III. Vision Chapter 6

A. Vision Different animal species depend more on some senses than on others. For humans, vision is the most important sense.

The cornea is the transparent covering that protects the eye The cornea is the transparent covering that protects the eye. The iris contracts the size of the pupil Each iris is so distinctive that an iris-scanning machine can confirm your identity.

The pupil is what allows light in and the lens adjusts to the distance of objects.

The retina contains the receptor cells responsible for vision The retina contains the receptor cells responsible for vision. This is where images are imprinted. There are two kinds of receptor cells in the retina—rods & cones.

Rods are mainly responsible for night vision   Cones allow us to see colors The retina has 3 types of color receptors: red, green, and blue. When we stimulate combinations of these cones, we see other colors.

Interesting Fact: People who are color blind have a malfunction in their cones Unlike our trichromatic vision (3 functioning types of cones), they have either mono or dichromatic vision.

In the case of vision, our sensory adaptation occurs as the sensitivity of rods and cones change according to how much light is available. The process by which rods and cones become more sensitive to light in response to lower levels of illumination is called dark adaptation.

  Problems with dark adaptation are a part of the reason that there is a greater incidence of highway accidents at night! While driving at night, your eyes shift from the darkened interior of the car, to the road illuminated by headlights, to the darker areas of the side of the road. We never completely adapt like we would if in a movie theatre. Adaptation to bright light happens much more quickly than adaptation to dark

Messages from the eye must travel to the brain in order for a visual experience to occur. The optic nerve is responsible for carrying these messages. The blind spot is the point where the optic nerve leaves the eye. There are no photoreceptors (neurons) here.

B. Visual Information Processing As you look at a tiger in the zoo, information enters your eyes, is transduced, and is sent to your brain as millions of neural impulses.

Finally, in some way, these separate teams pool their work to produce a meaningful image, which you compare with previously stored images and recognize: a crouching tiger.

C. Visual Organization 1. Form Perception   Figure and Ground: the perception of figures against a background Among the voices you hear at a party, the one you attend to becomes the figure; all others are part of the ground As you read, the words are the figure, the white paper is the ground.

Grouping: the perceptual tendency to organize stimuli into coherent groups   Proximity: tendency to group together events that are near each other We see three sets of two lines instead of six separate lines

Continuity: tendency to group stimuli into continuous patterns This patter could be a series of alternating semicircles, but we perceive it as two continuous lines—one straight, one wavy.

Closure: the tendency to perceive a complete or whole figure even when there are gaps in what your senses tell you   We assume that the circles are complete but partially blocked by the (illusory) triangle. Add some small line segments to close off the circles and your brain stops constructing a triangle.