Brain - Vision

Senses - Vision

This work was done as a part of Summer Program at Elio Academy of Biomedical Sciences.

Author: Riyaa Sri Ramanathan

Sense organs turn stimuli into electrical signals through a process called transduction. These electrical messages are carried through a network of cells and fibers to specialized areas of the brain. These impulses are then processed and integrated into a seamless perception of the surroundings.

 

Vision is one of the most complicated senses, involving many processes that work simultaneously enabling to see what is around. Vision has been studied intensively, and knowledge of how light energy is converted into electrical signals comes primarily from the studies of fruit flies, Drosophila Melanogaster and mice. Higher Level visual processing has mostly been studied in monkeys and cats.

Beauty of sight, the perception of image and translating it into objects involves the process of vision. And the organ eye does it via several functions. Light passes through the cornea, the most moist area of the eye, and enters the eye through the pupil. The iris regulates how much light enters by changing the size of the pupil. The lens then bends the light so that it focuses on the inner surface of the eyeball, on a sheet of cells called the retina. 

The rigid cornea does the initial focusing, and the lens thickens or flattens to better focus the objects on the retina. Visual input is mapped directly onto the retina as a two-dimensional reversed image. Objects to the right project images onto the left side of the retina and vice versa, while objects above are imaged at the lower part and vice versa. Signals are processed by specialized cells in several layers of the retina as they fall, and such signals travel via the optic nerves to other parts of the brain and undergo further integration and interpretation.

Retina is identified as three-layered consisting of Photoreceptors, Interneurons and Ganglion Cells. These cells communicate extensively with each other before sending information to the brain. The light waves entering through the cornea and the lens travels through the Ganglion Cells and interneurons before reaching the photoreceptors. Ganglion cells and interneurons do not respond directly to light, but rather process and relay the information received from the photoreceptors to the deeper layers of the eye. There are approximately 125 million photoreceptors in each human eye, responsible for turning light into electrical impulses through the process of transduction. 

Light sensitive photoreceptors are the rods and cones located in the peripheral layer of the retina. Rods, which make up about 95% percent of the photoreceptors in the human eye, are more sensitive, and mostly allow us to see images in dim light. Cones, however, pick up fine details and color, allowing to engage in activities that require visual acuity, or sharpness of vision. The human eye contains 3 types of cones, each sensitive to a different range of colors namely, red, green or blue. Three cones have different levels of sensitivity, and as the sensitivities overlap, information about different combinations of colors is conveyed, and allow humans to see the familiar color spectrum. 

The center of the Retina is called the fovea, a small pitted space where cones are more densely packed than other retinal areas. The fovea contains only red or green cones, and can resolve many fine details. Vision is sharper in the center than in the periphery. The area immediately around the fovea, called the macula, is critical for reading and driving, containing mostly red or green cones. The death or degeneration of photoreceptors in the macula is known as Macular Degeneration. In the US and other developed countries, macular degeneration is a leading cause of blindness in people later than 55 years old.

 

 

 

Eyes help in distinguishing shapes, colors, contrasts, speed and direction of movement. Information processes happen as the neurons pass on the inputs as it receives from the preceding layers. Inputs being received vary across the retina with visual acuity being highest in the macular region. As ganglion cells receive information through one or more interneurons from one or more cones, fine details are resolved. Signals received by the ganglion cells are from the photoreceptor cells near the margins of the retina. Convergence of these inputs explains that peripheral vision is less detailed. Portion of visual space providing input to a single ganglion cell is called its receptive field.

Visual processing compares the amount of light hitting on small, adjacent areas on the retina. Receptive fields of individual ganglion cells occupy the retina like a “tile”, providing a complete 2D representation of the visual scene. Receptive fields of retinal cells are activated as light hits a tiny region on the retina, and the region corresponds to the center of its field. 

However the activation is inhibited when the light strikes the donut-shaped area surrounding the center. Ganglion cell responds weakly when the light strikes the entire receptive field — The donut and its hole. In this way, object detection is ideally done as the visual system maximizes the perception of contrast. 

Neural activity taking place in the axons of ganglion cells is transmitted through the optic nerves, which exit at the back of each eye, and travel towards the back of the brain. Photoreceptors are absent, in the area of the retina where the optic nerve leaves the eye. In each eye, the exit point of the optic nerve results in a Blindspot. Area of the blindspot is filled with information from the other eye by the brain’s fortuitousness. Signals traveling along the nerve fibers from both the eyes first converge at a crossover junction called the Optic chiasm. 

Nerve fibers carry information from the left side of the retinas of both eyes to the left side of the brain and information from the right side of both retinas proceeds onto the right side of the brain. Visual information is then relayed to the lateral geniculate nucleus, and then to the primary visual cortex at the back of the brain.

 

 

 

A thin sheet of neural tissue located in the occipital lobe at the back of the brain, not larger than a half dollar is the primary visual cortex. Like the Retina, this region consists of many layers with densely packed cells. The middle layer which receives messages from the thalamus, has receptor fields similar to those in the retina and can preserve the Retina’s visual map. Cells above and below the middle layer have more complex receptive fields. Stimuli are registered with orientations by these cells and pass the information through new processing streams to other parts of the visual cortex. As visual information from the Primary Visual Cortex is combined in other areas, receptive fields become increasingly complex and selective. Neurons at higher levels of processing respond to specific objects and faces. After reaching the occipital lobe, visual signals are fed into several parallel, but interacting processing streams, the dorsal stream, which heads up toward the parietal lobe, and the ventral stream, which heads down to the temporal lobe. These streams were believed to carry out separate processing of unconscious vision, which guides behavior and conscious visual experiences. Recent research suggests that a conscious experience is created between streams.

Brain clearly extracts information at different stages, compares it with past experiences and passes to higher levels processing just to recognize an image.

Seeing with two eyes, known as binocular vision, allows humans to perceive depth or three dimensions. For a clear 3D image, pairs of eyes should be equally active, properly aligned and visual fields should overlap. Information from the perspective of each eye is processed and preserved in the primary visual cortex. The benefit of a pair of eyes is that, larger visual field is mapped on the primary visual cortex. Nerve fibers exiting each eye, crossing over at the optic chiasm, causes signals from the left visual field to be mapped to the right and vice versa.

Author: Riyaa Sri Ramanathan

This work was done as a part of Extended Research Program at Elio Academy of Biomedical Sciences.