The Marvel of Human Vision

Introduction

Indeed, there manifold marvels of nature that grace life on earth and always resuscitate endless debates behind the origin of the earth. As a matter of fact, it the marvel of life on the planet earth is pervasive to an extent that all that are artificial are modeled after the natural design. For instance, the state monopoly of force (the army, the police, and the prison systems) is fashioned after the immune system, while the camera, the computer, and the television, after the eye. Among these marvels, the human eye remains of immense and complicated beauty. The manner in which the eye works together with the brain is a marvel that is, therefore, to be discussed forthwith.

The brain works together with the eye to see objects through the means of a visual system. This visual system allows details from the environment to be translated. This process starts with the lens and the retina and ends in the cortex which is in the brain.

Schaeffel divulges that the process starts when the eye focuses light on its retina. From this juncture, the light is absorbed by one of the layers of the photoreceptor cells. It is these cells which convert the absorbed light into electrochemical signals. These electronic signals are divided into two cones and rods. The rod cells strengthen the night vision by giving a good response to dim light. These rods are mainly situated in the retina. It is because of this that people realize a better and sharper view when focusing their gaze on the side of the object that is being observed.

On the other hand, cone cells are concentrated in the fovea, the central region in the retina. Cones are responsible for high acuity tasks such as reading and color detection. Cones are divided into three types, depending on the manner in which they respond to blue, green and red light.

McLeod argues the signals relayed to the photoreceptor pass through interneuron situated in the second layer of the retina. The signal passes through to ganglion cells which are in the third layer. The same neurons in the two retinal layers readily exhibit the receptive fields. These receptive fields enable the neurons to read the changes in contrast in an image. These changes show the shadows and edges of an object. From this point, it is the ganglion cells which collect information along and send the same information to the brain, via the optic nerve. The information about color is one among those that are sent into the brain.

According to Merleau-Ponty, the optic nerve on the other hand primarily directs information through the thalamus and then to the cerebral cortex. It is in this cerebral cortex where the visual perception takes place. The nerve also carries information which is necessary for the sustenance of the mechanics of vision into two sites (pretectum and the superior colliculus) which are in the brainstem. The pretectum in respect and response to light intensity controls the papillary size. It is at this juncture that details which govern scanning by the eyes to the superior colliculus. The superior colliculus, on the other hand, takes responsibility for the saccades (the movement of the eye in short jumps). The saccades from this juncture permit the brain to detect or sense smooth scans by piecing together spates of images that are relatively still. The saccadic eye movements solve any problem that is related to extreme blurring.

Similarly, most of the projections travel from the retina through the optic nerve to the lateral geniculate nucleus (LGN). The LGN is a thalamus which is situated deep in the center of the brain. The LGN also separates the input if the retina into parallel streams; so that whereas one contains color and fine structure, the other contains motion and contrast. Those cells that process the fine structure and color comprise top four of the six LGN layers. The four layers are called parvocellular layers, given that their cells are small. Conversely, those cells that process motion and contrast account for the LGN bottom two layers so that they are known as magnocellular layers, due to their large cells.

Pinker explains that the cells of the parvocellular and magnocellular layers make projections as far as from the back of the brain, to the primary visual cortex (PVC). The cells in the PVC are arranged in many ways to allow the calculation of the location of objects in space by the visual system. The PVCs are arranged retinotopically so that the point to point map which is extant between the primary visual cortex and the retina and the neighboring areas of the PVC and the retina is acknowledged. The same allows the PVC to place objects in two dimensions in the visual world- the vertical and the horizontal. The depth which is the third dimension is mapped in the PVC. All these signals are processed in the ocular dominance columns which are made up of a stack of cells.