Active membrane properties and signal encoding in graded potential neurons.

We investigated the influence of active membrane properties on the precision by which the stimulus velocity is encoded in the membrane potential of a motion-sensitive interneuron in the blowfly. The so-called HS-cells respond to visual motion stimuli with a graded shift in membrane potential. Superimposed on this graded response are small spike-like events. This "mixed" visual response mode can be modified by current injection in two different ways. (1) By ongoing injection of hyperpolarizing current, the spike-like events are turned into full-blown action potentials, and (2) by injection of depolarizing current, the spike-like events become completely suppressed. The visual response then consists of a graded shift of membrane potential only. As a measure of the fidelity, we calculated the coherence between the motion stimulus and the response of the cell elicited with different electrical manipulations of the cell. We found that the coherence was highest for the cell at rest. Any electrical manipulation resulted in a reduced coherence. This was attributable partly to a lower signal-to-noise ratio and partly to an increased nonlinearity in the response. By applying a threshold operation we transformed the analog membrane response into an all-or-none spike train. A comparison between these two ways of signal representation revealed that more information about the stimulus velocity is inherent in the analog membrane potential than in the spike train.