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Chapter twelve: Visual and hydrodynamic flow perception
determine a threshold. However, double the number of trials is needed including the no
go-stimuli as go- and no go-stimuli need to be balanced in order to be able to approach
the threshold.
Please note that psychophysical experiments alone cannot describe the sensory abilities of an organism in total. Additional application experiments such as the hydrodynamic
trail-following experiment (Dehnhardt et al. 2001) need to be conducted to show what the
animals use their sense for and how they use it in their environment.
12.3.3 Particle image velocimetry
Particle image velocimetry (PIV) measurements are fundamental to all hydrodynamic
experiments in order to control qualitative and quantitative aspects of the water movements used as stimuli. With the help of PIV, water movements can be visualized and
water velocities can be measured, also over time (Figure 12.2c). During PIV measurements, tracer particles are added to the water. These particles are neutrally buyont and
thus keep position unless a water movement is generated. A fanned-out laser is illuminating the particles in one layer. A top view video camera is filming the light reflected from
particles in the region of interest (ROI) first without (background flow) and then with
hydrodynamic event such as induced by, for example, a paddle that is moved from left
to right through the ROI. The video raw material can then be analyzed offline in the PIV
software DaVis 7.2 (LaVision GmbH, Göttingen, Germany). With this software, velocity
vectors are calculated from correlating the displacement of tracer particles in subsequent
images (Keane and Adrian 1992).
12.3.4 Challenges associated with the investigation of senses underwater
A big challenge when investigating sensory abilities of (semi)aquatic mammals is that
often experiments need to be conducted underwater. Consequently, the equipment
needs to be water-proof which renders every purchase cost-intensive and often setups
need to be custom built as off-the-shelf setups are not available. Furthermore, setups
usually have to be quite large when working with marine mammals thus space might
be a limiting factor.
The presentation of optic flow stimuli underwater requires large projection screens or
monitors. Generally, for the presentation of high-quality stimuli, stimulus presentation has
to be in a shaded area, ideally in an experimental chamber (Hanke et al. 2008; Gläser et al.
2014; Weiffen et al. 2014). For using underwater back projections on a projection screen, the
projector needs to be lowered in a water-proof housing. Alternatively, the beamer stays in
air but then the water surface needs to be calmed in order to avoid distortion of the image
by surface waves. Still the stimuli might be affected by water quality, even if experiments
are conducted in an aquarium setting, as, for example, dissolved particles can lower contrast and intensity. This phenomenon can be reduced to some extent when monitors are
applied as then light is only attenuated between experimental subject and monitor and not
additionally in between projection screen and beamer.
Testing underwater usually requires one or multiple cameras to record the animal’s behavior. For optic flow experiments a camera is needed as stimulus presentation needs to take place in a shaded area. In hydrodynamic experiments, especially in
the application experiments, a top view camera is essential to be able to analyze the
animal’s and the trail generator’s trails. However, as a large area needs to be overseen
the camera has to be installed high above the experimental area which is challenging.
Chapter twelve: Visual and hydrodynamic flow perception
determine a threshold. However, double the number of trials is needed including the no
go-stimuli as go- and no go-stimuli need to be balanced in order to be able to approach
the threshold.
Please note that psychophysical experiments alone cannot describe the sensory abilities of an organism in total. Additional application experiments such as the hydrodynamic
trail-following experiment (Dehnhardt et al. 2001) need to be conducted to show what the
animals use their sense for and how they use it in their environment.
12.3.3 Particle image velocimetry
Particle image velocimetry (PIV) measurements are fundamental to all hydrodynamic
experiments in order to control qualitative and quantitative aspects of the water movements used as stimuli. With the help of PIV, water movements can be visualized and
water velocities can be measured, also over time (Figure 12.2c). During PIV measurements, tracer particles are added to the water. These particles are neutrally buyont and
thus keep position unless a water movement is generated. A fanned-out laser is illuminating the particles in one layer. A top view video camera is filming the light reflected from
particles in the region of interest (ROI) first without (background flow) and then with
hydrodynamic event such as induced by, for example, a paddle that is moved from left
to right through the ROI. The video raw material can then be analyzed offline in the PIV
software DaVis 7.2 (LaVision GmbH, Göttingen, Germany). With this software, velocity
vectors are calculated from correlating the displacement of tracer particles in subsequent
images (Keane and Adrian 1992).
12.3.4 Challenges associated with the investigation of senses underwater
A big challenge when investigating sensory abilities of (semi)aquatic mammals is that
often experiments need to be conducted underwater. Consequently, the equipment
needs to be water-proof which renders every purchase cost-intensive and often setups
need to be custom built as off-the-shelf setups are not available. Furthermore, setups
usually have to be quite large when working with marine mammals thus space might
be a limiting factor.
The presentation of optic flow stimuli underwater requires large projection screens or
monitors. Generally, for the presentation of high-quality stimuli, stimulus presentation has
to be in a shaded area, ideally in an experimental chamber (Hanke et al. 2008; Gläser et al.
2014; Weiffen et al. 2014). For using underwater back projections on a projection screen, the
projector needs to be lowered in a water-proof housing. Alternatively, the beamer stays in
air but then the water surface needs to be calmed in order to avoid distortion of the image
by surface waves. Still the stimuli might be affected by water quality, even if experiments
are conducted in an aquarium setting, as, for example, dissolved particles can lower contrast and intensity. This phenomenon can be reduced to some extent when monitors are
applied as then light is only attenuated between experimental subject and monitor and not
additionally in between projection screen and beamer.
Testing underwater usually requires one or multiple cameras to record the animal’s behavior. For optic flow experiments a camera is needed as stimulus presentation needs to take place in a shaded area. In hydrodynamic experiments, especially in
the application experiments, a top view camera is essential to be able to analyze the
animal’s and the trail generator’s trails. However, as a large area needs to be overseen
the camera has to be installed high above the experimental area which is challenging.