The Design of Compound Eyes and the Illumination of Natural Habitats
191
Laughlin, this Vol.). Instead, a wonderful compromise has evolved that maintains
eye size. Rather than scaling up the whole eye, some animals have a small part of
their eye, and thus a small part of their visual field, scaled up. A local increase in eye
radius may not only allow an increase in lens size (which reduces diffraction and
improves light capture) but if eye radius is increased sufficiently, it might even allow
a simultaneous reduction in Aif> (thus sharpening anatomical resolution).
Unfortunately, any benefits gained in one eye region necessarily come at the cost of
other regions. Despite this, it turns out that these specialized eye regions, known as
acute zones, are quite common, especially in apposition eyes from bright habitats
(Herridge 1978). Until very recently, they were thought impossible in superposition
eyes due to the optics of this type of eye, but at least one diurnal hawkmoth now
appears to possess them (Sect. 3.2).
The presence of an acute zone implies that one region of the visual world is more
important to an animal than others. Exactly which region depends on several factors,
many reflecting transient needs, such as the need to find a mate or prey, or the need
to detect the advance of a predator. Others reflect more permanent needs, not the
least of which is the structure of the habitats where animals live. In his terrain theory
of eye design, Hughes (1977) proposed that the spatial layout of sampling stations in
an eye is matched to the spatial layout of the habitat. As beautifully explained by
Rudiger Wehner ( 1987), such matched filtering greatly simplifies processing, "freeing
the brain from the need to perform more intricate computations to extract the
information finally needed for fulfilling a particular task". Another aspect of habitat
structure, the way in which scene details flow through an animal's visual field when
it moves, is also processed with the help of matched filtering. These three aspects of
an animal's habitat - its structure, its motion flow field and the locations of other
animals- have been identified by Land (1989a, 1999) as the main influences on the
evolution of acute zones. For a more comprehensive review, the interested reader is
referred to Land's beautiful accounts.
2.1.2 The Struggle to Capture Light
In dim habitats the luxury of an acute zone is rarely permitted and the capture of
light by the entire eye becomes the overriding priority. This does not mean to say
that matched filtering is absent. Dim habitats still have a structure, present a motion
flow field and contain mates and prey. Rather, the matched filtering found in animals
from dim habitats is not as obviously manifested in regional variations of eye structure.
As we shall soon see, matched filtering also occurs in the neural circuitry which
processes visual information, and this applies equally well to animals from dim
habitats as to those from bright ones.
As light levels fall, so too does the reliability of vision. There are several reasons
for this, not the least of which is the random nature of photon arrivals on the retina.
A photoreceptor that absorbs N photons during one integration time will experience
an uncertainty, or shot noise, of --iN photons associated with this sample, that is, N ±
--iN photons (Land 1981; Warrant and Mcintyre 1993). This noise reduces the
reliability of intensity measurements, and thereby discrimination of the contrasts
191
Laughlin, this Vol.). Instead, a wonderful compromise has evolved that maintains
eye size. Rather than scaling up the whole eye, some animals have a small part of
their eye, and thus a small part of their visual field, scaled up. A local increase in eye
radius may not only allow an increase in lens size (which reduces diffraction and
improves light capture) but if eye radius is increased sufficiently, it might even allow
a simultaneous reduction in Aif> (thus sharpening anatomical resolution).
Unfortunately, any benefits gained in one eye region necessarily come at the cost of
other regions. Despite this, it turns out that these specialized eye regions, known as
acute zones, are quite common, especially in apposition eyes from bright habitats
(Herridge 1978). Until very recently, they were thought impossible in superposition
eyes due to the optics of this type of eye, but at least one diurnal hawkmoth now
appears to possess them (Sect. 3.2).
The presence of an acute zone implies that one region of the visual world is more
important to an animal than others. Exactly which region depends on several factors,
many reflecting transient needs, such as the need to find a mate or prey, or the need
to detect the advance of a predator. Others reflect more permanent needs, not the
least of which is the structure of the habitats where animals live. In his terrain theory
of eye design, Hughes (1977) proposed that the spatial layout of sampling stations in
an eye is matched to the spatial layout of the habitat. As beautifully explained by
Rudiger Wehner ( 1987), such matched filtering greatly simplifies processing, "freeing
the brain from the need to perform more intricate computations to extract the
information finally needed for fulfilling a particular task". Another aspect of habitat
structure, the way in which scene details flow through an animal's visual field when
it moves, is also processed with the help of matched filtering. These three aspects of
an animal's habitat - its structure, its motion flow field and the locations of other
animals- have been identified by Land (1989a, 1999) as the main influences on the
evolution of acute zones. For a more comprehensive review, the interested reader is
referred to Land's beautiful accounts.
2.1.2 The Struggle to Capture Light
In dim habitats the luxury of an acute zone is rarely permitted and the capture of
light by the entire eye becomes the overriding priority. This does not mean to say
that matched filtering is absent. Dim habitats still have a structure, present a motion
flow field and contain mates and prey. Rather, the matched filtering found in animals
from dim habitats is not as obviously manifested in regional variations of eye structure.
As we shall soon see, matched filtering also occurs in the neural circuitry which
processes visual information, and this applies equally well to animals from dim
habitats as to those from bright ones.
As light levels fall, so too does the reliability of vision. There are several reasons
for this, not the least of which is the random nature of photon arrivals on the retina.
A photoreceptor that absorbs N photons during one integration time will experience
an uncertainty, or shot noise, of --iN photons associated with this sample, that is, N ±
--iN photons (Land 1981; Warrant and Mcintyre 1993). This noise reduces the
reliability of intensity measurements, and thereby discrimination of the contrasts