Abstract
Altogether, more than ninety species of animals, mostly arthropods but also some cephalopods have shown to possess the ability to detect linearly polarized light and determine its plane of polarization. (Waterman TH, 1965). Animals sustain their life by gaining the energy necessary from their surrounding environment, requiring the continuous acquisition of sensory information. The most important sensory instrument of many animals is the visual system, with the photoreceptors as the main element of the visual information.
Visual pigments are molecules in photoreceptors that initiate the process of vision. They are inherently dichroic and absorb light according to its axis of polarization. Many animals have taken advantage of this property and have built receptor systems capable of analyzing the polarization of incoming light. Polarized light is commonly produced by scatter or reflection and can be incorporated into a high level of visual perception like colour vision allowing segments of a visual area to have different regions of polarization. Animal polarization sensitivity can be associated with behavioural tasks like orientation or navigation. It can also be used for tasks such as contrast enhancement, camouflage breaking, object recognition, signal detection and discrimination. Polarized vision is very useful in terrestrial, shallow-water and deep water environments, and has consequently evolved in vertebrates and invertebrates, from marine species to terrestrial animals.
INTRODUCTION
The eyes are the most integral organs of the body. They enable us to see stable images of the world. The eye consists of refractive elements which include the cornea and the lens, with an aperture-like pupil, created by a contractile diaphragm; the iris which has control of the amount of light entering the eye. At the back of the eye contains the retina which receives light and sends signals along the visual pathway to the brain. These structures are separated by the aqueous and vitreous and contained within the transparent cornea and opaque sclera. (shown in Fig. 1.1.)
THE RETINA
Basic Retinal Anatomy
The retina is a thin layer of tissue lining the inner portion of the eye, lying between the vitreous and the choroid (Fatt & Wiessman 1992). Retinal thickness varies from approximately 0.1mm- 0.5mm, being thickest around the optic disc and thinning at the ora serrata, the equator, and the fovea (Hogan et. al. 1971). The retina is the image plane of the eye's optical system and is responsible for converting information from the image of the external environment into neural impulses that are transmitted to the brain for decoding and analysis. (Forrester et. al. 2002).
The Retinal Layers
The retina is composed of ten layers (figure 1.3), the innermost nine of which make up the neural retina and the outermost of which is the non-neural retinal pigment epithelium. The retinal layers in order from the outside in include, (10) inner limiting membrane; (9) retinal nerve fiber layer; (8) ganglion cell layer; (7) inner plexiform layer; (6) inner nuclear layer; (5) outer plexiform layer; (4) outer nuclear layer; (3) outer limiting membrane; (2) photoreceptor layer; and (1) retinal pigment epithelium (RPE) (Fatt & Wiessman 1992).
Photoreceptor layer
In particular, The Photoreceptor layer (figure 1.3) is made up of the outer segments of the rod and cone photoreceptors. The photoreceptor outer segments contain visual pigments responsible for the absorption of photons of light and initiation of the neuro-electrical impulse. The photopigment contained in rods is more active in scotopic conditions whereas those contained in cones are more active in photopic conditions. Cone photoreceptors are also colour sensitive with three types of cone cells being sensitive to long, medium, and short wavelengths of light respectively. The combination of rods and cones allows for the detection of virtually any visual signal (Forrester et. al. 2002).
While the human eye may detect any visual signal, it can only cope with two aspects of light, brightness and colour, for many animals polarization of light is a further source of visual information.
Polarized Light Vision
Polarized light vision is a common visual specialization found in both non-human vertebrates and invertebrates ( Waterman TH, 1981). Polarized light exists in the visual environments of many animals as a result of scattering from the atmosphere or reflection and transmission at different surfaces such as water. To detect different polarization states of light, the individual light-sensitive cells in an animal's eye must be able to exhibit a differential polarization response (Coughlin, D. J., and C. W. Hawryshyn. 1995). Several polarization detection mechanisms have been discovered among a variety of terrestrial and aquatic invertebrates (Waterman TH, 1981)).
Polarization cues have an intrinsic spatial component, and therefore the spatial layout of photoreceptive and neural structures will largely influence the way information about polarization vision is accepted and encoded by the nervous system.
History of Polarized light ( How it was discovered)
Polarization of light was discovered in the natural environment at the beginning of the nineteenth century. In 1809, Etienne Louis Malus, a French engineer used a double refracting birefringent calcite crystal whilst looking at the light of the setting sun reflected from the windows of the Luxemburg Palace in Paris. This is where he discovered the polarization of light by surface reflection. Soon after, a discovery from an astronomer, Dominique Arago described that scattered light from the sunlight sky reaching the earth is partially polarized. Both Malus (1809) and Arago (1811) had ascertained that light particles had poles and when passing through double refracting substances, these polar particles became aligned and ordered. Malus called the orderly aligned particles, polarized light (Wehner R, 1983).
Properties of polarization of light
In principle, Light has two almost mutually exclusive properties, those of particles and waves. Consider light as an assemblage of electromagnetic waves. As light propagates, its electric field as well as the associated magnetic field vibrates. the vibration direction of the electric field can be described by a vector called the e-vector. Since light waves are one kind of transverse wave, the e-vector angles are always perpendicular to the line along which light travels. If all the light waves in a beam of light have their e-vectors vibrating in the same plane, it is called polarized light. The electric field vector of a plane wave may be arbitrarily divided into two perpendicular components labeled x and y (with z indicating the direction of travel When projecting the traces of the e-vectors of a beam of polarized light onto a plane that is perpendicular to the direction of the propagation, the projections form a line, an ellipse, or a circle. From the pattern of the projected e-vector forms, they are called respectively linearly, elliptically, or circularly polarized light.( Feynman et al., 1963; and Shurcliff and Ballard, 1964).
Classification of Polarization
Light in the form of a plane wave in space is said to be linearly polarized. Light is a transverse electromagnetic wave, but natural light is generally unpolarized, all planes of propagation being equally probable. If light is composed of two plane waves of equal amplitude by differing in phase by 90°, then the light is said to be circularly polarized. If two plane waves of differing amplitude are related in phase by 90°, or if the relative phase is other than 90° then the light is said to be elliptically polarized.
Linear Polarization
A plane electromagnetic wave is said to be linearly polarized. The transverse electric field wave is accompanied by a magnetic field wave (shown below Fig.
Circular Polarization
Circularly polarized light consists of two perpendicular electromagnetic plane waves of
equal amplitude and 90° difference in phase. (Fig ) If light is composed of two plane waves of equal amplitude but differing in phase by 90°, then the light is said to be circularly polarized. When viewing a light beam traveling away from an observer, in both elliptically and circularly polarized light, the imaginary wave can rotate either in the clockwise or th counterclockwise direction, which are termed right-handed or left-handed circular polarization, respectively. (Feynman et al., 1963; and Shurcliff and Ballard, 1964).
Circularly polarized light may also be produced by passing linearly polarized light through a quarter-wave plate at an angle of 45° to the optic axis of the plate.
Elliptical Polarization
Elliptically polarized light consists of two perpendicular waves of unequal amplitude which differ in phase by 90°. (Fig.
Invertebrates polarized vision-
A compound eye consists of thousands of individual photoreceptor units. Compared with simple eyes, compound eyes possess the ability to detect polarized light (Volkel et al, 2003)
Most arthropods (insects and crustaceans) have compound eyes which are composed of different optical units known as ommatidia. Each ommatidium is a complete eye containing one or more lenses that capture and focus the incoming light. The optical waveguide is built from the contributions of several individual photoreceptors. Each photoreceptor or retinula cell has part of its cell membrane folded into extremely slender microvilli which are stacked to form a thin rod like structure called a rhabdomere. The rhabdomere of all retinula call cells within the ommatidiun collectively form the rhabdom which can either be "open" as in flies and many bugs or "fused" as in most cases of other compound eyes. Visual pigment rhodopsin molecules are densely packed within the microvillar membranes of the waveguide which are responsible for the generation of an electric response to light. (Hardie, 2006). The rhabdomere is analogous in function to the retinal rods of vertebrate eyes.
The chromophores of visual pigment ( retinal, hydroxyretinal, or dehydroretinal) absorb light with the electric vector aligned with the excitable double bond. In photoreceptor membranes of arthropods, the double bond is aligned with the membrane surface and parallel to the microvillar long axis. (Goldsmith and Wehner, 1977). This results in each microvillus having high sensitivity to light of e vector orientation parallel to the long axis of the microvillus and low sensitivity to light of orthogonal e vector orientation(Labhart and Meyer, 1999). This means, the membrane has a high dichroic ratio. Photoreceptors located within the same ommatidium receiving linearly polarized light through the same dioptric apparatus, differ in their preferred e vector direction.( Shaw, 1967).
In terrestrial insects, the photoreceptor cells have non overlapping microvilli (Labhart and Meyer,1999). Aquatic invertebrates microvilli are arranged mostly non overlapping (Schwind, 1955), or in stacks with alternating sections of orthogonal microvilli in cephalopods and crustaceans. (Waterman, 1981). Microvillar photoreceptors are inherently polarization sensitive. The tubular arrangement of the photoreceptor membrane already results in dichroism, even if the pigment molecules are randomly oriented(Fig 16.1c) within the tangent planes of the microvilli membrane. (Laughlin et al, 1975). This microvillar structure absorbs twice as much linearly polarized light if e vector is parallel to the microvilli.(Fig 16.1d,e)
In ommatidia with open rhabdoms (flies and bugs), increasing rhabdom length results in self screening and therefore decreasing polarization sensitivity. Due to waveguide effects, increasing rhabdom diameter has a similar effect. Therefore; increasing the sensitivity of a single rhabdomere decreases polarization sensitivity and vice versa. (Snyder, 1973), a compromise or an alternative solution could be a fused rhabdom
A measurement of polarization sensitivity of a photoreceptor is the sensitivity to light with the e vector parallel to microvillus orientation divided by the sensitivity to light with an orthogonal e vector orientation. (Fig 6 a). If all microvilli are straight and parallel over the entire length of the rhabdom, A receptor will have a high polarization sensitivity. The polarization sensitivity of a photoreceptor also depends on the length and width of the rhabdom, on the concentration of the visual pigment rhodopsin in the membrane, and on the presence, sensitivity, and arrangement of other receptors. (Snyder 1973; Nilsson et al ,1987)
The sensitivity of the eye to light, and the reliability and speed of the electrical signal, depend on the optical light gathering capacity of the ommatidium and the properties of individual ion channels in the photoreceptor membrane.
Whether an eye is capable of detecting polarized light depends on the structure and intrinsic dichroism of the photoreceptive waveguides and their visual pigment molecules. Thus the light stimulus that species see the best with its particular intensity, speed, size, colour and polarization is directly related to morphological and physiological adaptations that are a direct evolutionary consequence of the animals ecological needs.
Rhabdomeres with orthogonal polarization sensitivity
Most ommatidia have two sets of rhabdomeres, each set having microvillar orientations orthogonal to the other within a "fused" rhabdom. In decapod crustaceans, there are two sets of receptors that contribute to the rhabdom in alternating layers. (Fig 6b) (Eguchi and Waterman, 1966),( check dissertation document) In this way, the more distal layers function as filters for the more proximal layers enhancing polarization sensitivity. (Snyder, 1973). An example in flies, the distal two tiered central receptors R7 works as a filter for the proximal receptor R8, enhancing its polarized sensitivity considerably (Hardie, 1980). Also, rhabdomeres of orthogonal polarization with preferred angle can work as lateral filters for each other which can have an effect on receptors of both microvillar organisations without loss of polarization sensitivity. Comparing the two sets of rhabdomeres with orthogonal microvillar orientation makes polarized vision independent of absolute light intensity.(Wehner and Labbhart, 2006)
Spectral Sensitivity and Polarization
For polarization vision to be independent of spectral composition of light, photoreceptors for polarization vision should share the same spectral sensitivity. Most crustaceans use long wavelength receptors for polarization, but stomatopods use short wavelength, and in insects, all wavelengths spectral types are used (Labhart and Meyer, 1999). Receptors used for colour vision should ideally be insensitive to polarization (Wehner and Bernard, 1993) which is achieved by self screening in long receptors, by curved microvillar orientation( Nilson et al, 1987). In insects, ie, flies and bees, the rhabdoms twist during ommatidial development eliminate polarization sensitivity. In cricket receptors, microvillar orientation changes abruptly along the rhabdom length. (Labhart and Meyer, 1999). If receptors with diff spectral and polarization sensitivities are compared, colour vision will depend on polarization and vice versa.
True Polarization vision
Optical elements in midband rows 1-4 are used for color vision, those in rows 5 and 6 are used for detection of light polarization. Human eyes are poor at sensing light polarization, but very good at sensing its color and brightness; however, sensation of light polarization is found in many animals. It is found in some insects, crustaceans, fish, birds, and particularly in cephalopods, a class of mollusks that includes squids, octopuses and cuttlefish. A key requirement for polarization sensitivity is the excitation of two or more classes of visual pigments that have different alignments in the eye.
For true polarization vision where the degree and angle of polarization can be disentangled, three analysers are necessary (Warrant, 1999). This is only achieved by ommatidia from two angled rows in the eyes of stomatopods (Marshall et al, 1991). Each ommatidium in midband rows 5 and 6 have receptors sensitive to three different polarization angles. The distal short wavelength receptors in row 5 are sensitive to light polarized perpendicular to the midband, whereas those in row 6 are sensitive to light that is polarized parallel. The long wavelength receptors are similar to those in other crustaceans (Eguchi and Waterman, 1966) which consist of alternating layers of microvilli with orthogonal orientations (45 and 135 degrees) to those of the distal rhabdom respectively. Three analysers in each ommatidium or four in both rows will allow true polarization vision. (Marshall et al, 1999).
Non-human vertebrate polarized vision.-
In addition to intensity and colour, the retinas of many invertebrates are capable of light detection based on its linear polarization (Wehner, 1983). In vertebrates, however, except for anchovies (Fineran & Nicol, 1978), axial dichroism is absent rendering vertebrate outer segments insensitive to the polarization of axially incident light. However, there is evidence for polarization sensitivity in a few species of fish ie (goldfish, rainbow trout and sunfish).
In vertebrates, the underlying biophysical mechanisms of polarization sensitivity remain unknown. While several studies have proposed different experimental examples (Cameron, D. A., and E. N. Pugh. 1991), there has been no conclusive evidence analysing the mechanism of polarization sensitivity in vertebrate photoreceptors.
The long axis of the photoreceptors of vertebrates is usually oriented toward the pupil of the eye.( Laties et al, 1968). This orientation gives the maximum amount of light to the photoreceptors outer segment because off axis light is not absorbed by the inner segment. However, this orientation also minimises the receptors use of dichroism for polarization sensitivity. It is believed that the mechanism of polarization discrimination in vertebrate photoreceptors is not due to axial differential absorption in photoreceptor outer segments (Land, M. F. 1991). This is understood from experiments performed by (Cone, 1972). He discovered that in multiple rods of a frog, the visual pigment undergoes rotational diffusion within the outer segment membranes which implies that all axially incident polarized light will be absorbed identically in all directions. Nonetheless, it is known that only particular classes of cones, and not rods, provide the polarization sensitivity in the visual system (Flamarique, I. N., and C. W. Hawryshyn. 1998). Although the absorption of individual pigment molecule depends on the e vector direction of axially incident linearly polarized light, the randomly oriented molecules absorb the same amount of light independently of the e vector direction. The outer segments of vertebrate photoreceptors have membrane disks that are axially illuminated where the surface is perpendicular to the longitudinal axis of the receptor. (Laties and Enoch, 1971). Fig(16.3) The receptor is then sensitive to polarization if the incident light is transversally scattered or reflected onto the outer segment (fig 16.3, 31.3,28.3)( Waterman, 1975) or if the outer segment has axially oriented disks. (fig 28.2).
The technique of microspectrophotometry has proved to be the method for investigating how polarized light is absorbed by individual photoreceptor cells (Bowmaker, J. K. 1984). The results of this show a significant difference between the way axially orientated rods and cones of goldfish, a species with polarization vision by their ultraviolet, mid- and long-wavelength sensitive pigments (Hawryshyn, C. W., and W. N. McFarland. 1987), absorb linearly polarized light. The reported results illustrate that the mid-wavelength sensitive (MWS) part of the double cone photoreceptor, which is known to play a role in polarization vision (Hawryshyn, C. W., and W. N. McFarland. 1987), displays axial dichroism.
These double cones of vertebrates are composed of two independently developed photoreceptors which mediate polarization sensitivity. light polarized parallel to the axis joining the centers of the two halves of a double cone can potentially stimulate the receptors more strongly than light polarized in the direction perpendicular to both that axis and the normal axis of light propagation down the length of the double cone.
The retinas of anchovies have two unique photoreceptor types "bifid" and "long" cones (Fineran & Nicol, 1976). The outer segments of these cells contain multiple layers of membranes, lamellae oriented longitudinally (axially). This orientation is distinct from that in all other vertebrate rods and cones, where the lamellae are stacked transversely with their planes perpendicular to the incident light path. Although the common arrangement provides optimal absorption for normally incident light rays, it is also insensitive to the rays' direction of vibration/ polarization (Fineran & Nicol, 1976, 1978).
Human sensitivity to polarized light-
Humans are unable to perceive polarization of light. Even though this does not appear to affect our visual performance, polarization vision is a sensory augmentation that can significantly enhance both automated image understanding and even possibly human performance.(Wolff, 1997). This additional visual channel of reality is mostly invisible to us without the aid of instruments.
There are 3 entoptic phenomenons of the human visual system that enable us to detect polarized light. These include Haidinger brushes, Boehm brushes and Shurcliff brushes.
The entoptic phenomenon of Haidinger brushes was discovered by geologist and mineralogist William Karl von Haidinger in 1844. In 1846 Haidinger was studying minerals under polarized light and tried to discriminate a pattern in the refracted light. He then perceived a faint yellowish stain when he looked directly at the light without the crystal. The stain rotated together with the polarizer revealing that humans were able to perceive linear polarization of light. That stain is now known as the Haidinger's brush (Rothmayer,M, 2007). Because human photoreceptors are insensitive to e vector of paraxially incident light, polarization phenomenon in humans seem to come from dichroic properties of the ocular media and foveal regions of the retina. According to Haidingers phenomenon, if the observer gazes at an unstructured area for a few seconds that is emmiting polarized white light with horizontal e vector( ie. The sky), then glances at a white field of unpolarized light with vertical e vector, a yellowish horizontal bar or bowtie shape is visible with a fainter bluish area between the yellow brushes. Haidinger brushes are relatively small and subtend 3 to 5 degrees of vision (Dodt T, Kuba M, 1994). Contrast is best when the degree of linear polarization is 100%.
Many experiments have been performed by Lieberman et al (1974), Govardovski (1975), as to the exact mechanism of Haidinger brushes but could not explain the yellow and blue colours of the brushes. However, Weale (1975) explained that Haidinger brushes are due to light scattering in the retina, with photoreceptors detecting light passing laterally through. This scattering light occurs in the layers of Henle fibres (Hememger RP, 1982). Henle fibres extend radially from the cone photoreceptor outer segment and synapse in the outer plexiform layer. Lutein pigment molecules are attached to the radial Henle fibre framework giving intrinsic birefringence (Bone and Landrum, 1984). and demonstrate linear(dichroism. the structure of the henle fibre can account for haidinger brushes, Hememger, 1982) dichroism and explain haidinger brushes (Bone and Landrum, 1984).. These dichroic lutein molecules are aligned perpendicular to the radially oriented Henle fibres and are tangentially oriented along concentric circles centered at the fovea. Fig 32.3. Certain conditions, such as age related macular degeneration reduces the polarization effect of the macula and thus also leads to changes in the brush pattern ( Rothmayer M, 2007)
Boehms Brushes- The second entoptic phenomenon was discovered by Boehm (1940), these brushes depend on differential scattering in the retina (Vos and Bowman, 1940). This differential scattering is strongest near the axis of the beam and decreases with increasing distance from the axis. Broehm brushes are entoptic images seen when rotating e vector linearly polarized light is fixated at 15-20 degrees parafoveally.
Shurcliff Brushes- The final entoptic phenomenon discovered by Shurcliff in 1954 added to haidingers phenomenon that Haidingers brush can also detect circular polarized light to produce similar brushes like haidingers and determine the sense of rotation. Shurcliff noted that linear dichroism in the macula resulting in birefringent properties in the refracting cornea and lens.
Sources of polarized light vision
Three scenarios are described in which animals make use of polarized light: the underwater world, the water surface and the terrestrial environment (Wehner, 2001)
The primary source of linearly polarized light is sunlight, scattered by air molecules ie gases and aerosol in the atmosphere and light reflected from water surfaces. Underwater light is also polarized from scattering of water molecules and particulate matter (Waterman, 1975). Although water has a higher density than does air, and supports more particles in a given volume, light scattered in either environment will become polarized (Waterman, 1954; Wehner, 2001; Horvath and Varju, 2003). Reflection from from shiny surfaces (eg, leaves, wet surfaces, animal skin, scales, or cuticle) also produces strong polarization. This polarized light created by scattering and reflection provides animals with spatial information (Waterman 1981).
Solar radiation indirectly reaches the photoreceptors after being transmitted from objects. When light is produced, the e vector of an individual wave can vibrate randomly in any direction. The amount of linearly polarized light reaching the photoreceptors depends on overall intensity, degree of polarization, and e vector angle. The greater the angle, the greater the amount of polarized light reaches the eye. Rays of light reaching the eye parallel to direction of the incoming radiation will not be polarized whereas those reaching the eye at right angles to the direction will be 100% polarized; however, due to atmospheric turbulence and other secondary factors reduce maximum polarization to around 75-80%. Consequently, the sun moves across the sky during the day continuously changing the pattern of polarization and the degree of polarization will depend on the viewing direction reaching up to 75% in the upper part of the sky when the sun is close to the horizon and the air is clear and dry (Coulson, 1988). The polarization of the blue sky is described by the semi-empirical Rayleigh model. (fig.
Circular polarized light as mentioned before is formed by two mutually perpendicular, linearly polarized components of equal amplitude but differing in phase by 90 degrees. The vector sum of the two components has fixed amplitude, but rotates by 360 degrees for each wavelength of propagation. Circular polarized light is relatively uncommon in nature (Konnen,1985)
The primary feature of sunlight polarization of the sky can be explained by Raleigh scattering in the atmosphere, others include reflection explained from scattering at the Brewster angle, dichroism and birefringence. (Coulson, 1988).
Polarization of light in cloudy skies comes from the cloud itself. The white light illuminating the cloud becomes partially linearly polarized after scattering on particles of the cloud such as water droplets.
Light from the evening sky originates from the moonlight, stars, scattered light from earths atmosphere and light pollution from environment lighting (Wolstencroft and Brandt,1974). Moonlight has almost the same spectral components compared to sunlight but leans more towards the red wavelength (Kopal, 1969). The moonlight is partially linearly polarized but a full moon however is unpolarized like the sun (Pellicori, 1971). Even at night, the light from the sun reflected by the surface of the moon is scattered and linearly polarised within the earths atmosphere( Gal et al. 2001)
At sunset or sunrise, unpolarized incident light will become partially horizontally linearly polarized when reflected from a flat water surface . This is the main optical cue for habitat finding by insects living in, on, or near water (Horvath, 1995). The degree of polariztion depends on the structure of the surface ie. The ratio of diffuse to specular reflection ; however, the maximum amount of linear polarization of reflected skylight is located in Brewster zone where light is reflected with an angle of 53 degrees to the vertical known as Brewster angle. Fig 12.2.
Reflectivity patterns of a flat water surface are also seen for clear and overcast skies. Under clear sky skylight conditions, the pattern of horizontal polarization gets distorted by the reflection of the more complicated skylight pattern however the distortions occur away from the Brewster angle and do not cause major problems detecting water surface polarization. Insects in fact, when searching for bodies of water, even get attracted to horizontal polarized reflections from a wet asphalt road or an oily surface (Schwind, 1991). This entire behavioural sequence is elicited only in the ultraviolet range of the spectrum (Schwind, 1983) It is assumed that the reflection polarization of skylight at the water surface is governed by the Fresnel theory (Horvath, 1995).
Polarization in water is diff than in air, specifically at depths greater than a few meters. Due to refraction at the air/water interface, illumination from the sun or moon is confined to within 46° of overhead. The resulting polarization field, while variable, is predictably near horizontal most of the time (Waterman, 1954, Wehner, 2001, Cronin and Shashar, 2001), and the degree of polarization is almost always lower than in air (Novales Flamarique and Hawryshyn, 1997, Cronin and Shashar, 2001) Underwater polarization in fresh water ponds and in oceans is usually much less than 50% but it is always higher in the direction of the open water than in the vicinity of the shore ( Cronin and Shashar, 2001)
Aquatic animals are surrounded by complex polarized patterns. Underwater, there are two polarization patterns, inside and outside the Snell window. Snells window explains how an aquatic animal can observe everything above surface through a cone of light of width around 96 degrees (Martin Edge and Turner, 1999). The underwater polarization pattern forms a sphere that surrounds the animal and distribution of polarization will depend on the position of the sun and the calmness of the water surface. In addition, the maximum linear polarization occurs in a bandlike form along the sphere perpendicular to the refracted sunlight (Hawryshyn, 1992). Fig 14.1
Polarization is greatest closest to the water surface (Ivanoff and Waterman, 1958) and decreases within 40m of depth till a critical depth, where in reaches a maximum value horizontally. This critical depth ranges from 40m to 200m (Ivanoff and Waterman, 1958).
As light travels deeper into the water, any polarization patterns entering from the sky or that are dependent on the azimuth of the sun gradually diminish. A general advantage that aquatic animals receive from underwater polarization is the enhancement of contrast or reducing the obscuring effects of haze (Shashar and Cronin, 1996) which is important because the scattering of light within the hydrosphere decreases the contrast between an object and its background.
Although most of the natural light sources on earth produce outputs that are unpolarized, natural light can be converted into polarized light by scattering or reflection in both terrestrial and aquatic environments (Horvath and Varju, 2003; Wehner, 2001).
Chapter 3
Polarized light sensitivity is widespread in the animal kingdom. It exists in many species including crustaceans, arachnids, cephalopods, insects and vertebrates. Animals which utilize polarized light signals to communicate have been found in both terrestrial and aquatic environments (Cronin et al., 2003; Sweeney et al., 2003).
Polarized vision in terrestrial species. Arthropods- Insects (bees, flies , locusts)
Polarization vision has been developed in a variety of animal groups for use in navigation, contrast enhancement, communication , and intraspecific signalling(Horvath and Varju, 2003; Wehner and Labhart, 2006).
Terrestrial animals with polarized-light vision function ie.arthropods (insects), the sky displays a pattern useful for navigation, but unpredictable pattern of polarized light reflection can conceal true colours of an object (Wehner and Bernard, 1993, Kelber, 1999, Kelber et al., 2001). In addition, photoreceptors in some animals that are sensitive to polarized light tend to structurally destroy polarization sensitivity (Marshall et al., 1991,Wehner and Bernard, 1993), whereas other animals may recognise and assess objects of interest using their polarizational cues (Kelber, 1999, Kelber et al., 2001).
Insects can perceive the e vector pattern of polarized light in the sky and use it as a compass for spatial orientation or course control. This is used for problems such as navigation and homing. To accomplish these tasks, insects utilise polarization filters across the sky and translates the complex e-vector patterns into simple temporal modulations of summed receptor outputs (Wehner R, 1989).
Insect polarization vision relies on specialized photoreceptor cells in a small dorsal rim area of the compound eye called the POL area (Labhart and Meyer, 1999). It is adapted for reducing spatial resolution in the dorsal rim by increasing the acceptance angle of the receptor (Labhart and Meyer, 1999). Stages in the brain involved in polarized light signalling include specific areas in the lamina, medulla and lobula of the optic lobe, in the midbrain, the anterior optic tubercle, the lateral accessory lobe, and the central complex. Integration of polarized-light signals with information on solar position start in the optic lobe. In the central complex, polarization-opponent interneurons form a network of interconnected neurons. The organization of the central complex, its connections to thoracic motor centers, and its involvement in the spatial control of locomotion suggest that it serves as a spatial organizer within the insect brain, including the functions of compass orientation and path integration. Time compensation in compass orientation is possibly achieved through a neural pathway from the internal circadian clock in the accessory medulla to the protocerebral bridge of the central complex. (Homberg, 2004). Studies by Labhart 2008 show evidence using intracellular electrophysiology and cell marking.
Rhabdomeric twist and misalignment and their functional significance
Twists of rhabdomeres along their longitudinal axis have been found outside the dorsal rim area of the compound eyes of insects. The twist was proven to be an in vivo structure (Wehner and Bernard, 1993). Smola and Tscharntke, 1979 suggested that rhabdomere twist may be the improvement of absorption of unpolarized light and reduction of self screening. In nontwisted, long rhabdomeres with aligned dipoles, the total absorption of unpolarized light is less than in corresponding rhabdomeres with randomly oriented dipoles, due to self screening. However, when rhabdomeres having microvilli with aligned dipoles are twisted, self screening is prevented ( Wehner et al, 1975)
