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J. R. Dearworth, Jr., E. E. Brune, A. Boyd; The Partial Consensual Pupillary Light Response in the Turtle. Invest. Ophthalmol. Vis. Sci. 2007;48(13):5539.
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The turtle has a pupillary response to light, but it is not known if the response is consensual. The aim of this study was to determine if the response in the red-eared slider turtle, Trachemys scripta elegans, is consensual.
Four turtles were used as subjects in two separate experiments following the guidelines of the IACUC at Lafayette College. Turtles were secured in a restraint that immobilized their head movements with minimal discomfort. Heads were inserted into a light integrating sphere, and both pupils were monitored through small apertures by infrared sensitive cameras. Infrared LEDs illuminated pupils in darkness. The right eye of each animal was shielded from light stimulation throughout the experiments. Initially both eyes were in the dark for 40 minutes. Then, the left eye was exposed to light to elicit the direct pupillary light response. In experiment one, a light intensity of 6.51 x10-14J·µm-2·s-1 was tested in three turtles. In experiment two, three different intensities: 5.83 x10-15 (low), 5.83 x10-14 (medium), and 2.92 x10-12 J·µm-2·s-1 (high) were tested in one turtle. Pupil sizes of both eyes were tracked manually using images recorded onto tape and automatically using the ViewPoint EyeTracker® (Arrington Research). To test for the possibility of light bleeding through the orbits, an animal was euthanized and one of its eyes enucleated. The light coming through the intact eye into the empty orbit was measured.
In the first experiment using three turtles, the mean of the directly stimulated pupil diameter was reduced to 68.5% ± 1.8 (SEM), n=9, from its maximum size measured in the dark. In the eye not stimulated by light, the pupil was also reduced, but less, to 87.4% ± 1.9. In the second experiment using one turtle, pupils of both eyes were also reduced dependent on the intensity. In this experiment, pupil areas were calculated. For the directly stimulated eye, pupil areas were 72.4% ± 1.9, n=3 (low); 60.3% ± 1.7, n=3 (medium); and 53.6 %, n=1 (high). The eye not stimulated by light decreased, but not as much: 88.9% ± 2.8, n=3 (low); 83.2% ± 3.7, n=3 (medium); and 79.0%, n=1 (high). Light intensity measured in the empty orbit was zero except at the high intensity where it was 2.92 x10-17 J·µm-2·s-1.
Eyes that were not stimulated by light responded less, but otherwise similar to stimulated eyes. Since light intensity coming through the intact eye into the empty orbit was below the threshold of the pupillary light response, our results suggest that the turtle has a partial consensual pupillary light response.
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