A new study in mice shows that dopamine neuron activity plays a key role in judgment of time, slowing down the internal clock.
Sofia Soares et al investigated midbrain dopamine neurons during timing behavior in mice; when measuring and manipulating mouse activity, the team observed that dopamine neurons controlled temporal judgments on a time scale of seconds. Image credit: Geralt.
Organisms’ ability to accurately estimate periods of time is variable and depends on circumstances, including motivation, attention and emotions.
Dopamine neurons residing in the midbrain have been implicated as regulators of this complex process. However, a direct link between the signals carried by these neurons and timekeeping is lacking.
What’s more, current studies in which timing behavior is disrupted have demonstrated conflicting results – in some cases, increased dopamine release speeds up the subjective sense of time, while in other instances, it is slowed down or unaffected.
To make sense of dopamine’s involvement in time approximation, a team of researchers at the Champalimaud Centre for the Unknown in Lisbon, Portugal, tracked dopamine activity in mice performing timed tasks.
The mice were presented with two audible tones, and trained to classify the interval between each as shorter or longer.
“We trained mice to estimate whether the duration of the interval between two tones was shorter or longer than 1.5 seconds,” explained Dr. Joe Paton, corresponding author of the study, published in the Dec. 9 issue of the journal Science.
“After months of training, they became pretty good at it.”
“Mice indicated their choice but placing their snout at either a right (shorter) or left (longer) port,” he added.
“During the task, the interval between the tones was made to vary, and if the mice chose the right answer (they correctly estimated time), they were rewarded.”
The authors observed bursts of activity in mouse dopamine neurons that synchronized exclusively to the second noise, reflecting the rodents’ anticipation of an upcoming reward, combined with their surprise about the arrival time of the sound.
They discovered the transient activation or inhibition of dopamine neurons was sufficient to slow down or speed up time estimation, respectively.
“We found that if we stimulated the neurons, the mice tended to underestimate duration, and if we silenced them, they tended to overestimate it,” Dr. Paton said.
“This result, together with the naturally occurring signals we observed in the previous experiments, demonstrate that the activity of these neurons was sufficient to alter the way the animals judged the passage of time. This was the major result of our study.”
Can it be extrapolated to humans? According to the researchers, it is very likely that a similar circuit is at work in the human brain.
“But the problem is that what we now measured in mice cannot be said to be a percept, because the animals cannot tell us what they felt,” Dr. Paton noted.
“When we study animals, the only thing we can measure is the animal’s behavior. But we are never sure of what they perceive. We interpret this as ‘a subjective experience of the animal,’ but it’s no more than an interpretation. And that’s the best we can do.”
“Even so, we like to ‘wildly speculate.’ There’s this cliché about young lovers staying up all night talking, and not feeling time go by. It might be those dopamine neurons at work shrinking time in a spectacular way.”
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Sofia Soares et al. 2016. Midbrain dopamine neurons control judgment of time. Science 354 (6317): 1273-1277; doi: 10.1126/science.aah5234
