The neural basis of vision in dim light
Many animals have mastered a nocturnal lifestyle, using vision to navigate and forage, even with more than one million times less light available than on a sunny day. However, when only few photons are available vision becomes uncertain, or noisy: at any given time the rate of photons arriving at the eye varies slightly – and the relative variation is higher the lower light levels are. Moreover, photoreceptors themselves have a certain level of intrinsic noise that reduces visual reliability further. These sources of noise do not affect vision much during the day since the incoming signal is many times stronger than the noise. At night however, with only few photons available per photoreceptor, the intrinsic noise together with the photon arrival uncertainty can decrease photoreceptor performance drastically.
The solution seems easy: increase the number of photons an eye captures. Which is exactly what the eyes of nocturnal animals do: they have larger lenses or larger photoreceptors, to increase photon capture. However, ocular adaptations can only partly explain the astonishing sensitivity of nocturnal insects – we thus hypothesise that there are additional adaptations at higher levels of visual processing in the brain, in particular visual summation of photons in space and time. This summation – which our previous work suggests is likely to occur in the first optic ganglion of the optic lobe, the “lamina” – improves visual reliability by brightening the coarser and slower features of the visual world at the expense of finer and faster features. However exactly which cells are responsible, and which neural strategies they employ, remain unknown.
This project aims at describing and localizing adaptations for nocturnal vision in the optic lobes of nocturnal insects, particularly the tropical bee Megalopta genalis (work done in our electrophysiology lab in Panama) and the hovering hawkmoth Deilephila elpenor (work done in Lund). We are currently investigating spatial summation in the lamina monopolar cells (LMCs) of the lamina, and in wide-field motion detecting cells in the lobula plate. To identify the nature and the location of this summation, anatomical and physiological studies will be performed in our nocturnal model species as well as in closely related diurnal species.