about a small space in our visual field, our brain fills this in with the same scene that we see in the adjacent areas. Thus, if it is a missing spot in the sky, the brain fills in the appropriate blue and continues the cloud pattern that is immediately adjacent. This means that if there is a single spot in an otherwise uniform field, and if that spot is only imaged onto the blind spot, it is invisible to us. You can test this directly with the diagram below. Close your left eye and focus on the cross with your.right eye. Move this page closer and further from your eye until the mark on the right disappears. It should occur as you move this page slightly closer to your eye than reading distance.

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A great advantage of the optic disk and retinal arrangement is to give vertebrates higher acuity and speedier vision than would be otherwise possible. The reason is that the RPE, which provides essential biochemical regeneration necessary for photoreception, allows the photoreceptors to respond much more rapidly to changes in light than possible if they had to maintain their full biochemical function independently. And, the RPE provides that essential “black screen” behind our photoreceptors to retain our high visual acuity. Were the retina to be reversed, with photoreceptors in the inner retina, our acuity would likely be lessened and the speed of response reduced, and the profusion of blood vessels to provide nutrition for the added photoreceptor function would further limit this. Indeed, for invertebrates, for which the photoreceptors are first cells in the optical path, accommodations are made (such as having a compound eye, as seen in flies) that substantially restrict visual acuity.

RETINAL PATHWAYS

Through Pathway

Once a rod or cone has transduced the light to a change in membrane potential, it then conveys this by its synapse onto a bipolar cell, with one group of bipolar cells receiving rod information and another receiving cone information. The bipolar cells are so named because they have processes going in two directions from their cell body. Their dendrites receive information from photoreceptors, and their axons transmit information to the ganglion cells. Bipolar cells thus span the inner half of the retina (see Fig. 1), from the outer plexiform layer (OPL; where the photoreceptors form their synaptic contacts onto bipolar and horizontal cells) to the inner plexiform layer (IPL; where the bipolar cells form synaptic contacts onto amacrine and ganglion cells). The cell bodies of the bipolar cells comprise most of the inner nuclear layer (INL), while the cell bodies of the photoreceptors comprise the outer nuclear layer (ONL) [4, 8].

Bipolar cells then synapse onto retinal ganglion cells in the IPL. The ganglion cell bodies comprise the ganglion cell layer (GCL), and their axons make the nerve fiber layer (NFL).

This straight-through pathway also involves processing of the visual information. A significant part of this for the cone bipolar cells is to convert the signals into on and off signals, meaning responses to light onset or to light offset. So, half the bipolar cells

are dedicated to transmitting an increase in signal when there is an increase in light in the region they subtend, while the other half of the cone bipolar cells convey a larger signal when there is a decrement in the light in the region they subtend. The first group is called ON bipolar cells, and the latter group is termed OFF bipolar cells simply to indicate they turn on or off with light, respectively. This on-and-off feature is conserved in transmission to the ganglion cells, so they also exhibit the same types of signals [4, 8, 9].

It is the ON pathway that facilitates our perception of light on a dark background; the OFF pathway provides the perception of dark images on a light background. These parallel channels to convey visual information are key in providing one other feature of our senses: the exquisite sensitivity to change. Like our other sensory systems, our visual system has the feature of observing any change in the visual image. One can readily observe this by staring at a constant image for tens of seconds. As you stare at one point in the image, the periphery gradually disappears and with time even the central portion of the image will fade. Anyone who has watched for meteors is especially aware of this; when you fixate on one star, all the others gradually disappear, but when a meteor (or airplane) crosses the sky it is vividly seen. Indeed, in reference to another point, when we then turn to look directly at the meteor, we see that it is much dimmer than when we saw it with our peripheral vision [4].

While cone bipolar cells are either ON or OFF, rod bipolar cells are all of the ON variety. Thus, for dim light, we are keenly aware when the light is on or increasing but less so when it is decreasing.

Receptive Fields

The visual field or receptive field of each photoreceptor is quite small. As discussed above, it is about 1 arc minute in size or the size of one pixel on an LCD monitor at normal viewing distance. In the central part of our vision, there is a correspondence of cones with their respective bipolar cells, so that this level of acuity is maintained. Thus, the receptive fields of cone bipolar cells approximate the receptive fields of the cone photoreceptors in the central fovea. But, as one moves further out, more and more cones synapse onto each bipolar cell, resulting in a lessening of acuity with distance from the fovea.

Hence, the size of the receptive fields of bipolar cells is determined by the number of photoreceptors from which they receive input. Cone bipolar cell receptive fields are small, especially in the fovea, while rod bipolar cell receptive fields are much larger. One reason is that cones, in providing high-acuity vision, require each spot of light to be processed for us to maintain that resolution. The rod bipolars, which are used to determine if a dim light is present, combine the signals of many rods to increase the chance of seeing a very small light signal [1, 4].

Lateral Pathway

Now, if we simply had the straight-through pathway from photoreceptors to bipolar cells to ganglion cells, we would have vision that is somewhat grainy, and we would not have the excellent discrimination of edges that is inherent in our vision. The two lateral paths in the retina are essential in rectifying this. The horizontal cells make lateral interactions in the outer retina, while the amacrine cells make lateral interactions in the inner retina.

These laterally oriented cells are inhibitory, meaning that they provide negative feedback, or suppress the signals, to the adjacent cells. Thus, when a horizontal cell receives information of bright light from one photoreceptor, it emits a signal of darkness to the adjacent photoreceptors, lessening their light response. In any constant field of illumination, this simply decreases the overall response. But, in regions where there is a border of light and dark, this has the effect of increasing the lightness at the edge of the light region and increasing the darkness at the edge of the dark region. The result is contrast enhancement created by the outer retina, a feature that has been observed and re-created by many impressionist painters (one may readily call to mind the images of dancers by Edgar Dégas, in which the dark stage has a black line adjacent to a dancer, while the lightness of her arm or leg has a white line at its edge to denote the contrast enhancement perceived by the painter’s visual system).

The other feature these laterally acting cells have is to create receptive fields in the bipolar and ganglion cells that are ON in the center and OFF in the surround for ON bipolar cells (and OFF center, ON surround for OFF bipolar cells). This is referred to as a center-surround receptive field, and it is the type of receptive field present for bipolar cells and ganglion cells. These center-surround receptive fields are due to the action of the horizontal cells, which connect to a large number of photoreceptors and thus make the larger inhibitory “surround” for each bipolar cell receptive field. Thus, the inhibitory feedback of the horizontal cells allows a sharpening of perception, calling to attention the contrasts inherent in our visual scenes [1, 4, 8].

Retinal Ganglion Cells

The retinal ganglion cells, while being the third cell in the visual path (photoreceptor to bipolar to ganglion), also benefit from the lateral processing of the horizontal and amacrine cells. The horizontal feedback occurs in the outer retina, in the OPL, while the amacrine feedback occurs in the IPL. This IPL is a rich network of connections between bipolar, amacrine, and ganglion cells. And, it is spatially organized, with the OFF bipolar cells making synaptic contact in the outer half of the IPL and the ON bipolar cells making synaptic contact onto ganglion cell dendrites in the inner half of the IPL [4, 8, 9].

The ganglion cells that receive signals from ON bipolar cells are also ON center cells, having an OFF surround. The ones with OFF input are OFF center, ON surround ganglion cells. At this point in the retina, the visual information is much more processed, so the information that is transmitted to the vision center of the brain is not only of the color pixels we see, but also of the color contrast, contrast enhancement, and in some animals the information of the movement of objects.

The ganglion cells are the innermost part of the retina; their axons join together at the optic disk and leave the eye by passing through the retina and sclera, creating the optic nerve. This nerve travels to the lateral geniculate nucleus of the thalamus, located in the center of the head.

The ganglion cells are the first visual system cells to have a long axon, and they create action potentials to transmit their visual information from the eye. These action potentials thus carry information from the eye to the thalamus of the brain, where the information is then relayed to the visual cortex, which is located in the back of the brain in an area called the occipital lobe. It is in the visual cortex that we assemble the signals