Reliability of a fly motion-sensitive neuron depends on stimulus parameters

The variability of responses of sensory neurons constrains how reliably ani mals can respond to stimuli in the outside world. We show for a motion-sens itive visual interneuron of the fly that the variability of spike trains de pends on the properties of the motion stimulus, although differently for di fferent stimulus parameters. (1) The spike count variances of responses to constant and to dynamic stimuli lie in the same range. (2) With increasing stimulus size, the variance may slightly decrease. (3) Increasing pattern c ontrast reduces the variance considerably. For all stimulus conditions, the spike count variance is much smaller than the mean spike count and does no t depend much on the mean activity apart from very low activities. Using a model of spike generation, we analyzed how the spike count variance depends on the membrane potential noise and the deterministic membrane potential f luctuations at the spike initiation zone of the neuron. In a physiologicall y plausible range, the variance is affected only weakly by changes in the d ynamics or the amplitude of the deterministic membrane potential fluctuatio ns. In contrast, the amplitude and dynamics of the membrane potential noise strongly influence the spike count variance. The membrane potential noise underlying the variability of the spike responses in the motion-sensitive n euron is concluded to be affected considerably by the contrast of the stimu lus but by neither its dynamics nor its size.