Tuesday, January 20, 2015

Gecko Night Vision - Psychedelic Dude

By Katie Grady
Image credits can be found by clicking on the images.

If you think scaling a vertical wall in one meter per second is cool, imagine seeing colors at night or having technicolor night vision. Gecko vision makes these reptiles impressive beyond their super sticky feet. In recent years, researchers have been designing specialty cameras and multifocal contact lenses, in part, based on ‘gecko vision’… another case of nature’s designs inspiring some creative technology. Geckos are among an elite group of animals with this very special power to see colors at night. Most vertebrates sacrifice color vision at night in exchange for enhanced sensitivity to what little light is available. Geckos decided they were too cool for this sacrifice and opted for 24/7 color vision instead. This could just be an anomaly or perhaps there is some benefit to seeing colors while hunting at night. 

Let’s start with what we know - the human eye. For a brief review, we have a pupil that expands based on light availability, a lens that relaxes and contracts to focus on objects far away and most important for the topic at hand, a retina. The retina is loaded with millions of light-sensitive cells called rods and cones. As you may recall, rods are used for monochrome (black and white) vision in low light where as cones are used for seeing colors and finer detail. The optic nerve acts as the receiver for these electric signals feeding them to the neurons in our brain in order to get converted into images.


Geckos are a bit unique. Somewhere along the line, in evolutionary history, lizards and geckos (order: Squamata) lost the rods known to vertebrates and now only retain various types of cones. With weak surrounding light intensities, nocturnal geckos can access color vision because of very large, light-sensitive cones, unlike the color-blind rod vision used by humans (Roth and Kelber, 2004). "Researchers calculated (these cones) to be more than 350 times more sensitive than human cone vision at the human color vision threshold." - Science Daily



Researchers set out to study this little fella to your right… the nocturnal helmet gecko, Tarentola chazaliae, local to the coast of Morocco. The Swedish crew of scientists (Roth, et al 2009) wanted to see the extent of color information utilized by geckos at night. They compared helmet gecko to day gecko optics using two techniques: 1) qualitative photorefractometry and 2) a quantitative high resolution sensor (Hartmann-Shack wavefront sensor) to look at refractive or focussing power in small eyes. The latter is a useful, unobtrusive method for gathering optical data from live animals.


So what did these researchers find? First, they discovered that within the same species there were varying amounts of light going through parts of the pupil, which poses the question do some geckos have worse vision then others and need corrective lenses themselves? Secondly, they found that the helmet gecko possesses a multifocal optical system with distinct concentric zones. What does this mean exactly? Basically, these geckos have multiple focal zones unlike most vertebrates. Start by picturing an archery target with a bullseye in the middle…

Each circle (zone) operates at different refractive powers, which detect certain wavelengths of light. These ranges of light are then focused simultaneously on the retina. The results from this study show at least two zones of different refractive powers meaning these geckos have the ability to get sharp images of objects at varying depths of field. They can literally focus on at least two things at once! Diurnal geckos (those that are active during the day) are considered monofocal because they can only focus on objects at one distance (like humans), in part due to smaller photoreceptors. Multifocal optics is what sets the nocturnal helmet gecko apart from other animals, hence the whole next generation of cameras and multifocal contact lenses!


The three specific photopigments present in helmet geckos are sensitive to UV, blue and green wavelengths of light, which makes sense because most organisms in the surrounding environment (i.e. plants) are reflecting these colors of light. However, the presence of receptors is not enough to prove they are actually using this color vision at night. Remember the whole signal pathway from the retina to the brain? The question remains - are they utilizing these large photoreceptors to see color images at night?

In one study, with simulated dim moonlight, helmet geckos were able to differentiate blue from grey patterns (Roth and Kelber, 2004). Crickets were dusted with either blue ("tasty") powder or grey ("distasteful") powder. With color as the signal to be learned the geckos routinely chose the blue crickets.  Even though these geckos lack red-sensitive cones, they are still behaviorally choosing color vision in low light conditions. (Side note - this is why red light bulbs are used for reptile heating in aquaria.) There is still speculation as to why this predator would preserve color vision at night.

In addition, these geckos may have built in correctional abilities for blurred images. Having a larger pupil relative to body size then most vertebrates and a short focal length can lead to chromatic distortion in which the lens cannot focus all colors to the same point (Roth, et al 2009). When shorter wavelengths of light are refracted and unable to meet up with longer wavelengths, the retinal image can become blurred. The multifocal optical system so gracefully programmed in these geckos is suggested to correct for this defocusing.

With super impressive cones in their retinas and multifocality abilities, helmet geckos (among other nocturnal reptiles) represent model organisms for advancing optic technologies. Next time you purchase a camera or contact lenses think about what cute little critter that might have inspired the mechanics behind what we might take for granted on a daily basis.



References:

Roth, L.S.V., Lundstrom, L., Kelber, A., Kroger, R.H.H., and P. Unsbo. 2009. The pupils and optical systems of gecko eyes. Journal of Vision. 9(3):27, 1-11.

Kelber, A., and L.S.V. Roth. 2006. Nocturnal colour vision – not as rare as we might think. The Journal of Experimental Biology. 209, 781-788.


Roth, L.S.V., and A. Kelber. 2004. Nocturnal colour vision in geckos. Proceedings of the Royal Society of London B. 271, S485–S487.

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