The functional organisation of locust ocelli

Abstract

This paper examines the optical and physiological organisation of locust ocelli with a view to understanding their function. The approach taken in this work has largely been to correlate quantitative measurements of the spectral, angular and absolute sensitivities of large second order neurons with radiometric data from the natural environment. The resulting estimates of the natural performance of these neurons form the basis for a hypothesis of their functional significance. 1. In both median and lateral ocelli of adult locusts the image plane lies well behind the retina regardless of pupil diameter. It is pointed out that underfocussing confers positive advantages and should be seen as an integral part of ocellar design. 2. The ocellar nerves contain axons that carry graded hyperpolarisations and axons that carry trains of action potentials. Graded potentials are identified with the large (L) neurons. The spiking axons, of which some are light-inhibited and some are light-excited, are thought to be the small (S) neurons. 3. Constriction of the radial pupil during light adaptation causes little diminution of the total field of view of an ocellus. The horizontal extent of the visual fields of L neurons in lateral ocelli, measured electrophysiologically is about 140°, effectively that of the whole retina. These wide fields are consistent with the underfocussed dioptrics and the reported extensive branching of these cells in the subretinal neuropile. 4. Dark adaptation of L neurons following normal preparative procedures takes several hours and increases sensitivity by over 3 log units. Complete dark adaptation is associated with large (up to 10 mV average amplitude) hyperpolarising bumps clearly distinguishable from noise and presumed to result from single photon captures. 5. Using an index of sensitivity that relates to monochromatic stimulation of an eye with an axial point source it is estimated that L neurons are 5 × more sensitive than compound eye lamina neurons but to an extended source they would be 5000 × more sensitive. 6. Of 18 L neurons whose spectral sensitivity was examined, all had maximum sensitivity close to 370 nm. A secondary peak at 500 nm varied in height from cell to cell between 48% and 0.3% of the UV peak. One cell was found in the median ocellus that showed stronger inhibition from green than UV stimuli. It is concluded that color coding by the ocellus is possible. 7. It is suggested that the disposition of locust ocelli, the spectral sensitivity and the temporal and spatial filtering characteristics of their L neurons suit these cells well to the task of detecting instability in flight. This idea is discussed in relation to the anatomy and physiology of ocellar pathways. © 1978 Springer-Verlag.