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Abstract:
When people make decisions they often
face opposing demands for response speed and
response accuracy, a process likely mediated by
response thresholds. According to the striatal
hypothesis, people decrease response thresholds
by increasing activation from cortex to striatum,
releasing the brain from inhibition. According
to the STN hypothesis, people decrease response
thresholds by decreasing activation from cortex
to subthalamic nucleus (STN); a decrease in STN
activity is likewise thought to release the brain
from inhibition and result in responses that are
fast but error-prone. To test these hypotheses—
both of which may be true—we conducted two
experiments on perceptual decision making in
which we used cues to vary the demands for
speed vs. accuracy. In both experiments, behavioral
data and mathematical model analyses confirmed
that instruction from the cue selectively
affected the setting of response thresholds. In the
first experiment, we used ultra high-resolution
7T structural magnetic resonance imaging (MRI)
to locate the STN precisely. We then used 3T
structural MRI and probabilistic tractography
to quantify the connectivity between the relevant
brain areas. The results showed that participants
who flexibly change response thresholds
(as quantified by the mathematical model)
have strong structural connections between presupplementary
motor area and striatum. This
result was confirmed in an independent second
experiment. In general, these findings show that
individual differences in elementary cognitive
tasks are partly driven by structural differences
in brain connectivity. Specifically, these findings
support a cortico-striatal control account of how
the brain implements adaptive switches between
cautious and risky behavior.
