The Visual System (131-163)
Light Enters the Eye and Reaches the Retina
- From the tiny, distorted, upside-down, two-dimensional retinal images projected on the visual receptors that line
the backs of the eyes, the visual system creates an accurate, richly detailed, three-dimensional perception.
Understanding the visual system requires the integration of two types of research, that, that probes the visual
system with sophisticated neuroanatomical,neurochemical, and neurophysiological techniques, and research that
focuses on the assessment if what we see.
- (The Case of Mrs.Richards Fortification Illusions and the Astronomer) Mrs. Richards suffered from migraine
heaches and often experienced visual displays, called fortification illusions, prior to her attack. When she drew
her illusions she noticed their similarity to astonomer George Biddell Airy’s drawings.
- Some animals have special adaptations that allow them to see under very dim illumination but no animal can
see in complete darkness. Light can be thought of as discrete particles of energy, called photons , traveling
through space at about 300,000km per second, or as waves of energy. Light is sometimes definaed as waves of
electromagnetic energy that are between 380-760nm in length. Wavelength plays an important role in the
perception of colour, and intensity plays an important role in the perception of brightness.
- The amount of light reaching the retinas is regulated by the donut-shaped bands of contractile tissue, the irises
which gives our eyes their characteristic colour. Light enters the eye through the pupil the hole in the iris. The
adjustment of pupil size in response to changes in illumination represents a compromise between sensitivity (the
ability to detect the presence of dimly lit objects) and acuity (the ability to see the details of objects). when
illumination is high, the pupils contact and the image falling on each retina is sharper and there is a greater depth
of focus. When illumination is too low to adequately activate the receptors, the pupils dilate to let in more light,
thereby sacrificing acuity and depth of focus.
- Behind each pupil is a lens, which focuses incoming light on the retina. When we direct our gaze at something
near, the tension on the ligaments holding each lens in place is adjusted y the ciliary muscles, and the lens
assumes its natural cylindrical shape. The movements of your eyes are coordinated so that each point in your
visual world is projected to corresponding points on your two retina, therefore our eyes must converge. The
positions of the images on your twp retinas can never correspond exactly because your two eyes do not view the
world from exactly the same position. Binocular disparity is the difference in the position of the same image on
the two retins, this is greater for close objects than for distant objects; therefore your visual system can use the
degree of binocular disparity to construct one 3-D perception from two two-dimensional retinal images.
The Retina and Translation of Light into Neural Signals.