dopaminergic neurons in the ventral tegmental area (VTA) and substantia nigra (SN) in the midbrain
There are several different types of DA receptors (D1 to D5) – broadly categorised as D1-type and D2-type.
D1 receptors much more abundant in PFC
D2 receptors more abundant in striatum
Dopaminergic signalling occurs via several different pathways
Dopamine pathways
Mesolimbic pathway
Nigrostriatal pathway
Mesocortical pathway
Tuberoinfundibular pathway
Dopamine (DA) function
Role of DA in addiction is well established
Modern theories of DA emphasise learning and motivation over pleasure (‘hedonic impact’ of rewards).
Drugs of abuse enhance DA function by acting on midbrain neurons to transiently increase extracellular concentrations of DA
DA neurons in the striatum encode reward prediction error - discrepancy between expected and gained reward (Schultz, 1998)
Prediction errors do play a key role in the development of addiction
Brozoski (1979)
dopamine depletion in the PFC of monkeys causes a deficit in spatial working memory nearly as severe as complete ablation (removal)
Parkinson’s Disease
Gradual loss of dopamine neurons in the nigrostriatal pathway
Primarily a motor disorder - bradykinesia (slowed movement), akinesia (impairment of voluntary movement) and tremor
Currently no cure for PD although treatments include L-dopa (precursor of dopamine) and more recently deep brain stimulation
How does dopamine influence cognitive control?
One theory suggests that dopamine might play different roles depending on the site of action
D1 receptors are more prevalent in the PFC – function linked to stability of representations
D2 receptors are more prevalent in the striatum – function linked to flexible behavior
Dopamine might achieve a balance of stability and flexibility by exerting different effects on striatal and PFC activation
Effects of methylphenidate (‘Ritalin’) on brain activation during reversal learning
Dodds et al. (2008)
To test these new ideas about dopamine, we examined the effects of methylphenidate (commonly used to treat ADHD) on brain activation during reversal learning. Methylphenidate is a dopamine reputake inhibitor, which means that after dopamine is released into the synapse, it prevents its re-absorption, leading to an overall increase in extra-cellular dopamine levels.
We gave healthy subjects a probabilistic reversal learning task in the MRI scanner.
On each trial, subjects are presented with two abstract visual patterns. Using trial-and-error feedback, subjects must discover which of the two patterns is correct (the subject’s choice is indicated here with a small, black arrowhead). Feedback (a green smiley face or red sad face) is presented as soon as the subject has chosen one of the patterns with a left or right button press. After several correct trials, the contingencies switch and the subject must switch to selecting the other pattern.
The results showed a strikingly clear effect. We examined in which areas of the brain activation was modulated by methylphenidate when the subject switched their response from one pattern to another. The only region that showed such modulation was the putamen (part of the striatum). Thus, the results show a key role for striatal dopamine in mediating cognitive flexibility.
Gating role for basal ganglia in cognition - flexible updating of current goal states into PFC
Van Schouwenberg et al. (2010)
These studies have led to models of the basal ganglia (striatum) in which the basal ganglia act as a ‘gate’ allowing or preventing new goal states into the PFC.
The idea here is that current goal states (or tasks or stimulus-response contingencies) are represented by coalitions of neurons in the PFC. These representations remain stable while the subject is focused on the current task or goal. But when the reward contingencies change, or new information becomes available, requiring a switch in responding, dopamine signalling in the basal ganglia prevents responding to the previously relevant stimulus and allows the establishment of a new PFC representation, enabling a different stimulus to gain control of responding.
Cools et al examined effects of bromocriptine (D2 receptor antagonist) on switching behaviour and neural activation in the striatum
Colour of fixation cross instructed subjects to encode faces or scenes
- Trials were either switch (attend to different stimulus than the previous trial) or nonswitch (attend to the same stimulus as previous trial)
