A linear area centralis extending across the turtle retina and stabilized to the horizon by non-visual cues

Abstract

A common fresh-water turtle ( Pseudemys scripta elegans) is shown to have a linear area centralis extending horizontally almost all the way across the retina. Within this line the receptors are small and densely concentrated, as demonstrated by using the colored oil droplets to visualize the receptor mosaic in isolated inverted retinas. Below this line the increase of receptor size occurs more gradually than above the line. In the central portion of the line, there is an expansion of the high-density receptor area into the lower half of the retina. In a fresh eyecup the linear area centralis may be seen as a distinct red line that is parallel to a black line across the iris. During limited rotation of the turtle around its lateral or longitudinal axis, the black iris line (and hence the area centralis) is stabilized in the plane of the animal's horizon by reflexive head and eye movements, which occur without visual cues and hence are presumably under vestibular control. This linear type of area centralis, which is stabilized to the horizon, appears to be highly functional specialization for an animal living close to the earth's surface, for which the bulk of relevant visual stimuli must originate at or near the horizon.