Optometry

Optometry is the study of eyes. It deals with visual systems and visual information processing.

- The iris consists of pigmented tissue known as stromae. It is the most forward portion of the eye and the only one seen on superficial inspection. The stroma connects a sphincter muscle, which contracts the pupil, and a set of dilator muscles which open it. The high pigment content blocks light from passing through the iris and restricts it to the pupil. Depending on the amount of light, the iris makes the pupil larger or smaller and is similar in function to the iris of a camera.

 - The lens is a transparent structure in the eye that helps to focus light on to the retina. The lens, by changing shape, functions to change the focal distance of the eye so that it can focus on objects at various distances, thus allowing a sharp image of the object of interest to be formed on the retina. This function is similar to the focusing of a photographic camera via the adjustment of its lens.

-The vertebrate Retina is a light sensitive part inside the inner layer of the eye. The retina contains three types of light-sensitive photoreceptor cells: rods, cones, and ganglions. Two of these three types of photoreceptor cells, rods and cones, receive light and transform it into image-forming signals which are transmitted through the optic nerve to the brain. In this way, the retina is to the eye what film is to a camera.

-The Optic Nerve is composed of retinal ganglion cell axons and Portort cells. It leaves the eye via the optic canal running towards the optic chiasm where there is a partial crossing of fibers from the nasal visual fields of both eyes. From the lateral geniculate body, fibers of the optic radiation pass to the visual cortex in the occipital lobe of the brain. The opticnerve contains 1.2 million nerve fibers. The fibers from the retina run along the optic nerve to nine primary visual nuclei in the brain, whence a major relay inputs into the primary visual cortex.

-The Primary Visual Cortex is the best studied visual area in the brain. In all mammals studied, it is located in the
posterior pole of the occipital cortex (the occipital cortex is responsible for processing visual stimuli). It is the simplest, earliest cortical visual area. It is highly specialized for processing information about static and moving objects and is excellent in pattern recognition. From the visual cortex information is disseminated to other lobes of the brain.

-The Parietal Lobe is positioned above the occipital lobe and behind the frontal lobe. The parietal lobe integrates sensory information from different modalities, particularly determining spatial sense and navigation. This enables regions of the parietal cortex to map objects perceived visually into body coordinate positions.

-The Temporal Lobes are parts of the brain that are involved in speech, memory, and hearing. The temporal lobe is involved in auditory processing and is home to the primary auditory cortex. It is also heavily involved in semantics both in speech and vision. The temporal lobe is also involved in memory formation.

-The Hippocampus is a part of the brain located in the medial temporal lobe. It belongs to the limbic system and plays major roles in short term memory and spatial navigation.

-The Amygdalae are almond-shaped groups of neurons located deep within the medial temporal lobes of the brain in complex vertebrates, including humans. They are part of the limbic system that are shown in research to perform a primary role in the processing and memory of emotional reactions.

 

The Retina seen in the image above is located to the rear of the eye.                                                                    A human eye can have 120 million rods compared to an Owl which has over 10 billion.

It can distinguish over 10 million different colors, and is capable of                                                                       The average person’s eye has over 6 million cones. In the Image above The longer

seeing a single photon of light.                                                                                                                                hairlike nerves are the rods and the shorter sprout like nerves are the cones.  

 

Rods cannot distinguish colors, but are responsible for low-light black and white (scotopic) vision; they work well in dim light as they contain a pigment, visual purple, which is sensitive at low light intensity, but saturates at higher intensities. Rods are distributed throughout the retina. Rod density is greater in the peripheral retina than in the central retina.

Cones are responsible for seeing color. They require brighter light to function than rods require. There are three types of cones, maximally sensitive to long-wavelength, medium-wavelength, and short-wavelength light (often referred to as red, green, and blue, respectively, though the sensitivity peaks are not actually at these colors). The color seen is the combined effect of stimuli to, and responses from, these three types of cone cells. Cones are mostly concentrated in and near the fovea. Only a few are present at the sides of the retina. Objects are seen most sharply in focus when their images fall on this spot, as when one looks at an object directly. Cone cells and rods are connected through intermediate cells in the retina to nerve fibers of the optic nerve. When rods and cones are stimulated by light, the nerves send off impulses through these fibers to the brain.