Visual Sensation

  What is Visual Sensation?       

            The sensory experience which brings into awareness objects in the environment through the act of seeing is referred to as vision or visual sensation. The sense organ concerned with this is the human eye. The stimuli for visual sensation are the light rays.

Mechanism of Visual Sensation:

Light rays from external objects impinge on the human eye. The human eye is more or less similar to a camera. Just as the camera has a lens, the human eye also has a lens. The light rays pass through the lens and strike the inner layer of the eye known as the retina. The retina is comparable to the film in the camera. Just as in the camera, the area of exposure is controlled by varying the aperture in the lens; the lens in the human eye can also expand or contract through the actions of a set of muscles known as the ciliary muscles. The ciliary muscles act depending on the intensity of the light rays and thereby regulate the functioning of the lens.

The retina:

            The retina is the vitally sensitive part of the eye and receives light stimuli. The retina is made up of two types of neural structures known as rods and cones. The rods are elongated structures and are found in greater number in the peripheral or outer region of the retina, whereas the cones are rather conical in shape and are in large numbers in the central part of the retina.  The rods are sensitive to achromatic (black and white) light rays while the cones are sensitive to chromatic (colour) light rays. The retina actually is a continuation of a very important nerve, the optic nerve. The optic nerve which opens out as the retina at the rear end of the human eye, carries the visual stimulations from the retina to the occipital lobe which is situated at the hind-side of the brain. The visual system, therefore, consists of the occipital lobe, the optic nerve and the rods and cones in the retina. At the spot where the optic nerve enters the eye and opens out as the retina there are no rods or cones. This part is known as the blind spot. Any light stimulus reaching this part is not sensed. There is another point in the retina, the central spot known as the fovea centralis. This is the spot of maximum sensitivity and light stimuli striking this point enjoy the clearest vision.

Dimensions of Light:

            There are 3 dimensions of light hue, saturation and Brightness.

Hue: It refers to the color quality of the light and corresponds to the color names that we use such as yellow, green, red etc.

Saturation: It refers to the purity of the light. The more the saturated the stimulus, the stronger the color experience and the less saturated the more it appears white or grey or blackish. Pink is the combination of red light and white light. The more white light is added the less ‘’red” the pink is. Eventually the red may be so overwhelmed by the white that we barely notice the pink at all.

Brightness: It refers to the amount of light present. The more bright an object is, the easier it is to see and to notice the colors.  

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