The mechanics of vision

They are called respectively, the blue (/ ), green (7), and red (p) receptors. The different sensitivities to wavelength provide the fonndation for colour vision. Light of different wavelengths impinging on the retina induce responses in the / , 7, and p cones of magnitudes dictated by the spectral sensitivities. 

There are six specific areas, VI to V6, within the visual cortex responsible for the perception of different aspects of appearance. VI responds to orientation, real and imaginary boundaries, and has some wavelength response. It also detects overlapping features in the scene. Cells in V2 are sensitive to colour, motion, orientation and stereoscopic featnres. V3 is sensitive to form and depth. V4 is sensitive to colour and is the site responsible for the maintenance of colour constancy. V5 analyses motion and V6 is responsible for analysing the absolute position of an object in space (Lee 

Information from the retina is processed in two broad systems one is concerned with identification of the object, the other with relative spatial position (Tovee 1996). 

Moving objects can be tracked using pursuit eye movement. Considerable neuron intercoimections are required to follow objects that are continuously displaced from one point to another. Two other types of eye movement occur. Continual small movements are needed to destabilise the image and prevent the retina adapting to a continuous stimulus, and larger short movements permit the eye to scan the visual environment. Three distances appear to be relevant within this depth perception. These concern the personal space occupied by our body, peripersonal space within reach, and extrapersonal space beyond (Tovee 1996). 

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Dienes and polyenes have been a subject of great interest due to their important role in biology, materials science and organic synthesis. The mechanism of vision involves cis-trans photoisomerization of 11 -civ-retinal, an aldehyde formed from a linear polyene. Moreover, this kind of molecule exhibits high linear and non-linear electrical and optical properties. Short polyenes are also involved in pericyclic reactions, one of the most important classes of organic reactions. 

The analysis of CD and absorption data has been used since 1974, mainly by Nakanishi and his coworkers, to study the structure of the rhodopsins, pigments involved in the mechanism of vision, and of their chromophores50. 

The study of the mechanism of vision in vertebrates 23>24) has progressed to the point where the first consequence of photon absorption has been described as an activation of the isomerization of the 11 -cis retinal chromophore of rhodopsin to all-trans. That triggers a complex sequence of reactions leading to the mysterious inside of the brain. Brrr, I had better get back — it looks dark in there. But the brain can generate sensations of light. Maybe, one day, we will be able to see enough to understand, but we ll go back just the same to a safer subject. 

I spent a productive period. .. climbing down on my retina, walking around and thinking about certain problems relating to the mechanism of vision."... 

Since the mid-1950s biochemistry has painstakingly elucidated the workings of life at the molecular level. Darwin was ignorant of the reason for variation within a species (one of the requirements of his theory), but biochemistry has identified the molecular basis for it. Nineteenth-century science could not even guess at the mechanism of vision, immunity, or movement, but modern biochemistry has identified the molecules that allow those and other functions. 

Any one cone cell contains only one type of opsin and is sensitive to only one color of light. Color blindness results from loss or mutation of one or the other of the cone opsins. The combination of 11-c/s-retinaldehyde with cone opsin is sometimes called iodopsin, with rhodopsin meaning more specifically the holo-protein of rod opsin. Most studies of the mechanisms of vision shown in Figure 2.5 have been performed using rods by extrapolation, it is assumed that the same mechanisms are involved in cone vision. 

In the mechanism of vision, m-retinal (82) is isomerized under the influence of light into /ratf.s-retinal (83). Give the EiZ stereochemistry of the double bond that undergoes isomerization in (a) 82, before it is stimulated by light, and in (b) 83, after stimulation and isomerization. 

Scientific studies into the mechanism of vision and human colour perception began in the seventeenth century with the recognition that the eye s lens must somehow project an image of the object viewed onto the back of the eye. Newton s classic experiments on the refraction of light led him to conclude that... 

Attention is focused on carotenoids and polyenes, which are known to be chemically very unstable as isolated entities but to acquire great stability when they are suitably surrounded by a protein cage and become the active elements in the mechanism of vision and photosynthesis. The CL dependence of the in situ Raman spectra of the carotenoids as naturally occurring pigments in bird feathers was studied by Veronelli et al. [65]. Later attention was focused on the bacterial membrane protein bacteriorhodopsin (bR). a small protein (—26,000 daltons) whose potential application in optical and electro-optical devices has been explored by many authors. The justification of such interest lies in the fact that bR contains all-/rfl/ .v retinal, which acts as a lightabsorbing center and makes bR a naturally reversible photochromic system. All-optical switching can be achieved by proper illumination of bR with yellow or blue light. 

Nonadiabatic transitions definitely play crucial roles for molecules to manifest various functions. The theory of nonadiabatic transition is very helpful not only to comprehend the mechanisms, but also to design new molecular functions and enhance their efficiencies. The photochromism that is expected to be applicable to molecular switches and memories is a good example [130]. Photoisomerization of retinal is well known to be a basic mechanism of vision. In these processes, the NT type of nonadiabatic transitions play essential roles. There must be many other similar examples. Utilization of the complete reflection phenomenon can also be another candidate, as discussed in Section V.C. In this section, the following two examples are cosidered (1) photochromism due to photoisomerization between cyclohexadiene (CHD) and hexatriene (HT) as an example of photoswitching molecular functions, and (2) hydrogen transmission through a five-membered carbon ring. 

Thus far in this book we have discussed one- or two-component photochemical systems which because of their relative simplicity lend themselves quite well to laboratory study. Consequently the mechanisms of many of the photoreactions we have discussed have been elucidated in exquisite detail. As we turn our attention in this chapter to some photochemical aspects of living systems, we shall find much more complex situations in which mechanistic details are just now beginning to be obtained. In some systems, such as those which exhibit phototaxis or phototropism, so little is known that our treatment must as a consequence be limited to only a brief discussion of these phenomena. The topics we will consider here are photosynthesis, vision, phototaxis and phototropism, and damage and subsequent repair of damage by light. Due to space limitations, a discussion of the very fascinating area of bioluminescence must be omitted. 

The experimental values for. /V = 9 —> 15, compare extremely well with calculated spectra [74], The method is sufficiently accurate to elucidate the mechanism of colour vision in terms of the calculated spectrum of the related chromophore, retinal [76]. 

Shand J, Foster RG 1999 The extraretinal photoreceptors of non-mammaKan vertebrates. In Archer SN, Djamgoz MBA, Loew ER, Partridge JC, Vallerga S (eds) Adaptive mechanisms in the ecology of vision. Kluwer Academic Publishers, Dordrecht, Netherlands, p 197—222 Smyth RD, Saranak J, Foster KW 1988 Algal visual systems and their photoreceptor pigments. Prog Phycol Res 6 255-286... 

This Chapter begins with a comprehensive review of the quantum-mechanical properties of organic molecules and how this affects their photon excitation. A series of detailed definitions and concepts are presented that are not normally found in biological treatises. These concepts are vital to an understanding of the mechanisms involved in the photochemistry of vision. 

The global field of photochemistry has been defined in Coxon Halton as the study of chemical reactions initiated by light. 13 Although succinct, it may be too restrictive for the purposes of vision. The field might be better described by replacing the expression chemical reactions by chemical processes or chemical interactions. The revised definition would allow mechanisms, such as sensitization of one species by another without any change in species to occur. This... 

Areas of particular interest to the vision area were in defining the rules which determined the absorption effectiveness of a chromophore, the elucidation of the "liquid crystalline" state of matter, the electronic characteristics of useful chromophore and the mechanisms of interfacing between chromophores and their associated "signal" receivers. 

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