Ординатура / Офтальмология / Английские материалы / Small Animal Ophthalmology Secrets_Riis_2002

8 Color Vision in Animals

which stimuli that differ in color are used to excite the system, and the response is analyzed in terms of the spectral nature of the stimulus. Constant response criteria are preferred, but constant stimulus intensity series are also useful. If done at the level of the photoreceptor with either intracellular or suction electrodes, the data should be similar to that obtained using MSP, which seems to be the case. If done at higher levels the responses are much more complex, but information about color capabilities is still possible. Recently Jacobs and his collaborators have used a flicker method to isolate and number the spectral sensitivity channels in a number of vertebrates including companion animals. The paradigm involves alternating a colored stimulus with a broadband white one and asking the question, "what intensity colored stimulus is required to match the response to the white stimulus?" This is done for a series of colored stimuli from the red to the near ultraviolet (UV). The series is then repeated, only this time in the presence of a background light chosen to desensitize red-sensitive cells. If such cells are present, there will be a change in the spectral sensitivity curve, and by subtracting one from the other the spectral sensitivity of the red channel can be calculated. In this way, Jacobs has found evidence that the majority of mammals are dichromats-they have two spectral sensitivity channels. Trichromacy, as seen in humans and several other primates, is relatively rare.

Most recently, molecular techniques have been used to check for color vision. Starting with the excellent studies of Nathans (1986) on the human color vision system, steady progress has been made in examining many other species. The basic technique involves determining the opsins (the protein moiety of the visual pigment) expressed in the retina by isolating mRNAs, making cDNAs, using probes to pick out likely opsin candidates, and obtaining their nucleotide sequences. These can be translated into amino acid sequences. Once the amino acids present at specific "tuning" sites are known, the spectral absorption of the visual pigment that uses the opsin can be calculated. It is also possible to insert a constructed opsin gene into an expression system and then reconstitute the visual pigment by adding vitamin A aldehyde to the expressed opsin. The absorption spectrum of the reconstituted visual pigment can be measured directly. This technique has been most successfully applied to primates and rodents.

Of course, the best test for color vision is to demonstrate it behaviorally. That is, can an animal discriminate among objects based only on spectral characteristics as opposed to brightness differences? In a typical test, animals would be trained to always go to a red card verses a white card using classical conditioning techniques. Once the animal was operating at a reasonable confidence level, it would be asked to discriminate the red card from cards of other colors and brightnesses. The whole process would then be repeated after training to another color. In this way a color space could be constructed indicating which colors could be discriminated and how different they had to be to be discriminated. Although this sounds straightforward, it is fraught with difficulties. Training the animal is difficult, and there are problems controlling for other cues such as texture, position, or even odor. Only a few cases of behavioral testing have been unequivocal in demonstrating color vision.

S.What about color vision anomalies?

It is well known that humans can suffer from alterations to their color vision system ren-

dering them incapable of making color discriminations in particular spectral regions - that is, they are color blind. It is now known for certain that color blindness results when one of the normally present visual pigments is absent due to a mutation that eliminates one of the opsins. For example, a person lacking the opsin for the red photoreceptor cannot distinguish between reds and greens and is called a protanope. Loss of the green or blue opsins leads to discrimination loss in other spectral regions. It is to be expected that such mutations would be present in nonprimates. However, behavioral color blindness has not been confirmed in any other species.

Of greater interest are mutations that alter the opsin at the sites that spectrally tune the visual pigment. In these cases the normal number of opsins are present, but they are not in the "normal" spectral position. This leads to a distortion ofthe color space meaning that these individuals would see the world differently from their "normal" conspecifics. In humans such amino acid differences

lead to conditions such as protanomaly and deuteranomaly. There is behavioral evidence that this occurs in animals, particularly primates.

6.What do other animals see?

This is a common question and one that captures the imagination of animal owners and prac-

titioners. If asked within the context of television, it can be said with certainty that our color television sets do not "work" for dogs and cats. Color television is designed specifically around the human system and appears as color nonsense to other animals with different visual pigments and spectral sensitivities. This does not mean that animals cannot recognize things on a television screen, only that they cannot use color as a recognition cue. If one knew enough about the color vision system of an animal, a true color television system could be made that would provide the full richness of color we expect from our system. Only for the honeybee has such a system been constructed, but once the canine genome is available, some enterprising person will probably fill the void and offer a pet color television system complete with synthesized color movies for the pet's enjoyment. Cat and horse television cannot be far behind!

7.What about color vision in other vertebrates?

Remembering the minimum requirements for color vision, it is not surprising that nonmam-

malian vertebrates also have color vision systems. Among the best studied are fish, reptiles, and birds. However, whereas three color channels appears to be the maximum number for mammals, it is not uncommon for other animals to have four, five, or even six color channels. A good example from fish is the goldfish, which has four visual pigments in four separate classes of cones including double cones. There are three cones sensitive in the human visible range as well as an ultraviolet sensitive cone. In fact, most diurnal nonmammals have an ultraviolet cone leading to the potential for quadrachromatic vision at higher order color vision systems. The goldfish is also interesting because instead of using vitamin Al as the chromophore for making visual pigment, it uses vitamin A2 . The substitution of vitamin A2 for vitamin Al shifts the absorption spectrum for visual pigments toward the red end of the spectrum. In general, freshwater fish use vitamin A2 for making visual pigments, and marine species use vitamin AI. For example, a switch to vitamin A2 in humans would push the long-wave pigment from a Amax of 565 nm to approximately 625 nm. As a consequence, the spectral range over which the goldfish color vision system operates is greater than for humans and other mammals, extending from the ultraviolet to the infrared. The same is true for many tropical coral reef fish.

However, it is in reptiles and birds where vertebrate color vision really expands. Diurnal reptiles not only have up to five visual pigments, some using vitamin A2 as the visual pigment chrornophore, but they also can have a variety of highly colored oil droplets in their cones that act as filters that change their spectral sensitivity (Fig. 1). These droplets act as cut-off filters that can improve the spectral resolution in certain spectral regions. The combinations of pigments and droplets can lead to as many as six different chromatic channels before chromophore type even comes into play!

The presence of so many color channels in other vertebrates begs the question of what we are missing by having only trichromatic color vision that excludes the ultraviolet and infrared parts of the spectrum. Spectroradiometric measurements of many natural objects show reflectance differences extending into the UV and the infrared. For example, many fruits have substantial UV reflectance that not only could improve contrast, but also could add to the already available color information in the visible spectrum to increase recognition cues for frugivores. It is also known that many animal parts, such as the feathers in birds and the dewlaps in lizards, show variable UV reflectance. At the other end of the spectrum, chlorophyll-containing plant parts are highly reflective in the infrared with the spectral character ofthis reflectance depending on such factors as chlorophyll concentration, age, prior light exposure (plants can tan!), or disease. We can only guess at how rich in color the world would appear if we could look at it through the eyes of other animals.

Figure 1. Oil droplets from the eye of a lizard (Analis cristatellus). There are four classes of oil droplet in almost all fully diurnal lizards: a yellow, a green, and two colorless droplets, one of which absorbs in the ultraviolet. These are placed immediately in front of the outer segments and act as filters (800 X).

BIBLIOGRAPHY

1.Boynton RM: Human Color Vision. New York, Holt, Rinehart & Winston, 1979.

2.Cronly-Dillon JR, Gregory RL: Evolution of the Eye and Visual System. Boca Raton, FL, CRC Press, 1991.

3.Jacobs GH: Comparative Color Vision. New York, Academic Press, 1981.

4.Jacobs GH: Photopigments and seeing: Lessons from natural experiments. Invest Ophthalmol Vis Sci 39:2205-2216, 1998.

5.Loew ER: Determinants of visual pigment spectral location and photoreceptor cell spectral sensitivity. In Djamgoz MBA, Archer SN, Vallerga S (eds): The Outer Retina. London, Chapman & Hall, 1995, pp 57-78.

6.Lythgoe IN: The Ecology of Vision. Oxford, Clarendon Press, 1979.

7.Mollon JD, Sharpe LT: Colour Vision: Physiology and Psychophysics. London, Academic Press, London, 1983.

8.Nathans J. Thomas D, Hogness DS: Molecular genetics of human color vision: The genes encoding blue, green, and red pigments. Science 232: 193-203, 1986.

9.Smythe RH: Vision in the Animal World. London, MacMillan, London, 1975.

Figure 1. Late-venous phase fluorescein angiogram of a normal canine retina.

evaluating orbital and sinus disease. In recent years, MRI and CT scan have become more readily available and are considered routine in most specialty practices.

While currently only available in limited facilities, the newer imaging modalities are offering insight into disease pathogenesis and allowing accurate evaluation of extent of damage. Ultrasound biomicroscopy and confocal microscopy are applicable for anterior segment disease, and the nerve fiber analyzer, confocal scanning laser ophthalmoscope, and optical coherence tomographer examine the posterior segment and are most applicable in the understanding of glaucomatous retinal and optic nerve damage. Color Doppler ultrasonography allows evaluation of ophthalmic and orbital blood vessels to assess blood velocity and vascular resistance. This has applications in diseases with altered hemodynamics such as glaucoma and diabetes mellitus.

4.What is the goal of performing ophthalmic imaging?

The goal of the clinician should be to assess intraocular and periocular structures not visible

by routine biomicroscopy or indirect ophthalmoscopy, to evaluate the extent of intraand extraocular involvement in a disease process, and to examine structures that relate to the visual process such as optic nerve, optic chiasm, optic tracts, and visual cortex that cannot be adequately evaluated by routine examination. In addition, imaging techniques may help obtain a diagnosis, formulate a treatment plan, or give a more accurate prognosis of outcome.

S.What are the indications for skull radiographs in ophthalmology?

Radiographs of the skull are indicated to evaluate orbital trauma, to evaluate nasal, sinus, and

oral disease with periorbital or intraorbital extension, and to look for bony destruction (Fig. 2). The variation in the interand intraspecies (especially canine) anatomy of the skull combined with the superimposition of structures often makes interpretation difficult. Use of lateral, dorsoventral, ventrodorsal, oblique, skyline, and open-mouth views is often required to fully evaluate the orbital and periorbital anatomy.

Figure 2. A, Plain radiograph of a 10- year-old canine with a swelling ventral to the right eye. Lucency surrounding rostral and caudal roots of right maxillary PM4, compatible with apical abscessation is noted. B, Close-up of abnormal right rostral and caudal roots of maxillary PM4. C, Close-up of normal left premolars. (Courtesy of J. Reichle.)

6. When do I choose contrast as compared to plain radiographs?

Plain radiographs are indicated to evaluate the bony orbit, nasal and sinus cavities, and skull (Fig. 3). They are obtained following head trauma, with exophthalmos and orbital disease, to evaluate nasal and sinus cavity disease resulting in or affected secondarily to orbital disease and to

Figure3. Plain lateralradiograph of a dolicocephalic dog. A circularareaof bony sclerosis is visible anterior to the orbit above the molars.

evaluate for radioopaque foreign bodies such as pellets. General anesthesia is advised, and dorsal, lateral, and oblique views should be obtained. Evaluate the radiographs for fractures, bony lysis/proliferation, and increased soft tissue density.

Following plain radiographs, contrast radiographs can be used to evaluate the nasolacrimal apparatus, performing a dacryocystorhinography, and the vascular anatomy, using orbital venograms and arteriograms. Dacryocystorhinography is performed by cannulating the superior punctum and slowly injecting 1-2 ml of a radiopaque contrast agent into the nasolacrimal apparatus while occluding the inferior punctum (Fig. 4). The contralateral side is used as a normal control if possible. In general, use of other contrast radiographic techniques to further evaluate the orbit has been replaced by CT and MRI. In addition, CT and MRI are better suited to ascertain the extent of involvement of nasal, sinus, and orbital neoplasia prior to surgical or radiation therapy.

Figure 4. Contrast material has been used to performa dacryocystorhinogram of the dog in Figure 3. The radiopaque contrastagent is pooling in the area of previously noted bony sclerosis, identifying a cysticdilation of the nasolacrimal duct.

7, What are the indications for ophthalmic ultrasonography?

Ultrasonography is an inexpensive, noninvasive, safe procedure that allows evaluation of the intraocular and retrobulbar tissue without sedation or general anesthesia. Ocular ultrasonography is indicated whenever opacity of the transmitting media of the eye (cornea, aqueous humor, lens, vitreous humor) prevents a complete ophthalmic examination. Ultrasound aids in evaluation of intraocular mass lesions, differentiation between solid and cystic structures, evaluating the extent of damage following ocular trauma, examination for a foreign body, axial length determination,

and examination of retrobulbar orbital structures. The most common clinical indications for ocular ultrasound are to evaluate for the presence of a retinal detachment in eyes with a cataract, to assess posterior segment damage and examine for the presence of a foreign body following trauma, and to evaluate intraocular structures in eyes with severe corneal opacification. In addition, orbital evaluation can be performed in instances of exophthalmos or orbital trauma. New ultrasound technologies include three-dimensional imaging, tissue characterization, and very high frequency (50 MHz) ultrasound biomicroscopy. For a further discussion of ocular ultrasonography, please see Chapter 4.

8.What is ultrasound biomicroscopy?

Ultrasound biomicroscopy is similar to a conventional B-scan ultrasound, but uses an oper-

ating frequency of 40-100 MHz to provide a high-resolution image of the cornea and anterior segment. The axial resolution is 2-5 mm, but the image resolution is similar to a histopathologic section. The cornea, sclera, limbus, iris, anterior chamber, iridocorneal angle, lenticular zonules, and ciliary processes are all imaged. This technique is most useful in evaluation of the iridocorneal angle in glaucoma and assessment of anterior uveal neoplasms, and it may prove useful in the determination of the depth of corneal involvement of squamous cell carcinoma or other infiltrative corneal diseases.

9.What is the difference between CT and MRI?

CT, or CAT, stands for computer axial tomography. In general, CT scans provide high de-

tail cross-sectional images with excellent contrast resolution compared with conventional radiographs. In addition the information can be manipulated to view the anatomy in any desired plain of section. Like radiographs, CT scans rely on a radiation source to transmit x-rays, but do so by rotating the source to create cross-sectional, two-dimensional scans. The data is then digitized and analyzed to create a gray-scale reconstructed image. The lighter the appearance of the area imaged (e.g., bone, muscle) the greater the absorption of x-rays; conversely, the darker the image (e.g., fat, liquids), the lower the x-ray absorption. CT scans are excellent for evaluation of the orbit, nasal, and sinus cavities and are good for the intraorbital portion of the optic nerve. CT is not as good as MRI for soft tissue (globe, optic nerve) or intracranial examination. CT is most often used to evaluate the extent of neoplastic involvement of the orbit and adjacent structures prior to surgical and radiation therapy (Figs. 5 and 6).

Figure 5. A CT scan of an ll-year- old canine with progressive exophthalmos and strabismus of the left eye of I month duration. A soft tissue mass with bone production and destruction is evident in the left orbit/nasal cavity. Diagnosis: chondrosarcoma. (Courtesy of J. Reichle and Colorado State University.)

Figure 6. A CT scan of a 13-year-old canine. A soft tissue mass arising from right nasal cavity with bone destruction and invasion into orbit and is evident. Diagnosis: probable nasal adenocarcinoma (Courtesy of J. Reichle.)

MRI like CT allows information to be reformatted and viewed in any plain of section. Contrast and spatial resolution are excellent and permit differentiation of soft tissue and fluid. MRI images are obtained using a strong, static magnetic field that causes hydrogen protons to align with the magnetic field. Radiofrequency pulses are then applied, changing the axis of rotation of the protons, which then return to their previous alignment when the pulse is discontinued. This generates a signal that is used to create an image. The image depends on proton-density and pro- ton-relaxation dynamics representative of the chemical and physical tissue properties. With MRI, proton-density, Tl-weighted, and T2-weighted images are used with proton-density best for discriminating between gray and white matter, Tl-weighted images better for anatomic detail and contrast enhancement, and T2-weighted for identifying masses, edema, and other fluids. T2 images of the globe are easily identified by the white, high-density vitreous signal. In addition, contrast enhancement using gadolinium-diethylenetriaminepentaacetic acid (Gd-DTPA) may help to highlight areas of blood-brain barrier breakdown.

Appearance on MRI a/Various Tissues

10.When do I choose CT versus MRI?

CT is preferred for evaluation of orbital trauma, foreign body detection, and evaluation of os-

seous, cartilaginous, or calcified structures. CT is most often used to evaluate the extent of neoplastic involvement of the orbit and adjacent structures prior to surgical and radiation therapy. MRI is better suited for assessment of soft tissues (Fig. 7). It is indicated to assess extraocular extension of an intraocular neoplasm; evaluate orbital soft tissue, optic nerve, and optic chiasm; and examine for intracranial disease (neoplasia, inflammatory). Use of fat-suppression and contrastenhancement techniques makes MRI a superior technique over CT for intraocular and optic nerve images. MRI is generally considered to be contraindicated when a metallic foreign body is suspected. An additional difference is that CT is generally significantly less expensive than MRI and more widely available.

Figure 7. MRI of a 5-year-old canine, blind in the left eye with peripapillary retinal separation and retinal hemorrhage. Enlargement and increased intensity of the left optic nerve on the dorsal (coronal) view is seen. (Courtesy of J. Reichle and DC Davis.)

BIBLIOGRAPHY

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2.Byrne SF, Green RL: Ultrasound of the Eye and Orbit. St. Louis, Mosby, 1992.

3.Koplin RS, Gersten M, Hodes B: Real Time Ophthalmic Ultrasonography and Biometry: A Handbook of Clinical Diagnosis. Thorofare, NJ, Slack, 1985.

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dog. Vet Radiol Ultrasound 35: 102-108, 1994.

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(eds): Equine Diagnostic Ultrasonography. Baltimore, Williams & Wilkins, 1998, pp 637-643.

7.Williams J, Wilkie DA: An alternate modality for the examination of opaque eyes: Ophthalmic ultrasonography. Comp Cont Educ 18:667-677, 1996.

8.Williamson TH, Harris A: Color Doppler ultrasound imaging of the eye and orbit. Surv Ophthalmol 40:255-267,1996.