The effect of mean luminance on the size selectivity of identified target interneurons in the dragonfly

Summary

• 1. By penetrating axons in the ventral nerve cord of the dragonfly, Aeshna umbrosa, we measured the intracellular responses of target-selective visual interneurons to movement of black square ‘targets’ ranging from 1° to 32° visual angle at several levels of mean background luminance.

• 2. Neuronal responses, measured both in number of spikes and in the magnitude of integrated postsynaptic potentials, showed a preference for larger target size at lower mean luminance (Table 1, Figs. 1–3). The latency of postsynaptic potential (psp) and spike responses from onset of target movement increased with a decrease in mean luminance (Fig. 1).

• 3. A measure of mean target size preference (Eqn. 1) for one identified interneuron (MDT4) in both laboratory and outdoor lighting shows a continuous decrease of preferred size with increases of mean luminance over more than 4 orders of magnitude.

• 4. The time to reach the new steady state of cell response after the decrease of mean luminance was ordinarily less than 30 s, but sometimes longer (Fig. 4).