Indicate the location of the ventral striatum/nucleus accumbens and explain the functional role of ventral striatum/nucleus accumbens , give examples

NA = ventral stratum

NA is a very important area for decision making, function – evaluation of expected values of the option.

NA is connected with hippocampus and other areas such as Amygdala is. This area is connected to motivation and encodes potential costs of doing something.

Example one (Brian Knutson study)

A series of trials for subjects lying in fMRI. Before each trial a subject get cue – how much money for this trial he gets, then the trial comes. The results show that NA activation is proportional to anticipated gain, thus reflect the expected value. And the highest rate of activation is registered before trial.

Example two(J.Neurosci study)

Card game. The task is to indicate a red card. In condition A a subject can bid on 4 card per 8 given, in condition B on only 1 card. Thus the probability of getting reward is manipulated. NA activity in this study is proportional to expected rewards.

• Neuroeconomics theorem: Subjective value is the averaged firing rate of a population of neurons

coding behavioral preferences.

• Subcortical nucleus accumbens (NAc) activity codes valence and proportional to anticipated gain

magnitude.

• Certain populations of NAc neurons are more sensitive to expected reward probability and other

populations are more sensitive to reward magnitude. Hovewer these areas of NA are intersected by one another.

Explain phenomena of self-stimulation

Animal directly stimulate the brain and self-stimulation leads to starvation and death. The same refers to some patiencts which can stimulate brain arears.

Directly access to values in the brain lead to reject of internal sources of values. Emulate values in the brain, cause values are nothing more than activity of neurons in certain areas. The self-stimulation is stopped by blocking dopamine receptors.

Dopamine is critical to addiction.

What is dopamine?

Neurotransmiter, produced by two areas VTA (ventral tegmental area) and Substantia nigra.

Dopamine functions as a neurotransmitter—a chemical released by nerve cells to send signals to other nerve cells. The brain includes several distinct dopamine systems, one of which plays a major role in reward-motivated behavior. Most types of reward increase the level of dopamine in the brain, and a variety of addictive drugs increase dopamine neuronal activity. Other brain dopamine systems are involved in motor control and in controlling the release of several other important hormones.

Dopamine is distributed around the brain and the main area riched with dopamine is Nucleus accumbens (NA) located in ventral stratum. This area plays a crucial role in experience of pleasure and processes of evaluation, because it encodes values during decision making process.

With dopamine receptors blocked responding was initially normal but dropped to zero within a few minutes. Animals in this condition rarely approached the lever. Thus self-stimulation is stopped by blocking dopamine receptiors.

What does dopamine neurons’ activity code?

This codes expectation of reward. In the experiments with conditioning(Pavlov) the highest inflaw of dopamine registered at different time depending on the whole period of conditioning. At the beginning - when the reward is gotten, at the end – when conditional stimuli appears. This contributes to the fact that inflow of dopamine relates to expectation of value.

Explain results of the article Bechara, A., Damasio, H., Tranel, D., and Damasio, A. R. (1997). Deciding advantageously before knowing the advantageous strategy. Science 275, 1293-1295.

Explain the Iowa Gambling Task.

This experiment related to the issue of emotions. In brief, there are two systems of emotional reactions: 1) innate mechanisms which provides automatic actions and 2) learning mechanisms (NA, OFC, Amygdala). Emotions is a kind of heuristics which help us to make decisions. Structure’s which support emotional reactions are usually lie inside limbic system, because evolutionally this system appears in mammals as a results of the development of their social life. In the current course emotions are equal to subjective values.

Procedure: In the game 4 decks of cards given A-D. Subjects (3 groups: normal people,people with injury in OFC and people with injury in Amygdale) A and B are disadvantageous ( big win and big gain, but in sum a subject get no reward). C and D advantageous (moderate win and loose, but at the end overall gain is positive). Subjects have to learn which deck is advantageous as a result of picking cards from all types of decks.

Results: people with injuries choose A and B, normal choose C and D.

Why this happens?

T his unability to make wright choice can be explain by unability to use emotions while making decisions for patients with injuries in OFC.

The researchers recoded skin conductance responses which reflects the emotional arousal of subjects, playing the game. There data were recorded at two conditions:

Anticipatory time window - t skin conductance responses (SCRs) generated in reaction to anticipation of the outcome (when the subject was pondering which card to select).

Reward/Punishment (R/P) time window - skin conductance responses (SCRs) generated in reaction to the outcome of winning or losing a certain amount of money.

The clear difference between normal subjects and injured subject was discovered. We can see from the graph below that emotional reaction for normal people differentiate advantageous and disadvantageous conditions in both anticipatory and reward/punishment time windows. Subject were more aroused when they anticipated negative outcome, emotional system signals the risk.

All process of gambling was divided into 4 periods.

• “pre-punishment” period, when subjects sampled the decks, before they had yet encountered any punishment.

• “pre-hunch” period, when subjects began to encounter punishment, but still had no clue about what was going on in the game.

• “hunch” period, when subjects began to express a hunch about which decks were riskier, but they were not certain.

• “conceptual” period, when subjects knew very well that there were good and bad decks, and which decks were good and bad.