Sensory Biology of Sea Turtles
87
3.2.3.2 Electrophysiology
Electrophysiological techniques have also been employed to investigate the visual
acuity thresholds of sea turtles (Bartol et al., 2002). Electrical responses recorded
from the visual system provide an objective measure of a variety of visual phenomena, including the dependence of a response on the character of the stimulus (Riggs
and Wooten, 1972; Bullock et al., 1991). In the Bartol et al. (2002) study, the
technique of visual evoked potentials (VEPs) was used. VEPs are compound field
potentials of any neural tissue in the visual pathway and can be obtained from a
subject animal by the use of surface electrodes placed on the head directly above
the optic nerve and corresponding optic tectum. In this study, the researchers used
a modified goggle filled with seawater over the stimulated eye. This apparatus
allowed for the testing of underwater acuity. The stimuli were black and white striped
patterns of decreasing size, yet always of equal brightness. One peak in the VEP
recordings was found by the researchers to be present in all suprathreshold recordings, showing a dependence of peak amplitude on stimulus stripe size (Figure 3.4).
From this peak, Bartol et al. (2002) were able to identify an acuity threshold level
of 0.187 (visual angle = 5.34 min of arc) when data from all six turtles were pooled.
This level of acuity would permit loggerheads to discern prey, such as horseshoe
and blue crabs, as well as large predators, and is comparable to many species of
marine fishes. Interestingly, these researchers were unable to collect any discernible
VEP response when the turtles were tested with their eyes in air (i.e., without the
water-filled goggle), suggesting that the sea turtle eye operates much differently in
the two media (Bartol et al., 2002) (Figure 3.4).
3.2.3.3 Behavior
Psychophysical methods were used to investigate the visual acuity of juvenile loggerhead sea turtles ( C. caretta ) in the aquatic medium (Bartol, 1999). An operant
conditioning method was developed to train juvenile loggerheads in a tank environment to identify a striped stimulus. The tank was set up with two response keys:
one was located below a striped panel and the other below a gray panel. Turtles
were trained by receiving a food reward only when the response key was chosen
below the striped panel. Once training of these turtles was achieved, the stimulus
was reduced in size until the turtle could no longer respond correctly. These turtles
were found to be highly appropriate subject animals for an in-tank behavior study,
and retained their training over time. From these trials, Bartol (1999) found the
behavioral acuity threshold for juvenile loggerheads to be approximately 0.078
(visual angle of 12.89 min of arc), comparable to that found in the electrophysiology
study (Bartol et al., 2001) and similar to the visual acuity of other benthic shallowwater marine species.
3.2.4 V ISUAL B EHAVIOR ON L AND
The visual behavior of hatchling and nesting female sea turtles as they orient toward
water while on land also has been studied. Vision has been identified in numerous
articles as the primary sense used in sea-finding behavior of both hatchlings and
1123 book.book Page 87 Monday, November 11, 2002 11:11 AM
87
3.2.3.2 Electrophysiology
Electrophysiological techniques have also been employed to investigate the visual
acuity thresholds of sea turtles (Bartol et al., 2002). Electrical responses recorded
from the visual system provide an objective measure of a variety of visual phenomena, including the dependence of a response on the character of the stimulus (Riggs
and Wooten, 1972; Bullock et al., 1991). In the Bartol et al. (2002) study, the
technique of visual evoked potentials (VEPs) was used. VEPs are compound field
potentials of any neural tissue in the visual pathway and can be obtained from a
subject animal by the use of surface electrodes placed on the head directly above
the optic nerve and corresponding optic tectum. In this study, the researchers used
a modified goggle filled with seawater over the stimulated eye. This apparatus
allowed for the testing of underwater acuity. The stimuli were black and white striped
patterns of decreasing size, yet always of equal brightness. One peak in the VEP
recordings was found by the researchers to be present in all suprathreshold recordings, showing a dependence of peak amplitude on stimulus stripe size (Figure 3.4).
From this peak, Bartol et al. (2002) were able to identify an acuity threshold level
of 0.187 (visual angle = 5.34 min of arc) when data from all six turtles were pooled.
This level of acuity would permit loggerheads to discern prey, such as horseshoe
and blue crabs, as well as large predators, and is comparable to many species of
marine fishes. Interestingly, these researchers were unable to collect any discernible
VEP response when the turtles were tested with their eyes in air (i.e., without the
water-filled goggle), suggesting that the sea turtle eye operates much differently in
the two media (Bartol et al., 2002) (Figure 3.4).
3.2.3.3 Behavior
Psychophysical methods were used to investigate the visual acuity of juvenile loggerhead sea turtles ( C. caretta ) in the aquatic medium (Bartol, 1999). An operant
conditioning method was developed to train juvenile loggerheads in a tank environment to identify a striped stimulus. The tank was set up with two response keys:
one was located below a striped panel and the other below a gray panel. Turtles
were trained by receiving a food reward only when the response key was chosen
below the striped panel. Once training of these turtles was achieved, the stimulus
was reduced in size until the turtle could no longer respond correctly. These turtles
were found to be highly appropriate subject animals for an in-tank behavior study,
and retained their training over time. From these trials, Bartol (1999) found the
behavioral acuity threshold for juvenile loggerheads to be approximately 0.078
(visual angle of 12.89 min of arc), comparable to that found in the electrophysiology
study (Bartol et al., 2001) and similar to the visual acuity of other benthic shallowwater marine species.
3.2.4 V ISUAL B EHAVIOR ON L AND
The visual behavior of hatchling and nesting female sea turtles as they orient toward
water while on land also has been studied. Vision has been identified in numerous
articles as the primary sense used in sea-finding behavior of both hatchlings and
1123 book.book Page 87 Monday, November 11, 2002 11:11 AM