| Summary: | Perceptual decisions require the temporal integration of sensory evidence to a response threshold. How the brain performs this operation is unknown. In fruit flies, expression of the forkhead box P transcription factor (FoxP) in αβ core (αβ<sub>c</sub>) Kenyon cells of the mushroom bodies influences decision times in odour discrimination tasks. Flies with a hypomorphic mutation in the FoxP locus take longer to commit to a choice than wild-type flies, especially in difficult tasks. Using calcium imaging and patch clamp electrophysiology in vivo, I investigate how FoxP shapes the biophysical properties of αβ<sub>c</sub> neurons and link these properties to the flies' olfactory decision-making behaviour. I find that αβ<sub>c</sub> Kenyon cells integrate individual odour-evoked synaptic inputs to action potential threshold at time scales matching the speed of olfactory discrimination. FoxP, by controlling the abundance of the voltage-gated potassium channel Shal (K<sub>V</sub>4) in αβ<sub>c</sub> Kenyon cell dendrites, determines the integrative properties of these neurons and dictates decision times. Targeted expression of dominant-negative or functional Shal in αβ<sub>c</sub> Kenyon cells is sufficient to correct or reproduce, respectively, the FoxP mutant phenotype. Subthreshold dynamics in membrane voltage thus have a previously unrecognised influence on temporal aspects of decision-making.
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