Neural integration in the first optic neuropile of dragonflies

Summary

• 1. Intracellular recordings are made from second order interneurons in the visual system of the dragonfly,Hemicordulia tau. The dark-adapted spectral sensitivity functions of these large monopolar cells (LMC's) are measured by obtaining in tensity/response functions at 12 wavelengths between 317 and 614 nm.

• 2. The LMC response waveform depends only upon intensity and not upon wavelength (Fig. 1) and at all wavelengths except 614 nm the slopes of the V/logI curves are not significantly different (Fig. 2 and Table 1). It is concluded that dark-adapted LMC's are not spectrally opponent and the dark-adapted pathway shows no wavelength dependent inhibition.

• 3. The average spectral sensitivity function of LMC's is extremely broad (Fig. 5) and sensitivity is greater than 40% between 317 and 614 nm. Maximum sensitivity is at 475 nm and at this wavelength 1.2×10⁹ quanta/cm²/s are required to give a response of 50% maximum amplitude.

• 4. LMC's have no polarisation sensitivity in the green and low PS in the UV. This, together with the shape of the spectral sensitivity curve and the lack of response noise in the UV, suggests that the “linked pigment” retinula cells with broad spectral sensitivity functions form the major receptor input to LMC's.

• 5. The LMC's lack of colour sensitivity is considered along with their absolute sensitivity, dynamic range of response, angular sensitivity and lightadaptation properties. It is concluded that the LMC's function to provide the visual system with a high sensitivity, low noise contrast input that is well suited for high acuity movement perception.