Perceptual decision
How does the brain track fast-changing evidence during a decision?
Open access · cc by · source: Europe PMC
Each new piece of visual evidence travelled through the brain in three stages, from visual to parietal to motor areas, with the posterior cingulate cortex involved throughout.
Study at a glance
- Design
- Human experiment — Within-subject MEG reaction-time task: a single dot jumped every 100 ms and participants judged which of two targets was its centre; dot positions were regressed against sensor and source signals.
- N
- N=34 · 34 analysed adults (37 recruited; 1 dropped after training, 2 for eye artefacts/bad channels); each saw the same 480 trials.
- Population
- Healthy right-handed adults aged 20-35 from a Leipzig participant pool
- Outcome
- Trial-by-trial correlation of MEG signals (sensor and source level) with momentary evidence, accumulated evidence and choice
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Key findings
Participants averaged 78% accuracy, and their choices followed the running sum of evidence (correlation around 0.7) far more than any single dot. Brain signals tracked each new dot in three phases: around 120 ms in visual cortex, around 180 ms in parietal cortex, and from roughly 300 to 500 ms in motor and premotor areas. The irrelevant vertical position was represented much more weakly in the late phase, and accumulated evidence was represented persistently in motor, premotor and posterior cingulate regions. The build-up before the response resembled these evidence signals, differing from the response itself mainly in stronger motor-cortex correlations.
Methodology
While their brain activity was recorded with MEG, 34 adults watched a white dot jump to a new position every 100 ms and decided as quickly and accurately as they could which of two targets (framed as beehives) it was centred on. The horizontal position of each dot was the decision-relevant evidence, while its vertical position served as an irrelevant control. The researchers correlated these dot positions, and their running sum, with MEG signals and with reconstructed brain sources.
Limitations
The design used a pre-selected set of slow-response stimuli, so the pattern of dot influence partly reflects stimulus choice rather than natural decision behaviour. Momentary and accumulated evidence are mathematically linked, so their brain correlates cannot be fully separated. MEG source estimates have limited spatial precision, and the authors cannot rule out tiny eye or finger movements contributing to motor-area signals. The weak parietal accumulation signal may mean parietal neurons were too sparse to detect with MEG rather than that parietal cortex does not accumulate evidence.
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