Catching prey in darkness
Text source: http://www.bernstein-zentren.de/
Original work: A. Sichert, P. Friedel, and L. van Hemmen (2006) -- Snake's Perspective on Heat: Reconstruction of Input Using an Imperfect Detection System.
Phys. Rev. Lett. 97, 068105. (Abstract)
Catching prey in darkness, the sixth sense of snakes
In complete darkness they can go hunting and can detect their
warm-blooded prey very precisely,- some types of snakes have a so
called pit hole with which they can perceive infrared radiation. It
consists of a freely suspending "pit membrane" with heat sensitive
cells in a pit beside each eye. The opening of the pit organ is big
enough that sufficient infrared radiation can reach the membrane and
the snake can quickly recognize even prey which moves very quickly.
This however also has disadvantages. In the same way that a pinhole
camera can only take a very blurred picture when the aperture is set
wide open, the image of the heat distribution on the membrane is also
very unclear. That snakes can nevertheless clearly recognize their prey
with the help of the pit organ is the paradox of the Infrared detection
system- the organ seems to work better than is physically possible. The
scientists Andreas Sichert, Paul Friedel and Leo van Hemmen from the
BCCN and the Technical University Munich have now found an explanation
to this paradox.
(a) Pit hole of a temple viper is
located between eye and nostril. As the schematic drawing (b) shows, a
point of the input is projected onto the pit membrane by the wide
aperture of the pit organ as a disc-shaped image. [Image: Guido
Westhoff]
Every point that radiates warmth on the surface of the prey is mapped onto the pit membrane through the opening of the pit organ as a disc shaped image, every edge becomes blurred. The information about the original object of prey, however, is completely contained in the blurred picture on the pit membrane. Every point of the reconstructed image appears as a linear combination of all the measuring points of the heat distribution on the membrane. The scientists have now developed an algorithm, which allows them to infer the original image from the overlap of many spots and blurred edges. They call their reconstruction method a "virtual lens". This reconstruction of the original image is the exact equivalent of the calculation operation which a neural feedforward network can perform. Thus it has been shown that the brain of the snake can at least theoretically perform such a calculation.
Every point that radiates warmth on the surface of the prey is mapped onto the pit membrane through the opening of the pit organ as a disc shaped image, every edge becomes blurred. The information about the original object of prey, however, is completely contained in the blurred picture on the pit membrane. Every point of the reconstructed image appears as a linear combination of all the measuring points of the heat distribution on the membrane. The scientists have now developed an algorithm, which allows them to infer the original image from the overlap of many spots and blurred edges. They call their reconstruction method a "virtual lens". This reconstruction of the original image is the exact equivalent of the calculation operation which a neural feedforward network can perform. Thus it has been shown that the brain of the snake can at least theoretically perform such a calculation.
For the reconstruction tests Sichert et
al. used Albrecht Dürer's rabbit as "prey" (a). A screened version
served as an example of the distribution of warmth (b) which appears
blurred on the pit membrane (c). (d) shows the reconstruction of the
rabbit with the help of the presented algorithm.
By varying different parameters, the scientists found that the virtual lens reacts very sensitively to errors in measurements by the membrane. The prerequisite for the reconstruction of the image is therefore that the membrane can discern, very reliably, extremely small differences in the heat distribution. The membrane is only 15 micrometers thick and hangs freely suspended in the pit organ so that it is isolated by the air. Heat sensitive cells from the membrane react to differences in temperature of only a few millikelvins. How this sensitivity comes about is now the next question, which the scientists in the group of Sichert and van Hemmen want to address. Up to now the pit organ is at room temperature 10 times superior to any un-cooled technical system. If it were possible to construct a highly sensitive infrared detection system, the application would be considerable! For example if cars could use a similar system to sense pedestrians at night when they are crossing a street, this would be a great step forward road safety.
By varying different parameters, the scientists found that the virtual lens reacts very sensitively to errors in measurements by the membrane. The prerequisite for the reconstruction of the image is therefore that the membrane can discern, very reliably, extremely small differences in the heat distribution. The membrane is only 15 micrometers thick and hangs freely suspended in the pit organ so that it is isolated by the air. Heat sensitive cells from the membrane react to differences in temperature of only a few millikelvins. How this sensitivity comes about is now the next question, which the scientists in the group of Sichert and van Hemmen want to address. Up to now the pit organ is at room temperature 10 times superior to any un-cooled technical system. If it were possible to construct a highly sensitive infrared detection system, the application would be considerable! For example if cars could use a similar system to sense pedestrians at night when they are crossing a street, this would be a great step forward road safety.
