Perceptual decision
Do two frontal regions play different roles in perceptual decisions?
Open access · cc by · source: Europe PMC
Zapping the frontal eye field produced brain responses that changed with how close the person was to answering, while zapping ventral prefrontal cortex produced responses that changed with whether the answer would be right.
Study at a glance
- Design
- Human experiment — Single-pulse TMS over FEF or ventral PFC at variable times during a motion-discrimination task, with 60-channel EEG recording the TMS-evoked potentials.
- N
- Two separate experiments: 20 participants for FEF stimulation and 13 for ventral PFC stimulation.
- Population
- Healthy adults aged roughly 20-46 in Tokyo
- Outcome
- Scalp TMS-evoked potentials 20-40 ms after the pulse, split by time-to-response and by choice accuracy; source-localised spread of the evoked signal
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Key findings
Evoked potentials after frontal eye field stimulation differed between early and late pulses (mainly over left centro-parietal scalp) but not between correct and error trials. The reverse held for ventral prefrontal stimulation: responses over right parieto-occipital scalp differed between correct and error trials but not by timing, a double dissociation seen 20-40 ms after the pulse. The same factors had no effect on EEG from trials without a pulse, and source estimates suggested both regions sent signals to dorsolateral prefrontal and medial frontal areas, with the ventral site also reaching motion area MT. The pulses barely affected behaviour, lowering accuracy slightly and leaving reaction times unchanged.
Methodology
Healthy adults judged whether a cloud of moving dots drifted left or right, with the proportion of coherently moving dots varied from trial to trial. On half the trials a weak magnetic pulse was delivered either to the frontal eye field (20 people) or to a ventral prefrontal site near the inferior frontal sulcus (13 people) at a random moment before the response. EEG recorded how the pulse-evoked activity spread across the scalp, and trials were sorted afterwards by how long before the response the pulse came and by whether the choice was correct.
Limitations
The samples were small and different people were tested at each site, so the two regions were never compared within the same brains. The early-versus-late time windows were borrowed from monkey single-neuron recordings with eye-movement responses, which the authors admit may not transfer to humans pressing buttons. The FEF timing effect partly overlapped with time elapsed since the stimulus, and signal spread faster than 20 ms or source locations based on a template head cannot be pinned down precisely. Labelling the networks as 'accumulation' and 'selection' is an interpretation of which factor modulated the EEG, not a direct measurement of those computations.
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