Sight

For information on how the eye works, seeEye.

Thedorsal stream(green) andventral stream(purple) are shown.

Sight(also calledeyesightorvision) is one of thesenses.Having sight means to be able tosee.Seeing givesanimalsknowledgeof theworld.Some simple animals can only tell light from dark, but with vertebrates, the visual system is able to formimages.

The ability to interpretvisible lightinformation reaching theeyesis calledvisual perception.Sight is the resultingperception.The components that are necessary for vision are known as thevisual system.

Process

Light enters the animal'seyes,and a part of the eye called thelenssends information from the light to the back part of the eye called theretina.The retina is composed of light-sensitive cells which fire a signal down theoptic nervewhen light hits the cell. The optic nerve is a bundle ofnerve fibresfrom all over the retina.

When the information from the light leaves the retina, it goes to the brain. It travels along theoptic chiasmauntil it reaches theoptic cortexat the rear of the brain. The information is then processed to find out the shapes and colours of objects. From that and frommemory,it can tell of what kind the object is. For example, it can somehow tell atreefrom ahouse.The path on which this kind of information flows is calledventral stream.

The brain can also tell where objects are. For example, it can tell how far away an object is (this is calleddepth perception). This is needed when catching a ball. The path on which this kind of information flows is calleddorsal stream.

What is sense of sight

The major problem in visual perception is that what people see is not simply a translation of an image on theretina.For one thing, we see the world right side up, even though the image on the retina is upside down (because it has passed through the lens). Therefore, it is difficult to explain what happens to create what we actually see. The key, which took centuries to be appreciated, is that the brain works on thedatafrom the eyes, and marries it with memories and guesses, all at lightning speed. The result is an experience of the world which looks to each person as if it were simplereality.However, although based on reality, it is actually a mental construct, built by the brain.

History

Many figures in the ancient world had ideas about vision.Plato,Aristotle,Euclid,PtolemyandGalenall had their ideas, but most of these ideas werespeculation.They were not based on anyscientific method.

Alhazen(965–c. 1040) carried out investigations andexperimentson visual perception. He extended the work of Ptolemy onbinocular vision,and commented on the anatomical works of Galen.

Leonardo da Vinci(1452–1519) is believed to be the first to recognize the special optical qualities of the eye. He wrote "The function of the human eye... was described by a large number of authors in a certain way. But I found it to be completely different". His main experimental finding was that there is only a distinct and clear vision at the line of sight, the optical line that ends at the fovea. Although he did not use these words literally he actually is the father of the modern distinction between foveal andperipheral vision.

Hermann von Helmholtzexamined the human eye and concluded that it was, optically, rather poor. The poor-quality information gathered by the eye seemed to him to make vision impossible. So he thought vision could only be the result of some form ofunconsciousinferences.As well as information from the eyes, the brain used information from previous experiences. The world as experienced is built up from assumptions and conclusions from incomplete data, using prior experience of the world.

Examples of well-known assumptions, based on visual experience, are:

• light comes from above
• objects are normally not viewed from below
• faces are seen (and recognized) upright.
• closer objects can block the view of more distant objects, but not vice versa
• figures (i.e., foreground objects) tend to have convex borders

The study ofvisual illusions(cases when the inference process goes wrong) has yielded much insight into what sort of assumptions the visual system makes.

Related pages

• Optic tectum
• Neuroscience
• Ophthalmology
• Color blindness

Other websites

• Visual Perception 3 - Cultural and Environmental Factors
• Gestalt Laws
• The Organization of the Retina and Visual System
• Dr Trippy's SensoriumArchived2007-05-29 at theWayback MachineA website dedicated to the study of the human sensorium and organisational behaviour