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Chapter twelve: Visual and hydrodynamic flow perception
photon absorbance irrespective of the direction from which the photon is entering the
eye. The omnidirectionality of the tapeta might have evolved in marine mammals as
a response to the 3D underwater world in which a marine mammal can translate but
also rotate in any orientation and in which photons thus might arrive at the eye from
above, below, or from the sides.
Marine mammal eyes not only possess adaptations to increase sensitivity, but their
eyes can also quickly adapt to low light levels. Levenson and Schusterman (1999) could
show that pinnipeds can dark-adapt within several minutes in comparison to human eyes
that require more than 20 min to reach maximum sensitivity in darkness. Furthermore,
the rate of dark adaptation seems to correlate with diving depth as the northern elephant
seal dark adapts within only 6 min which corresponds with the time required to reach
depths of approximately 1500 m to which they were found to regularly dive (Le Boeuf and
Laws 1994). In contrast, it took the eyes of the shallow-diving harbor seals 18 min to darkadapt and humans even 22 min (Levenson and Schusterman 1999). It is also noteworthy
that a positive relationship between diving depth and orbit size has also been discovered
recently (Debey and Pyenson 2013).
12.2.2 Basis for optic flow perception
Among the marine mammals, motion vision has so far only been investigated in harbor seals. The first optic flow experiment in harbor seals (Gläser et al. 2014) was based
on two motion vision studies. Research in this field started in 2008, when Hanke et al.
(2008) investigated optokinetic eye movements in harbor seals (Figure 12.3a). The optokinetic nystagmus describes a basic motion stabilizing reflex consisting of a pursuit eye
movement during which the eye follows the whole-field motion stimulus. Optimally,
the visual motion is canceled during the pursuit eye movement which is the prerequisite for a sharp image on the retina. In order to re-center the eye in the orbit, the
eye performs a fast saccade against stimulus direction after the pursuit eye movement. Pursuit eye movement and saccade alternate as long as there is a visual motion
stimulus. Hanke et al. (2008) showed that harbor seals possess optokinetic eye movements and that the eye stabilizes image motion equally well irrespective of stimulus
movement direction. The latter phenomenon has never been described before and
might be an adaptation to the low structured, 3D underwater world in which harbor
seals operate and in which important visual information might arrive at the eye from
any direction. In conclusion, this study showed that harbor seals are indeed able to
perceive visual motion.
In a follow-up experiment, it was determined how sensitive harbor seals are in respect
to global motion stimuli (Weiffen et al. 2014). For his purpose, the seal was presented with
large random dot displays in which a specific number of dots are displayed in the display
area (Figure 12.3b). At the extremes, these dots move either randomly (0% coherence) or
all dots move into one direction (100% coherence). A threshold of motion sensitivity is
achieved by varying the coherence of the dots. When presented with random dot displays,
the seal quickly learned the task with these complex stimuli, and it was able to detect the
display with coherent motion out of two displays with the lowest threshold amounting to
4.7% coherence (Weiffen et al. 2014), a sensitivity to coherent motion equivalent to the
highest sensitivity assessed so far in humans and monkeys (see for example, Newsome
and Paré 1988). However, the experimental conditions used to assess this threshold might
have allowed the seal to use secondary cues that were eliminated in the second phase
of the experiment. In this phase, the seal’s threshold was determined at 23% coherence.
Chapter twelve: Visual and hydrodynamic flow perception
photon absorbance irrespective of the direction from which the photon is entering the
eye. The omnidirectionality of the tapeta might have evolved in marine mammals as
a response to the 3D underwater world in which a marine mammal can translate but
also rotate in any orientation and in which photons thus might arrive at the eye from
above, below, or from the sides.
Marine mammal eyes not only possess adaptations to increase sensitivity, but their
eyes can also quickly adapt to low light levels. Levenson and Schusterman (1999) could
show that pinnipeds can dark-adapt within several minutes in comparison to human eyes
that require more than 20 min to reach maximum sensitivity in darkness. Furthermore,
the rate of dark adaptation seems to correlate with diving depth as the northern elephant
seal dark adapts within only 6 min which corresponds with the time required to reach
depths of approximately 1500 m to which they were found to regularly dive (Le Boeuf and
Laws 1994). In contrast, it took the eyes of the shallow-diving harbor seals 18 min to darkadapt and humans even 22 min (Levenson and Schusterman 1999). It is also noteworthy
that a positive relationship between diving depth and orbit size has also been discovered
recently (Debey and Pyenson 2013).
12.2.2 Basis for optic flow perception
Among the marine mammals, motion vision has so far only been investigated in harbor seals. The first optic flow experiment in harbor seals (Gläser et al. 2014) was based
on two motion vision studies. Research in this field started in 2008, when Hanke et al.
(2008) investigated optokinetic eye movements in harbor seals (Figure 12.3a). The optokinetic nystagmus describes a basic motion stabilizing reflex consisting of a pursuit eye
movement during which the eye follows the whole-field motion stimulus. Optimally,
the visual motion is canceled during the pursuit eye movement which is the prerequisite for a sharp image on the retina. In order to re-center the eye in the orbit, the
eye performs a fast saccade against stimulus direction after the pursuit eye movement. Pursuit eye movement and saccade alternate as long as there is a visual motion
stimulus. Hanke et al. (2008) showed that harbor seals possess optokinetic eye movements and that the eye stabilizes image motion equally well irrespective of stimulus
movement direction. The latter phenomenon has never been described before and
might be an adaptation to the low structured, 3D underwater world in which harbor
seals operate and in which important visual information might arrive at the eye from
any direction. In conclusion, this study showed that harbor seals are indeed able to
perceive visual motion.
In a follow-up experiment, it was determined how sensitive harbor seals are in respect
to global motion stimuli (Weiffen et al. 2014). For his purpose, the seal was presented with
large random dot displays in which a specific number of dots are displayed in the display
area (Figure 12.3b). At the extremes, these dots move either randomly (0% coherence) or
all dots move into one direction (100% coherence). A threshold of motion sensitivity is
achieved by varying the coherence of the dots. When presented with random dot displays,
the seal quickly learned the task with these complex stimuli, and it was able to detect the
display with coherent motion out of two displays with the lowest threshold amounting to
4.7% coherence (Weiffen et al. 2014), a sensitivity to coherent motion equivalent to the
highest sensitivity assessed so far in humans and monkeys (see for example, Newsome
and Paré 1988). However, the experimental conditions used to assess this threshold might
have allowed the seal to use secondary cues that were eliminated in the second phase
of the experiment. In this phase, the seal’s threshold was determined at 23% coherence.