Neuroscience and Decision-Making: Key Brain Structures and Processes

Neuron

A brain cell that fires once inputs hit a threshold (-55 mV). Like a switch that flips when enough pressure builds.

Synapse

The connection between two neurons where signals pass. Like two people passing a note.

Neural cascade

One neuron firing triggers others like dominoes. This chain of activity is what thinking and deciding looks like in the brain.

Brain vs. computer

Brains process things in parallel and rewire through experience; computers work sequentially and need new code to change.

fMRI

Brain scanner that tracks blood flow as a proxy for activity. Good at where; bad at when.

Striatum

The brain's 'I want that' signal. Lights up when something rewarding is nearby and drives you toward it.

Knutson et al. (2007)

Striatum activation while looking at a product predicted whether people would buy it — the brain decided before the person consciously did.

Breiter et al. (1997)

Only cocaine (not saline) lit up the striatum in cocaine users — it responds to actual reward, not just the act of consuming.

Coutier et al. (2008)

Attractive faces activated the striatum more than unattractive ones — the same region that responds to food and money also responds to good-looking people.

Vo et al. (2014)

Striatum-damaged patients couldn't learn which option gave better rewards even after many tries. The striatum is necessary for updating value through experience.

Dopamine neurons

Fire when something is better than expected, not just when something good happens. Project from the brainstem to the striatum.

Reward Prediction Error (RPE)

The gap between what you expected and what you got. δ = actual − predicted.

Schultz et al. (1997)

Monkey juice experiment. (A) Unexpected juice → dopamine spike. (B) Expected juice → spike shifts to the cue predicting it. (C) Expected juice withheld → dopamine dip.

Pavlovian conditioning and RPE

Once a cue reliably predicts reward, dopamine fires at the cue instead of the reward itself.

Rescorla-Wagner rule

How expectations update after each experience: Vₜ₊₁ = Vₜ + α · δₜ.

Dopamine and addiction

Drugs artificially spike dopamine. Over time receptors downregulate so normal life feels flat.

Dopamine-related clinical conditions

Parkinson's: loss of dopamine neurons → motor impairment. Schizophrenia: dysregulated dopamine; antipsychotics block dopamine receptors.

Amygdala

The brain's alarm system. Fires for threats, fear, and unpleasant stimuli.

Hsu et al. (2006)

More uncertainty in a choice = stronger amygdala activation.

Incidental emotion effect

when your current emotional state influences decisions or judgments, even if that emotion is unrelated to the decision at hand.This phenomenon can lead to biased or irrational choices, impacting behavior in various contexts.

Charpentier et al.

Seeing fearful faces (unrelated to the task) activated the amygdala and made people more risk-averse in subsequent gamble choices.

Johnson & Tversky

Reading a sad story inflated probability estimates for bad events — incidental mood bled into unrelated risk judgments.

Whitman & Meinhof

Both had amygdala tumors found at autopsy, suggesting amygdala dysfunction may contribute to extreme aggression.

Amygdalotomies

Surgical removal of the amygdala, done in the 1970s to control severe aggression.

Insula

The brain's 'this is wrong or gross' signal. Handles disgust, pain, and moral outrage.

Sanfey et al.

Ultimatum Game — Unfair money offers activated the insula and caused people to reject them.

Prefrontal Cortex (PFC)

The brain's executive. Integrates value signals from the striatum, amygdala, and insula into a final decision.

Phineas Gage

PFC destroyed by an iron rod through his skull. Survived but became impulsive, erratic, and unable to follow through on plans.

Reason and emotion (Descartes' Error)

There is no pure reason separate from emotion. The PFC needs affective input from the striatum, amygdala, and insula to function.

Accumulation-to-threshold

The brain accumulates evidence for each option over time and decides once it crosses a confidence threshold.

Gold & Shadlen (2001)

LIP neurons ramped up activity as a monkey watched moving dots — once they hit a threshold, the monkey responded.

SPRT (Sequential Probability Ratio Test)

Sample evidence one piece at a time; stop when a confidence threshold is hit.

Speed-accuracy tradeoff

Lower threshold = faster but more errors. Higher threshold = slower but more accurate.