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H. Lee, K. Bumsted O'Brien; Theoretical and Behavioural Limit of Visual Resolution in Temperate and Tropical Seahorses. Invest. Ophthalmol. Vis. Sci. 2010;51(13):898.
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© ARVO (1962-2015); The Authors (2016-present)
Seahorses are visually guided feeders that prey upon small, fast moving crustaceans. Their feeding behaviour strongly suggests a mechanism for high resolution vision although the presence of a fovea has been debated. The purpose of this study was to determine whether seahorses have a fovea and to investigate the limit of their spatial resolution by determining the density of ganglion cells (GC) in order to calculate their theoretical limit of visual acuity, and to behaviourally measure their visual resolution.
Retinas from two seahorse species, a temperate H. abdominalis (Ha, n=8) and a tropical H. taeniopterus (Ht, n=4), were flat-mounted or processed for frozen sections. GC densities were determined in both sections and flat-mounts stained with propidium iodide. The theoretical limit of visual resolution was estimated based on GC density and a lens diameter based retinal magnification factor. Reactive distance, a behavioural measure of visual resolution, was tested in mature Ha (n=3) and Ht (n=10) seahorses.
Both species posses a rod free convexiclivatefovea in the ventro-temporal retina. GC density was highest along the foveal slope (40,000 cells/mm2 Ha and 55,000 cells/mm2 Ht). GC densities of both species declined in the foveal center (25,000 cells/mm2). In the far periphery, GC densities gradually decreased to 18,000cells/mm2. The calculated theoretical limit of visual resolution on the foveal slope was 2.62 cpd for Ha and 3.07 cpd for Ht. Therefore, at a distance of 10 cm, the theoretical limit of prey size resolution was 0.33 mm (Ha) and 0.28 mm (Ht). In the behavioural experiments, the limit of prey resolution for Ha was 3.28 cm while the limit for Ht was 2.04 cm at a distance of 10 cm.
Both Ha and Ht retinas possessed a fovea. Ht had a greater density of GC on the foveal slope and a higher behaviourally measured visual resolution compared to Ha, although in both species, the calculated theoretical limit of visual resolution was higher compared to behavioural measurements.
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