CHAPTER 4 SECTION 2 The Visual System Goal

CHAPTER 4 SECTION 2 The Visual System Goal Two: Explain how the visual system enables us to see, and by communicating with the brain, to perceive the world.

The Visual Stimulus and the Eye � � Light is a form of electromagnetic energy. Light travels through space in waves. The wavelength of light is the distance from the peak of one wave to the peak of the next. Amplitude is the height of the wave and it is associated with the brightness of a visual stimulus. Purity is the mixture of wavelengths in light.

The Structure of the Eye � � The eye is set up like a camera in that it gets the picture of the world. The sclera is the white outer part of the eye that gives the eye its shape and protects the eye from injury. The iris is the colored part of the eye. The pupil, which appears black, is the opening in the center of the iris. The iris contains muscles that allow the pupil to get larger or smaller depending on how much light is being let into the eye.

The Structure of the Eye � � The cornea is the clear membrane on the outer part of the eye. The curved surface on the cornea bends light on the surface of the eye in order to focus it to the back of the eye. The lens is transparent and somewhat flexible. When a person is looking at an object far away, the lens has a relatively flat shape. However, when a person is looking at an object that is closer, more bending of the light is needed.

Structure of the Eye © 2011 The Mc. Graw-Hill Companies, Inc.

Structure of the Eye: Retina Rods � sensitive to even dim light, but not color � function well in low illumination � humans have ≈ 120 million rods Cones � respond to color � operate best under high illumination � humans have ≈ 6 million cones © 2011 The Mc. Graw-Hill Companies, Inc.

Structure of the Eye: Retina Fovea densely populated with cones vital to many visual tasks Bipolar and Ganglion Cells Optic Nerve Blind Spot where optic nerve leaves the eyeball perception involves top-down processing © 2011 The Mc. Graw-Hill Companies, Inc.

Structure of the Eye: Retina © 2011 The Mc. Graw-Hill Companies, Inc.

Visual Processing Pathway of Visual Information � optic nerve optic chiasm … visual cortex Optic Chiasm: Optic Nerve Fibers Divide � left visual field right hemisphere � right visual field left hemisphere Primary Visual Cortex � occipital lobe � initial visual processing © 2011 The Mc. Graw-Hill Companies, Inc.

Visual Processing © 2011 The Mc. Graw-Hill Companies, Inc.

Visual Processing Feature Detectors � highly-specialized cells in the visual cortex � size, shape, color, movement, or combination � deprivation studies: brain “learns” perception Parallel Processing Binding © 2011 The Mc. Graw-Hill Companies, Inc.

Parallel Processing � � � What” and “where” are two questions that need to be answered in order for people to respond appropriately to a visual stimulus. The “what” pathway is in the temporal lobe and processes information about what the object is. The “where” pathway is located in the parietal lobe and processes information about an object’s location. Parallel processing is a simultaneous distribution of information across different neural pathways. It helps information move rapidly through the brain.

Binding � Binding is when the different pathways and cells bring together and integrate information. � Through binding a person can integrate information about various parts of an object. For example, if a person sees a chair, then through binding they not only see just the chair, but they also see the size, the color, the motion, etc. of the chair.

Color Vision: Theories Trichromatic Theory Three Types of Receptors � green, red, and blue cones Color Blindness � one or more cone types is inoperative © 2011 The Mc. Graw-Hill Companies, Inc.

Color Vision: Theories � � The term color blind refers to seeing some colors but not others. Color blindness depends on which of three kinds of cones (green, red, and blue) is not working. The opponent-process theory states that cells in the visual system respond to red-green and blue-yellow colors. A cell excited by red and green could be inhibited, or a cell excited by blue and the yellow could be inhibited.

Color Vision: Afterimages © 2011 The Mc. Graw-Hill Companies, Inc.

Color Vision: Theories Afterimage � sensation remains after a stimulus is removed � trichromatic theory cannot explain afterimages Opponent Process Theory � complementary color pairs © 2011 The Mc. Graw-Hill Companies, Inc.

Perceiving Shape, Depth, Motion, and Constancy � � � To perceive a visual stimulus, the fragments of information that the eye sends to the visual cortex must be organized and interpreted. The figure-ground relationship occurs when a person organizes the perceptual field into stimuli that stand out (figure) and those that are left over (ground). Gestalt psychology explains how people naturally organize their perceptions according to certain patterns. Closure is a gestalt principle; when a person sees a disconnected or complete figure, they see a whole. Proximity is a second gestalt principle; when individuals see objects as close to each other, they tend to group them as together. The third gestalt principle is similarity; when objects are similar, individuals tend to group them together.

Visual Perception � organizing and interpreting visual signals � dimensions - shape - depth - motion - constancy © 2011 The Mc. Graw-Hill Companies, Inc.

Visual Perception: Shape Gestalt Psychology perceptions are naturally organized according to certain patterns � whole is different from the sum of the parts � Gestalt Principles figure-ground relationship � closure � proximity � similarity � © 2011 The Mc. Graw-Hill Companies, Inc.

Figure-Ground Relationship © 2011 The Mc. Graw-Hill Companies, Inc.

Visual Perception: Shape © 2011 The Mc. Graw-Hill Companies, Inc.

Visual Perception: Depth � the brain constructs perception of 3 D from 2 D images processed by the retina � binocular cues - disparity - convergence © 2011 The Mc. Graw-Hill Companies, Inc.

Depth Perception � � � Depth perception is the ability to see objects in three dimensions. Binocular cues are depth cues that depend on the combination of the images in the left and right eyes and on the way the two eyes work together. For example, if a person holds their hand over one eye and focuses on an object and then switches to cover their other eye, the switching back and forth between the eyes will cause the object to jump back and forth. Monocular cues are depth cues that are available from the image in one eye, either the left or the right eye. Some examples of monocular cues are: familiar size, height in the field of view, linear perspective, overlap, shading, and texture gradient.

Visual Perception: Depth Monocular Cues – Pictorial Cues familiar size height in the field of view linear perspective overlap shading texture gradients © 2011 The Mc. Graw-Hill Companies, Inc.

Monocular Cues: Shading, Texture Gradient © 2011 The Mc. Graw-Hill Companies, Inc.

Monocular Cues: Linear Perspective, Height in Field © 2011 The Mc. Graw-Hill Companies, Inc.

Visual Perception: Motion � humans have specialized motion detectors � apparent movement © 2011 The Mc. Graw-Hill Companies, Inc.

Visual Perception: Motion © 2011 The Mc. Graw-Hill Companies, Inc.

Visual Perception: Constancy Perceptual Constancies � recognition that objects do not physically change despite changes in vantage point and viewing conditions � sensory information (retinal image) changes, but perceptual interpretation does not Size, Shape, and Color Constancies © 2011 The Mc. Graw-Hill Companies, Inc.

Motion Perception � � Apparent motion occurs when an object is stationary but it is perceived as moving. An example of apparent motion is watching an IMAX movie. Two forms of apparent motion are stroboscopic motion and movement aftereffects. Perceptual constancy refers to the recognition of objects as remaining stationary and unchanging even though sensory input about them is changing. There are three types of perceptual constancy: size constancy, shape constancy, and brightness constancy.