Visual processing

Carotenoids absorb visible light (Section 13 21) and dissipate its energy as heat thereby protecting the organism from any potentially harmful effects associated with sunlight induced photochemistry They are also indirectly involved m the chemistry of vision owing to the fact that p carotene is the biosynthetic precursor of vitamin A also known as retinol a key substance m the visual process... 

The structural chemistry of the visual process beginning with 3 carotene was de scribed in the boxed essay entitled Imines in Biological Chemistry in Chapter 17... 

Mueller-Limmroth, W., D. Berges et al. (1958). The effects of helenien and vitamin A on the primary visual processes. I. An electroretinographic study on the problem of the sites of helenien and vitamin A actions (German). Z. Biol. 110(5-6) 457 -75. 

Describe the essential photochemical features of the visual process. 

The a-, (3- and y-carotenes, which are found in most plants, are vitamin A provitamins and are converted to vitamin A alcohol (all- ran,v-retinol), which is usually called vitamin Aj (Figure 12.8) by oxidative mid-point cleavage. Retinol and its fatty acid esters are the main forms in which vitamin A is stored in animals and humans, and its oxidation product, 1 1-c/s-retinal (vitamin A, aldehyde), is required for the visual process. 

Opioids are sedating and cause a reduction in processing speed in clinical populations (e.g.. Digit Symbol Substitution Test) (Wood et al. 1998). However, a study in healthy subjects did not confirm these effects on digit substitution (Walker and Zacny 1998). Improvements are seen in choice reaction time after morphine administration (Hanks et al. 1995). Deficits have been reported in early-stage visual processing (O Neill et al. 1995 Hanks et al. 1995). By comparison, morphine s cognitive effects are lesser than those of lorazepam, but milder than hydromorphone (Rapp et al. 1996 Hanks et al. 1995). 

A vitamin A derivative, ll-cw-retinal, is central to the visual process in humans. It is derived from all-trani-retinol by converting the terminal hydroxyl group to an... 

The molecular event that triggers the visual process is the light-induced transformation of the 11-c form of retinal in rhodopsin to the aJi-trans form. 

Retinai a 20-carbon molecule critically involved in the visual process. 

Fig. 7 Transcriptome effects of T3 administration on the developing zebrafish embryo, (a) Heatmap of microarray results from 652 probes showing significant differences between T3-treated and control samples. Fold induction values (in log scale) are represented by different shades of color (scale shown in the far-right bar.), (b) Distribution of overrepresented (red), underrepresented (blue), and unchanged/undetected (ivory) transcripts in T3-treated embryos belonging to the functional categories ossification, visual processes, and oxygen transport. The significance of the observed variations (p values) was calculated by the hypergeometric distribution with the Bonferroni correction...

Figure 7 shows the effect of ectopic administration of T3 to the developing zebrafish embryo. At nontoxic concentration (50 nM), only a moderate fraction (less than 5%) of the zebrafish transcriptome shows significant changes. Ossification, visual processes, and the hematopoietic system were the physiological processes most affected by the treatment, in a pattern consistent with an advancement of the development in these particular functions (Fig. 7b). Genes involved in these three processes are known targets for TDCs during metamorphosis in amphibians, teleost fishes, and lampreys [54—60], and constitute molecular counterparts of different endpoints used to test for TDC in amphibians [56, 58]. Therefore, they are excellent candidates for markers of thyroid disruptors in zebrafish at early developmental stages. Chapter 14 provides a more in-deep description of the developmental effects of thyroid disruption in zebrafish embryos.

Signaling substances and many proteins are involved in visual processes. Initially, a light-induced cis-trans isomerization of the... 

Vitamin A (retinol) is the parent substance of the retinoids, which include retinal and retinoic acid. The retinoids also can be synthesized by cleavage from the provitamin (3-car-otene. Retinoids are found in meat-containing diets, whereas p-carotene occurs in fruits and vegetables (particularly carrots). Retinal is involved in visual processes as the pigment of... 

Metabohc derivatives of the fat-soluble vitamin retinol, which was first described in 1913. These metabolites play a central role in the visual process where the primary photoevent is attended by conversion of ll-cA-retinal to all-rrans-retinal. The oxidized metabolite retinoic acid is an agonist in cell growth and proliferation. 

Examination of brain circuitry mediating the visual processing of fearful faces has been extended to children and adolescents. Amygdala activation in youth during the viewing of fearful faces compared to control stimuli has been demonstrated in two studies (Baird et ah, 1999 Thomas et ah, 2001). 

In the visual process a light signal is received by the photoreceptors, rhodopsin, of the rod cells which are then converted to the activated state, R. The activated rhodopsin passes the signal on to the cognate G-protein, transducin, which in turn activates the next effector molecule, a cGMP phosphodiesterase. The phophodiesterase hydro-... 

As far as altered states of consciousness are concerned, it is often the formal aspects that are emphasized to have visions, internal stimuli must become predominant to have intense visions, they must become very predominant, and when visions become intense, they are more likely to become exotic, numinous, or preternatural, and thus to suggest otherworldliness. And so on. To understand the visions of altered states, we had better understand how the form of visual processing is altered at the level of the brain. We have at least an odds on chance of doing this, whereas we have no chance at all of knowing why the visionary brain sees exotic flowers. 

Retinene is another biologically important molecule which is known to form triplets in good yield.460 Again, the exact role of its triplet state in the visual process has not been elucidated, although the necessity for cis-trans isomerizations during the sight cycle and the known tendency of unsaturated carbonyl compounds to form triplets suggests that at least part of the process may involve triplet retinene. 

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