Biomedical Engineering Reference
In-Depth Information
implications for the development of bio-inspired
navigation systems for robots and unmanned
aerial vehicles. Through these observations,
Srinivasan has concluded that bees demonstrate
complex behavior derived from optical flow
[65]
. Specifically, bees:
lens with a complement of photoreceptors, as
shown in
Figure 1.18
[41, 66-68]
.
Figure 1.18
shows how six photoreceptors
(R1-R6) surround two coaxially aligned photo-
receptors (R7, R8) in an irregular pattern; R1-R6
terminate in the neurons of the lamina, whereas
R7 and R8 bypass the lamina and connect
directly to the medulla
[41, 66-68]
. The photo-
receptors function as transducers to convert
light energy into ionic current, and are thought
to be sensitive to both magnitude and angle of
the impinging light. The angular sensitivity of
each photoreceptor has a profile that is approxi-
mately Gaussian
[41, 69-74]
, as depicted in
Figure 1.18
.
Six photoreceptors (R1-R6) contribute to
the neural superposition effect; the remaining
two photoreceptors (R7, R8) bypass the lamina
and are therefore not directly involved in neu-
ral superposition. For the purposes of this dis-
cussion of neural superposition, we will
concentrate on R1-R6. As previously described
for a neural superposition compound eye,
each individual photoreceptor (R1-R6) is con-
nected in the lamina with five other common
view photoreceptors from adjacent omma-
tidia. This is depicted in the cross-sectional
view of six adjacent ommatidia on the far-left
side of
Figure 1.18
. Pick carefully studied this
arrangement and reported that the axis of the
receptors converge at a distance of 3-6 mm in
front of the corneal surface. This provides for
a substantial gain in light sensitivity. It also
results in a slight blurring of the image, but,
as Pick noted, it provides the fly with the abi-
lity to determine the distance from an object
by sensing the imbalance of response from the
photoreceptor grouping
[75]
. The resulting
overlapped Gaussian profile responses are
depicted in
Figure 1.18
.
Photoreceptor signals from R1-R6 are then
combined and processed in the lamina by
monopolar cells L1 and L2
[76]
. There is some
disagreement concerning the function of the L1
and L2 cells. Some hypothesize the cells could
• Gaugedistancetraversedenrouteby
integrating the apparent motion of the
visual scene. This integration appears to be
independent of the contrast and spatial
frequency content (structure) of the scene.
• Maintainequidistantseparationfromtunnel
walls by balancing the images as seen by
each eye as the tunnel is traversed. This is
accomplished by holding the average image
velocity constant between the two eyes.
• Maintainacontrolledlandingonahorizon-
tal surface by maintaining a constant
average image velocity for the surface
approach. This is accomplished by main-
taining a forward speed that is approxi-
mately proportional to altitude.
For a closer investigation of the visual system,
in the next section, we concentrate on a single
species:
M. domestica
.
1.4 CASE STUDY:
MUSCA
DO
MESTICA
VISION SENSO
R
Dipterans, such as
M. domestica
, are flying insects
equipped with two wings and two balancers;
they exhibit a highly parallel, compartmenta-
lized, analog vision system of the neural supers-
position type
[42, 66]
. The primary visual system
of the housefly consists of two compound eyes
equipped with approximately 3000 ommatidia
per eye. As previously discussed, an ommati-
dium is the major modular structural unit of the
compound eye. Each ommatidium of the house-
fly is equipped with a cutinous, hexagonal,
25
µ
m diameter facet lens and a cone-shaped
