How the Brain Encodes Reward
published: Aug. 9, 2010, recorded: May 2009, views: 3803
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As Ann Graybiel puts it, “basal ganglia were dark basement structures” until Okihide Hikosaka began his classic 1980s research demonstrating how these neuronal clusters influenced eye movements. Hikosaka has deepened and broadened his work in this once neglected area of the brain, and brings a McGovern audience up to date on his latest discoveries.
Hikosaka briefly sketches what is known about the basic pathways leading in, around and out of the basal ganglia, circuits that have been associated with stress, pain, mood, memory and arousal. This specialized cluster of neurons seems especially attuned to the neurotransmitter dopamine, and Hikosaka has been investigating “a number of unsolved questions,” including how dopamine neurons form circuits for movement control, whether such neurons encode “motivational values,” and what other parts of the brain guide them.
Hikosaka describes research demonstrating that certain dopamine neurons become excited if a visual cue indicates a future reward, and become inhibited with a visual cue indicating no reward. Dopamine also increases after an action delivers a reward and decreases when an action produces no reward. Research began to explore whether dopamine neurons “encode motivational values, including reward and punishment.” After others’ studies yielded contradictory or uncertain conclusions, Hikosaka designed a set of studies on monkeys involving classical Pavlovian conditioning, with juice rewards and air puffs as aversive stimuli.
Among Hikosaka’s findings: some dopamine neurons were excited primarily by positive, reward-predicting stimuli, others inhibited by air puff-predicting stimuli. But he also found another group of dopamine neurons excited both by positive and negative reward-predicting stimuli (as well as the stimuli themselves). Hikosaka posited two types of neurons that react in very different ways to motivational signals, which he described as value-coding and salience-coding. He also determined that the lateral habenula, a part of the brain sitting at one end of the thalamus, seems to regulate dopamine pathways involved in some motivational responses. By sending a weak electric pulse through the lateral habenula, Hikosaka saw a very strong inhibition of the dopamine neurons that “encode mostly motivational values.”
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