VISUAL PATHWAY
9.5.1 Accommodation
We are to make a comparative discussion on visualization with respect to certain media such as water. It is observed that it is difficult for terrestrial vertebrates to view under the water (Cronin, 1986).
This is because the major refractive material in terrestrial vertebrates’ eyes is the clear cornea. When the eyes are submerged in water (fresh, brackish, or marine), the aqueous and vitreous humor fluids lose their refractoriness to the water with similar specific gravity (Cronin, 1986). When the cornea does the necessary adjustments to produce direct rays presumed to be cast on the retina, the lens, serving a function similar to the smooth adjustment knob of a microscope, finishes the arrangement; this is termed accommodation. In situations where the lens fails to make adjustments, rays to be passed behind to produce blurred images behind the retina, a condition known as hyperopia (Sanes & Zipursky, 2010).9.5.2 Accommodation in Amphibious Eyes
Aquatic animals or amphibious animals that are amphibious in nature have lenses that are so powerful that they do not require corneal aids for adjustment to produce sharp images underwater. This is achieved as a result of two distinct lens modifications in these groups of animals (Gislen et al., 2003). One is that the lens is spherical, contrary to the lozenge-shaped terrestrial animal lens forms. The second is the intrinsic gradient of the lens refractive index that decreases with distance (Land & Nilsson, 2012). When in free space (air), aquatic animals’ eyes fail to make the necessary adjustment due to the combination of the spherical lens and the curved cornea combination. Rays are refracted by both the cornea and lens, making images focus in front of the retina. As a result, aquatic animals tend to have myopia in terrestrial environments (Martin, 1998).
The question is, are all aquatic and amphibious animals myopic in terrestrial environments?Some animals do show amphibious views, they maintain ultra-visual accommodation in aquatic environments and still have the ability to produce images with reasonable quality in terrestrial environments. This form of visual modification is seen in some marine mammals and aquatic birds (Martin, 1998). This is achieved by specialized modifications comprising the reduction of corneal refractive power in air through an increment in its radius, thus rendering it a bit myopic. Another way is by closing the iris to make the pupil shrink. The ultimate reduction in pupillary aperture brings about an increased depth of the field of view. These adjustments are believed to improve both hyperopic and myopic images, permitting the eye to retain some optimal visual functions in the strange environment (Gislen et al., 2003).
9.5.3 Avian Mechanisms of Accommodation
It is believed that avian species have more advanced accommodation techniques compared to other terrestrial vertebrates (Glasser & Howland, 1996). The process in avian is a holistic one and brings about good results. This comprises changes in the overall curvature and position of the lens, and involvement of the cornea or the iris (Glasser & Howland, 1996; Levy & Sivak, 1980). Though evolutionary terrestrial, birds still possess additional accommodation abilities seen in aquatic and amphibious animals. They have domed cornea (As in mammals) with a flattened lens, manipulated by the ciliary process and somewhat hemispherical retina. The striking difference is the presence of a ring of scleral ossicles (Os-sclera) surrounding the region of the iris and sometimes a highly muscular iris, incorporating independent central and peripheral sets of muscle. There exist variations from one avian species to another. For instance, albatrosses, penguins, and other seabirds, the lens is nearly spherical and the cornea flatter to reduce its refractive contribution in air (Martin, 1998).
This makes refractive adjustment to be solely carried out by the lens. An amphibious ocular architecture is also seen in some avian species, this relies on the strengthening ring of scleral ossicles (Os-sclera), the extrinsic muscles of the iris, and the comparatively powerful ciliary body that squeeze the lens and shove it forward through the pupillary aperture when the eye enters water. The resulting bulge in the lens’s anterior surface grants it increased refractive power, compensating for the loss of corneal refraction. This visual mechanism is seen in diving ducks (Levy & Sivak, 1980), cormorants (Katzir & Howland, 2003), other diving birds and to some extent penguins (Not avians though) (Howland & Sivak, 1984). A study conducted about the amphibious vision in australasian gannets (Morus serrator) (Machovsky-Capuska et al., 2012) using infrared video retinoscopy revealed that corneas of these birds are not unusually flattened, and eyes are properly focused in both air and water. It is revealed that it takes these species less than 100 milliseconds to achieve accommodation at air-water interphase (Schaeffel et al., 1987). In essence, vision underwater requires rapid adaptation to a dimmer, bluer light environment than in air (Litwiler & Cronin, 2001).9.5.4 Accommodation in Rhesus
Monkeys (Macaca mulatto)
Accommodation in most reptiles and birds (explained earlier) is aided by the iris which pinches the lens to produce a more highly refracting anterior surface in a condition known as iridogenic lenticonus (Duke-Elder, 1958; Walls, 1944). When the ciliary muscle is in a relaxed position, the iris is pulled posteriorly over the anterior surface of the lens. This possibly exerts enough posterior pressure on the malleable lens to account for the deeper anterior chamber and the thinner lens seen in the normal eye. During accommodation, there is possibility that contraction of the ciliary body could have an effect on corneal curvature (Bito et al., 1987).
As the ciliary muscle contracts and its apex moves forward and inward, the ciliary ring narrows and the zonular fibers slacken, allowing the lens to become more spherical (Goodchild, Ghosh, & Martin, 1996). As the lens thickens and the anterior chamber shallows, the iris is stretched forward over the anterior lens surface. There is a practical loss of accommodation with ageing in the rhesus monkeys either in vivo or in vitro (Neider et al., 1990)9.5.5 Accommodation in Racoon
The raccoon (Procyon lotor) is a small carnivore which eats in the upright position, using hand- and finger-like front paws and digits to wash, hold and examine its food at close range (Rohen et al., 1989). It has a prominent ciliary smooth muscle and zonular apparatus. The ciliary muscle and zonular apparatus exhibit a shift from longitudinal to reticular or circular orientation of some ciliary muscle bundles, anterior movement of the muscle as a whole, and more oblique crossing of the zonular fiber bundles in the zonular plexus (Kruse et al., 1990). During accommodation, the lens thickens slightly as it advances anteriorly, while the apparent cornea to retina distance increases slightly (Walls, 1944). These minimal accommodative modifications account for the greatest accommodative capability of racoons over any non-primate terrestrial mammal so far studied and the relative persistence of accommodative prowess even with advance age (Rohen et al., 1989).
9.5.6 Accommodation in Cats
Cats possess a relatively large eyeball with a broadly curved cornea, a disproportionately large pupil and a globular shaped lens situated rather deep posteriorly within the eye (Goodchild et al., 1996; Vakkur, Bishop, & Kozak, 1963).
This offers cats great light-gathering capacity, reaching an estimated object intensity, retinal illumination 5.2 times greater in the cat than in the human (Goodchild et al., 1996). Tapetum lucidum located in the choroid just behind the retina acts as a diffuse reflecting surface (Coles, 1971; Weale, 1953), further enhances the efficiency of the cat’s eye for dim-light vision by reflecting light back through the visual receptors.
There is an attributed decrease in image sharpness, owing to the job of the tapetum lucidum to ‘magnify’ images (Walls, 1944). The pupil size is an important asset for image processing in cats. It assumes an elliptical shape with increasing light intensities, and at modest photopic levels the pupil is reduced in shape to just a narrow slit. The rostral superior colliculus (SC) plays an important part in the control of accommodation (Centrally). While the lateral suprasylvian (LS) area, the cortical area surrounding the middle suprasylvian sulcus (MSS) of the cat, is related to the control of lens accommodation (Bando et al., 1988; Wakabayashi et al., 2017). The LS area receives visual inputs. Some neurons in the mentioned area respond to changes in ocular disparity and target size and to motion in depth, which are important visual cues for accommodation (Ohtsuka & Sato, 2000). Some LS neurons also exhibited burst discharges preceding the onset of spontaneous accommodation (Bando et al., 1984). Accommodation- related signals from the cortex project mainly to the rostral SC (Wakabayashi et al., 2017). Changes in lens power could have only minimal influence on the refractive state of the eat’s eye, because of the relatively large separation between the cornea and lens, as such, accommodation is more effectively produced by the forward and backward movement of the principal planes of the lens (Goodchild et al., 1996). The dynamic range of pupillary response in the cat, with over a 6 log-unit luminance range, and pupil diameter varying from 16 mm to less than 0.5 mm (Kappauf, 1943). The pupil size influences very significantly the quality of the image formed by the cat, with worst images produced using pupil fully dilated. The more sharp images are produced with an increasingly smaller pupil diameters (Abramov et al., 1972). optimal performance is achieved when the pupil reduces 2-3 mm in diameter, which gives an optical resolution limit exceeding 20 cycles/deg (Bonds, 1974). This level of accommodative quality is seen when kitten’s eye develops to adult optical standards over the 11∕2 months or so after birth (Bonds & Freeman, 1978).9.5.6.1 Cat Retina Architecture
We are emphasizing on distribution of rods and cones within the central and peripheral retina of the cat. It is believed that rods are more in number compared to cones in cats’ eyes even within the area centralis, the region attributed for highest cone density in most animals (Sterling, 1983). Steinberg, Reid, and Lacy (1973) used constructed detailed maps of receptor density over the entire cat retina from photomicrographs of sections through the inner segment of an excised retina. The outcome is that cone density is highest within the area centralis but that even here rods are more than 10 times as numerous, confirming the observations of Sterling (1983). Maximum rod density occurs within an anular region 10-15 degrees from the center of the area centralis and gradually decreases with increasing retinal eccentricity. By comparing these receptor densities with the ganglion cell counts of Stone (1972), Steinberg et al. (1973) estimated the degree of receptor/ganglion cell convergence for different portions of the retina. Within the area centralis, the ratio of cones to ganglion cells is at a minimum, suggesting that this would be the region of maximum photopic acuity. On the other hand, the minimum rod/ganglion cell ratio occurs 5 degrees into the periphery, where optimum scotopic acuity would likely occur. These ratio estimates may vary, depending upon the portion sampled, because of the horizontally elongated region of high ganglion cell density in the cat retina (Hughes, 1975; Stone, 1972).
The accounted convergence of receptors onto ganglion cells is believed to be mediated by an organised network of neural connections within the outer and inner plexiform layers, and the details of these connections in the cat, studied using the electron microscope. Cones in the cat retina synapse onto two types of bipolar cells, an invaginating type which connects with from 4 to 9 individual cones and a flat type which contacts 8-14 cones (Boycott & Kolb, 1973), there is no evidence for a bipolar, which contacts just a single cone, such as the midget bipolar cells seen in primate retina. These cone bipolars in the cat, in turn, synapse directly onto ganglion cells
(Kolb & Famiglietti, 1974). Rods, on the other hand, contact another type of bipolar, termed rod bipolars (Boycott & Kolb, 1973), and these do not synapse directly onto ganglion cells, but, instead, onto amacrine cells which then contact the ganglion cells (Kolb & Famiglietti, 1974). Also, the receptors are interconnected by horizontal cells, the dendritic terminals of which synapse only with cones and the axon terminal only with rods (Kolb, 1974). It is seen that rods can have direct input to cones, perhaps via electrically passive gap junctions (Nelson, 1977). Though the significance of this synaptic arrangement between receptors is not clear, but it could be that the horizontal cells are somehow involved in the control of retinal sensitivity at different background illuminations (Enroth-Cugell & Lennie, 1975; Enroth-Cugell & Shapley, 1973). Ganglion cells in the cat retina are believed not to have homogeneous morphology (Brown & Major, 1966; Honrubia & Elliot, 1970; Leicester & Stone, 1967). There are morphologically three forms of ganglion cells distinguished based on axon thickness and dendritic field size. These are α, β and γ, they are seen at all retinal eccentricities. Distribution wise, within the central region, β cells are predominant, and for each type, the spread of dendritic fields increases systematically with retinal eccentricity, but at any particular retinal locus the spread of a cells is usually greater than that of β cells. With respect to size, the y cells display large dendritic fields but possess small axons (Boycott & Wassle, 1974).
We are to begin with photochemical and neural events which transpire prior to the ganglion cells. It is reported that even in the dark, the ganglion cells in the cat retina generate frequent, irregularly timed spikes which some believe are of receptor origin (Barlow & Levick, 1969). Spontaneous activity of the ganglion cells could be modulated by presentation of visual targets within a limited region of the retina, this infers that in all light-adapted ganglion cells this region (the receptive field), could be subdivided into two zones. That is, an approximately circular central region surrounded entirely by an annular area. Stimulation of one zone increases the cell’s firing rate (ON response), while stimulation of the other causes a decrease in firing, followed by a burst of activity upon removal of the stimulus (OFF response). Some cells consisted of ON-centers, while others of OFF-centers. In each region, cells are always arranged in an antagonistic fashion (Kuffler, 1953). The receptive field of cat retina is composed of two distinct response mechanisms (center and surround) arranged concentrically, each with an approximately Gaussian sensitivity distribution and the response of the cell is determined by the subtractive interaction of these two mechanisms (Cleland & Enroth- Cugell, 1968; Enroth-Cugell & Robson, 1966; Rodieck, 1965; Rodieck & Stone, 1965). The majority of receptive fields for the cat ganglion cells follow this antagonistic center-surround arrangement and tend to display no selectivity for contour orientation or direction of movement. The receptive field centers of ganglion cells, (whether ON or OFF type), vary in diameter from 0.5 to 8-degree visual angle (Wiesel, 1960) and are smallest within the area centralis. The antagonistic surround portion of the field is several times larger than the field center, and there is evidence that the surround extends through the center (Cleland & Enroth-Cugell, 1968; Hammond, 1973; Rodieck & Stone,
1965). The inhibitory effect of this surround region on the center disappeared with dark adaptation, leaving only an ON or an OFF response throughout the field (Barlow, Fitzhugh, & Kuffler, 1957). In exceptional cases, some ganglion cells exhibit weak excitatory effects to visual stimulation many degrees removed from the conventional receptive field center (McIlwain, 1964).
Ganglion cells could also be classified along another dimension, such as based on different response properties, independent of the center-surround organization discussed earlier. One of such classifications is the finding that one class of ganglion cells, termed X cells, display approximately linear summation over the entire receptive field, while a second class, termed Y cells, which is clearly nonlinear. Evidence for linear summation consisted of finding a “null position” within the receptive field, such that repetitive phase reversal of a bipartite field (e.g., one complete light/dark cycle of grating) which does not produce a net change in background activity (Enroth-Cugell & Robson,
1966). Cells that fall under type X typically exhibit the following characteristics: field centers more of the ON type (Not all); field centers tend to be smaller than Y type; maintained activity usually higher than Y type; respond only to slow or moderate target speeds; show no periphery effect; predominate within the area centralis; surround portion of field smaller diameter; small stationary targets evoke sustained response; resolve higher spatial frequencies than Y type and possess slow axonal conduction (Cleland et al.,
1973). On the other hand, type Y cells: respond to very rapidly moved targets; show clear effects on the periphery; are insensitive to image defocusing; respond transiently to onset and offset of stimulus; possess fast conducting axons (Ikeda & Wright, 1972). There are proposals that these two classes of retinal ganglion cells constitute separate neuronal channels which subserve contrasting roles in visual sensation (Cleland et al., 1971). There are assumptions on possible involvement of X cells in analyzing the spatial features of stimuli, and possible abilities of Y cells to process information concerning movement and time varying stimulation (Ikeda & Wright, 1972). Other than the early discussed center-surround cell categories, Stone & Fukada, 1974; Stone & Hoffmann, 1972 described a third category tagged ‘W’. These cells have field centers which are same in size to Y cell centers (at a given retinal eccentricity), and that the axons of the W cells conduct more slowly than either type X or type Y axons (Stone & Fukada, 1974). These W cells appear to comprise the bulk of the ganglion cells in the ‘tagged’ visual streak of the cat retina (Wilson, Rowe, & Stone, 1976). The fact that this third category of cells project heavily to the superior colliculus suggests that they may play some role in visuomotor behavior (Stone & Fukada,
1974). Some similarities exist between the physiological properties of the X, Y, and W ganglion cells as well as with those for α,β and γ.
9.5.6.3 Projection
Here, we are to discuss what happens to the axons after exiting the retina. Axons of the retinal ganglion cells, upon exiting the retina through the optic disc, partially cross at the optic chiasm. In all species, fibers from the nasal retina of each eye cross to the contralateral side of the brain (Crossing over) while fibers originating from the temporal retinae project ipsilaterally. An exception to this is seen in cats, where limited region around the area centralis which gives rise to bilateral projections showing a combination of crossed and uncrossed fibers (Stone & Fukada, 1974). With respect to size of the receptive field centers, this composite strip of crossed-uncrossed axons is wider horizontally for Y cells than for X cells (Kirk, Levick, Cleland, & Wassle, 1976). The complete/partial crossing-over is followed by the optic fibers projection to several subcortical sites in the brain (Garey & Powell, 1968; Laties & Sprague, 1966). It is proven that most of the of ganglion cell axons innervate cells of the lateral geniculate nucleus (LGN), which primarily consists of two anatomically distinct portions, the more prominent dorsal lateral geniculate and the smaller ventral lateral geniculate. The majority of cells in the dorsal LGN receive input from either X or Y cells (Wilson, Rowe, & Stone, 1976), while the ventral LGN innervation appears to be largely from W cells (Spear et al., 1975). Owing to their diffuse receptive-field properties and their projections to pretectal regions, it is strongly suggested that cells of the ventral LGN are involved in the pupillary response to light, vestibular and oculomotor reflexes. Cells in the dorsal LGN, on the other hand, display more well- defined receptive field properties and project to the visual cortex. The remaining ganglion cell axons project to structures in the midbrain, including the most prominent one, superior colliculus. There are strong suggestions that these midbrain structures are involved in visuomotor coordination while the geniculocortical pathways play the major role in spatial resolution and pattern recognition (Autrum et al., 1973).
9.6