A Choroidal Sleight of Hand

1398 Br J Ophthalmol 2005;89:1398 Br J Ophthalmol: first published as 10.1136/bjo.2005.077966 on 18 October 2005. Downloaded from COVER ILLUSTRATION ......................................................... A choroidal sleight of hand octurnal mammalian flight need for a retinal vascular system, and began in the early Eocene night helps improve the image, since these Napproximately 52–55 million bats don’t have to look through their years ago when few competitors for own blood supply. the feast of nocturnal insects and But, what does this do to the retinal flowering plants existed. Bats! Highly image? Apparently nothing, or at least successful and almost pan-continental, the bats seem adjusted to it. Fruit bats these extraordinary creatures have have an area centralis, but no true fovea. radiated into two major families—the The visual acuity of fruit bats ranges microchiroptera (microbats) and mega- from 3–6 cycles per degree which is chiroptera (megabats). Considered approximately equivalent to that of a blind, perhaps, because they were cat. Although their diurnal vision is not thought to navigate successfully with- as good as ours, fruit bats do use vision out eyesight, bats are far from blind. to locate fruit and blossoms, as their Megabats are crepuscular (dusk and nocturnal vision is excellent. Even the Figure 1 Choroid exposed. dawn) or nocturnal in habit and micro- microbats are known to use visual cues bats are strictly nocturnal, with visual for larger prey, especially when echolo- adaptations that address those niches. genes. The visual pigments that provide cation would not be useful in prey While the microbats comprise the colour vision for fruit bats include a detection. Vision in the microbats, how- echolocating families, the megabats short wavelength opsin with sensitivity ever, is approximately 1–2 cycles per consist of frugivorous and nectarivorous that extends into the ultraviolet and a degree, somewhat less than a rodent. bats. The phylogeny of these two groups long wavelength opsin with sensitivities The visual image, and hence visual of bats remains controversial, with a that could clearly be described as red. processing, in fruit bats must be differ- wide range of hypotheses about their Fruit bats probably use their colour ent from other mammals since these relationship, including ‘‘flying pri- vision to help discern coloured fruit or papillae create an undulating retina. blossoms. In crepuscular light, using mates’’ and ‘‘deaf fruit bats.’’ The fruit This would serve to increase the number their rods as a third visual pigment, bats have better vision, and have of photoreceptors, improve light gather- there may even be some form of, or evolved olfactory skills that rival those ing ability, and perhaps increase depth of dogs. interpretation of, trichromacy since of field, but may require a different form The tube-nosed fruit bats, like there would be three different peaks of of retinal processing or cerebral integra- Nyctimene robinsoni (the eastern tube- sensitivity to wavelengths of light. tion. The dioptric difference between the nosed fruit bat), have evolved a stereo Evidence suggests that the ultraviolet http://bjo.bmj.com/ peaks and depressions of the choroidal olfaction system that will locate an opsin is active and that bats do see into papillae is approximately 1.5 dioptres, odour plume three dimensionally and this range, which is most unusual for a and early investigators suggested that follow it. Tube-nose fruit bats are mammal. For example, phakic humans this difference between the peaks and specialists, preferring figs, but will feed cannot see into this range because our on other fruit. Being specialists, they crystalline lenses, and those of most valleys allows the bat to be in focus at must be able to follow scent through the pseudophakes, block ultraviolet light. distance and near simultaneously. Other eastern Australian rainforest where they The eye of the fruit bat is distinct, early investigators suggested that the are native. Although it seems counter- though, as there is no retinal vascula- fruit bats could not accommodate. Both on September 28, 2021 by guest. Protected copyright. intuitive, odours tend to remain in ture, and the retinal layering is surpris- of these assertions are not true, as these distinct trails, called ‘‘odour plumes,’’ ingly thick, up to perhaps 250 mm, or bats can accommodate at least 3.5 diop- especially in a rain forest, which is less more. Choroidal oxygen and nutrients tres and don’t need these choroidal affected by wind currents. Odours do would not diffuse much more than papillae for near focusing. diffuse to some extent, but usually 140 mm. So, how do fruit bats nourish These choroidal papillae, then, appear provide distinct plumes that can be their inner retina? to be a sleight of hand for oxygen followed, especially by a creature such The choroid consists of spike-like delivery. as N robinsoni that is well equipped with projections that stud the retina much I R Schwab, a stereo nose and enlarged olfactory like rivets uniting two pieces of metal University of California Davis, 4860 Y Street, bulbs (fig 1). As can be seen from the right Suite 2400, Sacramento, CA 95817, USA; Vision in the bat illustrated on this side of the cover, this creates a texture [email protected] month’s cover has not been specifically or undulations in the retina. These J Pettigrew, evaluated, but vision in closely related projections are called papillations. The The Vision, Touch, and Hearing Research species has. The retina is composed tip of each choroidal papilla projects Centre, School of Biomedical Sciences, The University of Queensland, Brisbane, Australia primarily of rods, as you might expect 125–150 mm into the retina and is of a crepuscular animal, but cones do believed to nourish the surrounding tips Image of Nyctimene robinsoni on left of cover exist. Some megabats have been shown of the inner retina so that no portion of by Jack Pettigrew, MD. to have the potential for trichromatic retina is much more than 100 mm from Histological images on right of cover and this vision by molecular analysis of opsin its blood supply. This eliminates the page by William Lloyd, MD. www.bjophthalmol.com.

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